The ability to autonomically regulate changes in BP is an important indicator of cardiovascular health. BP is autonomically regulated by baroreflex mediated HR responses. Previous studies have shown that baroreflex control of HR is significantly lower in females than in males. This study explores whether estrogen has any effect on baroreflex sensitivity (BRS).
In this study, BRS was measured by administrating phenylephrine, a vasoactive drug that increases BP. MAP was plotted against HR – the slope of the line represents the BRS. There was a significant difference in BRS of almost 30% between males and females. This difference was even greater in ovariectomized rats. However, ovariectomized rats that were treated with estradiol expressed no significant difference in BRS as compared to the males.
Age-matched (11-12 week old) male and female Sprague-Dawley rats were used in this study. At 9-10 weeks, in one group of females, the ovaries were tied off and isolated. In the other female group, a sham operation was performed which exposed the ovaries but did not isolate them. Both groups were given two weeks to recover. The ovariectomized rats were further divided into two groups: those injected with estradiol and those injected with a "vehicle" drug (that could be used as a control).
According to the authors, one important point to consider is age-related differences in BRS. Studies have shown that BRS decreases as age increases. Thus, the results of this study may not be replicable in rats of different ages.
The following excerpt from the study provides some indication of how these finding relate to neurons found in regions of the brainstem:
"First, estrogen exerts facilitatory and inhibitory roles on central glutamatergic (Wong and Moss 1992) and GABAergic (Kelly et al. 1992) neurotransmission, respectively. Both types of neurotransmissions are essential for central baroreflex function (Lawrence and Jarrott 1996). Furthermore, estrogen receptor mRNA containing neurons have been identified in nucleus tractus solitarius and caudal ventrolateral medulla (Simerly et al. 1990), brainstem areas that are involved in the central processing of baroreceptor information (Chalmers and Pilowsky 1991; Lawrence and Jarrott 1996). Further studies are needed, however, to determine the exact mechanism of baroreflex enhancement by estrogen."
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