Submitted on December 19, 2008
Revised on February 19, 2009
Accepted on February 25, 2009
Baroreflex sensitivity varies during the rat estrus cycle: role of gonadal steroids
Robert K. Goldman1, Afaf S. Azar, Julia M. Mulvaney, Carmen Hinojosa-Laborde2, Joseph R. Haywood3, and Virginia L. Brooks4*
1 Portland Veteran Affairs Medical Center
2 University of Texas Health Science Center
3 Michigan State University
4 Oregon Health & Science University
* To whom correspondence should be addressed. E-mail: brooksv{at}ohsu.edu.
Baroreflex sensitivity (BRS) increases in women during the luteal phase of the menstrual cycle, when gonadal hormones are elevated, but whether a similar cycle-dependent variation in BRS occurs in rats is unknown. In addition, whether cyclic BRS changes depend on gonadal steroids has not been previously investigated. To test these hypotheses, BRS was determined in cycling female rats using two approaches: (1) baroreflex control of renal sympathetic nerve activity (RSNA) in anesthetized rats; (2) cardiovagal spontaneous BRS (sBRS) in conscious rats instrumented for continuous telemetric measurements of mean arterial pressure (MAP) and heart rate (HR). MAP, HR, and sBRS were also measured in rats 2-3 and 5-6 weeks following ovariectomy (OVX), to eliminate gonadal steroids. In anesthetized rats, RSNA BRS gain was increased (P<0.01) during proestrus (-4.8±0.5 % control/mmHg) compared to diestrus/estrus (-2.8±0.3 %control/mmHg). Similarly, a proestrus peak in sBRS was observed in conscious rats (1.66±0.07 msec/mmHg, proestrus; 1.48±0.06 msec/mmHg, diestrus/estrus; P<0.001). OVX eliminated estrus cycle induced variation in sBRS. In addition, OVX reduced (P<0.05) diurnal variations in MAP (5.9±0.3 vs. 3.9±0.5 mmHg) and HR (54±4 vs. 39±3 bpm), and abolished diurnal variations in sBRS. Finally, while MAP, HR and sBRS were decreased 2-3 weeks following OVX, ~3 weeks later, MAP and sBRS increased, and HR decreased further. No changes in MAP, HR or sBRS were seen with time in sham OVX controls. In summary, RSNA and cardiovagal sBRS vary during the rat estrus cycle, and this variation is abolished by OVX. We conclude that sex steroid hormones are required for both cyclic and diurnal changes in BRS in rats.
