Islet-activating protein blocks glucagon desensitization in intact hepatocytes

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Abstract
Treatment of intact hepatocytes with islet-activating protein, from Bordatella pertussis, led to a pronounced increase in the ability of glucagon to raise intracellular cyclic AMP concentrations. Islet-activating protein, however, caused no apparent increase in the intracellular concentration of cyclic AMP under basal conditions. These effects were attributed to an enhanced ability of adenylate cyclase, in membranes from hepatocytes treated with islet-activating protein, to be stimulated by glucagon. When forskolin was used to amplify the basal adenylate cyclase activity, elevated GTP concentrations were shown to inhibit adenylate cyclase activity in membranes from control hepatocytes. This inhibitory effect of GTP was abolished if the hepatocytes had been pre-treated with islet activating protein. In isolated liver plasma membranes, islet-activating protein caused the NAD-dependent ribosylation of a Mr-40000 protein, the putative inhibitory guanine nucleotide regulatory protein, Ni. This effect was inhibited if guanosine 5′-[beta-thio]diphosphate rather than GTP was present in the ribosylation incubations. The ability of glucagon to uncouple or desensitize the activity of adenylate cyclase in intact hepatocytes was also blocked by pre-treating hepatocytes with islet-activating protein. Islet-activating protein thus heightens the response of hepatocytes to the stimulatory hormone glucagon. It achieves this by both inhibiting the expression of desensitization and also removing a residual inhibitory input expressed in the presence of glucagon.