Pseudosubstrate inhibition of cyclic AMP-dependent protein kinase in intact pancreatic islets: effects on cyclic AMP-dependent and glucose-dependent insulin secretion.

Synthetic peptides derived from the endogenous protein kinase A inhibitor (PKI) offer a specific means of inhibiting cyclic AMP-dependent protein kinase A (PKA), but their use in whole cells is restricted by the plasma membrane. We have now modified PKI sequences by N-terminal myristoylation to enhance their membrane permeability, and have used the myristoylated (myr) peptides to investigate the role of PKA activation in glucose-induced insulin secretion from intact pancreatic beta-cells. The myristoylated PKI peptides, myr PKI14-22 and myrPKI6-22, were effective inhibitors in vitro of PKA activity extracted from rat islets of Langerhans. In experiments using intact islets, myr PKI14-22 caused a concentration-dependent inhibition of insulin secretion in response to the PKA activators dibutyryl cyclic AMP and forskolin, suggesting that it gained access to the cytosolic compartment of intact beta-cells and inhibited PKA in situ. However, these concentrations of myr PKI14-22 did not inhibit insulin secretion in response to glucose suggesting that the activation of PKA is not required for the initiation of glucose-induced insulin secretion.