Activation of protein kinase activity in pancreatic acini by calcium and cAMP

Regulation of protein kinase activity by calcium and cAMP was investigated in cytosolic and particulate preparations from isolated mouse pancreatic acini. In cytosol, three protein kinase activities could be distinguished: a calcium-activated kinase activity that was increased by exogenous calmodulin (CaM) and abolished by treatment of cytosol with a phenothiazine-coupled resin, a calcium-activated kinase activity dependent on phosphatidylserine (PS), and cAMP-activated kinase activity. Phosphorylation of a Mr = 92,000 cytosolic protein was greatly increased by both CaM-dependent and cAMP-activated kinases, whereas PS-dependent kinase activity most heavily phosphorylated proteins of Mr = 62,000 and 40,000. In addition, these kinase activities demonstrated differences in specificity for exogenous protein substrates. CaM-and PS-dependent kinases were completely blocked by trifluoperazine; the inhibitor protein of cAMP-activated protein kinase selectively inhibited cAMP-activated kinase activity. Exogenous CaM decreased the concentration of free calcium for half-maximal activation of CaM-dependent kinase activity from 5.5 +/- 0.5 to 1.2 +/- 0.3 microM Ca2+; half-maximal activation of the PS-dependent and cAMP-activated kinase activities was achieved at 12 microM Ca2+ and 40-50 nM cAMP, respectively. In a particulate fraction depleted of endogenous CaM, CaM-dependent and cAMP-activated kinase activities were detected. In conclusion, this study demonstrates the existence of protein kinases in pancreatic acini which may be involved in the action of pancreatic regulatory agents that use calcium or cAMP as an intracellular messenger.