DIFFERENTIAL DESENSITIZATION OF A1 ADENOSINE RECEPTOR-MEDIATED INHIBITION OF CARDIAC MYOCYTE CONTRACTILITY AND ADENYLATE-CYCLASE ACTIVITY - RELATION TO THE REGULATION OF RECEPTOR AFFINITY AND DENSITY

Effects of chronic exposure of cultured atrial myocytes to R-N6-(2-phenylisopropyl)-adenosine (R-PIA) on the A1 adenosine receptor-mediated inhibition of adenylate cyclase activity and myocyte contractility were examined. Chronic exposure of atrial myocytes cultured from 14-day-old chick embryos to R-PIA desensitized the myocyte to the inhibitory effects of R-PIA on contractility and adenylate cyclase activity in a time- and dose-dependent manner. Desensitization of the negative inotropic response was only partial, whereas the adenosine receptor-mediated inhibition of adenylate cyclase activity was almost completely absent after 24 hours of R-PIA (1 μM) exposure. Furthermore, the contractile response to R-PIA desensitized more slowly than the desensitization of A1 adenosine receptor-mediated inhibition of adenylate cyclase (t 1/2 = 11.4 ± 0.7 hours versus 7.5 ± 1 hours, mean ± SEM, n = 12 and 6, respectively). Thus, the two A1 adenosine receptor-linked functional responses desensitized differently in response to chronic exposure of the myocyte to R-PIA. Binding of the antagonist radioligand [3H]-8-cyclopentyl-1,3-dipropylxanthine ([3H]CPX) in membranes from myocytes preexposed to R-PIA demonstrated a time-dependent decrease in receptor density without any change in the affinity for the antagonist radioligand. Computer analyses of agonist competition with [3H]CPX binding in membranes from control and R-PIA-treated myocytes revealed a conversion of the high-affinity A1 adenosine receptor to a low-affinity form such that after 24 hours of 1 μM R-PIA exposure, all of the receptors were in low-affinity form. On the other hand, downregulation of the receptor was partial, with nearly 60 of the receptor remaining after 24-hour treatment with 1 μM R-PIA. These data indicate that the mechanism of desensitization of the A1 adenosine receptor-mediated negative inotropic response differs from that of the desensitization of the adenylate cyclase. Uncoupling of the A1 adenosine receptor from a high-affinity state was closely associated with desensitization of the inhibition of adenylate cyclase; downregulation of the A1 adenosine receptor appeared to parallel desensitization of the negative inotropic response.