Estrogen, nitric oxide, and hypertension differentially modulate agonist-induced contractile responses in female transgenic (mRen2)27 hypertensive rats

Abstract

Clinical trials revealed that estrogen may result in cardiovascular risk in patients with coronary heart disease, despite earlier studies demonstrating that estrogen provided cardiovascular protection. It is possible that the preexisting condition of hypertension and the ability of estrogen to activate the renin-angiotensin system could confound its beneficial effects. Our hypothesis is that the attenuation of estrogen to agonist-induced vasoconstrictor responses through the activation of nitric oxide (NO) synthase (NOS) is impaired by hypertension. We investigated the effects of 17β-estradiol (E2) replacement in normotensive Sprague-Dawley (SD) and (mRen2)27 hypertensive transgenic (TG) rats on contractile responses to three vasoconstrictors, angiotensin II (ANG II), serotonin (5-HT), and phenylephrine (PE), and on the modulatory role of vascular NO to these responses. The aorta was isolated from ovariectomized SD and TG rats treated chronically with 5 mg E2 or placebo (P). The isometric tension of the aortic rings was measured in organ chambers, and endothelial NOS (eNOS) in the rat aorta was detected using Western blot analysis. E2 treatment increased eNOS expression in the SD and TG aorta and reduced ANG II- and 5-HT- but not PE-induced contractions in SD and TG rats. The inhibition of NOS with Nω-nitro-l-arginine methyl ester enhanced ANG II-, 5-HT-, and PE-induced contractions in P-treated and ANG II and PE responses in E2-treated SD and TG rats. Only the responses to 5-HT were augmented in hypertensive rats. In conclusion, this study shows that the preexisting condition of hypertension augmented the vascular responsiveness of 5-HT, whereas the attenuation of estrogen by ANG II and 5-HT vascular responses was not impaired by hypertension. The adrenergic agonist was unresponsive to estrogen treatment. The contribution of NO as a factor contributing to the relative refractoriness of the vascular responses is dependent on the nature of the vasoconstrictor and/or the presence of estrogen.