A MYRISTOYLATED PSEUDOSUBSTRATE PEPTIDE INHIBITOR OF PROTEIN-KINASE-C- EFFECTS ON GLUCOSE-INDUCED AND CARBACHOL-INDUCED INSULIN-SECRETION

We have used synthetic pseudosubstrate peptide inhibitors of protein k inase C (PKC) to re-examine the role of conventional isoforms of PKC i n the insulin secretory response of intact rat islets of Langerhans to glucose and to the cholinergic agonist carbachol (CCh). One peptide w as modified by N-terminal myristoylation (PKC-myr(20-28)) to allow its use in intact beta-cells. Maximal inhibition of PKC activity in vitro required 10-fold less of this peptide (PKC-myr(20-28)) than of its no n myristoylated analogue. The maximum inhibitory concentration of PKC- myr(20-28) had little effect on islet protein kinase A or Ca2+/calmodu lin kinase activities. PKC-myr(20-28) (25-100 mu M) caused a dose-depe ndent inhibition of phorbol myristate acetate (PMA)-induced insulin se cretion from intact rat islets but non-myristoylated peptides had litt le effect on the secretory response to PMA. A concentration of PKC-myr (20-28) (100 mu M) which maximally inhibited PMA-induced insulin secre tion, also inhibited the secretory response to CCh, but did not affect glucose-stimulated insulin secretion from intact islets. These result s indicate that myristoylation of pseudosubstrate peptides increases t heir potency as inhibitors and that PKC-myr(20-28) is a selective and cell-permeant inhibitor of PMA-sensitive isoforms of PKC. They also su ggest that the activation of PMA-sensitive PKC isoforms mediates the s timulatory effects of CCh, but is not obligatory for glucose-induced i nsulin secretion from pancreatic beta-cells.