Agonist-independent tonic inhibitory influence of G(i) on adenylate cyclase activity in rabbit ventricular myocardium and its removal by pertussis toxin: A role of empty receptor-mediated G(i) activation

We evaluated whether G(i) has a tonic inhibitory influence on myocardial adenylate cyclase (AC) in an agonist-independent way, and, if so, whether this is attributable to substantial coupling between agonist-free, empty inhibitory receptors and G(i). Rabbits received pertussis toxin (PTX, 10 mu g/kg i.v.) 40 h before preparing ventricular myocardial membranes, which was associated with virtually complete in vivo ADP-ribosylation and inactivation of the 41-kDa substrate. Pretreatment with PTX had no influence on basal AC activity but significantly enhanced AC activity elicited by 100 mu M GTP. Furthermore, it markedly increased AC activity stimulated with 5'-guanylyl imidodiphosphate (GppNHp) and isoproterenol through a wide range of concentrations of these stimulants. These findings indicate that G(i) has a tonic influence on the stimulatory effects of guanine nucleotides and beta-adrenoceptor stimulation on AC, even in the absence of the inhibitory receptor agonists. The muscarinic receptor antagonists atropine and AF-DX 116 significantly enhanced isoproterenol-stimulated AC activity as PTX pretreatment did, except that statistically significant increasing effects of these antagonists on GppNHp-stimulated AC activity was observed only at higher concentrations of GppNHp. The enhancement by atropine was not detected in PTX-pretreated membranes. The selective beta(2)-adrenoceptor antagonist ICI 118,551 did not modify the stimulatory effects of guanine nucleotides and isoproterenol on AC in either control or PTX-pretreated membranes, excluding the possible involvement of beta(2)-adrenoceptors in tonic activation of G(i). We conclude that G(i) is tonically activated by agonist-free, empty muscarinic receptors, which leads to attenuation of G(i) mediated or beta-adrenoceptor-mediated activation of AC. The potentiating effect of PTX pretreatment on GppHNp-stimulated AC activity may be at least partially due to the direct action of PTX on the G(i) heterotrimeric complex, independently of the coupled receptors. (C) 1997 Academic Press Limited.