Interaction of guanine nucleotides with adenylate cyclase in normal and spontaneously transformed RL-RP-C cloned rat hepatocytes

Spontaneous transformation of RL-PR-C hepatocytes leads to alterations in the adenylate cyclase complex which include a lower than normal basal level of activity, a loss of sensitivity to exogenous GTP, and a decreased sensitivity to isoproterenol. Both normal and transformed membranes possess substantial GTPase activity. Treatment of transformed hepatocyte membranes with either isoproterenol plus GMP or with cholera toxin, under conditions that displace tightly bound GDP, restored the GTP effect on adenylate cyclase, and eliminated the lag in the activation by guanyl-5'-yl-imidodiphosphate. Such pretreatment also enhanced guanine nucleotide effects on the adenylate cyclase of normal hepatocytes. These results are explainable on the basis that transformation increases adenylate cyclase-associated GTPase activity, and increases occupancy of nucleotide regulatory sites by inactive or inhibitory guanine nucleotides, e.g., GDP. Seemingly, both catecholamines and cholera toxin promote an exchange reaction at the regulatory sites, resulting in clearance of these sites of inhibitory nucleotides.