Effect of estrogen on pressor responses to alpha1-adrenoreceptor agonist in conscious female rats.

We examined the effect of estrogen on pressor responses to an alpha1-adrenoreceptor agonist (phenylephrine) in conscious female Wistar-Kyoto rats. At the age of 11 weeks, rats underwent ovariectomy or a sham procedure. At the age of 15 weeks, ovariectomized (OVX) rats received intramuscular injection of estradiol valerate (EV) 5 microg (OVX+EV 5 microg group; n = 6), EV 25 microg (OVX+EV 25 microg group; n=7), or placebo (OVX group; n = 8), and sham-operated rats received placebo (sham group; n = 8). After 4 days, dose-pressor response curves to phenylephrine were examined under the condition where the renin-angiotensin, vasopressin and autonomic nervous systems were pharmacologically blocked. Ovariectomy shifted the dose-pressor response curve to phenylephrine leftward with a significantly decreased log ED50 (microg/kg) (the dose needed to reach 50% of the maximal response) (sham: 0.81 +/- 0.04; OVX: 0.57 +/- 0.05; p < 0.05). Supplementation with EV 25 mircog, but not EV 5 microg, reversed the dose-pressor response curve to phenylephrine in OVX rats to the level of the curve in sham-operated rats with a significantly increased log ED50 (microg/kg) (OVX+xEV 5 microg: 0.47 +/- 0.05; OVX+EV 25 microg: 0.75 +/- 0.08). These results suggest that the physiological level of estrogen seen in intact female rats attenuates pressor responses to alpha1-adrenoreceptor agonist, while supplementation with a moderate dose of estrogen is needed to restore such effects of physiological-level estrogen within a short-term period after chronic estrogen withdrawal.