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1 Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, University of Texas Southwestern Medical Center at Dallas, Dallas, TX, USA
* To whom correspondence should be addressed. E-mail: BenjaminLevine{at}texashealth.org.
We tested the hypothesis that women have blunted sympathetic neural responses to orthostatic stress compared to men, which may be elicited under hypovolemic conditions. Muscle sympathetic nerve activity (MSNA) and hemodynamics were measured in 8 healthy young women and 7 men supine, and during 6-min of 60° head-up tilt (HUT) under normovolemic and hypovolemic conditions (randomly), with ~4 wk interval. Acute hypovolemia was produced by administration of a diuretic (furosemide) ~2 h before testing. Orthostatic tolerance was determined by progressive lower body negative pressure to presyncope. We found that furosemide produced an ~13% reduction in plasma volume, causing a similar increase in supine MSNA in men and women (
MSNA, mean ± SD: 5 ± 7 vs. 6 ± 5 bursts/min, P = 0.895). MSNA increased during HUT, and was greater in the hypovolemic than normovolemic condition (32 ± 6 in normovolemia vs. 44 ± 15 bursts/min in hypovolemia in males, P = 0.055; 35 ± 9 vs. 45 ± 8 bursts/min in females, P < 0.001); these responses were not different between the genders (gender effect, P = 0.832 and 0.814 in normovolemia and hypovolemia). Total peripheral resistance increased proportionately with the increases in MSNA during HUT; these responses were similar between the genders. However, systolic blood pressure (BP) was lower, while diastolic BP was similar in females compared to males during HUT, associated with a smaller stroke volume or stroke index. Orthostatic tolerance was lower in females, especially under hypovolemic conditions. These results indicate that men and women have comparable sympathetic neural responses during orthostatic stress under both normovolemic and hypovolemic conditions. The lower orthostatic tolerance in women is predominantly because of a smaller stroke volume presumably due to less cardiac filling during orthostasis, especially under hypovolemic conditions, which may overwhelm the vasomotor reserve available for vasoconstriction, or precipitate neurally mediated sympathetic withdrawal and syncope.
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