Apamin inhibits NO-induced relaxation of the spontaneous contractile activity of the myometrium from non-pregnant women

Abstract

There is now considerable evidence for the involvement of K+ channels in nitric oxide (NO) induced relaxation of smooth muscles including the myometrium. In order to assess whether apamin-sensitive K+ channels play a role in NO - induced relaxation of the human uterus, we have studied the effect of specific blockers of these channels on the relaxation of myometrium from non-pregnant women. In vitro isometric contractions were recorded in uterine tissues from non-pregnant premenopausal women who had undergone hysterectomy. Apamin (10 nM) and scyllatoxin (10 nM) did not alter spontaneous myometrial contractions. However, 15-min pretreatment of the myometrium strips with apamin completely inhibited relaxation caused by diethylamine-nitric oxide (DEA/NO). The pretreatment with scyllatoxin significantly reduced (about 2.6 times) maximum relaxation of the strips induced by DEA/NO (p < 0.05). These results strongly suggest that, beside Ca2+ and voltage dependent charybdotoxin-sensitive (CTX-sensitive) K+ channels, apamin-sensitive K+ channels are also present in the human non-pregnant myometrium. These channels offer an additional target in the development of new tocolytic agents.

Figure 1

Original recordings showing typical effects of DEA/NO on spontaneous contractile activity of the myometrium from non-pregnant women: (A) spontaneous activity and the response to depolarization caused by the solution containing high concentration of K⁺ (80 mM); (B) the effect of a cumulative administration of DEA/NO; (C) the effect of DEA/NO on tissue pretreated with 10 nM apamin; (D) the effect of DEA/NO on tissue pretreated with 10 nM scyllatoxin

Figure 2

Concentration – response relationships of DEA/NO-induced relaxation before and after pretreatment with blockers of Ca²⁺-sensitive K⁺ channels: (A) DEA/NO alone (n = 10), DEA/NO after preincubation with 10 nM scyllatoxin (n = 5), or 10 nM apamin (n = 10); (B) DEA/NO in the absence and presence of 100 nM CTX (n = 10). Each point represents the mean ± SEM and * indicates effects significantly different from those observed in absence of the blockers (ANOVA). The spontaneous contractile activity was treated as a control.

Figure 3

Original recording showing effects of DEA/NO on spontaneous contractions of the myometrium from non-pregnant women followed by reappearing of the contractile activity after wash-out with the PSS solution.

Figure 4

Effects of Ca²⁺-sensitive K⁺ channel blockers on amplitude and frequency of spontaneous contractions. Each bar represents the mean ± SEM of 10 (apamin), 5 (scyllatoxin), and 10 (CTX) experiments.

Figure 5

DEA/NO-induced changes of amplitude and frequency of contractions observed in the absence and after incubation with Ca²⁺-sensitive K⁺ channel blockers. Each bar represents the mean ± SEM of 10 (apamin), 5 (scyllatoxin), and 10 (CTX) experiments