Ca2+ dependence and pharmacology of large-conductance K+ channels in nonlabor and labor human uterine myocytes.

Two populations, Ca(2+)-dependent (BKCa) and Ca(2+)-independent K+ (BK) channels of large conductance were identified in inside-out patches of nonlabor and labor freshly dispersed human pregnant myometrial cells, respectively. Cell-attached recordings from nonlabor myometrial cells frequently displayed BKCa channel openings characterized by a relatively low open-state probability, whereas similar recordings from labor tissue displayed either no channel openings or consistently high levels of channel activity that often exhibited clear, oscillatory activity. In inside-out patch recordings, Ba2+ (2-10 mM), 4-aminopyridine (0.1-1 mM), and Shaker B inactivating peptide ("ball peptide") blocked the BKCa channel but were much less effective on BK channels. Application of tetraethylammonium to inside-out membrane patches reduced unitary current amplitude of BKCa and BK channels, with dissociation constants of 46 mM and 53 microM, respectively. Tetraethylammonium applied to outside-out patches decreased the unitary conductance of BKCa and BK channels, with dissociation constants of 423 and 395 microM, respectively. These results demonstrate that the properties of human myometrial large-conductance K+ channels in myocytes isolated from laboring patients are significantly different from those isolated from nonlaboring patients.