Sympathetic neural and hemodynamic responses to head-up tilt during iso-osmotic and hyper-osmotic hypovolemia

We hypothesized that muscle sympathetic nerve activity (MSNA) during head-up tilt (HUT) would be augmented during exercise-induced (hyperosmotic) dehydration but not iso-osmotic dehydration via oral diuretic. We studied 26 young healthy subjects (7 F, 19 M) divided into 3 groups: euhydrated (EUH, n = 7), previously exercised in 40°C while maintaining hydration; dehydrated (DEH, n = 10), previously exercised in 40 °C who lost ~3% of body weight via sweat loss; and diuretic (DIUR, n = 9), who did not exercise and lost ~3% of bodyweight via diuresis (furosemide, 80 mg po). We measured MSNA, heart rate (HR), blood pressure (BP) during supine rest and 30° and 45° HUT. Plasma volume (PV) decreased similarly in DEH (-8.5 ± 3.3%) and DIUR (-11.4 ± 5.7%) (p> 0.05). Plasma osmolality was similar between DIUR and EUH (288 ± 4 vs 284 ± 5 mmol/kg, respectively), but was significantly higher in DEH (299 ± 5 mmol/kg) (p<0.05). Mixed-model ANOVA was used with repeated measures on position (HUT) and between-group analysis on condition. HR and MSNA increased in all subjects during HUT. There was a significant main effect of group, such that MSNA and HR were higher in DEH compared to DIUR (P < 0.05). Changes in HR with HUT were larger in both hypovolemic groups compared to EUH (P < 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.

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