Effects of nitric oxide on the Ca2+-activated potassium channels in smoothmuscle cells of the human corpus cavernosum

Relaxation of the corpus cavernosum smooth muscle is an absolute prerequisi te of penile erection. Potassium channels play a role in the physiologic re gulation of corporal smooth muscle tone. Among the several subtypes of pota ssium channels, Ca2+-activated potassium channel (K-Ca channel) subtypes ar e thought to be the most physiologically relevant in the regulation of corp oral smooth muscle tone. The purpose of this study was to investigate the e ffects of nitric oxide (NO) and sildenafil on the K-Ca channels and elucida te the mechanisms of action on the K-Ca, channels in smooth muscle cells of the human corpus cavernosum. The conventional patch-clamp technique was ap plied to short-term cultured smooth muscle cells of the human corpus cavern osum. Single-channel currents were recorded in cell-attached or inside-out patches, and whole-cell currents were recorded in perforated-patches. In ce ll-attached patches, sildenafil alone did not activate the K-Ca, channels b ut sildenafil enhanced the NO-induced activation of K-Ca channels. The open probability of K-Ca channels was increased significantly after application of NO donor, SIN-1 (100 muM) (47 +/- 7.1%, n = 10, P = 0.002). The applica tion of sildenafil (100 nM) with SIN-1 (100 LM) markedly increased the open probability of K-Ca channels (148 +/- 24%, n = 8, P < 0.001). The activati on by SIN-1 or sildenafil with SIN-1 was completely blocked by pretreatment of the soluble guanylate cyclase inhibitor, ODQ (10 <mu>M). In inside-out patches, SIN-1 or sildenafil with SIN-1 failed to activate K-Ca channels at pCa 7.5 (n = 5). SIN-1 increased the whole cell outward K+ currents in all holding potential. The increased I-K by SIN-1 was inhibited by charybdotox in (CTX) about 70%. These data provide compelling evidence consistent with the involvement of the K-Ca channel subtype in modulating NO-induced relaxa tion responses in human corporal smooth muscle. Furthermore, the activation of K-Ca channels is thought to be mediated by activation of soluble guanyl ate cyclase, leading to increased intracellular levels of cyclic GMP and th e subsequent activation of protein kinase rather than direct NO effect.