Running title: DEHYDRATION AND MSNA RESPONSES TO TILT IN WOMEN

Fu et al. MS#: R-00013-2005.R1 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 60º 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