Agonist-independent Tonic Inhibitory Influence of Gion Adenylate Cyclase Activity in Rabbit Ventricular Myocardium and its Removal by Pertussis Toxin:a Role of Empty Receptor-mediated Giactictivitation

We evaluated whether Gihas 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 Gi. Rabbits received pertussis toxin (PTX, 10μg/kg i.v.) 40 h before preparing ventricular myocardial membranes, which was associated with virtually completein vivoADP-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 100μ 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 Gihas a tonic influence on the stimulatory effects of guanine nucleotides andβ-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 selectiveβ2-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 ofβ2-adrenoceptors in tonic activation of Gi. We conclude that Giis tonically activated by agonist-free, empty muscarinic receptors, which leads to attenuation of Gs-mediated orβ-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 Giheterotrimeric complex, independently of the coupled receptors.