17β-Estradiol Inhibits Ca2+Influx and Ca2+Release Induced by Thromboxane A2in Porcine Coronary Artery

BACKGROUND: We wished to investigate the possible mechanism of the protective effect of estrogen replacement on coronary atherosclerosis observed in postmenopausal women. METHODS AND RESULTS: Cytosolic Ca2+ concentration (Ca2+i) and contraction were measured simultaneously in fura 2-loaded porcine coronary arterial strips stimulated by the thromboxane A2 analogue U46619 and high-K+ depolarization in the presence and absence of 17 beta-estradiol. Pretreatment with 17 beta-estradiol (30 nmol/L to 30 mumol/L) inhibited the sustained elevation of Ca2+i and the sustained contraction induced by 300 nmol/L U46619. Higher concentrations of 17 beta-estradiol (1 to 100 mumol/L) also inhibited the U46619-induced transient increase in Ca2+i and contraction in the absence of extracellular Ca2+. In the strips precontracted by 90 mmol/L K+, 17 beta-estradiol (30 mumol/L) inhibited the increases in Ca2+i and contraction to resting levels. In contrast, 30 mumol/L 17 beta-estradiol only partially inhibited the U46619-induced sustained contraction, despite complete inhibition of the sustained increase in Ca2+i. Verapamil (10 mumol/L) also strongly inhibited the sustained increase in Ca2+i induced by 300 nmol/L U46619, with a partial inhibition of the U46619-induced sustained contraction. A subsequent addition of 30 mumol/L 17 beta-estradiol did not show an additional inhibitory effect on either the Ca2+i or the tension after the addition of verapamil. 17 beta-Estradiol (10 mumol/L) also inhibited the increase in Ca2+i and the contraction induced by cumulative addition of Ca2+ in the strips pretreated with 90 mmol/L K+. However, 17 beta-estradiol did not change the slope of the Ca2+i-tension curves. 17 beta-Estradiol (10 mumol/L) had no effect on the levels of cAMP and cGMP in the coronary strips. CONCLUSIONS: 17 beta-Estradiol inhibits the contraction of coronary vascular smooth muscle mainly inhibiting Ca2+ influx without changing Ca2+ sensitivity of contractile elements. The Ca2+ channel blocker-like action of 17 beta-estradiol may explain at least a part of the antiatherosclerotic effect of estrogen.