TEA INHIBITS ACH-INDUCED EDRF RELEASE - ENDOTHELIAL CA2-DEPENDENT K+ CHANNELS CONTRIBUTE TO VASCULAR TONE()

The effects of K+-channel blockers on the acetylcholine (ACh)-induced relaxation of vascular smooth muscle, intracellular free Ca2+ concentr ation ([Ca2+](i)) elevation, and ACh-evoked outward K+ current of endo thelial cells of rabbit aorta were studied using bioassay, spectrofluo rimetry, and patch-clamp techniques, respectively. In bioassay experim ents, ACh caused relaxation of endothelium-denuded aortic rings in a c oncentration-dependent manner when perfused through an endothelium-int act donor segment of aorta but not when perfused directly onto the rec ipient aortic ring. ACh-induced relaxation was inhibited by perfusion of tetraethylammonium ions (TEA; 5 mM) through the donor but not by pe rfusion directly onto the recipient segment. Glibenclamide had no effe ct on ACh-induced relaxation of the bioassay ring in either situation. ACh increased [Ca2+](i) at the endothelial surface of aortic strips b ut not at the adventitial surface. TEA inhibited ACh-induced [Ca2+](i) elevation, whereas glibenclamide had no effect. In patch-clamp experi ments with freshly isolated endothelial cells, ACh evoked a biphasic o utward current which was completely abolished by TEA (3 mM). It is con cluded that Ca2+-dependent K+ channels are important for increasing [C a2+](i) during agonist stimulation and consequently for the synthesis/ release of endothelium-derived relaxing factors (EDRFs). Furthermore, endothelial ATP-sensitive K+ channels do not contribute to ACh-induced relaxation or evoke an increase in endothelial [Ca2+](i) of rabbit th oracic aorta.