Role of 17beta-estradiol in the modulation of baroreflex sensitivity in male rats.

Female mammals have an enhanced baroreflex sensitivity compared with their male counterparts, leading researchers to speculate that estrogen modulates autonomic tone. Therefore, this study tests the hypothesis that exogenous estrogen can enhance the baroreflex sensitivity of male rats. Male Sprague-Dawley rats anesthetized with thiobutabarbitol sodium (50 mg/kg) were instrumented to measure blood pressure and heart rate and for the intravenous injection of drugs. The baroreflex was tested using intravenous injections of phenylephrine (0.025, 0.05, and 0.1 mg/kg), and the cardiovascular responses were plotted to obtain a measure of the sensitivity of the cardiac baroreflex. Intravenous injection of estrogen produced dose-related increases in the baroreflex sensitivity due to an increase in the magnitude of the reflex bradycardia. In a separate group of animals, stimulation of the vagus nerve for 2 h resulted in a decrease in baroreflex sensitivity. This effect was blocked when estrogen (1 x 10(-2) mg/kg) was administered immediately before the end of stimulation. In conclusion, intravenous injection of estrogen in male rats significantly enhanced baroreflex sensitivity and blocked the attenuation in the baroreflex sensitivity observed after vagal stimulation.