Effects of 17β-estradiol on sympathetic activity and pressor response to phenylephrine in ovariectomized rats

The effects of 17 beta-estradiol (E-2) on sympathetic activity were examined in conscious unrestrained ovariectomized rats, instrumented under methohexital anesthesia to record mean arterial pressure (MABP), heart rate (HR), renal nerve activity (RNA), and splanchnic nerve activity (SNA) 1 day before the experiment. Injection of E-2 (150 mu g/kg iv) caused reductions (P < 0.01) in RNA(29 +/- 6%), SNA (25 +/- 2%), and HR (26 +/- 5 beats/min) within 20 min, but MABP remained unchanged. Ninety minutes after intravenous injection of E-2 or vehicle, intravenous infusion of phenylephrine (PE; 6.2 mu g.min(-1).kg(-1)) induced similar increases in MABP and decreases in HR, RNA, and SNA in both groups. By contrast, in rats chronically treated with E-2, the presser response to PE was smaller (P < 0.01; 22 +/- 5 mmHg) than in vehicle-treated rats (40 +/- 4 mmHg). The changes in HR, RNA, and SNA were similar in both groups, but the ratios of changes in HR and SNA to MABP, an index of baroreflex sensitivity, were greater in the E-2-treated rats. These findings suggest that E-2 can act centrally to modulate sympathetic function and thereby participate in cardiovascular regulation.