Endothelium-independent effect of estrogen on Ca²⁺-activated K⁺ channels in human coronary artery smooth muscle cells

Objective: Postmenopausal estrogen replacement therapy lowers the incidence of cardiovascular disease, suggesting that estrogens support cardiovascular function. Estrogens dilate coronary arteries; however, little is known about the molecular basis of how estrogen affects the human coronary circulation. The cellular/molecular effects of estrogen action on human coronary smooth muscle were investigated in the present study. Methods: Patch-clamp and fluorescent microscopy studies were performed on human coronary myocytes in the absence of endothelium. Results: Estrogen increased whole-cell currents over a range of membrane potentials, and further studies indicated that the large-conductance (186.5±3 pS), calcium- and voltage-activated potassium (BKCa) channel was the target of estrogen action. Channel activity was stimulated ∼15-fold by nanomolar concentrations of 17β-estradiol, and this stimulation was reversed >90% by inhibiting cGMP-dependent protein kinase activity with 300 nM KT5823. 17β-Estradiol increased the level of cGMP and nitric oxide in human myocytes, and the stimulatory effect of estrogen on channel activity and NO production was reversed by inhibiting NO synthase with 10 μM NG-monomethyl-L-arginine. Conclusions: Our cellular and molecular studies identify the BKCa channel as a target of estrogen action in human coronary artery smooth muscle. This response to estrogen involves cGMP-dependent phosphorylation of the BKCa channel or a closely associated regulatory molecule, and further evidence suggests involvement of the NO/cGMP signaling system in coronary smooth muscle. These findings are the first to provide direct evidence for a molecular mechanism that can account for endothelium-independent effects of estrogen on human arteries, and may also help explain why estrogens reduce myocardial ischemia and stimulate coronary blood flow in patients with diseased coronary arteries. © 2002 Elsevier Science B.V. All rights reserved.