Atrial Natriuretic Peptide (ANP)-Induced Inhibition of Glucagon Secretion: Mechanism of Action in Isolated Rat Pancreatic Islets

Abstract

ANP increases insulin levels in vivo. Because in vitro an ANP-induced increase in cGMP levels of islets of Langerhans was observed but no simultaneous increase in insulin release, secreted glucagon may be a candidate for this second messenger affected by ANP. The inhibitory effect of glucose on glucagon secretion was pronounced by 1.0 nM ANP at 3.0 mM glucose as well as at 5.6 and 8.3 mM glucose. Because in other tissues cGMP (the specific second messenger of ANP) inhibits Ca2+ channels, the uptake of 45Ca2+ was investigated. ANP (1.0 nM) inhibited 45Ca2+ uptake, which was nearly completely abolished by a pertussis toxin (PT) pretreatment. The inhibition of 45Ca2+ uptake fits to inhibitory ANP effects on glucagon secretion but does not fit to insulin secretion. The glucagon secretion coupling cascade affected by ANP probably involves an increase in cGMP because 8-Br-cGMP (a membrane-permeable cGMP analogue) also decreased glucagon secretion. ANP (4–23), a truncated form of ANP, which is selective for the ANP clearance receptor, also inhibited glucagon secretion. HS-142-1, a guanylate cyclase receptor antagonist, tended to reverse the effect of ANP on glucagon release. The data indicate that in the presence of ANP, the in vivo homeostasis of glucose, though plasma insulin levels are increased, is not due to an ANP-mediated increase in glucagon secretion; ANP has a complex inhibitory effect on glucagon release. The data further indicate that the ANP-induced inhibition of glucagon secretion probably involves the cGMP system, an inhibition of Ca2+ uptake and the involvement of PT-sensitive G-proteins. Moreover, an involvement of the clearance receptor seems to be likely, ANP is a valuable tool for investigating glucagon secretion from pancreatic islets because paracrine effects of insulin can be excluded.