Relaxation of the corpus cavernosum smooth muscle is an absolute prerequisite of penile erection. Potassium channels play a role in the physiologic regulation of corporal smooth muscle tone. Among the several subtypes of potassium channels, Ca2 +-activated potassium channel (KCa channel) subtypes are thought to be the most physiologically relevant in the regulation of corporal smooth muscle tone. The purpose of this study was to investigate the effects of nitric oxide (NO) and sildenafil on the KCa channels and elucidate the mechanisms of action on the KCa channels in smooth muscle cells of the human corpus cavernosum. The conventional patch-clamp technique was applied to short-term cultured smooth muscle cells of the human corpus cavernosum. Single-channel currents were recorded in cell-attached or inside-out patches, and whole-cell currents were recorded in perforated-patches. In cell-attached patches, sildenafil alone did not activate the KCa channels but sildenafil enhanced the NO-induced activation of KCa channels. The open probability of KCa channels was increased significantly after application of NO donor, SIN-1 (100 microM) (47 +/- 7.1%, n = 10, P=0.002). The application of sildenafil (100 nM) with SIN-1 (100 microM) markedly increased the open probability of KCa channels (148 +/- 24%, n = 8, P < 0.001). The activation by SIN-1 or sildenafil with SIN-1 was completely blocked by pretreatment of the soluble guanylate cyclase inhibitor, ODQ (10 microM). In inside-out patches. SIN-1 or sildenafil with SIN-1 failed to activate KCa channels at pCa 7.5 (n=5). SIN-1 increased the whole cell outward K+ currents in all holding potential. The increased IK by SIN-1 was inhibited by charybdotoxin (CTX) about 70%. These data provide compelling evidence consistent with the involvement of the KCa channel subtype in modulating NO-induced relaxation responses in human corporal smooth muscle. Furthermore, the activation of KCa channels is thought to be mediated by activation of soluble guanylate cyclase, leading to increased intracellular levels of cyclic GMP and the subsequent activation of protein kinase rather than direct NO effect.