Ca2+-induced contraction of cat esophageal circular smooth muscle cells

ACh-induced contraction of esophageal circular muscle (ESO) depends on Ca2+ influx and activation of protein kinase C epsilon (PKC epsilon). PKC epsilon, however, is known to be Ca2+ independent. To determine where Ca2+ is needed in this PKC epsilon -mediated contractile pathway, we examined successive steps in Ca2+-induced contraction of ESO muscle cells permeabilized by saponin. Ca2+ (0.2-1.0 muM) produced a concentration-dependent contraction that was antagonized by antibodies against PKC epsilon (but not by PKC beta II or PKC gamma antibodies), by a calmodulin inhibitor, by MLCK inhibitors, or by GDP betas. Addition of 1 muM 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 AACOCF(3) (cytosolic phospholipase A(2) inhibitor). These data suggest that Ca2+ may directly activate phospholipases, producing DAG and arachidonic acid (AA), and PKC epsilon, which may indirectly cause phosphorylation of MLC. In addition, direct G protein activation by GTP gammaS 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 epsilon -dependent mechanisms to cause contraction.