The concept that the cystic fibrosis (CF) transmembrane conductance regulator, the protein product of the CF gene, can conduct larger multivalent anions such as ATP as well as Cl− is controversial. In this review, I examine briefly past findings that resulted in controversy. It is not the goal of this review to revisit these disparate findings in detail. Rather, I focus intently on more recent studies, current studies in progress, and possible future directions that arose from the controversy and that may reconcile this issue. Important questions and hypotheses are raised as to the physiological roles that ATP-binding cassette (ABC) transporter-facilitated ATP transport and signaling may play in the control of epithelial cell function. Perhaps the identification of key biological paradigms for ABC transporter-mediated extracellular nucleotide signaling may unify and guide the CF research community and other research groups interested in ABC transporters toward understanding why ABC transporters facilitate ATP transport.
- cystic fibrosis transmembrane conductance regulator
- multidrug resistance protein
- ion transport
- cystic fibrosis
- ATP-binding cassette
many laboratories have observed ATP channels, ATP transport, and/or ATP release in nonepithelial and epithelia cell systems expressing an ATP-binding cassette (ABC) transporter (3,29, 33, 34, 36, 41, 43). These ATP transport phenotypes were observed upon transient or stable expression of epithelial and/or heterologous cells with wild-type cystic fibrosis transmembrane conductance regulator (CFTR) or wild-type multidrug resistance transporter (MDR) (3, 29, 33, 36, 41, 43). These studies also observed 8–10 pS CFTR Cl− channel activity in the same recordings (3, 29, 33, 36, 41, 43). For CFTR, cAMP was the stimulus for both Cl− and ATP channels. Similar data were published concerning P-glycoprotein or MDR1; however, a stimulus was not required in this study (3). When CFTR or MDR was not present, ATP channels and Cl− channels were lacking (3, 29, 33, 36, 41). The simplest interpretation of these results was that ABC transporters conducted ATP (Fig.1). An alternative interpretation was that ABC transporters, as “conductance regulators,” may stimulate the activity of a separate ATP channel protein (Fig. 1). This latter conclusion is also valid, considering the wealth of published results showing effects of CFTR expression on epithelial Na+ channel (ENaC) function (39), outwardly rectifying Cl− channel (ORCC) regulation (36), and renal outer medulla K+ channel (ROMK) sensitivity to sulfonylureas (27). The concept of CFTR as a conductance regulator has been reviewed in detail elsewhere (35, 37).
Other laboratories, however, could not reproduce these results in CFTR-expressing airway cells (31), in sweat duct (31), in heterologous cells overexpressing CFTR (16-18, 31), or in planar lipid bilayers containing purified CFTR protein (24, 31). In these studies, CFTR Cl− channel activity was observed routinely without any appearance of ATP channels (16-18, 24, 31, 32). The only conclusion that could be made from these studies at the time was that CFTR itself cannot conduct ATP. These studies were performed well and cast doubt on the positive results listed above. In fact, most, if not all, experiments on both sides of the controversy have been controlled carefully, and only some of the laboratories have observed CFTR-associated ATP channels. Rather than revisit and debate all of the studies on both sides of this issue in detail, they are referenced in this review (2, 3, 16-18, 24,26, 29, 33, 36, 41). These findings have been debated within “Technical Comments” of the journalScience and have been reviewed recently (4, 8, 13, 35, 37, 38).
Laboratories that have and have not observed ATP channels on transduction of cells with wild-type CFTR have each performed valid experiments and have arrived at the appropriate conclusions. Two laboratories arrived at opposite conclusions using the same stably transfected Chinese hamster ovary (CHO) cells and single-channel patch-clamp recordings (18, 29). Both laboratories used bi-ionic conditions in which Cl− and ATP were on opposite sides of the membrane (18, 29). Technical differences in the way patch-clamp recordings are achieved may explain these disparate results. Some patch-clamp electrophysiologists use negative pressure or suction when obtaining seals and some do not. The degree of negative pressure, used to obtain gigaohm seals, may be critical for the appearance of ATP channels, especially if they are mechanosensitive ATP channels. Often, differences in cell culture conditions may change the phenotype of the cell or clone. Differences in the source of serum, in the lot number of serum, growth factors, or other media additives, or even in the basal medium can change the phenotype of a cell. Moreover, whether or not cells are grown on an extracellular matrix like collagen, fibronectin, or both during passage culture or while on substrates used for patch clamping can also influence results.
However, enough laboratories have observed ATP channels and ATP release that they may indeed exist and may be of physiological relevance. Perhaps a key cell culture condition is necessary to trigger CFTR-induced ATP transport out of the cell. A cofactor, the ATP channel or a regulatory protein, may be required to uncover the ATP release phenotype facilitated by ABC transporters. Perhaps stimuli alternative to or in addition to cAMP may be required to observe a robust ATP channel activity. It is possible that the laboratories that failed to observe CFTR-associated ATP channels did not dig deeply enough. Alternatively, conductive transport of ATP may exist; but, is it the most physiologically relevant source of ATP transport or release to fuel extracellular ATP signaling? From the phenomena observed in other systems such as platelets, sensory neurons, or neuroendocrine cells, exocytic release of ATP may be as important a mechanism or a more important mechanism of ATP release. These are issues that are directly relevant to this controversy and must be addressed.
CONTROVERSY BRINGS NEW DIRECTIONS
Recently published studies may bring us closer to an answer. Foskett and colleagues were among those that observed CFTR-associated ATP channels along with Cl− channel activity. They observed this activity in both intracellular membranes as well as plasma membranes of CHO cells stably transfected with wild-type CFTR. They argued that this function of CFTR may not only allow ATP transport across membranes but may also allow the “loading” of sulfated nucleotides (adenosine 3′-phosphate 5′-phosphosulfate) that may act as sulfate donors for posttranslational modification of proteins (29). Recently, they have added to this work and have shown that ATP channel activity observed in parental and wild-type CFTR-transfected MDCK cells does not correlate fully with CFTR expression, suggesting that CFTR may gate a separate yet closely associated ATP channel (41). Parental Madin-Darby canine kidney (MDCK) cells thought to lack CFTR did have ATP channel activity. Although different clones of MDCK cells do and do not have cAMP-stimulated Cl− secretion and express varied amounts of CFTR, these results suggest, albeit indirectly, that the ATP channel is a separate protein from CFTR. The clone of MDCK cell used in this study was not identified. An elaborate study of many CF disease-associated mutations in CFTR known to affect Cl− channel activity also revealed that protein kinase A (PKA)- and ATP-dependent gating of ATP channels is observed as well as similar gating of Cl− channels. Thus these authors concluded that CFTR is involved in gating or regulating the opening of an ATP channel that is likely, but not proven, to be separate from CFTR (Fig. 1) (41).
Hanrahan and colleagues (18, 31) were among those whose laboratories failed to observe ATP channel activity. They used the same stably transfected cells expressing wild-type CFTR that were used by Foskett and co-workers to characterize CFTR-associated ATP channels. Recently, however, Linsdell and Hanrahan (26) have shown that wild-type CFTR expressed in either baby hamster kidney (BHK) or CHO cells is permeant to a plethora of large organic anions including gluconate, pyruvate, formate, and isothionate. This permeation was asymmetrical; in other words, PKA- and ATP-stimulated CFTR only transported these large anions when the large anion was present on the cytoplasmic side of the membrane patch (26). ATPase inhibitors that prevent ATP hydrolysis and “lock open” CFTR Cl−channels (pyrophosphate, 5′-adenylylimidodiphosphate) allowed permeation to become symmetrical (26). Both Cl− and gluconate currents were inhibited by insertion of two well-known Cl− conduction mutations, K335E and R347D, suggesting that CFTR itself is transporting gluconate (26). These results suggest a new function for CFTR, the asymmetrical transport of large organic anions out of epithelial cells. The authors are careful to point out that the transport rates for these large anions are significantly lower than for conduction of Cl− and that this may reflect a transport on the threshold between conductive and nonconductive transport in terms of the rate of ion movement or conductance (in ions per second). The authors conclude that CFTR may function as a Cl− channel and as an anion pump (Fig. 1) (26). Recent suggestions from Hanrahan’s group point to the “pumping” of glutathione and glutathione conjugates as a possible function of CFTR analogous to its ABC transporter relative, the multidrug resistance protein (MRP). The reader may ask, How does this work relate to ATP transport? This study also reopens the possibility that ATP is transported by CFTR itself in a manner similar to that of these large organic anions. Before this study, the authors argued that the reason for this was because the hydrated radius of ATP was much larger than Cl− and, as such, could not possibly permeate the same anion pore (18, 31). However, the hydrated radius of ATP falls in the range of these permeant, large organic anions. Although they did not test the permeability of ATP relative to these other large anions in this more recent study, the authors did suggest that it is possible that ATP could be permeant under these conditions (26).
Wang et al. (47) have demonstrated an important role for conductive ATP release in liver cell volume regulation. Previously, they showed that hypotonicity stimulates an ATP whole cell conductance in rat hepatoma cells (HTC cells) that was silent under isotonic conditions (47). They showed elegantly that this conductive release of ATP was essential for autocrine control of cell volume regulation, because ATP scavengers and purinergic receptor antagonists blocked regulatory volume decrease (RVD) following cell swelling (47). ATP signaling antagonists also blocked stimulation of swelling-activated Cl− channels in these HTC hepatoma cells (47). More recently, these investigators showed that a subclone of HTC cells grown chronically in a synthetic bile acid and thus made to overexpress multiple MDR isoforms had a threefold upregulated ATP conductance and a sixfold more efficient RVD response than parental HTC cells on hypotonic challenge (34). These results suggest that MDR isoforms may modulate ATP release to potentiate hepatocyte volume regulatory processes in liver (34). These results also introduce a new paradigm for ABC transporter modulation of ATP channels, ATP release, and extracellular ATP signaling. This paradigm is autocrine control of cell volume regulation, and the role of extracellular ATP signaling in cell volume regulatory processes is revisited below (see Fig. 3).
CONTROVERSY FORGES NEW PARADIGMS FOR ABC TRANSPORTER-FACILITATED ATP TRANSPORT
New hypotheses concerning the physiological role(s) and cell biological paradigm(s), in which ABC transporter-facilitated ATP transport is involved, are required to aid the CF research community in their focus on this important issue. Fundamental physiological questions are essential to solving this puzzle, such as: Why would a cell release its ATP? What are the physiological roles of extracellular ATP release and signaling? Or, Why would ABC transporters facilitate ATP release and signaling in the apical epithelial microenvironment to affect epithelial cell function? Again, rather than debate the merits of past data, more recent and novel data and the paradigms supported by the data (both published and preliminary) are the focus of the rest of this review.
Many other laboratories are using epithelial and/or heterologous cell expression systems to study the impact of CFTR, MDR, or other ABC transporters on ATP transport, ATP release, and extracellular ATP signaling. Schwiebert and colleagues (43) have adapted the highly sensitive luciferase/luciferin-based bioluminescence detection assay for ATP in solution to study the release of ATP by epithelial monolayers adherent to their substrate and measured inside a luminometer on a platform in real time. This assay can measure the release of ATP into the apical medium or the basolateral medium surrounding an epithelium grown on a permeable filter support. Although this assay has been devised to study extracellular ATP signaling important for any and all physiological paradigms, it can also be used to identify, define, and elucidate the dynamics, cellular mechanisms, regulation, and physiological roles of ABC transporter-facilitated ATP transport, release, and signaling (43). The design of this assay is unique, and there are many differences between this assay design and the luminometers used by Schwiebert and co-workers compared with all past luminometry studies addressing the issue of ABC transporter-facilitated ATP release and signaling (2, 3,17, 23, 30, 48). Although some authors have argued that ATP release only occurs because of mechanically induced artifacts or cell damage (17) or because of physiologically relevant mechanical stimuli such as shear stress produced by air flow in the airways or blood flow through blood vessels (23, 48), ATP release can be measured under basal or unstimulated conditions in epithelial monolayers derived from the lung and airways, the intestine, the pancreas, the liver, and the kidney; basal ATP release from endothelial cell monolayers can also be studied (unpublished observations). This assay was also used to compare non-CF and CF epithelial monolayers and their capacity to release ATP. In non-CF epithelia, the ABC transporter, CFTR, potentiates ATP release across the apical membrane of non-CF epithelial cells under basal conditions (43). Epithelia homozygous for the ΔF508 CFTR mutation fail to release ATP across the apical membrane under basal conditions (43). Basolateral ATP release under unstimulated conditions is not different between non-CF and CF epithelia (43). This potentiation of ATP release has also been demonstrated in numerous other epithelial and heterologous systems (unpublished observations). Potentiation of basal ATP release has also been demonstrated for MDR isoforms in rat hepatocytes by Fitz and colleagues (34, 47). Collaboration between Schwiebert and co-workers and Fitz and colleagues has also shown recently that hypotonicity triggers ATP release (as measured by bioluminescence detection) in all epithelial, endothelial, and heterologous cells tested to date (unpublished observations). Most importantly, when an ABC transporter is expressed, hypotonicity-induced ATP release is potentiated dramatically (44).
In parallel studies, the planar lipid bilayer has proven to be an ideal system in which to decipher whether CFTR or a closely associated ATP-permeable channel conducts ATP. Studies performed by Ismailov, Schwiebert, and Benos have revealed that an ATP channel is present in a protein preparation derived from bovine tracheal epithelial membrane vesicles whether or not CFTR is immunodepleted from this preparation (44). Moreover, both this ATP channel and hypotonicity-induced ATP release are inhibited by the mechanosensitive ion channel blocker, gadolinium chloride (44). In sharp contrast, highly purified CFTR conducts Cl− but fails to conduct ATP (44), consistent with the work of Bear and co-workers (24) and of Gunderson and Kopito (31). In cell volume assays designed to study the impact of CFTR on RVD following a hypotonic challenge and performed with Fitz and co-workers, expression of wild-type CFTR was shown to accelerate and potentiate the RVD response, while cells lacking CFTR or expressing ΔF508-CFTR had an attenuated RVD response (44). Interestingly, gadolinium chloride as well as ATP scavengers attenuates RVD in heterologous fibroblasts and hepatoma cells (R. Roman, J. G. Fitz, and E. M. Schwiebert, unpublished observations). Together, the above cited laboratories of Schwiebert, Fitz, and Benos are pursuing the hypothesis that CFTR or MDR-facilitated ATP transport triggers extracellular ATP signaling for autocrine “self-control” of epithelial cell volume. In support of this hypothesis, P-glycoprotein or MDR was implicated as a volume-activated Cl− channel involved in cell volume regulation (46). Subsequent studies showed, however, that MDR was not necessarily a Cl−channel itself but rather was a regulator of ubiquitous volume-activated Cl−channels (14). There is irony in how similar the MDR story is compared with the current CFTR-ATP channel controversy (49). The molecular mechanisms whereby MDR regulates volume-activated Cl− channels are not understood; however, this story provides precedence for studying the roles of CFTR as well as other ABC transporters in epithelial cell volume regulation. In RVD, Cl− channels distinct from CFTR are recruited to promote Cl− efflux from cells. These findings also suggest that other stimuli such as hypotonicity or increases in intracellular Ca2+may be more robust for ABC transporter-associated ATP conductive transport than cAMP agonists or cAMP-dependent protein kinase.
Engelhardt and colleagues are studying CFTR-facilitated ATP release inXenopus oocytes injected with wild-type and mutant forms of CFTR using an adapted form of the luciferase detection assay designed by Schwiebert and co-workers. They have shown that only wild-type CFTR-expressing oocytes display an ATP-release phenotype in response to cAMP. However, only a subset of oocytes respond, and a precise protocol of incubating oocytes in 0 Cl−, followed by 0 Cl− plus cAMP agonists, and then followed by a change in Cl− to 100 mM or more in the continued presence of cAMP agonists is required to stimulate ATP release, as measured by the same luciferase/luciferin detection assay alluded to above (Q. Jiang, E. M. Schwiebert, W. Guggino, J. K. Foskett, and J. F. Engelhardt, unpublished observations). The current working hypothesis is that CFTR Cl− channel activity (especially Cl− transport into the oocyte) is essential for driving ATP release out of the oocyte. The fact that only some batches of oocytes from specific frogs display the CFTR-facilitated ATP release phenotype suggests that the ATP release mechanism is separate from but regulated by CFTR.
Eaton and co-workers have also generated preliminary data showing that CFTR modulates ATP release fromXenopus A6 renal epithelial cells under unstimulated conditions (D. C. Eaton, personal communication). CFTR-facilitated ATP transport into the extracellular medium may also be important for CFTR-mediated inhibition of ENaC-mediated Na+ absorption, observed by Stutts and co-workers (39). In CF, Na+absorptive transport is heightened. Preliminary studies by Eaton, Ling, Kleyman, and co-workers have found that A6Xenopus kidney epithelia release ATP under basal conditions. In parallel studies, they showed that ATP, through phospholipase C and protein kinase C, inhibits ENaC Na+ channel open probability (11,25). Antisense oligonucleotide inhibition of CFTR quiets ATP release from A6 cells, and ENaC Na+channel activity increases in parallel (D. C. Eaton, personal communication). A connection between CFTR, ATP, and ENaC activity remains to be elucidated; however, the parallels are intriguing. These results suggest that ATP release and signaling under basal conditions may contribute to normal autocrine control of transepithelial ion transport in renal epithelia.
WHY WOULD ANY CELL RELEASE ITS ATP?
If ATP transport and release are modulated by CFTR or other ABC transporters, CFTR-facilitated ATP transport out of cells down a favorable gradient would trigger extracellular ATP signaling. It is important to emphasize here that the gradient for ATP exit is >100,000-fold (3–5 mM intracellular ATP concentration to 10 nM extracellular ATP concentration). It is a log order of magnitude greater than but analogous to the Ca2+ entry gradient from extracellular stores (1 mM extracellular Ca2+ concentration to 100 nM intracellular Ca2+ concentration). Cells express a multitude of different ATP or purinergic receptors that receive released ATP signals transmitted from the same cell, neighboring cells, or a cell downstream from the “releasing” cell. For example, ATP could be released from proximal tubule epithelia and bind to distal tubule epithelia downstream to affect function. ATP could be released from submucosal glands of the airway and bind to airway surface epithelia to affect function. ATP release in gastrointestinal tract may modulate secretory processes in the liver, pancreas, and intestinal crypts. Relevant to extracellular ATP signaling in epithelial and nonepithelial settings, receptors that bind ATP and/or its metabolites are expressed abundantly. G protein-coupled purinergic (P2Y) receptors and cation channel-forming purinergic (P2X) receptors are expressed in many, if not all, tissues (1, 5, 7, 45). Transduction of purinergic signals within the cell would affect cell function.
Many reviews and book chapters have reviewed purinergic receptor cell and molecular biology recently (1, 5, 7, 9, 10, 15, 28, 45). It is important to emphasize, however, that purinergic signaling likely occurs in normal physiology in an autocrine or paracrine manner within a tissue. ATP is degraded rapidly by ecto-ATPases in the bloodstream as it passes through many tissues such as the lung and the liver (10, 15). Nevertheless, nucleotide metabolites (ADP) and nucleosides (adenosine) are also potent autocrine and paracrine agonists within tissues. Numerous nonepithelial cell systems also utilize ATP release and signaling to regulate function. Platelets release ADP and ATP to promote self-aggregation. ATP is released from presynaptic nerve terminals and binds to the postsynaptic nerve terminal in sensory ganglia of the spinal cord involved in nociception. ATP and its metabolites have profound effects on vascular tone in capillary beds. In fact, ATP release is triggered in endothelial cells by many agonists that trigger increases in intracellular Ca2+ (50). ATP and its metabolites are released by macrophages and mast cells in inflammatory responses and by purinergic and autonomic presynaptic nerve terminals alone or together with other neurotransmitters (10, 15). As such, autocrine and paracrine nucleotide and nucleoside agonists act within a tissue for platelet aggregation, neurotransmission, pain perception, modulation of vascular tone, modulation of skeletal muscle and heart contractility, mast cell and immune cell activity, and cell volume regulation (10, 15,47). Concerning the latter physiological process, ATP, released by a conductive transport mechanism, has been implicated recently as an essential autocrine regulator of cell volume in rat hepatoma cells (12,34, 47). Finally, in epithelia freshly isolated from frog palate and esophagus, extracellular ATP induces a depolarization of the apical membrane and enhances the frequency of ciliary beat in these epithelia (42). This latter finding may have profound implications for the control of mucociliary clearance in the airway, a function that is impaired in CF and in ciliary dyskinesia (Fig.2).
The take-home message of this review is that, if ABC transporters facilitate ATP transport, ATP release, and/or extracellular ATP signaling, CFTR and MDR could augment many of the physiological functions listed above. Different mixtures of ABC transporters may be relevant, depending on what tissue or cell type is of interest. One example is the proximal tubule of the kidney, which may express as many as four different ABC transporters, CFTR, MDR, MRP, as well as epithelial basolateral Cl−conductance regulator.
EXTRACELLULAR NUCLEOTIDE SIGNALING HAS RELEVANCE TO CF
Extracellular nucleotide signaling already has profound relevance to CF and is being exploited by Knowles, Boucher, Stutts, and co-workers to design therapies for CF using UTP and UTP analogs (6, 20, 22, 40). In normal and CF epithelia, extracellular ATP, via ATP or purinergic receptors, stimulates Cl−secretion through epithelial Cl− channels alternative to CFTR and stimulates fluid secretion (19). Extracellular ATP also inhibits amiloride-sensitive Na+channels in epithelia derived from many tissues (11, 25). Interestingly, without CFTR, Cl− and fluid transport are diminished, and Na+ transport is augmented in CF epithelia. This is corrected not only by adding back CFTR but also by adding exogenous nucleotides as agonists. ATP in the apical airway surface fluid (ASF) could control the activity of these epithelial conductances. If CFTR does control ATP release and signaling through modulation of a secretory ATP channel or through additional release mechanisms (nonconductive transport, exocytosis), CFTR, via ATP, could control multiple conductances simultaneously in the apical membrane of epithelia (Fig. 2). Moreover, other ABC transporters are conductance regulators. P-glycoprotein or MDR regulates volume-activated Cl−channels through an indirect signaling mechanism that is not fully understood (14). SUR is a physical part of the ATP-sensitive K+ channel conductance that modulates insulin secretion in pancreatic β-cells. CFTR, as a member of the ABC transporter superfamily, may act in similar direct and indirect mechanisms to modulate other channel proteins. Parallel study of these ABC transporters may provide insights into the conductance regulator capacity of any and all ABC transporters.
ATP IN THE APICAL EPITHELIAL MICROENVIRONMENT
The possible roles of extracellular nucleotide signaling in cell volume regulation and mucociliary clearance have been proposed above. What are some other possible roles of ABC transporter-facilitated ATP release surrounding epithelial barriers? Differences have been observed in the ionic strength of apical ASF in CF and non-CF airway epithelial monolayers. Osmotic strength was not measured in these studies; however, ASF is hypotonic with respect to NaCl on the apical surface of non-CF epithelia in a subset of these studies. Could ATP control these ionic or osmotic parameters in ASF or could the ionic or osmotic environment in the ASF control ATP release? Could extracellular ATP signaling modulate bile acid formation or enzyme secretion in the gastrointestinal tract? The exact ionic or osmotic strength of non-CF vs. CF ASF is also a matter of much debate in the CF research community. The important point in this debate is that the NaCl concentrations are different and that a defect underlying this difference may be present. The role of extracellular ATP modulation of salt and water transport in this microenvironment should be addressed (Fig. 2).
CONCLUSIONS AND UNANSWERED QUESTIONS
There are many questions that still guide this work, and other questions that have arisen from this work that are pertinent to normal physiology and to diseases such as CF. First, what are the mechanisms of ATP release and how are they regulated? Not only may ATP conductive channels be expressed in cells but nonconductive transporters and exocytic mechanisms may also promote ATP release from cells. How is ATP release regulated? Cyclic nucleotides are one stimulus; however, there may be others such as hypotonic challenge or increases in intracellular Ca2+. What is the molecular identity of the ATP channel regulated by ABC transporters? We must develop creative methods to answer this question. This ATP channel may be a novel family of ATP-selective channels or an anion channel that is permeable to multiple anions including Cl−. What role or roles do ABC transporters play in cell volume regulation (Fig.3)? P-glycoprotein or MDR has been implicated in the modulation of RVD mechanisms following a hypotonic challenge (Fig. 3). But, how does it perform this function and can other ABC transporters substitute for MDR? Could CFTR stimulation of ORCCs and CFTR inhibition of ENaCs relate as much to turning on Cl− efflux during RVD (and quieting Na+ influx involved in regulatory volume increase) during the RVD limb of cell volume control in epithelia as it does in modulating transepithelial ion transport?
ABC transporter-facilitated ATP conductive transport is becoming more clear in terms of mechanism, regulation, and physiological relevance. It is still an open question, however, if ABC transporters can conduct ATP themselves and/or regulate a closely associated but separate ATP channel. The answer may be that both mechanisms of ATP transport may occur. In simplest terms, CFTR, MDR, and other ABC transporters may act as ATP conductance regulators as well as transport ATP at rates at the threshold of nonconductive and conductive transport. Both functions would result in facilitation of extracellular ATP signaling within tissues. As such, ABC transporters may orchestrate their own activity, the activity of other ion channels, and, perhaps, the activity of other cell functions that we do and do not yet appreciate, at least in part, to maintain and regulate cell volume.
I thank Lisa M. Schwiebert, Douglas C. Eaton, William B. Guggino, Dale J. Benos, John F. Engelhardt, and Eric J. Sorscher for helpful comments, personal communications, and editorial assistance on this review. Work related to this subject is performed in Dr. Schwiebert’s laboratory by himself as well as by collaborating students including Brian Kudlow, Amanda Taylor, Gavin Braunstein, Kevin Marrs, Jeffrey Hovater, James Fortenberry, and Cash Casey.
Address for reprint requests: E. M. Schwiebert, Research Scientist in the Gregory Fleming James Cystic Fibrosis Research Center, Univ. of Alabama at Birmingham, BHSB 740, 1918 Univ. Blvd., Birmingham, AL 35294-0005.
This work is supported by a New Investigator award from the Cystic Fibrosis Foundation as well as New Investigator grants from the American Heart Association Southern Research Consortium and the Polycystic Kidney Research Foundation.
- Copyright © 1999 the American Physiological Society