Mechanism of heterologous desensitization of the adenylate cyclase system by glucagon in primary cultures of adult rat hepatocytes

During treatment of primary cultured hepatocytes with either glucagon or isoproterenol for several hours, the stimulations of cAMP formation by these hormones decreased time dependently. Glucagon treatment also reduced the response to isoproterenol, but isoproterenol treatment did not decrease the response to glucagon. Treatment with isoproterenol caused more rapid desensitization than treatment with glucagon. Assays of glucagon and beta-adrenergic receptors showed that the receptor number of only the hormone with which the cells were treated decreased and that dissociation constants of the receptors did not change. Moreover, in glucagon-desensitized cells, the effect of GTP on competition of the bindings of antagonist and agonist for the beta-adrenergic receptor did not change. After treatment with isoproterenol, stimulation of adenylate cyclase activity by the agonist was decreased without any decrease in the stimulations of activity by other effectors. In contrast, glucagon treatment greatly decreased the stimulations of activity by glucagon, isoproterenol, and guanyl-5'-yl imidodiphosphate and slightly decreased that by fluoride. However, after glucagon treatment, the cells showed normal responses to cholera toxin of activation of adenylate cyclase and ADP-ribosylation of guanine nucleotide binding regulatory protein (Ns). The maximal response of glucagon-treated cells to forskolin was about two-thirds that of untreated cells and this treatment also impaired the shift toward a low Kact value for forskolin observed in the presence of either glucagon or isoproterenol. These results indicate that isoproterenol caused homologous desensitization consisting of only a “down regulation” of the beta-adrenergic receptor, whereas glucagon caused heterologous desensitization, mainly by alteration of the Ns component, as well as “down regulation” of glucagon receptor. This altered Ns seems to be coupled normally to the beta-adrenergic receptor, but to have impaired coupling to the catalytic component of adenylate cyclase.