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(Stroke. 2002;33:1685.)
© 2002 American Heart Association, Inc.

Original Contributions

Estrogen Increases Endothelial Nitric Oxide Synthase via Estrogen Receptors in Rat Cerebral Blood Vessels

Effect Preserved After Concurrent Treatment With Medroxyprogesterone Acetate or Progesterone

Anne Marie McNeill, PhD; Chunying Zhang, MD; Frank Z. Stanczyk, PhD; Sue P. Duckles, PhD Diana N. Krause, PhD

From the Department of Pharmacology, College of Medicine, University of California at Irvine (A.M.M., S.P.D., D.N.K.), and Department of Obstetrics and Gynecology, Women’s and Children’s Hospital, University of Southern California, Los Angeles (C.Z., F.Z.S.).

Reprint requests to Sue P. Duckles, PhD, Department of Pharmacology, College of Medicine, University of California at Irvine, Irvine, CA 92697-4625. E-mail spduckle{at}uci.edu

Background and Purpose— In vivo and in vitro rat models of hormone therapy were used to test the following hypotheses: (1) estrogen acts directly on cerebrovascular estrogen receptors to increase endothelial nitric oxide synthase (eNOS); (2) increased protein correlates with higher NOS activity; and (3) effects of estrogen on eNOS are altered by concurrent treatment with either medroxyprogesterone acetate (MPA) or progesterone.

Methods— Blood vessels were isolated from brains of ovariectomized female rats; some were treated for 1 month with estrogen, estrogen and progesterone, or estrogen and MPA. Isolated cerebral vessels were also treated in vitro with estrogen in the absence and presence of progesterone, MPA, tamoxifen, and the estrogen receptor antagonist ICI 182 780. Levels of eNOS were measured by Western blot, and NOS activity was measured by [¹⁴C]arginine-[¹⁴C]citrulline conversion.

Results— Chronic hormone treatment in vivo resulted in plasma levels of 17ß-estradiol, progesterone, and MPA in the range of values found in humans. Estrogen treatment resulted in higher levels of cerebrovascular NOS activity that paralleled increases in eNOS protein. In vitro estrogen treatment for 18 hours also resulted in a concentration-dependent increase in eNOS protein (EC₅₀ 300 pmol/L) that was completely prevented by estrogen receptor antagonists tamoxifen or ICI 182 780. However, cotreatment with progesterone or MPA, either in vivo or in vitro, did not alter the effect of estrogen on eNOS protein.

Conclusions— Estrogen receptor activation in cerebrovascular tissue results in increased eNOS activity and protein levels. The latter effect persists in the presence of either progesterone or MPA. Thus, increased NO production by eNOS may contribute to the neuroprotective effects of estrogen.

Key Words: cerebral vessels • estrogens • nitric oxide synthase • rats

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