INVITED REVIEW
Regulation of cardiac and smooth muscle Ca²⁺ channels (Ca_V1.2a,b) by protein kinases

¹ Department of Physiology and Cell Biology and ² Department of Pharmacology, University of Nevada School of Medicine, Reno, Nevada 89557

	ABSTRACT

TOP ABSTRACT INTRODUCTION STRUCTURE OF CARDIAC AND... PKG REGULATION OF CA2+... PKA REGULATION OF CA2+... PKC REGULATION OF CAV1.2... G PROTEIN SUBUNIT REGULATION... REGULATION OF CAV1.2 CHANNELS... REFERENCES

High voltage-activated Ca²⁺ channels of the Ca_V1.2 class (L-type) are crucial for excitation-contraction coupling in both cardiac and smooth muscle. These channels are regulated by a variety of second messenger pathways that ultimately serve to modulate the level of contractile force in the tissue. The specific focus of this review is on the most recent advances in our understanding of how cardiac Ca_V1.2a and smooth muscle Ca_V1.2b channels are regulated by different kinases, including cGMP-dependent protein kinase, cAMP-dependent protein kinase, and protein kinase C. This review also discusses recent evidence regarding the regulation of these channels by protein tyrosine kinase, calmodulin-dependent kinase, purified G protein subunits, and identification of possible amino acid residues of the channel responsible for kinase regulation.

cGMP-dependent protein kinase; cAMP-dependent protein kinase; protein kinase C; G proteins

	INTRODUCTION

TOP ABSTRACT INTRODUCTION STRUCTURE OF CARDIAC AND... PKG REGULATION OF CA2+... PKA REGULATION OF CA2+... PKC REGULATION OF CAV1.2... G PROTEIN SUBUNIT REGULATION... REGULATION OF CAV1.2 CHANNELS... REFERENCES

CA_V1.2 channels are members of the superfamily of voltage-dependent Ca²⁺ channels that includes low voltage-activated, rapidly inactivating channels and high voltage-activated channels. The nomenclature for voltage-dependent Ca²⁺ channels has recently changed from alphabetic descriptors, i.e., L-, N-, P/Q-, and R-type, to names based on a system previously developed to describe K⁺ channels. This system uses numerals to define families and subfamilies of channels based on similarities in amino acid sequences. Thus the Ca_V1 family includes Ca²⁺ channel ₁-subunits encoded by four separate genes that give rise to Ca_V1.1, Ca_V1.2, Ca_V1.3, and Ca_V1.4 (formerly _1S, _1C, _1D, and _1F, respectively) (see Ref. 43). This review focuses specifically on signaling pathways involved in the regulation of cardiac and smooth muscle Ca_V1.2 channels. Ca_V1.2 channels in both cardiac and smooth muscle contribute to contraction by delivering extracellular Ca²⁺ to the cytoplasm and as a trigger for Ca²⁺-induced Ca²⁺ release. A number of excellent reviews have been published in recent years on Ca²⁺ channels (7, 37, 40, 55, 73, 74, 85, 114, 162, 163). The specific focus of this review, therefore, is on the most recent advances in our understanding of Ca_V1.2a and Ca_V1.2b channel regulation in cardiac and smooth muscle by cGMP-dependent protein kinase (PKG), cAMP-dependent protein kinase (PKA), and protein kinase C (PKC).

	STRUCTURE OF CARDIAC AND SMOOTH MUSCLE CA2+ CHANNELS

TOP ABSTRACT INTRODUCTION STRUCTURE OF CARDIAC AND... PKG REGULATION OF CA2+... PKA REGULATION OF CA2+... PKC REGULATION OF CAV1.2... G PROTEIN SUBUNIT REGULATION... REGULATION OF CAV1.2 CHANNELS... REFERENCES

The Ca²⁺ channels examined in this review are multisubunit protein complexes composed of a pore-forming ₁-subunit and several auxiliary subunits including an intracellularly located -subunit and an extracellularly located, disulfide-linked ₂/-subunit (Fig. 1). The cardiac and smooth muscle ₁-subunits are splice variants of the same gene and are designated Ca_V1.2a and Ca_V1.2b, respectively. Ca_V1.2a and Ca_V1.2b exhibit 93% homology in amino acid sequence. The molecular mass of each is ~240 kDa, and each ₁-subunit contains ~2,170 amino acids (11, 37, 73, 81, 90, 92, 119, 160, 185). The ₁-subunit defines the ionic pore of the channel and contains four repeats, each with six transmembrane segments. Cardiac and smooth muscle ₁-subunits are distinguished by a minor difference in the NH₂ terminus, in the hydrophobic segments IS6 and IVS3, and by an insertion in the I-II cytoplasmic linker in the smooth muscle subunit (Fig. 1) (9, 37, 92, 119). Splice variants of Ca_V1.2 have been reported to exist within a single tissue (10, 44). An interesting new twist in the story are recent reports suggesting that the COOH-terminal end of Ca_V1.2a is cleaved in vivo but remains colocalized with the "body" of the ₁-subunit. A proline-rich domain between residues 1973 and 2001 of the ₁-subunit has been identified as the mediator of the COOH-terminal membrane association (49, 51). Because the COOH-terminal region suppresses Ca_V1.2 channel activity (182), the channel could be regulated by changes in the association of the COOH-terminal fragment with the body of the ₁-subunit.

View larger version (32K): [in this window] [in a new window]   Fig. 1.   Model of the cardiac and smooth muscle CaV1.2 channel (1) and associated auxiliary subunits. In addition to the pore-forming 1-subunit, there is an intracellularly localized -subunit and an extracellularly located, disulfide-linked 2/-subunit. Sites of divergence between the cardiac and smooth muscle 1-subunits are indicated at the NH2 terminus, in the hydrophobic segments IS6 and IVS3 and by an insertion between I and II cytoplasmic linker. Evidence points to a functional role for cGMP-dependent protein kinase (PKG) phosphorylation of CaV1.2a at serine 533 (82). The same PKG phosphorylation site exists in CaV1.2b (i.e., serine 528). Evidence also suggests a functional role for cAMP-dependent protein kinase (PKA) phosphorylation of CaV1.2a at serine 1928 (34, 50, 131). The same PKA phosphorylation site exists in CaV1.2b (i.e., serine 1923). Additional studies suggest that phosphorylation of serine residues 478 and 479 on the 2a-subunit also are involved in functional regulation of channels by PKA (18). Although ~18 potential protein kinase C (PKC) phosphorylation sites exist on the 1-subunit, no studies to date have clearly identified a role for these sites in channel activation. However, recent studies suggest that threonine-27 and threonine-31 on CaV1.2a are involved in PKC-induced inhibition of the channel (115). These sites are not present on CaV1.2b.

Four different genes encode mammalian voltage-gated Ca²⁺ channel -subunits (₁-₄; Ref. 11). In addition, each of the four gene products can be alternatively spliced, giving rise to two to four splice variants per -subunit. Overall, these -subunits vary from ~53 to 71 kDa (for review, see Refs. 12 and 37). In cardiac muscle the ₂-subunit predominates (49, 59, 77, 134), whereas in smooth muscle at least three different -subunits (i.e., _1b, ₂, and ₃, 54-68 kDa molecular mass) have been identified (10, 31, 54, 77, 176). In contrast to the ₁-subunit, the -subunit does not contain putative transmembrane domains, although there are hydrophobic regions. The -subunit tightly binds to a highly conserved 18-amino acid sequence in the cytoplasmic linker between repeats I and II of the ₁-subunit (136). -Subunits target the ₁-subunit to the plasma membrane (12, 48, 53, 196) and facilitate Ca_V1.2 channel currents (12, 32, 53, 76, 122, 154). The _2a-subunit is unique in that it is posttranslationally palmitoylated by addition of a 16-carbon palmitic acid group to cysteine residues of the -subunit through a labile thioester linkage (26, 27). This palmitoylation confers unique regulatory properties on Ca_V1 channels (138), although it is uncertain whether such palmitoylated subunits actually exist in either cardiac or smooth muscle cells.

The ₂/ complex consists of an extracellularly located ₂-subunit linked via a disulfide bond to a membrane-spanning -subunit. The ₂ and  proteins are encoded by a single gene (35). ₂/-Subunits (~175 kDa) have been identified in both cardiac (37, 49, 183) and smooth muscle (3). This subunit appears to increase Ca²⁺ channel currents (91, 154) but also has been associated with prevention of prepulse facilitation (33).

	PKG REGULATION OF CA2+ CHANNELS

TOP ABSTRACT INTRODUCTION STRUCTURE OF CARDIAC AND... PKG REGULATION OF CA2+... PKA REGULATION OF CA2+... PKC REGULATION OF CAV1.2... G PROTEIN SUBUNIT REGULATION... REGULATION OF CAV1.2 CHANNELS... REFERENCES

PKG catalyzes the phosphorylation of a number of intracellular proteins that modulate muscle contraction. Two main types of PKG are present in eukaryotic cells, type I and type II (104). Type I PKG is a homodimer, and each subunit has a mass of ~78 kDa. Type II PKG also exists as a dimer whose subunit mass is ~86 kDa. The regulatory and catalytic domains of this molecule are both contained within a single polypeptide sequence. PKG type I is widely distributed and is isolated from soluble extracts of tissues, whereas PKG type II is a particulate form of the enzyme and has a limited tissue distribution. PKG type I is present in both cardiac and smooth muscle cells, although the level of PKG is much greater in smooth muscle than in cardiac muscle (106). This difference is attributable to some degree to a developmental decline in PKG levels from newborn to adult heart (95).

Role of PKG in Cardiac Muscle

In cardiac muscle there is continued controversy concerning the role of the cGMP/PKG pathway in the regulation of myocardial function (94). Although a number of studies have reported cGMP/PKG-mediated inhibition of Ca_V1.2a channels (13, 61, 64, 82, 103, 116, 145, 169, 170, 179), others have reported the opposite effect, particularly when cAMP levels are elevated (62, 88, 96, 128, 145). However, in most cases where stimulation has been observed, the proposed mechanism was not direct PKG-mediated activation of Ca_V1.2a but, rather, indirect mechanisms. For example, cGMP inhibits phosphodiesterase 3 (PDE3) (177). A reduction in PDE3 activity could lead to enhanced cAMP levels and, consequently, Ca²⁺ channel stimulation (88, 128). However, in the case of newborn rabbit ventricular cells, the stimulatory effect of cGMP on Ca_V1.2a appeared to be directly related to PKG, because it was abolished by the PKG inhibitor KT-5823 but not by the PKA inhibitor 5-22 or the PDE inhibitor IBMX (96).

Three different mechanisms have been suggested to account for cGMP-induced inhibition of Ca_V1.2 channels, i.e., 1) direct phosphorylation of the channel by PKG, 2) PKG-induced activation of a phosphatase, leading to dephosphorylation of the channel, and 3) cGMP stimulation of phosphodiesterase 2 (PDE2), leading to a reduction in cAMP levels. The implication of the second and third possibilities listed is that cGMP reduces the phosphorylation of Ca_V1.2 produced by PKA. Each of these possibilities is discussed below and is illustrated in Fig. 2.

View larger version (27K): [in this window] [in a new window]   Fig. 2.   Mechanisms proposed for cGMP-induced inhibition of CaV1.2 channels. In this schematic, guanylyl cyclase (G cyclase) is activated by nitric oxide (NO) to give rise to an increase in cGMP levels. cGMP then activates PKG to directly phosphorylate the 1-subunit of the Ca2+ channel. Another possibility is that PKG activates a protein phosphatase (PP), which then dephosphorylates the channel. Finally, cGMP may stimulate phosphodiesterase 2 (PDE2), which reduces cAMP levels and thus reduces stimulation of the channel by PKA. AKAP, A-kinase anchoring protein; A cyclase, adenylyl cyclase.

Evidence to support the notion that PKG directly phosphorylates Ca²⁺ channels has been derived from functional studies of Ca_V1.2a channels expressed in oocytes (82). In this study, PKG was activated by addition of the membrane-permeant species 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP). This cyclic nucleotide inhibited wild-type Ca²⁺ channel currents. In contrast, when serine residue 533 of the ₁-subunit was replaced with alanine, 8-Br-cGMP no longer had an effect on channel activity, suggesting that PKG produces inhibition by phosphorylating the channel at serine 533 (82). The actions of 8-Br-cGMP appeared to be independent of either the - or the ₂/-subunit of Ca_V1.2a because inhibition still occurred in the absence of these subunits.

Protein phosphorylation represents a balance between kinase and phosphatase activity. Thus second messenger pathways that modify this balance can upregulate or downregulate channel activity to meet physiological demands. An additional mechanism by which PKG, in particular, could inhibit Ca²⁺ channel activity is through stimulation of phosphatase activity, leading to dephosphorylation of the channel. When inhibitors of either protein phosphatase 1 (PP1) or 2A (PP2A) are applied to cardiac myocytes, Ca²⁺ channel stimulation generally has been observed, suggesting that there is both basal phosphorylation and dephosphorylation of the channel (for review, see Ref. 69). The notion that PKG may exert its effect by activating a phosphatase is not new. Smooth muscle myosin phosphatase is a known target of PKG, and activation of this phosphatase contributes to the well-known phenomenon of myofilament "desensitization" (72, 180). In addition, there is evidence suggesting that the actions of cGMP on Ca²⁺-activated K⁺ channels in rat pituitary tumor cells are due to PKG-induced stimulation of PP2A (187). In a more recent study, 8-Br-cGMP was reported to inhibit IBMX-stimulated Ca²⁺ channel currents in guinea pig ventricular myocytes, and this effect was abolished by the type 1 and 2A phosphatase inhibitor okadaic acid, leading to the conclusion that PKG inhibited the channels by activation of a phosphatase (145).

Another means by which Ca²⁺ channel phosphorylation could be reduced is to increase the rate of breakdown of cAMP by PDEs. A number of different isoenzymes of PDEs exist in cells, and these have been subdivided into seven families. At least four of these families (i.e., PDE1-4) are present in cardiac muscle (177). PDE1 and PDE2 hydrolyze both cAMP and cGMP, whereas PDE3 and PDE4 hydrolyze cAMP. PDE2 is stimulated by cGMP, whereas PDE3 is inhibited by cGMP and PDE4 is insensitive to cGMP (135). In some tissues, cGMP may stimulate Ca_V1.2a channel currents via inhibition of PDE3 (88, 128), whereas in others, cGMP may inhibit Ca_V1.2a channel current by stimulating PDE2 (45, 117). This opposing effect of cGMP on PDE2 vs. PDE3 serves to underscore the complexity of interpreting studies in which cGMP is applied to cells.

Role of PKG in Smooth Muscle

In contrast to its role in cardiac muscle, the role of PKG as an important mediator of vascular relaxation is well established (for review, see Ref. 19), particularly with regard to the actions of the endothelium-relaxing factor nitric oxide (NO) and nitrovasodilators such as sodium nitroprusside. PKG reduces cytoplasmic Ca²⁺ concentration via several mechanisms, and a number of studies suggest that one of these mechanisms is the inhibition of Ca_V1.2b channels. For example, Ca_V1.2b currents are reduced by NO and by NO donors such as sodium nitroprusside and by the membrane-permeable analog 8-Br-cGMP (1, 14, 80, 93, 109, 111, 139, 144, 167, 195). Current inhibition is reversed with PKG blockers (144, 167). However, the mechanism by which PKG reduces Ca_V1.2b channel activity is still unknown. Mutation experiments such as those described for the ₁-subunit of Ca_V1.2a (82) have yet to be repeated for Ca_V1.2b.

The notion that PKG inhibits Ca_V1.2b via the activation of a phosphatase is an intriguing one. Application of inhibitors that block both PP1 and PP2A (e.g., okadaic acid and calyculin A) have generally increased Ca²⁺ channel activity (46, 56, 100, 123, 124), suggesting that basal channel activity is determined by a balance of kinase and phosphatase activity. Some studies in smooth muscle have directly applied activated phosphatases to isolated patches or to the whole cell via cell dialysis. These studies have generally revealed inhibition of channel activity, i.e., the opposite effect [e.g., PP2A application (56, 70) or PP2B application (148)]. To date, there have been no studies directly linking activation of a phosphatase to the PKG-induced inhibition of Ca_V1.2b channels observed in smooth muscle.

Five of the seven PDE isoenzyme families have been identified in vascular smooth muscle, including PDE1-4 and PDE5, which is a cGMP-hydrolyzing PDE (for review, see Ref. 135). PDE3 accounts for approximately half of the cAMP-hydrolyzing activity in the pulmonary artery and aorta (140, 141). To date, there are no studies investigating the possible role of cGMP activation of PDE2 to account for the inhibitory effect of cGMP on Ca_V1.2b.

	PKA REGULATION OF CA2+ CHANNELS

TOP ABSTRACT INTRODUCTION STRUCTURE OF CARDIAC AND... PKG REGULATION OF CA2+... PKA REGULATION OF CA2+... PKC REGULATION OF CAV1.2... G PROTEIN SUBUNIT REGULATION... REGULATION OF CAV1.2 CHANNELS... REFERENCES

PKA is a tetramer consisting of two catalytic subunits bound to a regulatory subunit dimer. Like PKG, PKA is ubiquitously expressed in smooth muscle and cardiac muscle. Initially, two different forms of the regulatory subunit were defined, i.e., R-I and R-II, where the PKA-RI complex is essentially cytosolic and the PKA-RII complex almost exclusively membrane bound. However, recent molecular cloning techniques have revealed significant heterogeneity in both the catalytic and regulatory subunits of PKA (165).

Role of PKA in Cardiac Muscle

The cardiac Ca_V1.2a channel has long been recognized as a target of the adenylyl cyclase/PKA pathway. Stimulation of -adrenoceptors leads to enhanced single-channel activity as well as a three- to sevenfold increase in whole cell current in cardiac cells (114, 150, 171, 173) (see Fig. 3). Recent reports suggest that ₁-adrenergic receptors, which couple exclusively to the G protein G_s, produce a more widespread increase in cAMP levels in the cell (i.e., diffuse activation), whereas ₂-adrenergic receptors, which can be coupled to both G_s and G_i, produce a more localized activation of Ca_V1.2a channels (24). Early studies directed toward determining whether PKA phosphorylates Ca_V1.2 channels were complicated by the fact that the majority of biochemically isolated ₁-subunits undergo a proteolytic cleavage that removes a substantial portion of the COOH-terminal tail (22, 36, 68, 99, 202). In 1992, Yoshida and coworkers (202) showed that PKA-induced phosphorylation occurred in the 250-kDa form of the ₁-subunit but not in the truncated 200-kDa form. Later studies confirmed the importance of the COOH-terminal tail and identified serine 1928 as the site of PKA-induced phosphorylation (34, 50, 68, 120, 131). Interestingly, although the COOH-terminal of the ₁-subunit is cleaved, Hosey and colleagues (49, 51) have provided evidence that the cleaved portion of the ₁-subunit remains associated with the "body" and thus may still be involved in PKA-induced regulation of the channel. Other studies have shown that the cardiac -subunit also is phosphorylated during activation of the adenylyl cyclase/PKA pathway (50, 58, 137), and the sites of phosphorylation appear to be three loose consensus sites for PKA in the COOH terminus rather than the two strong consensus sites identified for PKA (52). In a later study, two of these three sites (i.e., serine-478 and serine-479) were shown to be critical for PKA-induced regulation of the ₁-subunit (18). The relative contribution of the ₁- vs. ₂-subunit phosphorylation to PKA regulation of Ca_V1.2a in vivo has yet to be determined and, indeed, may change with development and/or other conditions present in the cell. There are reports that PKA-induced phosphorylation of the ₁-subunit is dependent on localizing PKA to the vicinity of the channel via A-kinase anchoring protein (AKAP) (Fig. 3), whereas phosphorylation of the _2a-subunit does not require AKAP (50). However, another study failed to reproduce a PKA-dependent increase in current amplitude in HEK-293 cells expressing the cardiac ₁- and _2a-subunits, even when these subunits were coexpressed with AKAP79 (33).

View larger version (23K): [in this window] [in a new window]   Fig. 3.   Mechanisms proposed for -adrenergic stimulation of CaV1.2 channels. -Adrenergic receptors (-AR) are stimulated with agonists such as isoproterenol. The -receptor is coupled to the G protein Gs, which leads to activation of the G protein subunits Gs and G. Gs stimulates A cyclase, leading to enhanced levels of cAMP that activate PKA localized to the region of CaV1.2 via an AKAP. PKA is then proposed to activate the Ca2+ channel via phosphorylation of one or more of the Ca2+ channel subunits. In contrast, G is proposed to lead to stimulation of PKC, possibly via phosphatidylinositol 3-kinase, leading to phosphorylation of 1 or more of the Ca2+ channel subunits. Finally, evidence exists for direct activation of CaV1.2 via a direct membrane-delimited action of Gs on the channel.

Additional complications have arisen in attempts to demonstrate a functional correlate of PKA-induced phosphorylation in expression systems. Although some studies have reported PKA-induced stimulation of cardiac Ca_V1.2a currents (50, 90, 150, 199, 202), others have failed to show any effect of PKA (33, 118, 209). A variety of explanations have been offered for the negative results, including the importance of auxiliary subunits, particularly the -subunit (90) and, more recently, the need for channel targeting of PKA by AKAP (50). One report suggested that an additional, as yet unidentified, component of the pathway is important for PKA-induced regulation of the Ca²⁺ channel, because PKA failed to stimulate Ca_V1.2a channels unless oocytes were injected with mRNA from cardiac cells (23). Yet another study suggested that PKA leads to phosphorylation of a 700-kDa protein that couples to the cardiac -subunit (60). Despite studies that suggest an obligatory role for additional regulatory subunits, several studies have reported PKA-induced enhancement of expressed ₁-subunits of Ca_V1.2a in the absence of other regulatory proteins (150, 202). In summary, evidence exists in both native cells and expression systems to suggest that PKA can stimulate cardiac Ca_V1.2a channels via direct phosphorylation of serine 1928 on the ₁-subunit. However, additional sites appear to contribute to this process. The variable results reported from expression systems may be related to differences in the cell types used, differences in the regulatory proteins coexpressed with the ₁-subunit, and the activity of other enzymes in the cells (e.g., PDEs, phosphatases) that modify the basal levels of phosphorylation of the channel. Indeed, in some studies, although activated PKA was reported to be without effect on channel activity, inhibition of endogenous PKA decreased channel activity, suggesting that the Ca_V1.2a channels were maximally phosphorylated under basal conditions (131, 132, 159).

Role of PKA in Smooth Muscle

In contrast to the general agreement concerning the actions of the adenylyl cyclase/PKA pathway on cardiac Ca_V1.2a channels, the details of how PKA modulates Ca_V1.2b channels in smooth muscle, and, indeed, even the direction of this modulation has remained somewhat controversial. Over the past 12 years, the adenylyl cyclase/PKA pathway has been reported to inhibit (109, 161, 195), to enhance (47, 80, 84, 93, 113, 144, 151, 164, 166, 206), or to have no effect (121, 127, 184) on smooth muscle Ca_V1.2b channels. However, a preponderance of recent evidence has gradually accumulated to suggest that PKA can modulate smooth muscle Ca_V1.2b channels in a similar manner to that described for cardiac Ca_V1.2a channels. Specifically, studies of various smooth muscle cells have shown that 1) the membrane-permeable analog 8-Br-cAMP enhances Ca_V1.2b channel currents (80, 144), an effect blocked by PKA inhibitors (87, 206); 2) the adenylyl cyclase activator forskolin enhances Ca_V1.2b channel currents (80, 151), an effect blocked by PKA inhibitors (200); 3) activation of adenylyl cyclase with isoproterenol enhances Ca_V1.2b channel currents (80, 93, 151, 161, 194), an effect blocked by PKA inhibitors (206); 4) activation of adenylyl cyclase with the G protein subunit G_s enhances Ca_V1.2b currents (193, 205), an effect blocked by PKA inhibitors (205); 5) the catalytic subunit of PKA enhances whole cell Ca_V1.2b currents, an effect blocked by PKA inhibitors (144); and 6) application of the catalytic subunit of PKA to the cytosolic surface of inside-out patches increases the open probability of Ca_V1.2b channels (166). In addition, the smooth muscle ₁-subunit shares 93% homology with the cardiac ₁-subunit (92, 185) and contains the same consensus PKA phosphorylation site (73). Also, as in cardiac muscle, there is evidence that the actions of PKA on Ca_V1.2b channels requires targeting via an AKAP (206). All of these observations strongly suggest that PKA is likely to have a similar action on both Ca_V1.2b and Ca_V1.2a channels (Fig. 3). However, the response to PKA differs in two respects. First, most studies to date have not reported a significant effect of cAMP on the shape of the current-voltage (I-V) relationship of Ca_V1.2b channels in smooth muscle cells (80, 151). In contrast, a number of studies of cardiac muscle have noted a shift to the left of the I-V relationship with cAMP (see, for example, Ref. 6). Second, the cAMP pathway in cardiac muscle leads to a 3- to 7-fold increase in Ca_V1.2 channel currents, whereas in smooth muscle the stimulation is more modest, i.e., ranging between a 0.5- and 2-fold increase.

The stimulatory effect of PKA on cardiac Ca_V1.2a channels corresponds well with the known positive inotropic effect of -adrenoceptor stimulation in this muscle. However, in smooth muscle, agonists that activate adenylyl cyclase typically cause relaxation, making it more difficult to reconcile a stimulatory effect of PKA on Ca_V1.2b currents. It is possible that the Ca²⁺ entry associated with PKA stimulation is specifically targeted to events that are unrelated to global cytoplasmic Ca²⁺ concentration; e.g., an increase in the subsarcolemmal Ca²⁺ concentration may be coupled to enhancement of Ca²⁺-activated K⁺ channel activity (57). Alternatively, the enhanced Ca²⁺ entry may serve to fill stores in the sarcoplasmic reticulum (SR).

Some of the controversy surrounding the actions of cAMP on Ca_V1.2b channels may be related to the higher levels of PKG present in smooth muscle compared with cardiac muscle (106). Neither cAMP nor cGMP display absolute specificity for their respective kinases, i.e., the equilibrium dissociation constant for activation of PKG by cAMP is ~10-fold greater than that for PKA (see, for example, Ref. 39). Thus some of the actions of cAMP that have been attributed to PKA on Ca_V1.2b channels may, in fact, be due to PKG. In support of this hypothesis are studies showing that higher concentrations of 8-Br-cAMP and forskolin lead to inhibition of Ca_V1.2b channel currents and that this effect is blocked by PKG inhibitors (80, 144). These data suggest that Ca_V1.2b channels can be inhibited by "cross-over" activation of PKG by cAMP. Cross-over activation of PKG by cAMP also has been proposed by others to contribute to the effects of cAMP (see, for example, Refs. 105 and 186). Interestingly, in the presence of PKG inhibitors the reverse cross-over activation can be demonstrated as well, i.e., cGMP activation of PKA, leading to stimulation of Ca_V1.2b channel current (144). A similar action of cGMP on PKA has been proposed to underlie the NO-induced relaxation in PKG-I-deficient mice (146). This "cross talk" between cyclic nucleotides and protein kinases is illustrated in Fig. 4.

View larger version (25K): [in this window] [in a new window]   Fig. 4.   Mechanisms proposed for cGMP stimulation and cAMP inhibition of CaV1.2 channels (red arrows). Cross talk between the cyclic nucleotides may lead to cAMP-induced activation of PKG to inhibit the channel or cGMP-induced activation of PKA to stimulate the channel. Cross talk is more likely to occur when higher levels of cyclic nucleotide are achieved in the cell. Finally, cGMP also has been reported to inhibit PDE3, which leads to an increase in cAMP levels and, hence, stimulation of the channel via PKA.

Only one study to date has demonstrated enhancement of Ca_V1.2b channel currents with raised cAMP in an expression system (90). In this study, the ₁-subunit of Ca_V1.2b was coexpressed with the skeletal muscle _1a-subunit. cAMP produced current stimulation only in the presence of the -subunit. The authors suggested that the kind of -subunit present in a tissue may underlie tissue-specific effects of cAMP/PKA. However, other -subunits (e.g., ₂ or ₃) were not tested in this study. A recent study of A7r5 cells also suggests that cAMP-dependent stimulation of the channel requires a functional -subunit (87). Two other studies have failed to show enhancement of the Ca_V1.2b ₁-subunit by cAMP/PKA in an expression system (89, 209). In one case the ₁-subunit was expressed with ₂- and ₂/-subunits in HEK-293 cells (209), and in the other case the ₁-subunit was expressed alone in Chinese hamster ovary cells (89). This variability between studies of expressed smooth muscle Ca_V1.2b channels is similar to the variability reported for the actions of PKA on expressed cardiac Ca_V1.2a channels.

	PKC REGULATION OF CAV1.2 CHANNELS

TOP ABSTRACT INTRODUCTION STRUCTURE OF CARDIAC AND... PKG REGULATION OF CA2+... PKA REGULATION OF CA2+... PKC REGULATION OF CAV1.2... G PROTEIN SUBUNIT REGULATION... REGULATION OF CAV1.2 CHANNELS... REFERENCES

Eleven isoforms of PKC have been identified in mammalian tissues, and these are divided into three groups: 1) classic or conventional PKCs (cPKC) that are activated by diacylglycerol or phorbol ester and are Ca²⁺ sensitive (including , I, II, ), 2) novel or new PKCs (nPKC) that are activated by diacylglycerol or phorbol ester but are not Ca²⁺ dependent (, , , , L, µ), and 3) atypical PKCs (aPKC) that are not activated by diacylglycerol, phorbol ester, or Ca²⁺ (/, ) (for review, see Ref. 75).

Role of PKC in Cardiac Muscle

In cardiac muscle PKC phosphorylates a variety of different proteins that contribute to myocardial excitability and contraction. Among these is the Ca_V1.2a channel. Both the ₁- and ₂-subunits of Ca_V1.2a are phosphorylated by PKC in vitro with a stoichiometry of 2-3 moles of phosphate per mole of ₁-subunit and 1-2 moles of phosphate per mole of _2a-subunit (137). The first 46 amino acid residues at the NH₂-terminal end of the cardiac ₁-subunit have been shown to inhibit channel activity, and it has been suggested that PKC may enhance Ca_V1.2a channel activity by relieving NH₂-terminal inhibition (153). In this scheme, the site of PKC phosphorylation is left undefined except to exclude the NH₂-terminal region per se from phosphorylation (155). PKC generally has been reported to activate Ca_V1.2a channels in heterologous systems (17, 153, 158), but there are exceptions (115). Some studies in native cells have reported PKC-mediated stimulation of Ca_V1.2a (41, 67, 110), whereas others report inhibition (204). Current stimulation with phorbol esters sometimes has been reported to be transient and followed by later current inhibition (97, 172), with the later effect being PKC independent (5, 17, 97, 172). Furthermore, there is preliminary evidence to suggest that different PKC isoforms may produce opposing actions on Ca_V1.2a, with cPKCs producing inhibition of Ca_V1.2a but nPKCs enhancing channel current (63). A recent study by McHugh et al. (115) of Ca²⁺ channels expressed in TSA-201 cells suggests that inhibition of channel activity by PKC occurs through phosphorylation of threonine-27 and threonine-31 at the NH₂-terminal region of the ₁-subunit. These data underscore the need for additional studies to clarify the possibly complex nature of PKC-induced effects on Ca_V1.2a.

Role of PKC in Smooth Muscle

PKC has been reported to enhance Ca_V1.2b channel currents in various smooth muscle preparations (28, 101, 102, 125, 147, 152, 175, 178, 205). As with Ca_V1.2a, stimulation of Ca_V1.2b is sometimes followed by current inhibition (147). A number of different agonists, including norepinephrine, endothelin, angiotensin II, and serotonin, have been suggested to stimulate Ca_V1.2b channels via a PKC-dependent mechanism (for review, see Ref. 55). Because agonists that activate PKC often produce inositol 1,4,5-trisphosphate-induced release of Ca²⁺ from the SR, the PKC-induced channel stimulation can be masked by Ca²⁺-induced inactivation of channels (71, 86). Little is known about the mechanisms underlying Ca_V1.2b channel modulation by PKC.

	G PROTEIN SUBUNIT REGULATION OF CAV1.2 CHANNELS

TOP ABSTRACT INTRODUCTION STRUCTURE OF CARDIAC AND... PKG REGULATION OF CA2+... PKA REGULATION OF CA2+... PKC REGULATION OF CAV1.2... G PROTEIN SUBUNIT REGULATION... REGULATION OF CAV1.2 CHANNELS... REFERENCES

The PKA and PKC pathways discussed above are generally linked to agonist-induced activation of G protein subunits. Activated G protein subunits can regulate different ionic channels not only through intracellular second messenger pathways but also through a direct membrane-delimited gating of channels. For example, neuronal Ca_V2.2 channels (formerly N-type or _1B) are inhibited by G protein-activated PKC and by the direct binding of G to the ₁-subunit (78). Direct G binding takes place in the intracellular loop between repeat I and II as well as in a second downstream sequence (38). However, the requisite binding motif for this interaction is absent in Ca_V1.2 channels. In the case of Ca_V1.2 channels, evidence has been presented to suggest a direct membrane-delimited activation of Ca_V1.2a and Ca_V1.2b channels by the G protein subunit G_s; however, these effects are controversial (29, 170).

Role of G Protein Subunits in Cardiac Muscle

The first evidence suggesting a direct membrane-delimited action of G_s on Ca_V1.2a channels was from studies showing that the GTP analog guanosine 5'-O-(3-thiotriphosphate) (GTPS) prolonged the survival of excised Ca²⁺ channels and that Ca²⁺ channels incorporated into a lipid bilayer could be activated independently of cAMP, ATP, PKA, and the PKC activator phorbol ester. Preactivated G_s protein and the G_s subunit also were shown to mimic the effects of GTPS on channels in excised patches or in lipid bilayers (79, 197). Additional studies have reported that -adrenergic stimulation of Ca_V1.2a channels could not be explained solely by a PKA-dependent pathway, i.e., isoproterenol still increased Ca_V1.2a current by 35-80% in the presence of inhibitors of the adenylyl cyclase/PKA pathway in guinea pig ventricular myocytes (20, 130, 156), whereas forskolin, which is not G protein linked, was blocked entirely by the same maneuvers. In contrast, other studies have reported that the stimulatory effect of isoproterenol on Ca_V1.2a is blocked entirely by PKA inhibitors in several different animal species (29, 65, 66, 83).

Recent studies have provided further evidence for a membrane-delimited gating of Ca_V1.2a by G_s subunits. The whole cell Ca²⁺ channel current in cardiac myocytes from transgenic mice overexpressing cardiac G_s is 490% higher than in cells from wild-type control animals (98). Furthermore, in Xenopus oocytes, antisense knockdown of endogenous G_s reduced currents of expressed Ca_V1.2a channels, whereas coexpression of G_s with Ca_V1.2a enhanced currents (15). Interestingly, PKA inhibitors did not have any detectable action on the effects of G_s in either the transgenic mice or the oocyte model. In fact, a small cAMP-dependent decrease of current was observed in oocytes coexpressed with Ca_V1.2a and the ₂-adrenergic receptor (15). The authors concluded that coexpression of G_s, but not its acute activation via -adrenergic receptors, enhances Ca_V1.2a currents via a PKA-independent pathway. A recent study of Ca_V1.2a and ₂-subunits expressed in Chinese hamster fibroblasts with -adrenoceptors also suggested the presence of an additional cAMP-independent mechanism of channel activation (198). Thus controversy continues to surround the issue of direct membrane-delimited effects of G_s on Ca_V1.2a, although a high-affinity binding site for G_s on Ca_V1.2a has yet to be identified.

In our recent studies of G protein activation of native guinea pig cardiac Ca_V1.2a channel currents, we did not obtain any evidence to suggest the presence of a direct membrane-delimited pathway for either G_s or G activation of the channel (208). Rather, the pathways involved appeared to be very similar in nature to those described below for G protein regulation of smooth muscle Ca_V1.2b channels (see also Ref. 205 as well as Fig. 3), leaving open the possibility that the PKA-independent effect of -adrenergic stimulation in cardiac myocytes identified by others may actually be due to G-induced activation of PKC, rather than to a direct membrane-delimited effect of G_s.

Role of G Protein Subunits in Smooth Muscle

In 1994, Xiong and colleagues (195) reported that dialysis of cells with activated G_s gave rise to stimulation of Ca²⁺ channel currents in rabbit portal vein myocytes. It was suggested that this effect represented a direct membrane-delimited action of G_s on the channel. We directly explored this issue by comparing the effect of dialyzing cells with either activated G_s or G₁₂. Both G_s and G stimulated Ca_V1.2b currents in rabbit portal vein myocytes, whereas inactive subunits [i.e., G_s-guanosine 5'-O-(2-thiodiphosphate) and nonprenylated G] were without effect. Of even greater interest was the observation that different second messenger pathways appeared to be involved in the actions of the two G protein subunits. Thus G_s enhanced current via the adenylyl cyclase/PKA pathway, whereas G activated channels via a PKC-dependent pathway (205). Recently, we determined that both of these pathways also are present when endogenous G_s and G are stimulated with isoproterenol (Fig. 3) (207). To date, G dimers from four different sources all have been shown to stimulate Ca_V1.2b channels in a PKC-dependent manner. These include endogenous G₁₃ coupled to G₁₃ in rat portal vein (112), G purified from rat brain G_i/G_o (178), G₁₂ purified from Sf9 cells (205), and endogenous G coupled to G_s in rabbit portal vein cells (207). These data suggest that G-induced stimulation of Ca_V1.2b channels may be a fairly ubiquitous feature of agonist-induced G protein stimulation. Under these circumstances, G protein specificity still could be conferred by the G subunit coupled to G. The notion that many different G dimers exert similar effects certainly is not new. This was suggested early on (see, for example, Ref. 174) and has received continued support in more recent times (30, 143). The question of how G leads to activation of PKC in this pathway has been only recently addressed. Studies of rat portal vein myocytes suggest that at least one mechanism involved is G-induced stimulation of phosphatidylinositol 3-kinase (178). In most of the recent work on G protein subunit regulation, there has been little evidence to support the idea that either G_s or G activates Ca_V1.2b channels via a direct membrane-delimited pathway.

	REGULATION OF CAV1.2 CHANNELS BY OTHER KINASES

TOP ABSTRACT INTRODUCTION STRUCTURE OF CARDIAC AND... PKG REGULATION OF CA2+... PKA REGULATION OF CA2+... PKC REGULATION OF CAV1.2... G PROTEIN SUBUNIT REGULATION... REGULATION OF CAV1.2 CHANNELS... REFERENCES

Recent studies suggest that the activity of Ca_V1.2 channels in cardiac and smooth muscle cells also is regulated by other kinases, especially by protein tyrosine kinase (PTK) and calmodulin-dependent kinase (CaMK). Although this review focuses mainly on Ca_V1.2 channel regulation by PKG, PKA, and PKC, a summary of recent reports of channel regulation by PTK and CaMK follows.

Role of PTK

Tyrosine phosphorylation is an important regulator of cell function not only for growth-related responses such as gene transcription and cell division but also for rapid cellular responses such as cell adhesion and muscle contraction. Early studies on possible regulation of cardiac Ca²⁺ channels by PTK indicated that genistein, a specific PTK inhibitor, dose-dependently reduced Ca_V1.2 channel currents in rat (201) and guinea pig ventricular myocytes (25). However, in these studies, daidzein, an inactive analog of genistein, exerted the same inhibitory effect on Ca²⁺ channel current as genistein. In contrast, several other studies have reported inhibition of Ca²⁺ channel currents with PTK blockers that was not mimicked by daidzein (126, 181). In feline atrial myocytes, genistein was reported to have a biphasic effect on Ca²⁺ channel currents when examined with the whole cell perforated-patch recording method (181). Both inhibition and stimulation with genistein were not mimicked by daidzein but were abolished by the tyrosine phosphatase inhibitor vanadate. Interestingly, current stimulation was absent when the conventional whole cell patch recording method was used. The authors concluded that genistein inhibits Ca²⁺ channel current by blocking membrane-bound PTKs and stimulates Ca²⁺ channel current by blocking cytosolic PTKs. Others have reported that PTK inhibition leads only to current stimulation and have suggested a PKC-dependent mechanism (16). A recent report further proposes that PTK also may directly antagonize -adrenergic receptors (157). Together, these studies suggest that PTK may play a significant role in the regulation of cardiac Ca_V1.2 channels, but at present even the direction of this regulation appears to be controversial. Additional studies are required to better understand the possibly complicated role(s) that various PTKs play in modulating Ca_V1.2a activity. Caution is advisable as well when interpreting results using PTK antagonists, which may have more than one site of action.

Evidence that vascular Ca_V1.2b channels are regulated by PTK also has been provided. Wijetunge et al. (188) reported that Ca_V1.2b current in rabbit ear artery myocytes was significantly reduced by both genistein and tyrphostin 23 but not by daidzein. Inhibition of tyrosine phosphatases (190) or activation of endogenous c-Src tyrosine kinase (189) increased current in the same cell preparation. PTK inhibitors antagonized these stimulatory effects, suggesting a possible role for endogenous c-Src tyrosine kinase in the modulation of vascular Ca²⁺ channels (189, 191). In rat portal vein myocytes, genistein also reduced whole cell Ca²⁺ channel currents, whereas daidzein was without effect (107). Further studies of these cells revealed that genistein also decreased the mean open time and prolonged closed time of single channels (108).

Although the evidence to date suggests a possible role of PTK in the regulation of Ca_V1.2 channels, no studies have yet extended this work to PTK regulation of expressed cardiovascular Ca_V1.2 channels. A recent study of neuronal Ca_V1.2 channels indicated that potentiation of current by insulin-like growth factor 1 required phosphorylation of the ₁-subunit by PTK (8). It is possible that PTK phosphorylates a similar site on the ₁-subunit of cardiovascular Ca_V1.2 channels.

Role of CaMK

Ca²⁺-dependent inactivation and facilitation of Ca_V1.2 channels are well-known phenomena, and both forms of channel autoregulation appear to involve calmodulin (CaM). A number of different studies suggest that CaMK may be responsible for the positive-feedback, Ca²⁺-dependent facilitation of Ca_V1.2 channels (42, 168, 192, 203). An early study by Yuan and Bers (203) indicated that repetitive membrane depolarization from 90 to 0 mV in rabbit and ferret ventricular myocytes induced a staircase increase in Ca²⁺ current. This effect was completely abolished by dialyzing cells with either the CaMKII inhibitor CaMKII_(290-309) or CaMKII_(273-302). Similar results have been observed in rabbit ventricular myocytes (2). Constitutively active CaMK also markedly increases the open probability of single Ca²⁺ channel currents in murine ventricular myocytes recorded in inside-out patch configuration (42). The stimulatory effect of constitutively active CaMK required ATP and was not mimicked by CaM alone. It was blocked by the CaMK inhibitory peptide AC3-1 but not by either PKA or PKC inhibitory peptides. The authors suggested that CaMK phosphorylates a cell membrane-associated target protein, which gives rise to frequent, long openings of Ca²⁺ channels.

In contrast to CaM-dependent facilitation, some studies have proposed that CaM-dependent inactivation may result from channel dephosphorylation by the CaM-dependent phosphatase calcineurin (4, 21, 148). On the other hand, others have proposed that CaM directly interacts with the Ca²⁺ channel ₁-subunit and that channel phosphorylation is not involved (see, for example, Refs. 129, 133, 142, and 211). Some studies have gone so far as to suggest that direct binding of CaM to the ₁-subunit is a key step in both Ca²⁺-dependent inactivation and facilitation of Ca_V1.2 channels. Recently, it has been shown that mutations in a consensus CaM-binding IQ motif in the COOH-terminal tail of ₁ eliminates both forms of Ca²⁺-dependent automodulation of Ca²⁺ channels (210, 211). Whether this direct interaction between CaM and the channel ₁-subunits is sufficient to induce both Ca²⁺-dependent inactivation and facilitation requires further study, as does the question of the role of CaMK in the process of autoregulation.

	ACKNOWLEDGEMENTS

We thank Dr. James Kenyon for careful reading of this manuscript and valuable suggestions.

	FOOTNOTES

This work was supported by National Heart, Lung, and Blood Institute Grants HL-40399 (to K. D. Keef and J. R. Hume) and HL-49254 (to J. R. Hume) and by an American Heart Association Western Affiliate Grant-in-Aid (to J. Zhong).

Address for reprint requests and other correspondence: K. Keef, Dept. of Physiology and Cell Biology/MS 352, Univ. of Nevada School of Medicine, Reno, NV 89557 (E-mail: kathy{at}physio.unr.edu).

	REFERENCES

TOP ABSTRACT INTRODUCTION STRUCTURE OF CARDIAC AND... PKG REGULATION OF CA2+... PKA REGULATION OF CA2+... PKC REGULATION OF CAV1.2... G PROTEIN SUBUNIT REGULATION... REGULATION OF CAV1.2 CHANNELS... REFERENCES

1.   Akbarali, HI, and Goyal RK. Effect of sodium nitroprusside on Ca²⁺ currents in opossum esophageal circular muscle cells. Am J Physiol Gastrointest Liver Physiol 266: G1036-G1042, 1994[Abstract/Free Full Text].

2.   Anderson, ME, Braun AP, Schulman H, and Premack BA. Multifunctional Ca²⁺/calmodulin-dependent protein kinase mediates Ca²⁺-induced enhancement of the L-type Ca²⁺ current in rabbit ventricular myocytes. Circ Res 75: 854-861, 1994[Abstract/Free Full Text].

3.   Angelotti, T, and Hofmann F. Tissue-specific expression of splice variants of the mouse voltage-gated calcium channel 2/ subunit. FEBS Lett 397: 331-337, 1996[Web of Science][Medline].

4.   Armstrong, DL. Calcium channel regulation by calcineurin, a Ca²⁺-activated phosphatase in mammalian brain. Trends Neurosci 12: 117-122, 1989[Web of Science][Medline].

5.   Asai, T, Shuba LM, Pelzer DJ, and McDonald TF. PKC-independent inhibition of cardiac L-type Ca²⁺ channel current by phorbol esters. Am J Physiol Heart Circ Physiol 270: H620-H627, 1996[Abstract/Free Full Text].

6.   Bean, BP, Nowycky MC, and Tsien RW. Beta-adrenergic modulation of calcium channels in frog ventricular heart cells. Nature 307: 371-375, 1984[Medline].

7.   Beech, DJ. Actions of neurotransmitters and other messengers on Ca²⁺ channels and K⁺ channels in smooth muscle cells. Pharmacol Ther 73: 91-119, 1997[Web of Science][Medline].

8.   Bence-Hanulec, KK, Marshall J, and Blair LA. Potentiation of neuronal L calcium channels by IGF-1 requires phosphorylation of the ₁ subunit on a specific tyrosine residue. Neuron 27: 121-131, 2000[Web of Science][Medline].

9.   Biel, M, Ruth P, Bosse E, Hullin R, Stuhmer W, Flockerzi V, and Hofmann F. Primary structure and functional expression of a high voltage activated calcium channel from rabbit lung. FEBS Lett 269: 409-412, 1990[Web of Science][Medline].

10.   Bielefeldt, K. Molecular diversity of voltage-sensitive calcium channels in smooth muscle cells. J Lab Clin Med 133: 469-477, 1999[Web of Science][Medline].

11.   Birnbaumer, L, Campbell KP, Catterall WA, Harpold MM, Hofmann F, Horne WA, Mori Y, Schwartz A, Snutch TP, and Tanabe T. The naming of voltage-gated calcium channels. Neuron 13: 505-506, 1994[Web of Science][Medline].

12.   Birnbaumer, L, Qin N, Olcese R, Tareilus E, Platano D, Costantin J, and Stefani E. Structures and functions of calcium channel beta subunits. J Bioenerg Biomembr 30: 357-375, 1998[Web of Science][Medline].

13.   Bkaily, G, and Sperelakis N. Injection of guanosine 5'-cyclic monophosphate into heart cells blocks calcium slow channels. Am J Physiol Heart Circ Physiol 248: H745-H749, 1985.

14.   Blatter, LA, and Wier WG. Nitric oxide decreases [Ca²⁺]_i in vascular smooth muscle by inhibition of the calcium current. Cell Calcium 15: 122-131, 1994[Web of Science][Medline].

15.   Blumenstein, Y, Ivanina T, Shistik E, Bossi E, Peres A, and Dascal N. Regulation of cardiac L-type Ca²⁺ channel by coexpression of G_s in Xenopus oocytes. FEBS Lett 444: 78-84, 1999[Web of Science][Medline].

16.   Boixel, C, Tessier S, Pansard Y, Lang-Lazdunski L, Mercadier JJ, and Hatem SN. Tyrosine kinase and protein kinase C regulate L-type Ca²⁺ current cooperatively in human atrial myocytes. Am J Physiol Heart Circ Physiol 278: H670-H676, 2000[Abstract/Free Full Text].

17.   Bourinet, E, Fournier F, Lory P, Charnet P, and Nargeot J. Protein kinase C regulation of cardiac calcium channels expressed in Xenopus oocytes. Pflügers Arch 421: 247-255, 1992[Web of Science][Medline].

18.   Bunemann, M, Gerhardstein BL, Gao T, and Hosey MM. Functional regulation of L-type calcium channels via protein kinase A-mediated phosphorylation of the ₂ subunit. J Biol Chem 274: 33851-33854, 1999[Abstract/Free Full Text].

19.   Carvajal, JA, Germain AM, Huidobro-Toro JP, and Weiner CP. Molecular mechanism of cGMP-mediated smooth muscle relaxation. J Cell Physiol 184: 409-420, 2000[Web of Science][Medline].

20.   Cavalie, A, Allen TJ, and Trautwein W. Role of the GTP-binding protein G_s in the -adrenergic modulation of cardiac Ca channels. Pflügers Arch 419: 433-443, 1991[Web of Science][Medline].

21.   Chad, JE, and Eckert R. An enzymatic mechanism for calcium current inactivation in dialysed Helix neurones. J Physiol (Lond) 378: 31-51, 1986[Abstract/Free Full Text].

22.   Chang, FC, and Hosey MM. Dihydropyridine and phenylalkylamine receptors associated with cardiac and skeletal muscle calcium channels are structurally different. J Biol Chem 263: 18929-18937, 1988[Abstract/Free Full Text].

23.   Charnet, P, Lory P, Bourinet E, Collin T, and Nargeot J. cAMP-dependent phosphorylation of the cardiac L-type Ca channel: a missing link? Biochimie 77: 957-962, 1995[Medline].

24.   Chen-Izu, Y, Xiao RP, Izu LT, Cheng H, Kuschel M, Spurgeon H, and Lakatta EG. G_i-dependent localization of ₂-adrenergic receptor signaling to L-type Ca²⁺ channels. Biophys J 79: 2547-2556, 2000[Web of Science][Medline].

25.   Chiang, CE, Chen SA, Chang MS, Lin CI, and Luk HN. Genistein directly inhibits L-type calcium currents but potentiates cAMP-dependent chloride currents in cardiomyocytes. Biochem Biophys Res Commun 223: 598-603, 1996[Web of Science][Medline].

26.   Chien, AJ, Carr KM, Shirokov RE, Rios E, and Hosey MM. Identification of palmitoylation sites within the L-type calcium channel _2a subunit and effects on channel function. J Biol Chem 271: 26465-26468, 1996[Abstract/Free Full Text].

27.   Chien, AJ, Gao T, Perez-Reyes E, and Hosey MM. Membrane targeting of L-type calcium channels. Role of palmitoylation in the subcellular localization of the _2a subunit. J Biol Chem 273: 23590-23597, 1998[Abstract/Free Full Text].

28.   Chik, CL, Li B, Ogiwara T, Ho AK, and Karpinski E. PACAP modulates L-type Ca²⁺ channel currents in vascular smooth muscle cells: involvement of PKC and PKA. FASEB J 10: 1310-1317, 1996[Abstract].

29.   Clapham, DE. Direct G protein activation of ion channels? Annu Rev Neurosci 17: 441-464, 1994[Web of Science][Medline].

30.   Clapham, DE, and Neer EJ. G protein beta gamma subunits. Annu Rev Pharmacol Toxicol 37: 167-203, 1997[Web of Science][Medline].

31.   Collin, T, Lory P, Taviaux S, Courtieu C, Guilbault P, Berta P, and Nargeot J. Cloning, chromosomal location and functional expression of the human voltage-dependent calcium-channel ₃ subunit. Eur J Biochem 220: 257-262, 1994[Web of Science][Medline].

32.   Costantin, J, Noceti F, Qin N, Wei X, Birnbaumer L, and Stefani E. Facilitation by the _2a subunit of pore openings in cardiac Ca²⁺ channels. J Physiol (Lond) 507: 93-103, 1998[Abstract/Free Full Text].

33.   Dai, S, Klugbauer N, Zong X, Seisenberger C, and Hofmann F. The role of subunit composition on prepulse facilitation of the cardiac L-type calcium channel. FEBS Lett 442: 70-74, 1999[Web of Science][Medline].

34.   De Jongh, KS, Murphy BJ, Colvin AA, Hell JW, Takahashi M, and Catterall WA. Specific phosphorylation of a site in the full-length form of the alpha 1 subunit of the cardiac L-type calcium channel by adenosine 3',5'- cyclic monophosphate-dependent protein kinase. Biochemistry 35: 10392-10402, 1996[Medline].

35.   De Jongh, KS, Warner C, and Catterall WA. Subunits of purified calcium channels. Alpha 2 and delta are encoded by the same gene. J Biol Chem 265: 14738-14741, 1990[Abstract/Free Full Text].

36.   De Jongh, KS, Warner C, Colvin AA, and Catterall WA. Characterization of the two size forms of the alpha 1 subunit of skeletal muscle L-type calcium channels. Proc Natl Acad Sci USA 88: 10778-10782, 1991[Abstract/Free Full Text].

37.   De Waard, M, Gurnett CA, and Campbell KP. Structural and functional diversity of voltage-activated calcium channels. Ion Channels 4: 41-87, 1996[Medline].

38.   De Waard, M, Liu H, Walker D, Scott VE, Gurnett CA, and Campbell KP. Direct binding of G-protein complex to voltage-dependent calcium channels. Nature 385: 446-450, 1997[Medline].

39.   Dhanakoti, SN, Gao Y, Nguyen MQ, and Raj JU. Involvement of cGMP-dependent protein kinase in the relaxation of ovine pulmonary arteries to cGMP and cAMP. J Appl Physiol 88: 1637-1642, 2000[Abstract/Free Full Text].

40.   Dolphin, AC. L-type calcium channel modulation. Adv Second Messenger Phosphoprotein Res 33: 153-177, 1999[Web of Science][Medline].

41.   Dosemeci, A, Dhallan RS, Cohen NM, Lederer WJ, and Rogers TB. Phorbol ester increases calcium current and simulates the effects of angiotensin II on cultured neonatal rat heart myocytes. Circ Res 62: 347-357, 1988[Abstract/Free Full Text].

42.   Dzhura, I, Wu Y, Colbran RJ, Balser JR, and Anderson ME. Calmodulin kinase determines calcium-dependent facilitation of L-type calcium channels. Nat Cell Biol 2: 173-177, 2000[Web of Science][Medline].

43.   Ertel, EA, Campbell KP, Harpold MM, Hofmann F, Mori Y, Perez-Reyes E, Schwartz A, Snutch TP, Tanabe T, Birnbaumer L, Tsien RW, and Catterall WA. Nomenclature of voltage-gated calcium channels. Neuron 25: 533-535, 2000[Web of Science][Medline].

44.   Feron, O, Octave JN, Christen MO, and Godfraind T. Quantification of two splicing events in the L-type calcium channel alpha-1 subunit of intestinal smooth muscle and other tissues. Eur J Biochem 222: 195-202, 1994[Web of Science][Medline].

45.   Fischmeister, R, and Hartzell HC. Cyclic guanosine 3',5'-monophosphate regulates the calcium current in single cells from frog ventricle. J Physiol (Lond) 387: 453-472, 1987[Abstract/Free Full Text].

46.   Frace, AM, and Hartzell HC. Opposite effects of phosphatase inhibitors on L-type calcium and delayed rectifier currents in frog cardiac myocytes. J Physiol (Lond) 472: 305-326, 1993[Abstract/Free Full Text].

47.   Fukumitsu, T, Hayashi H, Tokuno H, and Tomita T. Increase in calcium channel current by -adrenoceptor agonists in single smooth muscle cells isolated from porcine coronary artery. Br J Pharmacol 100: 593-599, 1990[Web of Science][Medline].

48.   Gao, T, Chien AJ, and Hosey MM. Complexes of the _1C and  subunits generate the necessary signal for membrane targeting of class C L-type calcium channels. J Biol Chem 274: 2137-2144, 1999[Abstract/Free Full Text].

49.   Gao, T, Puri TS, Gerhardstein BL, Chien AJ, Green RD, and Hosey MM. Identification and subcellular localization of the subunits of L-type calcium channels and adenylyl cyclase in cardiac myocytes. J Biol Chem 272: 19401-19407, 1997[Abstract/Free Full Text].

50.   Gao, T, Yatani A, Dell'Acqua ML, Sako H, Green SA, Dascal N, Scott JD, and Hosey MM. cAMP-dependent regulation of cardiac L-type Ca²⁺ channels requires membrane targeting of PKA and phosphorylation of channel subunits. Neuron 19: 185-196, 1997[Web of Science][Medline].

51.   Gerhardstein, BL, Gao T, Bunemann M, Puri TS, Adair A, Ma H, and Hosey MM. Proteolytic processing of the C terminus of the _1C subunit of L-type calcium channels and the role of a proline-rich domain in membrane tethering of proteolytic fragments. J Biol Chem 275: 8556-8563, 2000[Abstract/Free Full Text].

52.   Gerhardstein, BL, Puri TS, Chien AJ, and Hosey MM. Identification of the sites phosphorylated by cyclic AMP-dependent protein kinase on the beta 2 subunit of L-type voltage-dependent calcium channels. Biochemistry 38: 10361-10370, 1999[Medline].

53.   Gerster, U, Neuhuber B, Groschner K, Striessnig J, and Flucher BE. Current modulation and membrane targeting of the calcium channel _1C subunit are independent functions of the beta subunit. J Physiol (Lond) 517: 353-368, 1999[Abstract/Free Full Text].

54.   Gollasch, M, Haase H, Ried C, Lindschau C, Morano I, Luft FC, and Haller H. L-type calcium channel expression depends on the differentiated state of vascular smooth muscle cells. FASEB J 12: 593-601, 1998[Abstract/Free Full Text].

55.   Gollasch, M, and Nelson MT. Voltage-dependent Ca²⁺ channels in arterial smooth muscle cells. Kidney Blood Press Res 20: 355-371, 1997[Web of Science][Medline].

56.   Groschner, K, Schuhmann K, Mieskes G, Baumgartner W, and Romanin C. A type 2A phosphatase-sensitive phosphorylation site controls modal gating of L-type Ca²⁺ channels in human vascular smooth-muscle cells. Biochem J 318: 513-517, 1996.

57.   Guia, A, Wan X, Courtemanche M, and Leblanc N. Local Ca²⁺ entry through L-type Ca²⁺ channels activates Ca²⁺-dependent K⁺ channels in rabbit coronary myocytes. Circ Res 84: 1032-1042, 1999[Abstract/Free Full Text].

58.   Haase, H, Bartel S, Karczewski P, Morano I, and Krause EG. In-vivo phosphorylation of the cardiac L-type calcium channel -subunit in response to catecholamines. Mol Cell Biochem 163-164: 99-106, 1996.

59.   Haase, H, Kresse A, Hohaus A, Schulte HD, Maier M, Osterziel KJ, Lange PE, and Morano I. Expression of calcium channel subunits in the normal and diseased human myocardium. J Mol Med 74: 99-104, 1996[Web of Science][Medline].

60.   Haase, H, Podzuweit T, Lutsch G, Hohaus A, Kostka S, Lindschau C, Kott M, Kraft R, and Morano I. Signaling from -adrenoceptor to L-type calcium channel: identification of a novel cardiac protein kinase A target possessing similarities to AHNAK. FASEB J 13: 2161-2172, 1999[Abstract/Free Full Text].

61.   Haddad, GE, Sperelakis N, and Bkaily G. Regulation of the calcium slow channel by cyclic GMP dependent protein kinase in chick heart cells. Mol Cell Biochem 148: 89-94, 1995[Web of Science][Medline].

62.   Han, J, Kim E, Lee SH, Yoo S, Ho WK, and Earm YE. cGMP facilitates calcium current via cGMP-dependent protein kinase in isolated rabbit ventricular myocytes. Pflügers Arch 435: 388-393, 1998[Web of Science][Medline].

63.   Hantash, BM, Thomas AP, and Reeves JP. Dual effect of protein kinase C on L-type calcium activity in L6 cells (Abstract). Biophys J 78: 203A, 2000.

64.   Hartzell, HC, and Fischmeister R. Opposite effects of cyclic GMP and cyclic AMP on Ca²⁺ current in single heart cells. Nature 323: 273-275, 1986[Medline].

65.   Hartzell, HC, and Fischmeister R. Direct regulation of cardiac Ca²⁺ channels by G proteins: neither proven nor necessary? Trends Pharmacol Sci 13: 380-385, 1992[Medline].

66.   Hartzell, HC, Mery PF, Fischmeister R, and Szabo G. Sympathetic regulation of cardiac calcium current is due exclusively to cAMP-dependent phosphorylation. Nature 351: 573-576, 1991[Medline].

67.   He, JQ, Pi Y, Walker JW, and Kamp TJ. Endothelin-1 and photoreleased diacylglycerol increase L-type Ca²⁺ current by activation of protein kinase C in rat ventricular myocytes. J Physiol (Lond) 524: 807-820, 2000[Abstract/Free Full Text].

68.   Hell, JW, Yokoyama CT, Wong ST, Warner C, Snutch TP, and Catterall WA. Differential phosphorylation of two size forms of the neuronal class C L-type calcium channel alpha 1 subunit. J Biol Chem 268: 19451-19457, 1993[Abstract/Free Full Text].

69.   Herzig, S, and Neumann J. Effects of serine/threonine protein phosphatases on ion channels in excitable membranes. Physiol Rev 80: 173-210, 2000[Abstract/Free Full Text].

70.   Hescheler, J, Kameyama M, Trautwein W, Mieskes G, and Soling HD. Regulation of the cardiac calcium channel by protein phosphatases. Eur J Biochem 165: 261-266, 1987[Web of Science][Medline].

71.   Hirakawa, Y, Kuga T, Kobayashi S, Kanaide H, and Takeshita A. Dual regulation of L-type Ca²⁺ channels by serotonin 2 receptor stimulation in vascular smooth muscle cells. Am J Physiol Heart Circ Physiol 268: H544-H549, 1995[Abstract/Free Full Text].

72.   Hofmann, F, Ammendola A, and Schlossmann J. Rising behind NO: cGMP-dependent protein kinases. J Cell Sci 113: 1671-1676, 2000[Abstract].

73.   Hofmann, F, Biel M, and Flockerzi V. Molecular basis for Ca²⁺ channel diversity. Annu Rev Neurosci 17: 399-418, 1994[Web of Science][Medline].

74.   Hofmann, F, Lacinova L, and Klugbauer N. Voltage-dependent calcium channels: from structure to function. Rev Physiol Biochem Pharmacol 139: 33-87, 1999[Web of Science][Medline].

75.   Hofmann, J. The potential for isoenzyme-selective modulation of protein kinase C. FASEB J 11: 649-669, 1997[Abstract].

76.   Hohaus, A, Poteser M, Romanin C, Klugbauer N, Hofmann F, Morano I, Haase H, and Groschner K. Modulation of the smooth-muscle L-type Ca²⁺ channel ₁ subunit (1C-b) by the _2a subunit: a peptide which inhibits binding of  to the I-II linker of ₁ induces functional uncoupling. Biochem J 348: 657-665, 2000.

77.   Hullin, R, Singer-Lahat D, Freichel M, Biel M, Dascal N, Hofmann F, and Flockerzi V. Calcium channel  subunit heterogeneity: functional expression of cloned cDNA from heart, aorta and brain. EMBO J 11: 885-890, 1992[Web of Science][Medline].

78.   Ikeda, SR. Voltage-dependent modulation of N-type calcium channels by G-protein subunits. Nature 380: 255-258, 1996[Medline].

79.   Imoto, Y, Yatani A, Reeves JP, Codina J, Birnbaumer L, and Brown AM. -Subunit of G_s directly activates cardiac calcium channels in lipid bilayers. Am J Physiol Heart Circ Physiol 255: H722-H728, 1988[Abstract/Free Full Text].

80.   Ishikawa, T, Hume JR, and Keef KD. Regulation of Ca²⁺ channels by cAMP and cGMP in vascular smooth muscle cells. Circ Res 73: 1128-1137, 1993[Abstract/Free Full Text].

81.   Itagaki, K, Koch WJ, Bodi I, Klockner U, Slish DF, and Schwartz A. Native-type DHP-sensitive calcium channel currents are produced by cloned rat aortic smooth muscle and cardiac ₁ subunits expressed in Xenopus laevis oocytes and are regulated by alp. FEBS Lett 297: 221-225, 1992[Web of Science][Medline].

82.   Jiang, LH, Gawler DJ, Hodson N, Milligan CJ, Pearson HA, Porter V, and Wray D. Regulation of cloned cardiac L-type calcium channels by cGMP-dependent protein kinase. J Biol Chem 275: 6135-6143, 2000[Abstract/Free Full Text].

83.   Kameyama, M, Hescheler J, Hofmann F, and Trautwein W. Modulation of Ca current during the phosphorylation cycle in the guinea pig heart. Pflügers Arch 407: 123-128, 1986[Web of Science][Medline].

84.   Kamimura, N, Suga S, Wada J, Mio Y, Suzuki T, and Wakui M. Excitatory and inhibitory actions of norepinephrine on the Ba²⁺ current through L-type Ca²⁺ channels of smooth muscle cells of guinea-pig vas deferens. J Cell Physiol 169: 373-379, 1996[Web of Science][Medline].

85.   Kamp, TJ, and Hell JW. Regulation of cardiac L-type calcium channels by protein kinase A and protein kinase C. Circ Res 87: 1095-1102, 2000[Abstract/Free Full Text].

86.   Khoyi, MA, Ishikawa T, Keef KD, and Westfall DP. Ca²⁺-induced inhibition of ⁴⁵Ca²⁺ influx and Ca²⁺ current in smooth muscle of the rat vas deferens. Am J Physiol Cell Physiol 270: C1468-C1477, 1996[Abstract/Free Full Text].

87.   Kimura, M, Osanai T, Okumura K, Suga S, Kanno T, Kamimura N, Horiba N, and Wakui M. Involvement of phosphorylation of -subunit in cAMP-dependent activation of L-type Ca²⁺ channel in aortic smooth muscle-derived A7r5 cells. Cell Signal 12: 63-70, 2000[Web of Science][Medline].

88.   Kirstein, M, Rivet-Bastide M, Hatem S, Benardeau A, Mercadier JJ, and Fischmeister R. Nitric oxide regulates the calcium current in isolated human atrial myocytes. J Clin Invest 95: 794-802, 1995.

89.   Kleppisch, T, Pedersen K, Strubing C, Bosse-Doenecke E, Flockerzi V, Hofmann F, and Hescheler J. Double-pulse facilitation of smooth muscle ₁-subunit Ca²⁺ channels expressed in CHO cells. EMBO J 13: 2502-2507, 1994[Web of Science][Medline].

90.   Klockner, U, Itagaki K, Bodi I, and Schwartz A. Beta-subunit expression is required for cAMP-dependent increase of cloned cardiac and vascular calcium channel currents. Pflügers Arch 420: 413-415, 1992[Web of Science][Medline].

91.   Klugbauer, N, Lacinova L, Marais E, Hobom M, and Hofmann F. Molecular diversity of the calcium channel 2 subunit. J Neurosci 19: 684-691, 1999[Abstract/Free Full Text].

92.   Koch, WJ, Ellinor PT, and Schwartz A. cDNA cloning of a dihydropyridine-sensitive calcium channel from rat aorta. J Biol Chem 265: 17786-17791, 1990[Abstract/Free Full Text].

93.   Koh, SD, and Sanders KM. Modulation of Ca²⁺ current in canine colonic myocytes by cyclic nucleotide-dependent mechanisms. Am J Physiol Cell Physiol 271: C794-C803, 1996[Abstract/Free Full Text].

94.   Kojda, G, and Kottenberg K. Regulation of basal myocardial function by NO. Cardiovasc Res 41: 514-523, 1999[Free Full Text].

95.   Kumar, R, Joyner RW, Komalavilas P, and Lincoln TM. Analysis of expression of cGMP-dependent protein kinase in rabbit heart cells. J Pharmacol Exp Ther 291: 967-975, 1999[Abstract/Free Full Text].

96.   Kumar, R, Namiki T, and Joyner RW. Effects of cGMP on L-type calcium current of adult and newborn rabbit ventricular cells. Cardiovasc Res 33: 573-582, 1997[Abstract/Free Full Text].

97.   Lacerda, AE, Rampe D, and Brown AM. Effects of protein kinase C activators on cardiac Ca²⁺ channels. Nature 335: 249-251, 1988[Medline].

98.   Lader, AS, Xiao YF, Ishikawa Y, Cui Y, Vatner DE, Vatner SF, Homcy CJ, and Cantiello HF. Cardiac G_s overexpression enhances L-type calcium channels through an adenylyl cyclase independent pathway. Proc Natl Acad Sci USA 95: 9669-9674, 1998[Abstract/Free Full Text].

99.   Lai, Y, Seagar MJ, Takahashi M, and Catterall WA. Cyclic AMP-dependent phosphorylation of two size forms of alpha 1 subunits of L-type calcium channels in rat skeletal muscle cells. J Biol Chem 265: 20839-20848, 1990[Abstract/Free Full Text].

100.   Larsson, O, Barker CJ, Sj-oholm A, Carlqvist H, Michell RH, Bertorello A, Nilsson T, Honkanen RE, Mayr GW, Zwiller J, and Berggren PO. Inhibition of phosphatases and increased Ca²⁺ channel activity by inositol hexakisphosphate. Science 278: 471-474, 1997[Abstract/Free Full Text].

101.   Lepretre, N, and Mironneau J. Alpha 2-adrenoceptors activate dihydropyridine-sensitive calcium channels via G_i-proteins and protein kinase C in rat portal vein myocytes. Pflügers Arch 429: 253-261, 1994[Web of Science][Medline].

102.   Lepretre, N, Mironneau J, and Morel JL. Both _1A- and _2A-adrenoreceptor subtypes stimulate voltage-operated L-type calcium channels in rat portal vein myocytes. Evidence for two distinct transduction pathways. J Biol Chem 269: 29546-29552, 1994[Abstract/Free Full Text].

103.   Levi, RC, Alloatti G, Penna C, and Gallo MP. Guanylate-cyclase-mediated inhibition of cardiac I_Ca by carbachol and sodium nitroprusside. Pflügers Arch 426: 419-426, 1994[Web of Science][Medline].

104.   Lincoln, TM, and Cornwell TL. Intracellular cyclic GMP receptor proteins. FASEB J 7: 328-338, 1993[Abstract].

105.   Lincoln, TM, Cornwell TL, and Taylor AE. cGMP-dependent protein kinase mediates the reduction of Ca²⁺ by cAMP in vascular smooth muscle cells. Am J Physiol Cell Physiol 258: C399-C407, 1990[Abstract/Free Full Text].

106.   Lincoln, TM, and Keely SL. Regulation of cardiac cyclic GMP-dependent protein kinase. Biochim Biophys Acta 676: 230-244, 1981[Medline].

107.   Liu, H, Li K, and Sperelakis N. Tyrosine kinase inhibitor, genistein, inhibits macroscopic L-type calcium current in rat portal vein smooth muscle cells. Can J Physiol Pharmacol 75: 1058-1062, 1997[Web of Science][Medline].

108.   Liu, H, and Sperelakis N. Tyrosine kinases modulate the activity of single L-type calcium channels in vascular smooth muscle cells from rat portal vein. Can J Physiol Pharmacol 75: 1063-1068, 1997[Web of Science][Medline].

109.   Liu, H, Xiong Z, and Sperelakis N. Cyclic nucleotides regulate the activity of L-type calcium channels in smooth muscle cells from rat portal vein. J Mol Cell Cardiol 29: 1411-1421, 1997[Web of Science][Medline].

110.   Liu, QY, Karpinski E, and Pang PK. Comparison of the action of two protein kinase C activators on dihydropyridine-sensitive Ca²⁺ channels in neonatal rat ventricular myocytes. Biochem Biophys Res Commun 191: 796-801, 1993[Web of Science][Medline].

111.   Lorenz, JN, Bielefeld DR, and Sperelakis N. Regulation of calcium channel current in A7r5 vascular smooth muscle cells by cyclic nucleotides. Am J Physiol Cell Physiol 266: C1656-C1663, 1994[Abstract/Free Full Text].

112.   Macrez, N, Morel JL, Kalkbrenner F, Viard P, Schultz G, and Mironneau J. A dimer derived from G₁₃ transduces the angiotensin AT₁ receptor signal to stimulation of Ca²⁺ channels in rat portal vein myocytes. J Biol Chem 272: 23180-23185, 1997[Abstract/Free Full Text].

113.   Marks, TN, Dubyak GR, and Jones SW. Calcium currents in the A7r5 smooth muscle-derived cell line. Pflügers Arch 417: 433-439, 1990[Web of Science][Medline].

114.   McDonald, TF, Pellzer S, Trautwein W, and Pelzer D. Regulation and modulation of calcium channels in cardiac, skeletal, and smooth muscle cells. Physiol Rev 74: 365-507, 1994[Free Full Text].

115.   McHugh, D, Sharp EM, Scheuer T, and Catterall WA. Inhibition of cardiac L-type calcium channels by protein kinase C phosphorylation of two sites in the N-terminal domain. Proc Natl Acad Sci USA 97: 12334-12338, 2000[Abstract/Free Full Text].

116.   Mery, PF, Lohmann SM, Walter U, and Fischmeister R. Ca²⁺ current is regulated by cyclic GMP-dependent protein kinase in mammalian cardiac myocytes. Proc Natl Acad Sci USA 88: 1197-1201, 1991[Abstract/Free Full Text].

117.   Mery, PF, Pavoine C, Pecker F, and Fischmeister R. Erythro-9-(2-hydroxy-3-nonyl)adenine inhibits cyclic GMP-stimulated phosphodiesterase in isolated cardiac myocytes. Mol Pharmacol 48: 121-130, 1995[Abstract].

118.   Mikala, G, Klockner U, Varadi M, Eisfeld J, Schwartz A, and Varadi G. cAMP-dependent phosphorylation sites and macroscopic activity of recombinant cardiac L-type calcium channels. Mol Cell Biochem 185: 95-109, 1998[Web of Science][Medline].

119.   Mikami, A, Imoto K, Tanabe T, Niidome T, Mori Y, Takeshima H, Narumiya S, and Numa S. Primary structure and functional expression of the cardiac dihydropyridine-sensitive calcium channel. Nature 340: 230-233, 1989[Medline].

120.   Mitterdorfer, J, Froschmayr M, Grabner M, Moebius FF, Glossmann H, and Striessnig J. Identification of PK-A phosphorylation sites in the carboxyl terminus of L-type calcium channel alpha 1 subunits. Biochemistry 35: 9400-9406, 1996[Medline].

121.   Muraki, K, Bolton TB, Imaizumi Y, and Watanabe M. Effect of isoprenaline on Ca²⁺ channel current in single smooth muscle cells isolated from taenia of the guinea-pig caecum. J Physiol (Lond) 471: 563-582, 1993[Abstract/Free Full Text].

122.   Neely, A, Olcese R, Baldelli P, Wei X, Birnbaumer L, and Stefani E. Dual activation of the cardiac Ca²⁺ channel _1C-subunit and its modulation by the -subunit. Am J Physiol Cell Physiol 268: C732-C740, 1995[Abstract/Free Full Text].

123.   Neumann, J, Boknik P, Herzig S, Schmitz W, Scholz H, Wiechen K, and Zimmermann N. Biochemical and electrophysiological mechanisms of the positive inotropic effect of calyculin A, a protein phosphatase inhibitor. J Pharmacol Exp Ther 271: 535-541, 1994[Abstract/Free Full Text].

124.   Obara, K, and Yabu H. Dual effect of phosphatase inhibitors on calcium channels in intestinal smooth muscle cells. Am J Physiol Cell Physiol 264: C296-C301, 1993[Abstract/Free Full Text].

125.   Obejero-Paz, CA, Auslender M, and Scarpa A. PKC activity modulates availability and long openings of L-type Ca²⁺ channels in A7r5 cells. Am J Physiol Cell Physiol 275: C535-C543, 1998[Abstract/Free Full Text].

126.   Ogura, T, Shuba LM, and McDonald TF. L-type Ca²⁺ current in guinea pig ventricular myocytes treated with modulators of tyrosine phosphorylation. Am J Physiol Heart Circ Physiol 276: H1724-H1733, 1999[Abstract/Free Full Text].

127.   Ohya, Y, Kitamura K, and Kuriyama H. Modulation of ionic currents in smooth muscle balls of the rabbit intestine by intracellularly perfused ATP and cyclic AMP. Pflügers Arch 408: 465-473, 1987[Web of Science][Medline].

128.   Ono, K, and Trautwein W. Potentiation by cyclic GMP of beta-adrenergic effect on Ca²⁺ current in guinea-pig ventricular cells. J Physiol (Lond) 443: 387-404, 1991[Abstract/Free Full Text].

129.   Pate, P, Mochca-Morales J, Wu Y, Zhang JZ, Rodney GG, Serysheva II, Williams BY, Anderson ME, and Hamilton SL. Determinants for calmodulin binding on voltage-dependent Ca²⁺ channels. J Biol Chem 275: 39786-39792, 2000[Abstract/Free Full Text].

130.   Pelzer, S, Shuba YM, Asai T, Codina J, Birnbaumer L, McDonald TF, and Pelzer D. Membrane-delimited stimulation of heart cell calcium current by -adrenergic signal-transducing G_s protein. Am J Physiol Heart Circ Physiol 259: H264-H267, 1990[Abstract/Free Full Text].

131.   Perets, T, Blumenstein Y, Shistik E, Lotan I, and Dascal N. A potential site of functional modulation by protein kinase A in the cardiac Ca²⁺ channel _1C subunit. FEBS Lett 384: 189-192, 1996[Web of Science][Medline].

132.   Perez-Reyes, E, Yuan W, Wei X, and Bers DM. Regulation of the cloned L-type cardiac calcium channel by cyclic-AMP-dependent protein kinase. FEBS Lett 342: 119-123, 1994[Web of Science][Medline].

133.   Peterson, BZ, Lee JS, Mulle JG, Wang Y, de Leon M, and Yue DT. Critical determinants of Ca²⁺-dependent inactivation within an EF-hand motif of L-type Ca²⁺ channels. Biophys J 78: 1906-1920, 2000[Web of Science][Medline].

134.   Pichler, M, Cassidy TN, Reimer D, Haase H, Kraus R, Ostler D, and Striessnig J. Beta subunit heterogeneity in neuronal L-type Ca²⁺ channels. J Biol Chem 272: 13877-13882, 1997[Abstract/Free Full Text].

135.   Polson, JB, and Strada SJ. Cyclic nucleotide phosphodiesterases and vascular smooth muscle. Annu Rev Pharmacol Toxicol 36: 403-427, 1996[Web of Science][Medline].

136.   Pragnell, M, De Waard M, Mori Y, Tanabe T, Snutch TP, and Campbell KP. Calcium channel -subunit binds to a conserved motif in the I-II cytoplasmic linker of the ₁-subunit. Nature 368: 67-70, 1994[Medline].

137.   Puri, TS, Gerhardstein BL, Zhao XL, Ladner MB, and Hosey MM. Differential effects of subunit interactions on protein kinase A- and C-mediated phosphorylation of L-type calcium channels. Biochemistry 36: 9605-9615, 1997[Medline].

138.   Qin, N, Platano D, Olcese R, Costantin JL, Stefani E, and Birnbaumer L. Unique regulatory properties of the type 2a Ca²⁺ channel  subunit caused by palmitoylation. Proc Natl Acad Sci USA 95: 4690-4695, 1998[Abstract/Free Full Text].

139.   Quignard, JF, Frapier JM, Harricane MC, Albat B, Nargeot J, and Richard S. Voltage-gated calcium channel currents in human coronary myocytes. Regulation by cyclic GMP and nitric oxide. J Clin Invest 99: 185-193, 1997[Web of Science][Medline].

140.   Rabe, KF, Tenor H, Dent G, Schudt C, Nakashima M, and Magnussen H. Identification of PDE isozymes in human pulmonary artery and effect of selective PDE inhibitors. Am J Physiol Lung Cell Mol Physiol 266: L536-L543, 1994[Abstract/Free Full Text].

141.   Rascon, A, Lindgren S, Stavenow L, Belfrage P, Andersson KE, Manganiello VC, and Degerman E. Purification and properties of the cGMP-inhibited cAMP phosphodiesterase from bovine aortic smooth muscle. Biochim Biophys Acta 1134: 149-156, 1992[Medline].

142.   Romanin, C, Gamsjaeger R, Kahr H, Schaufler D, Carlson O, Abernethy DR, and Soldatov NM. Ca²⁺ sensors of L-type Ca²⁺ channel. FEBS Lett 487: 301-306, 2000[Web of Science][Medline].

143.   Ruiz-Velasco, V, and Ikeda SR. Multiple G-protein combinations produce voltage-dependent inhibition of N-type calcium channels in rat superior cervical ganglion neurons. J Neurosci 20: 2183-2191, 2000[Abstract/Free Full Text].

144.   Ruiz-Velasco, V, Zhong J, Hume JR, and Keef KD. Modulation of Ca²⁺ channels by cyclic nucleotide cross activation of opposing protein kinases in rabbit portal vein. Circ Res 82: 557-565, 1998[Abstract/Free Full Text].

145.   Sakai, R, Shen JB, and Pappano AJ. Elevated cAMP suppresses muscarinic inhibition of L-type calcium current in guinea pig ventricular myocytes. J Cardiovasc Pharmacol 34: 304-315, 1999[Web of Science][Medline].

146.   Sausbier, M, Schubert R, Voigt V, Hirneiss C, Pfeifer A, Korth M, Kleppisch T, Ruth P, and Hofmann F. Mechanisms of NO/cGMP-dependent vasorelaxation. Circ Res 87: 825-830, 2000[Abstract/Free Full Text].

147.   Schuhmann, K, and Groschner K. Protein kinase-C mediates dual modulation of L-type Ca²⁺ channels in human vascular smooth muscle. FEBS Lett 341: 208-212, 1994[Web of Science][Medline].

148.   Schuhmann, K, Romanin C, Baumgartner W, and Groschner K. Intracellular Ca²⁺ inhibits smooth muscle L-type Ca²⁺ channels by activation of protein phosphatase type 2B and by direct interaction with the channel. J Gen Physiol 110: 503-513, 1997[Abstract/Free Full Text].

150.   Sculptoreanu, A, Rotman E, Takahashi M, Scheuer T, and Catterall WA. Voltage-dependent potentiation of the activity of cardiac L-type calcium channel alpha 1 subunits due to phosphorylation by cAMP-dependent protein kinase. Proc Natl Acad Sci USA 90: 10135-10139, 1993[Abstract/Free Full Text].

151.   Shi, QY, and Cox RH. GTP requirement for isoproterenol activation of calcium channels in vascular myocytes. Am J Physiol Heart Circ Physiol 269: H195-H202, 1995[Abstract/Free Full Text].

152.   Shimamura, K, Kusaka M, and Sperelakis N. Protein kinase C stimulates Ca²⁺ current in pregnant rat myometrial cells. Can J Physiol Pharmacol 72: 1304-1307, 1994[Web of Science][Medline].

153.   Shistik, E, Ivanina T, Blumenstein Y, and Dascal N. Crucial role of N terminus in function of cardiac L-type Ca²⁺ channel and its modulation by protein kinase C. J Biol Chem 273: 17901-17909, 1998[Abstract/Free Full Text].

154.   Shistik, E, Ivanina T, Puri T, Hosey M, and Dascal N. Ca²⁺ current enhancement by ₂/ and  subunits in Xenopus oocytes: contribution of changes in channel gating and ₁ protein level. J Physiol (Lond) 489: 55-62, 1995[Abstract/Free Full Text].

155.   Shistik, E, Keren-Raifman T, Idelson GH, Blumenstein Y, Dascal N, and Ivanina T. The N terminus of the cardiac L-type Ca²⁺ channel _1C subunit. The initial segment is ubiquitous and crucial for protein kinase C modulation, but is not directly phosphorylated. J Biol Chem 274: 31145-31149, 1999[Abstract/Free Full Text].

156.   Shuba, YM, Hesslinger B, Trautwein W, McDonald TF, and Pelzer D. Whole-cell calcium current in guinea-pig ventricular myocytes dialysed with guanine nucleotides. J Physiol (Lond) 424: 205-228, 1990[Abstract/Free Full Text].

157.   Sims, C, Chiu J, and Harvey RD. Tyrosine phosphatase inhibitors selectively antagonize -adrenergic receptor-dependent regulation of cardiac ion channels. Mol Pharmacol 58: 1213-1221, 2000[Abstract/Free Full Text].

158.   Singer-Lahat, D, Gershon E, Lotan I, Hullin R, Biel M, Flockerzi V, Hofmann F, and Dascal N. Modulation of cardiac Ca²⁺ channels in Xenopus oocytes by protein kinase C. FEBS Lett 306: 113-118, 1992[Web of Science][Medline].

159.   Singer-Lahat, D, Lotan I, Biel M, Flockerzi V, Hofmann F, and Dascal N. Cardiac calcium channels expressed in Xenopus oocytes are modulated by dephosphorylation but not by cAMP-dependent phosphorylation. Receptors Channels 2: 215-226, 1994[Web of Science][Medline].

160.   Snutch, TP, and Reiner PB. Ca²⁺ channels: diversity of form and function. Curr Opin Neurobiol 2: 247-253, 1992[Medline].

161.   Sperelakis, N, Xiong Z, Haddad G, and Masuda H. Regulation of slow calcium channels of myocardial cells and vascular smooth muscle cells by cyclic nucleotides and phosphorylation. Mol Cell Biochem 140: 103-117, 1994[Web of Science][Medline].

162.   Stea, A, Soong TW, and Snutch TP. Voltage gated calcium channels. In: Handbook of Receptors and Channels. Boca Raton, FL: CRC, 1995, p. 113-150.

163.   Striessnig, J. Pharmacology, structure and function of cardiac L-type Ca²⁺ channels. Cell Physiol Biochem 9: 242-269, 1999[Web of Science][Medline].

164.   Taguchi, K, Ueda M, and Kubo T. Effects of cAMP and cGMP on L-type calcium channel currents in rat mesenteric artery cells. Jpn J Pharmacol 74: 179-186, 1997[Medline].

165.   Tasken, K, Skalhegg BS, Tasken KA, Solberg R, Knutsen HK, Levy FO, Sandberg M, Orstavik S, Larsen T, Johansen AK, Vang T, Schrader HP, Reinton NT, Torgersen KM, Hansson V, and Jahnsen T. Structure, function, and regulation of human cAMP-dependent protein kinases. Adv Second Messenger Phosphoprotein Res 31: 191-204, 1997[Web of Science][Medline].

166.   Tewari, K, and Simard JM. Protein kinase A increases availability of calcium channels in smooth muscle cells from guinea pig basilar artery. Pflügers Arch 428: 9-16, 1994[Web of Science][Medline].

167.   Tewari, K, and Simard JM. Sodium nitroprusside and cGMP decrease Ca²⁺ channel availability in basilar artery smooth muscle cells. Pflügers Arch 433: 304-311, 1997[Web of Science][Medline].

168.   Tiaho, F, Piot C, Nargeot J, and Richard S. Regulation of the frequency-dependent facilitation of L-type Ca²⁺ currents in rat ventricular myocytes. J Physiol (Lond) 477: 237-251, 1994[Abstract/Free Full Text].

169.   Tohse, N, Nakaya H, Takeda Y, and Kanno M. Cyclic GMP-mediated inhibition of L-type Ca²⁺ channel activity by human natriuretic peptide in rabbit heart cells. Br J Pharmacol 114: 1076-1082, 1995[Web of Science][Medline].

170.   Tohse, N, and Sperelakis N. cGMP inhibits the activity of single calcium channels in embryonic chick heart cells. Circ Res 69: 325-331, 1991[Abstract/Free Full Text].

171.   Trautwein, W, Cavalie A, Allen TJ, Shuba YM, Pelzer S, and Pelzer D. Direct and indirect regulation of cardiac L-type calcium channels by -adrenoreceptor agonists. Adv Second Messenger Phosphoprotein Res 24: 45-50, 1990[Web of Science][Medline].

172.   Tseng, GN, and Boyden PA. Different effects of intracellular Ca and protein kinase C on cardiac T and L Ca currents. Am J Physiol Heart Circ Physiol 261: H364-H379, 1991[Abstract/Free Full Text].

173.   Tsien, RW, Bean BP, Hess P, Lansman JB, Nilius B, and Nowycky MC. Mechanisms of calcium channel modulation by beta-adrenergic agents and dihydropyridine calcium agonists. J Mol Cell Cardiol 18: 691-710, 1986[Web of Science][Medline].

174.   Ueda, N, Iniguez-Lluhi JA, Lee E, Smrcka AV, Robishaw JD, and Gilman AG. G protein subunits. Simplified purification and properties of novel isoforms. J Biol Chem 269: 4388-4395, 1994[Abstract/Free Full Text].

175.   Usuki, T, Obara K, Someya T, Ozaki H, Karaki H, Fusetani N, and Yabu H. Calyculin A increases voltage-dependent inward current in smooth muscle cells isolated from guinea pig taenia coli. Experientia 47: 939-941, 1991[Web of Science][Medline].

176.   Varadi, G, Mori Y, Mikala G, and Schwartz A. Molecular determinants of Ca²⁺ channel function and drug action. Trends Pharmacol Sci 16: 43-49, 1995[Medline].

177.   Verde, I, Vandecasteele G, Lezoualc'h F, and Fischmeister R. Characterization of the cyclic nucleotide phosphodiesterase subtypes involved in the regulation of the L-type Ca²⁺ current in rat ventricular myocytes. Br J Pharmacol 127: 65-74, 1999[Web of Science][Medline].

178.   Viard, P, Exner T, Maier U, Mironneau J, Nurnberg B, and Macrez N. G dimers stimulate vascular L-type Ca²⁺ channels via phosphoinositide 3-kinase. FASEB J 13: 685-694, 1999[Abstract/Free Full Text].

179.   Wahler, FM, and Dollinger SJ. Nitric oxide donor SIN-1 inhibits mammalian cardiac calcium current through cGMP-dependent protein kinase. Am J Physiol Cell Physiol 268: C45-C54, 1995[Abstract/Free Full Text].

180.   Walsh, MP, Kargacin GJ, Kendrick-Jones J, and Lincoln TM. Intracellular mechanisms involved in the regulation of vascular smooth muscle tone. Can J Physiol Pharmacol 73: 565-573, 1995[Web of Science][Medline].

181.   Wang, YG, and Lipsius SL. Genistein elicits biphasic effects on L-type Ca²⁺ current in feline atrial myocytes. Am J Physiol Heart Circ Physiol 275: H204-H212, 1998[Abstract/Free Full Text].

182.   Wei, X, Neely A, Lacerda AE, Olcese R, Stefani E, Perez-Reyes E, and Birnbaumer L. Modification of Ca²⁺ channel activity by deletions at the carboxyl terminus of the cardiac alpha 1 subunit. J Biol Chem 269: 1635-1640, 1994[Abstract/Free Full Text].

183.   Wei, X, Pan S, Lang W, Kim H, Schneider T, Perez-Reyes E, and Birnbaumer L. Molecular determinants of cardiac Ca= channel pharmacology. Subunit requirement for the high affinity and allosteric regulation of dihydropyridine binding. J Biol Chem 270: 27106-27111, 1995[Abstract/Free Full Text].

184.   Welling, A, Felbel J, Peper K, and Hofmann F. Hormonal regulation of calcium current in freshly isolated airway smooth muscle cells. Am J Physiol Lung Cell Mol Physiol 262: L351-L359, 1992[Abstract/Free Full Text].

185.   Welling, A, Ludwig A, Zimmer S, Klugbauer N, Flockerzi V, and Hofmann F. Alternatively spliced IS6 segments of the alpha 1C gene determine the tissue-specific dihydropyridine sensitivity of cardiac and vascular smooth muscle L-type Ca²⁺ channels. Circ Res 81: 526-532, 1997[Abstract/Free Full Text].

186.   White, RE, Kryman JP, El Mowafy AM, Han G, and Carrier GO. cAMP-dependent vasodilators cross-activate the cGMP-dependent protein kinase to stimulate BK_Ca channel activity in coronary artery smooth muscle cells. Circ Res 86: 897-905, 2000[Abstract/Free Full Text].

187.   White, RE, Lee AB, Shcherbatko AD, Lincoln TM, Schonbrunn A, and Armstrong DL. Potassium channel stimulation by natriuretic peptides through cGMP-dependent dephosphorylation. Nature 361: 263-266, 1993[Medline].

188.   Wijetunge, S, Aalkjaer C, Schachter M, and Hughes AD. Tyrosine kinase inhibitors block calcium channel currents in vascular smooth muscle cells. Biochem Biophys Res Commun 189: 1620-1623, 1992[Web of Science][Medline].

189.   Wijetunge, S, and Hughes AD. Activation of endogenous c-Src or a related tyrosine kinase by intracellular (pY)EEI peptide increases voltage-operated calcium channel currents in rabbit ear artery cells. FEBS Lett 399: 63-66, 1996[Web of Science][Medline].

190.   Wijetunge, S, Lymn JS, and Hughes AD. Effect of inhibition of tyrosine phosphatases on voltage-operated calcium channel currents in rabbit isolated ear artery cells. Br J Pharmacol 124: 307-316, 1998[Web of Science][Medline].

191.   Wijetunge, S, Lymn JS, and Hughes AD. Effects of protein tyrosine kinase inhibitors on voltage-operated calcium channel currents in vascular smooth muscle cells and pp60^c-src kinase activity. Br J Pharmacol 129: 1347-1354, 2000[Web of Science][Medline].

192.   Wu, Y, MacMillan LB, McNeill RB, Colbran RJ, and Anderson ME. CaM kinase augments cardiac L-type Ca²⁺ current: a cellular mechanism for long Q-T arrhythmias. Am J Physiol Heart Circ Physiol 276: H2168-H2178, 1999[Abstract/Free Full Text].

193.   Xiong, Z, and Sperelakis N. Regulation of L-type calcium channels of vascular smooth muscle cells. J Mol Cell Cardiol 27: 75-91, 1995[Web of Science][Medline].

194.   Xiong, Z, Sperelakis N, and Fenoglio-Preiser C. Isoproterenol modulates the calcium channels through two different mechanisms in smooth-muscle cells from rabbit portal vein. Pflügers Arch 428: 105-113, 1994[Web of Science][Medline].

195.   Xiong, Z, Sperelakis N, and Fenoglio-Preiser C. Regulation of L-type calcium channels by cyclic nucleotides and phosphorylation in smooth muscle cells from rabbit portal vein. J Vasc Res 31: 271-279, 1994[Web of Science][Medline].

196.   Yamaguchi, H, Hara M, Strobeck M, Fukasawa K, Schwartz A, and Varadi G. Multiple modulation pathways of calcium channel activity by a  subunit. Direct evidence of  subunit participation in membrane trafficking of the _1C subunit. J Biol Chem 273: 19348-19356, 1998[Abstract/Free Full Text].

197.   Yatani, A, Codina J, Imoto Y, Reeves JP, Birnbaumer L, and Brown AM. A G protein directly regulates mammalian cardiac calcium channels. Science 238: 1288-1292, 1987[Abstract/Free Full Text].

198.   Yatani, A, Tajima Y, and Green SA. Coupling of -adrenergic receptors to cardiac L-type Ca²⁺ channels: preferential coupling of the ₁ versus ₂ receptor subtype and evidence for PKA-independent activation of the channel. Cell Signal 11: 337-342, 1999[Web of Science][Medline].

199.   Yatani, A, Wakamori M, Niidome T, Yamamoto S, Tanaka I, Mori Y, Katayama K, and Green S. Stable expression and coupling of cardiac L-type Ca²⁺ channels with ₁-adrenoceptors. Circ Res 76: 335-342, 1995[Abstract/Free Full Text].

200.   Yokoshiki, H, Katsube Y, and Sperelakis N. Regulation of Ca²⁺ channel currents by intracellular ATP in smooth muscle cells of rat mesenteric artery. Am J Physiol Heart Circ Physiol 272: H814-H819, 1997[Abstract/Free Full Text].

201.   Yokoshiki, H, Sumii K, and Sperelakis N. Inhibition of L-type calcium current in rat ventricular cells by the tyrosine kinase inhibitor, genistein and its inactive analog, daidzein. J Mol Cell Cardiol 28: 807-814, 1996[Web of Science][Medline].

202.   Yoshida, A, Takahashi M, Nishimura S, Takeshima H, and Kokubun S. Cyclic AMP-dependent phosphorylation and regulation of the cardiac dihydropyridine-sensitive Ca channel. FEBS Lett 309: 343-349, 1992[Web of Science][Medline].

203.   Yuan, W, and Bers DM. Ca-dependent facilitation of cardiac Ca current is due to Ca-calmodulin-dependent protein kinase. Am J Physiol Heart Circ Physiol 267: H982-H993, 1994[Abstract/Free Full Text].

204.   Zhang, ZH, Johnson JA, Chen L, El Sherif N, Mochly-Rosen D, and Boutjdir M. C2 region-derived peptides of -protein kinase C regulate cardiac Ca²⁺ channels. Circ Res 80: 720-729, 1997[Abstract/Free Full Text].

205.   Zhong, J, Dessauer CW, Keef KD, and Hume JR. Regulation of L-type Ca²⁺ channels in rabbit portal vein by G protein _s and subunits. J Physiol (Lond) 517: 109-120, 1999[Abstract/Free Full Text].

206.   Zhong, J, Hume JR, and Keef KD. Anchoring protein is required for cAMP-dependent stimulation of L-type Ca²⁺ channels in rabbit portal vein. Am J Physiol Cell Physiol 277: C840-C844, 1999[Abstract/Free Full Text].

207.  Zhong J, Hume JR, and Keef KD. -Adrenergic stimulation of vascular L-type Ca²⁺ channels involves both G-protein _s and subunits. J Physiol. In press.

208.   Zhong, J, Keef KD, and Hume JR. -Adrenergic stimulation of cardiac L-type Ca²⁺ channels involves both PKA and PKC pathways (Abstract). Biophys J 78: 101A, 2000.

209.   Zong, X, Schreieck J, Mehrke G, Welling A, Schuster A, Bosse E, Flockerzi V, and Hofmann F. On the regulation of the expressed L-type calcium channel by cAMP-dependent phosphorylation. Pflügers Arch 430: 340-347, 1995[Web of Science][Medline].

210.   Zuhlke, RD, Pitt GS, Deisseroth K, Tsien RW, and Reuter H. Calmodulin supports both inactivation and facilitation of L-type calcium channels. Nature 399: 159-162, 1999[Medline].

211.   Zuhlke, RD, Pitt GS, Tsien RW, and Reuter H. Ca²⁺-sensitive inactivation and facilitation of L-type Ca²⁺ channels both depend on specific amino acid residues in a consensus calmodulin-binding motif in the _1C subunit. J Biol Chem 275: 21121-21129, 2000[Abstract/Free Full Text].