Dual calcium-dependent protein phosphorylation systems in pancreas and their differential regulation by polymyxin B1

Both phospholipid/calcium (PL/Ca2+) activated and calmodulin/Ca2+ (CaM/Ca2+)activated protein kinase systems were found in rat pancreatic extracts treated with Sephadex G-25. At least four substrate proteins for PL/Ca2+-activated kinase and one for a CaM/Ca2+-activated kinase were noted. Polymyxin B, an amphipathic antibiotic, was over 100-fold more potent as an inhibitor of PL/Ca2+-dependent protein phosphorylation than of the CaM/Ca2+-dependent system (Ki = app. 7 microM v. 950 microM). Fluphenazine inhibited both PL/Ca2+- and CaM/Ca2+-dependent protein kinases with equal potency, as did dibucaine. Inhibition by polymyxin B of PL/Ca2+-dependent phosphorylation could be overcome by increased amounts of phosphatidylserine. Low concentrations (10(-5)M) of polymyxin B completely inhibited carbachol-stimulated amylase release from intact pancreatic acini. These results indicate that polymyxin B may be useful in delineating the relative roles of PL/Ca2+-dependent and CaM/Ca2+-dependent protein phosphorylation in biological systems and suggest a potential role for the PL/Ca2+-activated kinase in regulation of pancreatic exocrine function.