Gender Differences in Baroreflex Sensitivity after Bed Rest

Abstract

Two potential contributing factors to post-spaceflight orthostatic intolerance are decreases in baroreflex sensitivity (BRS) and sympathetic nervous system response. PURPOSE: The purpose of this study was to examine the shape of the BRS curve and sympathetic response to a wide range of blood pressures (BP) before and during 6° head-down bed rest (BR). METHODS: Normal volunteers were tested 1 day before BR (20M, 10W) and near BR days 30 (20M, 10W), 60 (16M, 8W), and 90 (10M, 5W). BP was pharmacologically manipulated by 10-min infusions of phenylephrine (PE) and sodium nitroprusside (SNP) at 3 increasing concentrations, with a 20-min rest between PE and SNP. Electrocardiogram and continuous finger blood pressure were recorded. A blood sample was drawn at the end of each infusion to measure plasma norepinephrine levels. The spontaneous baroreflex slope (SBS), a measure of BRS, was calculated as the slope of a sequence of 3 or more beats in which the systolic BP (SBP) and following R-R interval (RR) both increased or decreased. The data included saturated responses at the upper but not the lower end of the BP range. Mean response curves were constructed using second-order mixed model analysis. Results are based on term significance in the models. RESULTSRR: RR was lower during BR than before BR (p<0.001). Before BR, men were modeled by a linear RR response to SBP (p=0.000) whereas women had a quadratic response that saturated at high SBP (p=0.019). By day 30, both genders were modeled by a linear response; compared to men, women had an attenuated (lower slope) RR response to changes in SBP (p=0.031). SBS: SBS vs SBP analysis showed a lower SBS during BR (p<0.001) than before BR. Before BR, women had a higher SBS than men (p=0.006). Women exhibited saturating SBS at higher SBP (p=0.016) on day 30, whereas men were modeled by a linear SBS response to SBP (p=0.035). NE: The NE response of women to diastolic BP was different from that of men before BR (p=0.035) and on day 30 (p=0.005). CONCLUSION: NE, RR, and SBS responses to BP are affected by gender and BR. Not only are there main effects of gender and BR, but gender and BR also influence the slope and saturation of the BRS curves. Attenuated and saturating RR and SBS responses, as well as differences in baseline values, may contribute to the higher rates of orthostatic intolerance in women and after bed rest.