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1 Department of Medicine, Rhode Island Hospital and Brown Medical School, Providence, Rhode Island 02903; 2 Department of Pharmacology, College of Pharmacy, Chung Ang University, Seoul 156-756, and 3 Department of Internal Medicine, Kangnam General Hospital, Public Corporation, Seoul 135-090, Korea
ACh-induced contraction of esophageal circular
muscle (ESO) depends on Ca2+ influx and activation of
protein kinase C
(PKC
). PKC
, however, is known to be
Ca2+ independent. To determine where Ca2+ is
needed in this PKC
-mediated contractile pathway, we examined successive steps in Ca2+-induced contraction of ESO muscle
cells permeabilized by saponin. Ca2+ (0.2-1.0 µM)
produced a concentration-dependent contraction that was antagonized by
antibodies against PKC
(but not by PKC
II or PKC
antibodies),
by a calmodulin inhibitor, by MLCK inhibitors, or by GDP
s. Addition
of 1 µM Ca2+ to permeable cells caused myosin light chain
(MLC) phosphorylation, which was inhibited by the PKC inhibitor
chelerythrine, by D609 [phosphatidylcholine-specific phospholipase C
inhibitor], and by propranolol (phosphatidic acid phosphohydrolase
inhibitor). Ca2+-induced contraction and diacylglycerol
(DAG) production were reduced by D609 and by propranolol, alone or in
combination. In addition, contraction was reduced by
AACOCF3 (cytosolic phospholipase A2
inhibitor). These data suggest that Ca2+ may directly
activate phospholipases, producing DAG and arachidonic acid (AA), and
PKC
, which may indirectly cause phosphorylation of MLC. In addition,
direct G protein activation by GTP
S augmented Ca2+-induced contraction and caused dose-dependent
production of DAG, which was antagonized by D609 and propranolol. We
conclude that agonist (ACh)-induced contraction may be mediated by
activation of phospholipase through two distinct mechanisms (increased
intracellular Ca2+ and G protein activation), producing DAG
and AA, and activating PKC
-dependent mechanisms to cause contraction.
calcium; smooth muscle; protein kinase C; phospholipase C; phospholipase D; myosin phosphorylation
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