Cell-surface estrogen receptors mediate calcium-dependent nitric oxide release in human endothelia

Background-Although estrogen replacement therapy has been associated with r eduction of cardiovascular events in postmenopausal women, the mechanism fo r this benefit remains unclear. Because nitric oxide (NO) is considered an important endothelium-derived relaxing factor and may function to protect b lood vessels against atherosclerotic development, we investigated the acute effects of physiological levels of estrogen on NO release from human inter nal thoracic artery endothelia and human arterial endothelia in culture. Methods and Results-We tested the hypothesis that estrogen acutely stimulat es constitutive NO synthase activity in human endothelial cells by acting o n a cell-surface receptor. NO release was measured in real time with an amp erometric probe. 17 beta-Estradiol exposure to internal thoracic artery end othelia and human arterial endothelia in culture stimulated NO release with in seconds in a concentration-dependent manner. 17 beta-Estradiol conjugate d to bovine serum albumin also stimulated NO release, suggesting action thr ough a cell-surface receptor. Tamoxifen, an estrogen receptor inhibitor, an tagonized this action. We further showed with the use of dual emission micr ofluorometry that 17 beta-estradiol-stimulated release of endothelial NO wa s dependent on the initial stimulation of intracellular calcium transients. Conclusions-Physiological doses of estrogen immediately stimulate NO releas e from human endothelial cells through activation of a cell-surface estroge n receptor that is coupled to increases in intracellular calcium.