Demonstration of adenylate cyclase coupled adenosine receptors in guinea pig ventricular membranes

Adenylate cyclase in homogenates of guinea pig ventricles was inhibited by the stable adenosine analogs N6-phenylisopropyladenosine (PIA) and 5(1)-N-ethylcarboxamidoadenosine (NECA). Inhibition required GTP and was enhanced by sodium ion. The maximum inhibition observed was 35.1 +/- 1.1%, the EC50 (95% confidence limits, n) for PIA and NECA were 0.20 microM (0.17-0.25 microM, 6) and 0.66 microM (0.26-1.7 microM, 4) respectively. 8-Phenyltheophylline (10 and 100 microM) and isobutylemethylxanthine (100 microM) antagonized the inhibitory effects of the adenosine analogs. These results indicate that adenosine receptors of the inhibitory type (RI or A1) are present in guinea pig myocardium and may mediate some of the cardiac responses to adenosine.