GTP-DEPENDENT INHIBITION OF CARDIAC ADENYLATE-CYCLASE BY MUSCARINIC CHOLINERGIC AGONISTS

In membrane-containing particles of rabbit myocardium, inhibition of adenylate cyclase (EC 4.6.1.1) by cholinergic agonists was studied. The cholinergic agonists, in the potency order oxotremorine > acetylcholine > carbachol, reduced basal adenylate cyclase activity with half-maximal effects at about 0.3, 1 and 3 μM, respectively; maximal inhibition observed was about 40%. The enzyme inhibition occurred without apparent lag phase and was reversed by the muscarinic cholinergic antagonist, atropine, which finding indicates that muscarinic cholinergic receptors are involved in this process. As for stimulation of cardiac adenylate cyclase by the β-adrenergic agonist, isoproterenol, the addition of GTP was necessary for maximal enzyme inhibition by the cholinergic agonists. The effects of GTP were half-maximal at about 0.2 and 1 μM and maximal at about 3 and 30 μM GTP for stimulation and inhibition, respectively. NaCl, which increased cardiac adenylate cyclase activity, facilitated the GTP-dependent cholinergic inhibition; the NaCl effect was maximal between 50 and 100 mM. In the presence of GTP, the cholinergic agonist, carbachol, not only reduced basal adenylate cyclase activity, but also inhibited adenylate cyclase stimulated by isoproterenol (100 μM) or NaF (10 mM). In the presence of cholera toxin (40 μg/ml), the GTP-induced activation of the enzyme was counteracted by carbachol. However, the stable GTP-analogues, guanylyl-5′-imidodiphosphate and guanosine-5′-O-(3-thiotriphosphate), which caused a persistent adenylate cyclase activation, reversed or prevented the carbachol-induced inhibition. The data indicate that cholinergic agonists inhibit cardiac adenylate cyclase by a process that requires (the hydrolyzable) GTP and involves sodium ions. © 1979 Springer-Verlag.