ADENOSINE-INDUCED HYPERPOLARIZATION IN GUINEA-PIG CORONARY-ARTERY INVOLVES A(2B) RECEPTORS AND K-ATP CHANNELS

The role of P-1 purinoceptor subtypes, the adenylyl cyclase (AC) pathw ay, and ATP-sensitive K+ (K-ATP) channels in adenosine (Ado)-induced m embrane hyperpolarization was investigated in isolated segments of the guinea pig coronary artery using conventional microelectrode techniqu es. Ado (1-100 mu M) elicited concentration-dependent hyperpolarizatio n (half-maximal effective concentration 7.5 +/- 0.5 mu M) that average d 28.6 +/- 2.9 mV at 100 mu M Ado. The A(1) selective agonist N-G-cycl opentyladenosine (CPA), the A(1)/A(2) agonist 2-chloroadenosine, and t he A(2a)/A(2b) agonist 5-(N-ethylcarboxamido)adenosine (NECA) each ind uced glibenclamide (3 mu M)sensitive hyperpolarization at 10 mu M. How ever, the selective A(2a)-receptor agonists CGS-21680 and dimethoxyphe nyl)-2-(2-methoxyphenyl]ethyladenosine (10 mu M each) were without eff ect. Responses to CPA and NECA were significantly reduced by the AC in hibitor SQ-22,536 (100 mu M). Activation of the AC-adenosine 3',5'-cyc lic monophosphate (cAMP)-protein kinase A (PKA) pathway by four additi onal methods, i.e., 1) forskolin (0.3-1 mu M), 2) isoproterenol (0.1-1 mu M), 3) combined milrinone (0.4 mu M) and rolipram (30 mu M), and 4 ) combined N-6-phenyladenosine 3',5'-monophosphate, 8-(6-aminohexyl)am inoadenosine 3',5'-cyclic monophosphate, and the Sp-isomer of 5,6-dich loro-1-D-ribofuranosylbenzimid azole-3',5'-cyclic monophosphothioate ( 100 mu M each), also gave rise to glibenclamide-sensitive hyperpolariz ation. These results suggest that stimulation of A(2b) receptors coupl ed to AC represents the predominant mechanism by which Ado elicits hyp erpolarization in this vessel. The ensuing increase in cAMP activates PKA, which then increases the activity of K-ATP channels. Our results further suggest that K-ATP channels are an important target for numero us pathways that raise cAMP levels in the coronary artery.