Differential effects of steroids on vascular tone and smooth muscle cell Ca2+-homeostasis

Abstract

The protective effects of 17p oestradiol against the onset of atherosclerosis in women can only partly be explained by altered lipid metabolism. The vessel wall itself is also a target organ for sex steroids. 17p oestradiol-induced increases in the expression of endothelial cell nitric oxide synthase [l] and proliferation of endothelial cells [2], as well as the decrease in the proliferation of smooth muscle cells [3] require de now protein synthesis. These effects of 17p oestradiol appear to be mediated by the classical ICI 182,780-sensitive nuclear oestrogen receptors. In contrast, 17p oestradiol induces a rapid and reversible endotheliumindependent relaxation of preconstricted vascular rings [4]. This latter response to 17p oestradiol is independent of changes in protein synthesis and has been attributed to an alteration of smooth muscle cell Ca2'-homeostasis, presumably involving a reduction in Ca2' influx through voltage operated L-type channels [5]. Recent studies have shown that progesterone also acts as a vasodilator [6], suggesting that the novel steroid receptor in the plasmalemma of cells is less selective than classical, nuclear ones. In the present study we have investigated the effects of 17poestradiol and progesterone on coronary vascular tone in isolated perfised Langendofl rat hearts [7] and on intracellular Ca2'-homeostasis in a rat fetal aortic smooth muscle cell line A7r5. Isolated perfused hearts: Male Sprague-Dawley rats (>350g) were killed by cervical dislocation, the heart excised and the aorta cannulated. The coronary vasculature was perfised retrogradely through the aorta at a constant flow (10 ml.min-') with a heated (37°C) Krebs-Henseleit bicarbonate buffer [7]. Coronary perfision pressure (CPP) was recorded as an index of vascular resistance. As CPP is generally low in isolated hearts perfised with a blood-free solution, CPP was raised with the thromboxane A2 mimetic U46619 (5 nM). Bolus injections (10 pl) of bradykinin and sodium nitroprusside (a donor of nitric oxide in aqueous solution) were then administered into the coronary perfusate. Close-arterial injections of bradykinin (BK, 0.1 nmol) and sodium nitroprusside (SNP, 10 nmol) caused a rapid and transient fall in CPP: 4 1 s mmHg (meankSE, n=15) for BK and 41k3 mmHg (n=15) for S N P . These vasodilator responses to BK and S N P confirmed the integrity of endothelial and smooth muscle cell layers [7]. 17p oestradiol and progesterone caused a rapid, dose-dependent (Fig. 1) and transient reduction in CPP. At the maximal dose used (1 00 nmol), 17p oestradiol and progesterone reduced CPP by 34+3 mmHg (n=15) and 19M mmHg (n=8), respectively. Disruption of the endothelium with 1.5 ml air reversed the normal vasodilator response to BK (0.1 nmol) to a vasoconstriction (754 mmHg, n=4), but did not alter the vasodilator responses to SNP (10 nmol, 50+7 mmHg), 17p oestradiol (100 nmol, 3 7 H mmHg, n=4) or progesterone (100 nmol, 20*6 mmHg, n=4). These findings suggest that 17p oestradiol and progesterone are acting directly on coronary vascular smooth muscle cells 40 1