Unoccupied beta-adrenoceptor-induced adenylyl cyclase stimulation in turkey erythrocyte membranes

The influence of beta-adrenoceptor ligands and a beta-adrenoceptor-derived peptide was studied on stimulation of adenylyl cyclase in membrane preparations of turkey erythrocytes. In the absence of receptor ligands, the hydrolysis-resistant GTP analogs, guanosine 5'-[gamma-thio]triphosphate and guanosine 5'-[beta,gamma-imino]triphosphate, caused a slow, time-dependent increase in adenylyl cyclase activity, which was accelerated and potentiated by the additional presence of the agonist, isoproterenol. In contrast, the beta-adrenoceptor antagonists, propranolol and pindolol, almost completely prevented stimulation of adenylyl cyclase by the GTP analogs alone, i.e. in the absence of an agonist, in a concentration-dependent and stereo-selective manner. This antagonist action was mimicked by a peptide corresponding to the second cytoplasmic loop of the turkey erythrocyte beta-adrenoceptor. On the other hand, GTP analog-preactivated cyclase activity and enzyme stimulation by fluoride and forskolin were not or only slightly reduced by beta-adrenoceptor antagonists. The data presented suggest that in turkey erythrocyte membranes agonist-free beta-adrenoceptors can cause significant Gs protein and subsequent adenylyl cyclase activation, and that this unoccupied receptor action can be blocked by beta-adrenoceptor antagonists, thus exhibiting by themselves a negative intrinsic activity.