Adenosine receptor-mediated inhibition of cardiac adenylyl cyclase activity may involve multiple receptor subtypes.

We previously reported that the adenosine receptor agonist N6-phenylisopropyladenosine (R-PIA) inhibits adenylyl cyclase in detergent-permeabilized embryonic chick ventricular myocytes in the presence of the adrenergic receptor agonist, isoproterenol (Ma and Green 1992). The slope of the dose response curve of this inhibition is very shallow (nH 0.3–0.4). The present studies on detergent-permeabilized chick myocytes evaluate the mechanisms underlying this shallow inhibition curve. We find that in contrast to R-PIA, two additional adenosine receptor agonists, N6-cyclopentyladenosine (CPA) and 2-chloro-N6-cyclopentyladenosine (CCPA), inhibit cardiac adenylyl cyclase activity in a monophasic, dose-dependent manner (nH∼1). Two A1 adenosine receptor antagonists, 8-cyclopentyl-1,3,-dipropylxanthine (CPX) and 3-(4-amino)phenethyl-l-propyl-8-cyclopentylxanthine (BW A884U) affect the R-PIA responses differently. BW A884U shifts the R-PIA dose response curve to the right in a parallel fashion while CPX both shifts the R-PIA response curve and increases its steepness. Cardiac A1 adenosine receptors were further characterized using one antagonist ([3H]CPX) and two agonist ([3H]R-PIA and [3H]CCPA) radioligands. [3H]CPX binds to the adenosine receptors in detergent-permeabilized ventricular myocytes with a Kd value of 3.3±0.2 nM and a BMAX value of 30.1 ± 2.4 fmol/mg protein (means ± SEM; N = 4). [3H]R-PIA detects more sites than [3H]CCPA (22.8 ±4.0 and 8.3 ± 1.3 fmol/mg protein, respectively; GTP-free conditions). CPA and CCPA inhibit [3H]R-PIA binding in a shallow, dose-dependent manner (nH ∼ 0.4), while R-PIA and CPA inhibit [3H]CCPA binding with a nH∼1. These results suggest that more than one subtype of adenosine receptor is present and can couple to the inhibition of adenylyl cyclase in embryonic chick cardiac myocytes.