Binding to Ax Adenosine Receptors of Intact Rat Ventricular Myocytes

The purpose of the present study was the identification of A, adenosine receptors in intact rat ventricular myocytes, which are thought to mediate the negative inotropic effects of adenosine. The adenosine receptor antagonist [H]-8-cydopentyl-l,3-dipropylxanthine was used as radioligand. Binding of the radioligand to intact myocytes was rapid, reversible, and saturable with a binding capacity of 40,000 binding sites per cell. The dissociation constant of the radioligand was 0.48 nM. The adenosine receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine, "xanthine amine congener," and theophyUine were competitive inhibitors with affinities in agreement with results obtained for A, receptors in other tissues. Competition experiments using the adenosine receptor agonists A-N-phenylisopropyladenosine, S'-JV-ethylcarboxamidoadenosine, and S-N'-phenylisopropyladenosine gave monophasic displacement curves with A, values of 50 nM, 440 nM, and 4,300 nM, which agreed well with the GTP-inducable low affinity state in cardiac membranes. The low affinity for agonists was not due to agonist-induced desensitization, and correlated well with the corresponding ICM values for the inhibition of cyclic AMP accumulation by isoprenaline. It is suggested that only a low affinity state of A, receptors can be detected in intact rat myocytes due to the presence of high concentrations of guanine nucleotides in intact cells. (Circulation Research 1988;63:613-620)