Action of VIP and secretin on adenylate cyclase activity in acinar cells from guinea pig pancreas

Abstract

In membranes from dispersed acinar cells from guinea pig pancreas, adenylate cyclase activity was increased by vasoactive intestinal peptide, secretin, and 5′-guanylimidodiphosphate, a nucleotidase-resistant analogue of GTP. The time-course for stimulation of adenylate cyclase activity in membranes incubated with vasoactive intestinal peptide was rapid compared with that in membranes incubated with 5′-guanylimidodiphosphate. In membranes incubated with vasoactive intestinal peptide plus the nucleotide, the time-course for stimulation of enzyme activity was the same as that with the nucleotide alone. The guanyl nucleotide potentiated the increase in adenylate cyclase activity caused by secretin or vasoactive intestinal peptide in that the increase in enzyme activity caused by the nucleotide plus either of the peptides was significantly greater than the sum of the increase caused by each agent acting alone. Without the guanyl nucleotide the threshold concentration for stimulation of adenylate cyclase activity by vasoactive intestinal peptide or secretin was greater than 1 nM whereas with the nucleotide a significant increase in enzyme activity could be detected with concentrations of vasoactive intestinal peptide or secretin as low as 10 pM. The dose response curves for peptide-stimulated adenylate cyclase activity were monophasic without 5′-guanylimidodiphosphate but were biphasic with the nucleotide. These actions of 5′-guanylimidodiphosphate were temperature-dependent, could not be reversed by washing, and did not reflect changes in membrane binding of the peptide or of the nucleotide. Furthermore, the effects of 5′-guanylimidodiphosphate could be inhibited by GTP or GDP, and once established, could be reversed by GTP or GDP. These results indicate that, in acinar cells from guinea pig pancreas, interactions of guanyl nucleotides with the GTP-regulatory site of adenylate cyclase may be an important determinant of the cellular response to vasoactive intestinal peptide and secretin.