Smooth muscle relaxation and activation of the large conductance Ca++ – activated K+ (BKCa) channel by novel oestrogens

Abstract

Background and Purpose Oestrogens can interact directly with membrane receptors and channels and can activate vascular BK_Ca channels. We hypothesized that novel oestrogen derivatives could relax smooth muscle by an extracllular effect on the α and β₁ subunits of the BK_Ca channel, rather than at an intracellular site. Experimental Approach We studied the effects of novel oestrogens on the tension of pre-contracted isolated rat aortic rings, and on the electrophysiological properties of HEK 293 cells expressing the hSloα or hSloα+β₁ subunits. Two of the derivatives incorporated a quaternary ammonium side-chain making them membrane impermeable. Key Results Oestrone, oestrone oxime and Quat DME-oestradiol relaxed pre-contracted rat aorta, but only Quat DME-oestradiol-induced relaxation was iberiotoxin sensitive. However, only potassium currents recorded in HEK 293 cells over-expressing both hSloα and hSloβ₁ were activated by oestrone, oestrone oxime and Quat DME-oestradiol. Conclusion and Implications The novel oestrogens were able to relax smooth muscle, but through different mechanisms. In particular, oestrone oxime required the presence of the endothelium to exert much of its effect, whilst Quat DME-oestradiol depended both on NO and BK_Ca channel activation. The activation of BK_Ca currents in HEK 293 cells expressing hSloα+β₁ by Quat DME-oestradiol is consistent with an extracellular binding site between the two subunits. The binding site resides between the extracellular N terminal of the α subunit and the extracellular loop between TM1 and 2 of the β₁ subunit. Membrane-impermeant Quat DME-oestradiol lacks an exchangeable hydrogen on the A ring obviating antioxidant activity.