Prolonged ovarian hormone deprivation impairs the protective vascular actions of estrogen receptor alpha agonists.

The vascular consequences of estrogen treatment may be driven by its initiation timing. We tested the hypothesis that the duration of ovarian hormone deprivation before estrogen reintroduction affects the role of estrogen as mediator of endothelial function and vascular relaxation in nondiseased vessels. Rats were ovariectomized and implanted with 17beta-estradiol (E(2)) or oil capsules 1, 4, and 8 months after surgery. After the longest hypoestrogenicity period, acetylcholine-mediated aortic relaxation was attenuated and insensitive to E(2) administration despite endothelial integrity. Whereas no rapid vasorelaxant responses were elicited by an estrogen receptor (ER) beta-selective agonist, responses to E(2) and an ERalpha selective agonist waned postovariectomy at any given time and were restored by E(2) treatment after 1 and 4 months but not 8 months postovariectomy. Accordingly, endothelial ERalpha mRNA and protein expression declined approximately 6-fold after prolonged hypoestrogenicity and was restored by estrogen replacement starting 1 month but not 8 months postovariectomy. Furthermore, the amount of active phosphorylated endothelial NO synthase rose significantly after E(2) replacement after 1 and 4 months but not 8 months postovariectomy. The present findings document that the functional impairment of the ERalpha/endothelial NO synthase signaling network after an extended period of hypoestrogenicity was not restored by E(2) administration, providing experimental support to early initiation of estrogen replacement with preferential ERalpha targeting to improve cardiovascular outcomes.