ABSTRACT
Acetylcholine-mediated relaxation of blood vessels from endothelial nitric oxide synthase "knockout" mice (eNOS - / - ) was insensitive to a combination of cyclooxygenase, nitric oxide synthase, and soluble guanylyl cyclase inhibitors and possessed the characteristics of being mediated by an endothelium-dependent hyperpolarizing factor ( E D H F ) . The release and/or cellular action of this putative E D H F was inhibited by a combination of apamin and charybdotoxin and thus assumed to involve calcium-activated charybdotoxin-and apamin-sensitive potassium channels. Patch clamp studies of myocytes from the small mesenteric arteries revealed differences in whole cell current density between myocytes obtained from eNOS —Iversus eNOS +/+ mice that may reflect differences in potassium channel subtype and thus contribute to the alterations in endothelium-dependent relaxation that have previ ously been reported in eNOS - / - mice. Four components of outward current were identified: (a) a 4-aminopyridine-sensitive transient outward current; (b) a 4-aminopyridine-sensitive delayed rectifier current; (c) a 4-aminopyridine-resistant delayed rectifier current; and (d) a tetraethylammonium-sensitive calcium-activated and delayed rectifier potassium ( K C a and K D R ) currents. A n apamin and charybdotoxin-sensitive current could not be detected in the myocytes. These data indicate that the apamin and charybdotoxin-sensitive component of the EDHF-mediated response observed in the mesenteric artery does not reflect the cellular action of E D H F at the level of the vascular smooth muscle cell and that, most likely, the site of action of apamin and charybdotoxin is on the endothelium.
