(Xeno)estrogen sensitivity of smooth muscle BK channels conferred by the regulatory beta 1 subunit - A study of beta 1 knockout mice

Estrogen and xenoestrogens (i.e. agents that are not steroids but possess e strogenic activity) increase the open probability (P-o) of large conductanc e Ca2+-activated K+ (BK) channels in smooth muscle. The mechanism of action may involve the regulatory beta1 subunit. We used beta1 subunit knockout ( beta1(-/-)) mice to test the hypothesis that the regulatory beta1 subunit i s essential for the activation of BK channels by tamoxifen, 4-OH tamoxifen (a major biologically active metabolite), and 17 beta -estradiol in native myocytes. Patch clamp recordings demonstrate BK channels from beta1(-/-) mi ce were similar to wild type with the exception of markedly reduced Ca2+/vo ltage sensitivity and faster activation kinetics. In wild type myocytes, (x eno)estrogens increased NPo (P-o x the number of channels, N), shifted the voltage of half-activation (V-1/2) to more negative potentials, and decreas ed unitary conductance. These effects were non-genomic and direct, because they were rapid, reversible, and observed in cell-free patches. None of the (xeno)estrogens increased the NP. of BK channels from beta1(-/-) mice, but all three agents decreased single channel conductance. Thus, (xeno)estroge ns increase BK NP. through a mechanism involving the beta1 subunit. The dec rease in conductance did not require the beta1 subunit and probably reflect s an interaction with the pore-forming a subunit. We demonstrate regulation of smooth muscle BK channels by physiological (steroid hormones) and pharm acological (chemotherapeutic) agents and reveal the critical role of the be ta1 subunit in these responses in native myocytes.