Inhibitory action of adenosine on histamine- and dopamine-stimulated cardiac contractility and adenylate cyclase in guinea pigs.

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Abstract
Infusion of adenosine into the coronary arteries of isolated guinea pig hearts produced a dose-dependent inhibition of cardiac contractile force development elicited by bolus injections of histamine (7.5 X 10(-9) mol) or dopamine (1.5 X 10(-8) mol). Threshold concentration of adenosine was 10(-7) M and maximal inhibition (90%) was obtained at 3 X 10(-5) M. Adenosine in the effective concentration range did not alter Ca2+-induced increases in contractile force. The rise in tissue levels of cAMP induced by equieffective doses of histamine (7.5 X 10(-9) mol) and dopamine (1.5 X 10(-8) mol) was inhibited by adenosine (3.5 X 10(-5) M) by about 60%. In a particular membrane preparation of guinea pig ventricles the adenylate cyclase activity stimulated by the histamine (10(-5) M) and dopamine (10(-4) M) was inhibited in a dose-dependent manner by adenosine. This effect could be reversed by theophylline (5 X 10(-5) M) in a competitive manner. The hormone-insensitive adenylate cyclase of a Lubrol-PX solubilized membrane preparation stimulated by NaF or 5'-guanylylimido-diphosphate (GppNHp) was also inhibited by adenosine (40% and 90% inhibition at 10(-5) M and 10(-4) M, respectively). Adenosine did not influence the Km value of the adenylate cyclase for ATP, but markedly lowered Vmax of the enzyme. From additional studies with purine-substituted (N6-methyl-adenosine, N6-phenylisopropyl-adenosine) and ribose-substituted (2'-deoxy-adenosine and arabino-furanosyl-adenine) adenosine analogues, we conclude that adenosine may inhibit the inotropic responses to hormones as well as the adenylate cyclase activity by specifically interacting with at least two different sites associated with the adenylate cyclase.