Properties of large-conductance K$^{+}$ channels in human myometrium during pregnancy and labour

The conversion of the electrically silent pregnant uterus to highly excitable at term represents a dramatic physiological event which is poorly understood. Here we provide the first description, from single-channel recordings, of a large conductance (212 pS) calcium-activated potassium channel (BK$_{\text{Ca}}$) in human pregnant myometrium which, in labour tissue, is either absent or has been considerably altered in its physiological and pharmacological properties. In the latter, the K$^{+}$ channels have an identical conductance (221 pS) and K$^{+}$ selectivity to BK$_{\text{Ca}}$ channels but exhibit no Ca$^{2+}$ or voltage sensitivity. We have termed these BK channels. Furthermore, the activity of the BK$_{\text{Ca}}$ channel from pregnant tissue is inhibited by internal application of Ba$^{2+}$ but not tetraethylammonium (TEA), whereas the activity of the BK channel is sensitive to internal TEA but not Ba$^{2+}$. The role of the BK$_{\text{Ca}}$ channel may be to suppress myometrial activity during gestation whereas BK channel activity may be important in providing a Ca$^{2+}$-independent K$^{+}$ conductance which would allow cytoplasmic Ca$^{2+}$ levels to rise without activating a counteracting Ca$^{2+}$-dependent outward current, normally provided by the BK$_{\text{Ca}}$ channels which, by its very nature, would tend to oppose depolarization. The findings suggest that K$^{+}$ channels may have an important role in determining the functional activity of the myometrium.