Different actions of protein kinase C isoforms α and ε on gastric acid secretion

1 The phorbol ester TPA, an activator of protein kinase C (PKC), inhibits cholinergic stimulation of gastric acid secretion but increases basal H+ secretion. 2 Since these contradictory findings suggest the action of different PKC isozymes we analysed the role of calcium-dependent PKC-alpha, and calcium-independent PKC-epsilon in gastric acid secretion. 3 Inhibition of PKC-alpha by the indolocarbazole Go 6976 revealed that about 28% of carbachol-induced acid secretion was inhibited by PKC-alpha. In the presence of Go 6976 approximately 64% of the carbachol-induced signal transduction is mediated by Ca2+/calmodulin-dependent protein kinase II (CaMKII), and 14% is conveyed by PKC-epsilon as deduced from the inhibition with the bisindolylmaleimide Ro 31-8220. 4 Inhibition of carbachol-induced acid secretion by TPA was accompanied by a decrease in CaMKII activity. 5 The stimulation of basal acid secretion by TPA was biphasic with a peak at a very low concentration (10 pm), resulting in an activation of the calcium-sensor CaMKII. The activation was determined with a phosphospecific polyclonal antibody against active CaMKII. The TPA-induced increase of H+ secretion was sensitive to the cell-permeable Ca2+-chelator BAPTA/AM, Ro 31-8220, and the CaMKII-inhibitor KN-62, but not to Go 6976, 6 Since TPA induced the translocation of PKC-epsilon but not of PKC-alpha in resting parietal cells, PKC-epsilon seems to be at least responsible for an initial elevation of free intracellular calcium to initiate TPA-induced acid secretion. 7 Our data indicate the different roles of two PKC isoforms: PKC-epsilon activation appears to facilitate cholinergic stimulation of H+-secretion likely by increasing intracellular calcium. In contrast, PKC-alpha activation attenuates acid secretion accompanied by a down-regulation of CaMKII activity.