Challenge of hepatocytes by glucagon triggers a rapid modulation of adenylate cyclase activity in isolated membranes

Abstract

Membrane fractions obtained from [rat] hepatocytes treated with glucagon exhibited a decreased glucagon (with or without GTP)-stimulated adenylate cyclase activity. A maximum effect was seen in around 5 min. No change in the rate of cAMP production was observed for the basal, NaF-, p[NH]ppG (guanosine 5''-[.beta.,.gamma.-imido]-triphosphate)- and GTP-stimulated states of the enzyme. The lag observed in the p[NH]ppG-stimulated adenylate cyclase activity of native membranes was abolished when membranes from glucagon-pretreated cells were used. When Mn2+ replaced Mg2+ in the assays, the magnitude of the apparent desensitization was decreased. Mn2+ abolished the lag of onset of p[NH]ppG-stimulated activity in native membranes. The desensitization process was dose-dependent on glucagon, which exhibited a Ka of 4 .times. 10-10 M. Depletion of intracellular ATP did not affect this process. Apparently, this desensitization occurs at the level of the guanine nucleotide-regulatory protein.