We evaluated whether Gi 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. Rabbits received pertussis toxin (PTX, 10 micrograms/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 microM 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 Gi has a tonic influence on he 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 Gi. We conclude that Gi is tonically activated by agonist-free, empty muscarinic receptors, which leads to attenuation of Gs-mediated or beta-adrenoceptor-mediated activation of AC. The potentiating effect of PTX pretreatment on GppNHp-stimulated AC activity may be at least partially due to the direct action of PTX on the Gi heterotrimeric complex, independently of the coupled receptors.