Adenosine-induced hyperpolarization in guinea pig coronary artery involves A2b receptors and KATP channels

The role of P1 purinoceptor subtypes, the adenylyl cyclase (AC) pathway, and ATP-sensitive K+ (KATP) channels in adenosine (Ado)-induced membrane hyperpolarization was investigated in isolated segments of the guinea pig coronary artery using conventional microelectrode techniques. Ado (1-100 microM) elicited concentration-dependent hyperpolarization (half-maximal effective concentration 7.5 +/- 0.5 microM) that averaged 28.6 +/- 2.9 mV at 100 microM Ado. The A1 selective agonist N6-cyclopentyladenosine (CPA), the A1/A2 agonist 2-chloroadenosine, and the A2a/A2b agonist 5-(N-ethylcarboxamido)adenosine (NECA) each induced glibenclamide (3 microM)-sensitive hyperpolarization at 10 microM. However, the selective A2a-receptor agonists CGS-21680 and N6-[2-(3,5-dimethoxyphenyl)-2-(2-methoxyphenyl]ethyladenosine (10 microM each) were without effect. Responses to CPA and NECA were significantly reduced by the AC inhibitor SQ-22,536 (100 microM). Activation of the AC-adenosine 3',5'-cyclic monophosphate (cAMP)-protein kinase A (PKA) pathway by four additional methods, i.e., 1) forskolin (0.3-1 microM), 2) isoproterenol (0.1-1 microM), 3) combined milrinone (0.4 microM) and rolipram (30 microM), and 4) combined N6-phenyladenosine 3',5'-monophosphate, 8-(6-aminohexyl)aminoadenosine 3',5'-cyclic monophosphate, and the Sp-isomer of 5,6-dichloro-1-D-ribofuranosylbenzimidazole-3', 5'-cyclic monophosphothioate (100 microM each), also gave rise to glibenclamide-sensitive hyperpolarization. These results suggest that stimulation of A2b receptors coupled to AC represents the predominant mechanism by which Ado elicits hyperpolarization in this vessel. The ensuing increase in cAMP activates PKA, which then increases the activity of KATP channels. Our results further suggest that KATP channels are an important target for numerous pathways that raise cAMP levels in the coronary artery.