NITRIC-OXIDE AND CGMP CAUSE VASORELAXATION BY ACTIVATION OF A CHARYBDOTOXIN-SENSITIVE K-CHANNEL BY CGMP-DEPENDENT PROTEIN-KINASE

Nitric oxide (NO)-induced relaxation is associated with increased leve ls of cGMP in vascular smooth muscle cells. However, the mechanism by which cGMP causes relaxation is unknown. This study tested the hypothe sis that activation of Ca-sensitive K (K-Ca) channels, mediated by a c GMP-dependent protein kinase, is responsible for the relaxation occurr ing in response to cGMP. In rat pulmonary artery rings, cGMP-dependent , but not cGMP-independent, relaxation was inhibited by tetraethylammo nium, a classical K-channel blocker, and charybdotoxin, an inhibitor o f K-Ca channels. Increasing extracellular K concentration also inhibit ed cGMP-dependent relaxation, without reducing vascular smooth muscle cGMP levels. In whole-cell patch clamp experiments, NO and cGMP increa sed whole cell K current by activating K-Ca channels. This effect was mimicked by intracellular administration of (Sp)-guanosine cyclic 3',5 '-phosphorothioate, a preferential cGMP-dependent protein kinase activ ator. Okadaic acid, a phosphatase inhibitor, enhanced whole cell K cur rent, consistent with an important role for channel phosphorylation in the activation of NO-responsive K-Ca channels. Thus NO and cGMP relax vascular smooth muscle by a cGMP-dependent protein kinase-dependent a ctivation of K channels. This suggests that the final common pathway s hared by NO and the nitrovasodilators is cGMP-dependent K-channel acti vation.