Regulation of protein kinases in exocrine secretory cells during agonist-induced exocytosis.

Stimulation of exocytosis in exocrine glands is associated with an increased phosphorylation of several particulate proteins. Irrespective of the type of secretagogue (cAMP-dependent agonists, calcium-dependent agonists, calcium ionophores, phorbol esters) exocytosis is always accompanied by an enhanced phosphorylation of the ribosomal protein S6. It is shown by an analysis of the phosphopeptide pattern of the in vivo and the in vitro phosphorylated S6 protein that the protein kinase responsible for phosphorylation of the S6 protein during enhanced exocytosis is protein kinase C. This is so irrespective of whether the agonist uses cAMP or calcium as second messenger. Experiments with isolated guinea pig parotid gland lobules reveal that not only the acetylcholine analog carbamoylcholine, but also the beta-agonist isoproterenol lead within seconds to an increased formation of diacylglycerol. As diacylglycerol increases the affinity of protein kinase C for calcium this finding would explain why the phosphorylation pattern of the S6 protein reflects activation of protein kinase C also under conditions where (as in the case of stimulation with beta-agonists) cAMP is the primary second messenger. It would further explain why the changes of the phosphorylation of individual histones observed during agonist-induced exocytosis in the parotid gland are quite similar for isoproterenol on one hand and carbamoylcholine on the other. A 22 K protein which becomes phosphorylated only when cAMP serves as second messenger is located in the membrane of the endoplasmic reticulum. A possible relationship of this protein with the calcium transport ATPase of the endoplasmic reticulum is under investigation.