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(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(Ca) channels but exhibit no Ca²⁺ or voltage sensitivity. We have termed these BK channels. Furthermore, the activity of the BK(Ca) channel from pregnant tissue is inhibited by internal application of Ba²⁺ but not tetraethylammonium (TEA), whereas the activity of the BK channel is sensitive to internal TEA but not Ba²⁺. The role of the BK(Ca) channel may be to suppress myometrial activity during gestation whereas BK channel activity may be important in providing a Ca²⁺- independent K⁺ conductance which would allow cytoplasmic Ca²⁺ levels to rise without activating a counteracting Ca²⁺-dependent outward current, normally provided by the BK(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.