MEDIATION BY NITRIC-OXIDE OF THE INDIRECT EFFECTS OF ADENOSINE ON CALCIUM CURRENT IN RABBIT HEART PACEMAKER CELLS

1 Adenosine (ADO) is a potent negative chromotropic agent in the mamma lian myocardium. We have used single myocytes from rabbit sino-atrial node (SAN) to examine whether nitric oxide (NO) is a significant media tor of the effects of ADO on the pacemaker activity, or the underlying Ca2+ and K+ currents. 2 SAN pacemaker cells were isolated from rabbit hearts by enzymatic dispersion, and CA(2+) and K+ currents were recor ded by the nystatin-perforated patch voltage clamp method. ADO was app lied in the presence of the beta-adrenoceptor agonist, isoprenaline (I so) to mimic the adrenergic tone which the SAN is subjected to in vivo . 3 Control experiments confirmed that isolated SAN cells responded to ADO (10-100 mu M) with the expected (i) small increase in background inwardly rectifying K+ current, I-K-ADOi and (ii) pronounced decrease in L-type Ca2+ current, I-Ca-L. These effects were mimicked by a selec tive A(1) purinoceptor agonist, N-6-cyclopentyladenosine (CPA, 10 mu M ), which selectively blocks A(1) purinoceptors. DMPX (10 mu M), a bloc ker of A(2) purinoceptor, had no effect on the actions of ADO. 4 A nit ric synthase inhibitor, L-NMMA (100 mu M), abolished the inhibitory ef fect of ADO on I-CA-L but did not alter activation of I-K-ADO. After L -NMMA washoff, it was possible to obtain the normal response (inhibiti on) of I-Ca-L to ADO in the same cell. 5 To evaluate whether the obser ved effect of nitric oxide (NO) on I-Ca-L was mediated by an increase in guanylyl cyclase (GC) activity and cyclic GMP formation, the guanyl yl cyclase inhibitor, LY 83583 (40 mu M) was applied prior to ADO. Und er these conditions, the inhibitory effect of ADO on I-Ca-L was abolis hed, but the activation of I-K-ADO was still observed. 6 In combinatio n, these findings strongly suggest that in mammalian primary pacemaker tissue which is under adrenergic tone, the effects of ADO on I-Ca-L a re mediated by NO.