EFFECTS OF 17-BETA-ESTRADIOL ON SYMPATHETIC ACTIVITY AND PRESSOR-RESPONSE TO PHENYLEPHRINE IN OVARIECTOMIZED RATS

The effects of 17 beta-estradiol (E-2) on sympathetic activity were ex amined in conscious unrestrained ovariectomized rats, instrumented und er methohexital anesthesia to record mean arterial pressure (MABP), he art rate (HR), renal nerve activity (RNA), and splanchnic nerve activi ty (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. N inety minutes after intravenous injection of E-2 or vehicle, intraveno us infusion of phenylephrine (PE; 6.2 mu g.min(-1).kg(-1)) induced sim ilar increases in MABP and decreases in HR, RNA, and SNA in both group s. By contrast, in rats chronically treated with E-2, the presser resp onse to PE was smaller (P < 0.01; 22 +/- 5 mmHg) than in vehicle-treat ed 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 in dex of baroreflex sensitivity, were greater in the E-2-treated rats. T hese findings suggest that E-2 can act centrally to modulate sympathet ic function and thereby participate in cardiovascular regulation.