Downregulation of protein kinase C in guinea pig pancreatic acini: effects on secretion

Pretreatment of guinea pig pancreatic acini with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) induced a time- and concentration-dependent down-regulation of protein kinase C. In control acini almost all of the protein kinase C activity was present in a cytosolic fraction. Incubation with TPA initially shifted protein kinase C activity to a particular fraction which then disappeared over the following 24-h incubation with TPA. To study the role of protein kinase C in stimulus-secretion coupling, acini were pretreated with TPA and then amylase release was studied in response to various secretagogues. Preincubation of acini with TPA led to a time- and concentration-dependent decrease in TPA-stimulated amylase release that correlated with protein kinase C downregulation. Preincubation of acini with 1 microM TPA for 24 h, resulting in complete loss of protein kinase C activity, abolished the secretory effect of subsequently added TPA. By contrast, the secretory effects of cholecystokinin octapeptide (CCK-8) and carbamylcholine chloride (CCh) were only inhibited by 44 and 34%, respectively, and amylase release stimulated by the Ca2+ ionophore A23187 and an adenosine 3',5'-cyclic monophosphate-mediated agonist, vasoactive intestinal peptide, was unaffected. Dose-response curves for CCK-8- or CCh-stimulated amylase release in TPA-pretreated acini revealed attenuation of both maximal efficacy and sensitivity. However, the CCh-stimulated intracellular Ca2+ increase as determined by use of the fluorescent probe fura-2 was not affected by the long-term TPA pretreatment of acini. This study strongly suggests that both protein kinase C and intracellular Ca2+ play a significant role in CCK-8- and CCh-stimulated amylase release.