Estrogen regulates β₁-subunit expression in Ca ²⁺-activated K⁺ channels in arteries from reproductive tissues

Daily estradiol-17β (E₂β) increases basal uterine blood flow (UBF) and enhances acute E₂β-mediated increases in UBF in ovariectomized nonpregnant ewes. The acute E₂β-mediated rise in UBF involves vascular smooth muscle (VSM) large-conductance Ca ²⁺-activated K⁺ channels (BK_Ca). BK _ca consist of pore-forming α-subunits and regulatory β₁-subunits that modulate channel function and E ₂β responsiveness. It is unclear whether E₂β also alters subunit expression and thus channel density and/or function, thereby contributing to the rise in basal UBF and enhanced UBF responses that follow daily E₂β. Therefore, we examined BK_ca subunit expression by using reverse transcription-PCR and immunoblot analysis of arterial VSM from reproductive and nonreproductive tissues and myometrium from ovariectomized nonpregnant ewes after daily E₂β (1 μg/kg iv) or vehicle without or with acute E₂β (1 μg/kg). Tissue distribution was determined by immunohistochemistry. Acute E₂β did not alter α- or β₁-subunit expression in any tissue (P > 0.1). Daily E₂β also did not affect α-subunit mRNA or protein in any tissue (P > 0.1) or mesenteric arterial VSM β₁-subunit. However, daily E₂β increased uterine and mammary arterial VSM β₁-subunit mRNA by 32% and 83% (P < 0.05), uterine VSM protein by 30%, and myometrial β₁-subunit mRNA and protein by 74% (P < 0.005). Immunostaining of uterine arteries, myometrium, and intramyometrial arteries paralleled immunoblot analyses for both subunits. Although BK_Ca density is unaffected by daily and acute E₂β, daily E₂β increases β₁- subunit in proximal and distal uterine arterial VSM. Thus prolonged E ₂β exposure may alter BK_ca function, estrogen responsiveness, and basal vascular tone and reactivity in reproductive arteries by modifying α:β₁ stoichiometry.