Evidence against adenylate cyclase-coupled adenosine receptors in the guinea pig heart.

(--)-N6-(Phenylisopropyl)adenosine (PIA, 10 microM) and 5'-N-(ethylcarboxamide)adenosine (NECA, 50 microM), potent agonists at the adenylate cyclase-coupled R-site adenosine receptor, were investigated for adenylate cyclase inhibition in a guinea pig ventricular membrane preparation by means of the dATP assay method. Neither compound influenced basal or isoproterenol-stimulated cyclase activity, irrespective of whether Na ions were present or not. These data suggest that R-site receptors may not be involved in the cardiac adenylate cyclase system of the guinea pig.