Baroreflex sensitivity varies during the estrus cycle of the rat

Abstract

Baroreflex sensitivity (BRS) fluctuates during the menstrual cycle; it reaches its peak during the luteal phase, when gonadal steroids are elevated. However, whether BRS varies during the estrus cycle of the rat is unclear. To test this hypothesis, female rats (n=5) were instrumented for continuous telemetric (24 hr/day; 1000 Hz) measurement of blood pressure (BP) and were housed with a daily light:dark cycle of 12 hr:12 hr. After recovery (10 days), the estrus cycle was established by daily cytologic evaluation of vaginal smears for at least 2 cycles (~10 days). During the following 5–10 days, cardiovagal BRS (in ms/mmHg) was estimated using the sequence method, which selects spontaneous relationships between ramps in systolic BP and associated changes in pulse interval. On each day of the estrus cycle, BRS was increased during lights on (L‐on), compared to lights off (L‐off) (P<0.05). In addition, during L‐on, BRS was elevated on proestrus compared to estrus (1.77±0.11 vs. 1.59±0.08, P<0.05). During L‐off, BRS was increased during protestrus compared to both estrus and metestrus (1.54±0.06 vs. 1.39±0.15 and 1.29±0.04; P<0.05). To test if increases in gonadal hormones underlie the elevated gain on proestrus, rats (n=3) were studied for 4 consecutive days 1–2 weeks following ovariectomy (OVX). In OVX rats, no daily variations in BRS were observed (P>0.50). BRS on each of the 4 days during L‐on was: 1.37±0.20, 1.31±0.18, 1.37±0.20, 1.39±0.07; during L‐off: 1.23±0.06, 1.24±0.05, 1.24±0.01, 1.33±0.06. In summary, cardiovagal BRS fluctuates during the estrus cycle in rats and reaches its peak during proestrus. Because OVX eliminates this daily BRS variation, we conclude that changes in ovarian gonadal steroids mediate this BRS fluctuation. Supported by HL70962, AHA and Collins Fdn.