Stimulation and inhibition of cAMP accumulation by glucagon in canine hepatocytes

We have examined, by use of isolated canine hepatocytes and selected hormone analogs, the mechanisms by which glucagon modifies the accumulation of cellular cAMP. Low concentrations of glucagon (≤ 3 nM) enhanced the accumulation of hepatocyte cAMP, whereas higher concentrations of the hormone diminished the effectiveness of lower ones. This biphasic concentration dependence was observed as well for some glucagon analogs, but not for others, and was apparent for cells incubated in the presence or absence of theophylline. Glucagon at high concentrations (≥ 10 nM) also inhibited the accumulation of cAMP induced by isoproterenol. The inhibitory effect of glucagon in both of these systems was reversed or attenuated by cell incubations involving the use of pertussis toxin (islet-activating protein) or a peptide antagonist of the glucagon-adenylyl cyclase system. We conclude that (a) glucagon, through its interaction with high and low affinity binding sites, can either stimulate or inhibit the production of hepatocyte cAMP; (b) the inhibitory action of the hormone appears to arise from interactions of ligand with a subset of these binding sites and to require structural characteristics in addition to those that determine receptor binding affinity per se; and (c) the glucagon and adrenergic systems involved in stimulating cAMP accumulation are linked, at least with regard to the negative effect induced by high concentrations of glucagon.