Nongenomic modulation of the large conductance voltage- and Ca2+-activated K+ channels by estrogen: A novel regulatory mechanism in human detrusor smooth muscle

Estrogens have an important role in regulating detrusor smooth muscle (DSM) function. However, the underlying molecular and cellular mechanisms by which estrogens control human DSM excitability and contractility are not well known. Here, we used human DSM specimens from open bladder surgeries on 27 patients to elucidate the mechanism by which 17-estradiol regulates large conductance voltage- and Ca2+-activated K+ (BK) channels, the most prominent K+ channels in human DSM. We employed single BK channel recordings on inside-out excised membrane patches, perforated whole-cell patch-clamp on freshly isolated DSM cells, and isometric tension recordings on DSM-isolated strips to investigate the mechanism by which 17-estradiol activates BK channels. 17-Estradiol (100nmol/L) rapidly increased depolarization-induced whole-cell K+ currents in DSM cells. The 17-estradiol stimulatory effects on whole-cell BK currents were completely abolished by the selective BK channel inhibitor paxilline (1mol/L), clearly indicating that 17-estradiol specifically activates BK channels. 17-Estradiol also increased the frequency of ryanodine receptor-mediated transient BK currents. Single BK channel recordings showed that 17-estradiol (100nmol/L) significantly increased the BK channel open probability of inside-out excised membrane patches, revealing that 17-estradiol activates BK channels directly. 17-Estradiol reduced spontaneous phasic contractions of human DSM-isolated strips in a concentration-dependent manner (100nmol/L-1mol/L), and this effect was blocked by paxilline (1mol/L). 17-Estradiol (100nmol/L) also reduced nerve-evoked contractions of human DSM-isolated strips. Collectively, our results reveal that 17-estradiol plays a critical role in regulating human DSM function through a direct nongenomic activation of BK channels.