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1 Department of Biomedical Sciences, University of Missouri-Columbia, Columiba, MO, USA; Dalton Cardiovascular Research Center, University of Missouri-Columbia, Columiba, MO, USA
2 Department of Biomedical Sciences, University of Missouri-Columbia, Columiba, MO, USA; Department of Medical Pharmacology and Physiology, University of Missouri-Columbia, Columiba, MO, USA; Dalton Cardiovascular Research Center, University of Missouri-Columbia, Columiba, MO, USA
* To whom correspondence should be addressed. E-mail: foleycm{at}missouri.edu.
Exposure to a period of microgravity or bedrest produces several physiologic adaptations. These changes which include an increased incidence of orthostatic intolerance have an impact when people return to a 1 G environment or resume an upright posture. Compared to males, females appear more susceptible to orthostatic intolerance after exposure to real or simulated microgravity. Decreased arterial baroreflex compensation may contribute to orthostatic intolerance. We hypothesized that female rats would exhibit a greater reduction in arterial baroreflex function following hindlimb unloading (HU) compared to male rats. Mean arterial pressure (MAP), heart rate (HR) and renal sympathetic nerve activity (RSNA) were recorded in conscious animals after 13-15 days of HU. Baseline HR was elevated in female rats, and HU increased HR in both genders. Consistent with previous results in males, baroreflex mediated activation of RSNA was blunted by HU in both genders. The maximum RSNA in response to decreases in MAP was reduced by HU (male control 513±42% n=11; male HU 346±38% n=13; female control 359±44% n=10; female HU 260±43% n=10). The maximum baroreflex increase in RSNA was lower in females as compared to males in both control and HU rats. Both female gender and HU attenuated baroreflex mediated increases in sympathetic activity. The combined effects of HU and gender resulted in reduced baroreflex sympathetic reserve in females as compared to males and could contribute to the greater incidence of orthostatic intolerance in females following exposure to spaceflight or bedrest.
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