Demonstration of specific receptors for adenosine in guinea-pig myocardium.

Adenosine analogues N6-phenylisopropyladenosine and 5'-N-ethylcarboxamidoadenosine inhibit cardiac adenylate cyclase in a concentration dependent manner. Inhibition of adenylate cyclase required the presence of guanosine-triphosphate and Mg2+ ion. The concentration of Na+ ion was critically important: maximal inhibition was observed at 50 mmol/l Na+. 8-Phenyltheophylline and isobutylmethylxanthine antagonized the inhibition of adenylate cyclase mediated by adenosine analogues. Adenosine receptors of the inhibitory type (R1 or A1) are present in guinea-pig myocardium and may mediate some actions of adenosine.