Activation of Ca²⁺-Activated K⁺ Channels in Human Myometrium by Nitric Oxide

To investigate the mechanism of human uterine smooth muscle relaxation, the activation of Ca²⁺-activated K⁺ channels in cultured myometrial cells obtained from human pregnant myometrium at term by nitric oxide was evaluated at the single cell level using the patch-clamp technique. The open probability of the K⁺ channel after the addition of 3 × 10–³ M isosorbide dinitrate, a nitric oxide donor (0.116 ± 0.048) was significantly higher than that before the addition (0.059 ± 0.032; n = 9, p < 0.01). In myometrial cells pretreated with lipopolysaccharide, activation of K⁺ channels was also noted after the addition of L-arginine (10–⁴ M; open probability increased from 0.179 ± 0.076 to 0.380 ± 0.105, n = 9, p < 0.01: 10–³ M; open probability increased from 0.073 ± 0.050 to 0.242 ± 0.098, n = 12, p < 0.01). Either 10–³ M N-nitro-L-arginine-methyl-ester, an inhibitor of nitric oxide synthase, or 10–⁶ M methylene blue, an inhibitor of guanylate cyclase, abolished activation of the K⁺ channel by 10–³ M L-arginine in pretreated myometrial cells with lipopolysaccharide. Application of 10–³ M L-arginine to the intracellular surface of an excised inside-out patch in the myometrial cells pretreated with lipopolysaccharide failed to increase Ca²⁺-activated K⁺ channel activity, suggesting that the activation was mediated by intracellular messengers. These results indicate that nitric oxide should control human myometrial relaxation during pregnancy via activation of Ca²⁺-activated K⁺ channels.