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: euhydrated (EUH, n = 7), previously exercised in 40°C while maintaining hydration; dehydrated (DEH, n = 10), previously exercised in 40°C during which ~3% of body weight was lost via sweat loss; and diuretic (DIUR, n = 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 (–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 (main effect of position; P < 0.05). There was also a significant main effect of group, such that MSNA and HR were higher in DEH compared with DIUR (P < 0.05). Changes in HR with HUT were larger in both hypovolemic groups compared with EUH (P < 0.05). The differential HUT response "strate...
Source: Journal of Neurophysiology - Category: Neurology Authors: Tags: Research Articles Source Type: research