Sex Difference In Muscle Sympathetic Nerve Activity During Passive Leg Raising

The cardiopulmonary baroreflex regulates sympathetic vasomotor outflow. Reduced central blood volume (CBV) due to mild head-up tilt or lower body negative pressure heightens sympathetic activity to skeletal muscles (muscle sympathetic nerve activity: MSNA) by unloading cardiopulmonary baroreceptors. Conversely, increasing CBV during mild head-down tilt, lower body positive pressure, or passive leg raising (PLR) inhibits sympathetic activity by loading cardiopulmonary baroreceptors. Any sex-based differences in MSNA responses to CBV change remain unexplored. PURPOSE: To elucidate sex-based distinction in the degree of sympathoinhibiton during the loading of cardiopulmonary baroreceptors in young individuals. METHODS: Healthy young females (YF, n = 9, 20 ± 1 yrs, means±SE) and age-matched males (YM, n = 9, 22 ± 1 yrs) participated in two experimental visits on separate days. On the first day, the participants performed a passive leg raising (PLR) test with MSNA measurement. MSNA was recorded via microneurography of the left radial nerve. On the second day, they repeated the PLR test with estimated central venous pressure (eCVP) measurement as an index of CBV. eCVP was measured based on peripheral venous pressure and monitored using a cannula in the right large antecubital vein. For the PLR test, the participants rested in a supine position (0°), and then the lower limbs were lifted passively and gradually at 10°, 20°, 30°, and 40°. The participant’s legs were supported at each angle for 3 minutes. RESULTS: Baseline mean arterial blood pressure (MAP) and MSNA burst frequency (BF) and burst incidence (BI) were lower in YF than YM. MSNA BF and BI decreased during the PLR test in both YF (ΔMSNA BF, -6.3 ± 0.5 bursts/min at 40°) and YM (ΔMSNA BF, -6.0 ± 0.4 bursts/min at 40°), but did not differ between YM and YF (BF, P = 0.64). The eCVP increased during the PLR test in both YF (ΔeCVP, +2.8 ± 0.9 mmHg at 40°) and YM (ΔeCVP, +3.0 ± 0.5 mmHg at 40°), but did not differ by sex (P = 0.92). Heart rate and MAP remained unchanged throughout the PLR test in each group. CONCLUSIONS: These results suggest that sex has minimal influence on the sympathoinhibitory effects caused by loading of the cardiopulmonary baroreceptors in young individuals. Supported by JSPS KAKENHI #22H03479. JSPS KAKENHI #22H03479