Posch AM, Luippold AJ, Mitchell KM, Bradbury KE, Kenefick RW, Cheuvront SN, Charkoudian N. Sympathetic neural and hemodynamic responses to head-up tilt during isoosmotic and hyperosmotic hypovolemia. We hypothesized that muscle sympathetic nerve activity (MSNA) during head-up tilt (HUT) would be augmented during exercise-induced (hyperosmotic) dehydration but not isoosmotic dehydration via an oral diuretic. We studied 26 young healthy subjects (7 female, 19 male) divided into three groups:

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... rated (EUH, n ϭ 7), previously exercised in 40°C while maintaining hydration; dehydrated (DEH, n ϭ 10), previously exercised in 40°C during which 3% of body weight was lost via sweat loss; and diuretic (DIUR, n ϭ 9), a group that did not exercise but lost~3% of body weight via diuresis (furosemide, 80 mg by mouth). We measured MSNA, heart rate (HR), and blood pressure (BP) during supine rest and 30°and 45°HUT. Plasma volume (PV) decreased similarly in DEH (Ϫ8.5 Ϯ 3.3%) and DIUR (Ϫ11.4 Ϯ 5.7%) (P Ͼ 0.05). Plasma osmolality was similar between DIUR and EUH (288 Ϯ 4 vs. 284 Ϯ 5 mmol/kg, respectively) but was significantly higher in DEH (299 Ϯ 5 mmol/kg) (P Ͻ 0.05). Mixedmodel ANOVA was used with repeated measures on position (HUT) and between-group analysis on condition. HR and MSNA increased in all subjects during HUT (main effect of position; P Ͻ 0.05). There was also a significant main effect of group, such that MSNA and HR were higher in DEH compared with DIUR (P Ͻ 0.05). Changes in HR with HUT were larger in both hypovolemic groups compared with EUH (P Ͻ 0.05). The differential HUT response "strategies" in each group suggest a greater role for hypovolemia per se in controlling HR responses during dehydration, and a stronger role for osmolality in control of SNA.