Background-Our goal was to test sympathetic and cardiovagal baroreflex sensitivity and the transduction of sympathetic traffic into vascular resistance during the early follicular (EF) and midluteal (ML) phases of the menstrual cycle. Methods and Results-Sympathetic baroreflex sensitivity was assessed by lowering and raising blood pressure with intravenous bolus doses of sodium nitroprusside and phenylephrine. It was defined as the slope relating muscle sympathetic nerve activity (MSNA; determined by microneurography) and diastolic blood pressure. Cardiovagal baroreflex sensitivity was defined as the slope relating R-R interval and systolic blood pressure. Vascular transduction was evaluated during ischemic handgrip exercise and postexercise ischemia, and it was defined as the slope relating MSNA and calf vascular resistance (determined by plethysmography). Resting MSNA (EF, 1170Ϯ151 U/min; ML, 2252Ϯ251 U/min; PϽ0.001) and plasma norepinephrine levels (EF, 240Ϯ21 pg/mL; ML, 294Ϯ25 pg/mL; Pϭ0.025) were significantly higher in the ML than in the EF phase. Furthermore, sympathetic baroreflex sensitivity was greater during the ML than the EF phase in every subject (MSNA/diastolic blood pressure slopes: EF, Ϫ4.15; FL, Ϫ5.42; Pϭ0.005). No significant differences in cardiovagal baroreflex sensitivity or vascular transduction were observed. Conclusions-The present study suggests that the hormonal fluctuations that occur during the normal menstrual cycle may alter sympathetic outflow but not the transduction of sympathetic activity into vascular resistance. (Circulation. 2000;101:862-868.)

Background-We tested sympathetic and cardiovagal baroreflex sensitivity during the placebo or "low-hormone" phase (LH) and 2 to 3 weeks later during the "high-hormone" phase (HH) of oral contraceptive (OC) use in 9 women. Methods and Results-Sympathetic baroreflex sensitivity was assessed by intravenous doses of sodium nitroprusside and phenylephrine and defined as the slope relating muscle sympathetic nerve activity (by microneurography) and diastolic blood pressure. Cardiovagal baroreflex sensitivity was defined as the slope relating R-R interval and systolic blood pressure. No difference was observed for resting muscle sympathetic nerve activity or plasma norepinephrine levels. However, sympathetic baroreflex sensitivity was greater and mean arterial pressure was higher during the LH than in the HH phase. Similarly, cardiovagal baroreflex sensitivity was greater in the LH than in the HH phase. Conclusions-Sympathetic and cardiovagal baroreflex sensitivities change during the 28-day course of OC use.Furthermore, changes in baroreflex sensitivity with OC differ from changes in baroreflex sensitivity during the normal menstrual cycle. (Circulation. 2000;102:1473-1476.)Key Words: estrogens Ⅲ progesterone Ⅲ blood pressure Ⅲ nervous system, autonomic W e recently reported that blood pressure regulation by the sympathetic nervous system changes during the menstrual cycle. 1 Resting muscle sympathetic nerve activity (MSNA) and sympathetic baroreflex sensitivity, but not cardiovagal baroreflex sensitivity, were greater during the midluteal (high hormone) phase compared with the early follicular (low hormone) phase of the menstrual cycle.Our goal in the present study was to extend these observations to women taking oral contraceptives (OC). We compared resting MSNA, sympathetic baroreflex sensitivity, and cardiovagal baroreflex sensitivity in young women during the placebo or "low hormone" (LH) phase of OC use and during the active OC phase ("high hormone"; HH). We hypothesized that our results would parallel our previous findings, ie, resting MSNA and sympathetic baroreflex sensitivity would be greater when women were taking exogenous estrogen and progestin than when taking the placebo. Methods SubjectsThe Institutional Review Board approved all procedures. Nine women were recruited and gave written informed consent. Subjects were young (30Ϯ2 years), healthy (body mass index, 24.2Ϯ1.0 kg/m 2 ) nonsmokers who had not taken medications except monophasic OC, which provided 30 to 35 g of ethinyl estrogen and low-dose progestin for 21 days and placebo for 7 days. Experimental ProtocolSubjects were studied twice, once during the LH phase (4 to 5 days after starting placebo pill, 3 to 6 days after onset of menstruation) and once during the HH phase of OC use (17 to 20 days after starting the ethinyl estrogen-progestin pills). The order of testing was counterbalanced.Subjects were placed in the supine position, and a catheter was placed in an antecubital vein for drug injections. Heart rate was determined from an ECG, and bea...