Water deprivation does not augment sympathetic or pressor responses to sciatic afferent nerve stimulation in rats or to static exercise in humans

Excess dietary salt intake excites central sympathetic networks, which may be related to plasma hypernatremia. Plasma hypernatremia also occurs following water deprivation (WD). The purpose of this study was to test the hypothesis that WD induces hypernatremia and consequently augments sympathetic and pressor responses to sympathoexcitatory stimuli in rats and humans. Sympathetic nerve activity (SNA) and arterial blood pressure (ABP) responses to sciatic afferent nerve stimulation (2–20 Hz) and chemical stimulation of the rostral ventrolateral medulla (RVLM) were assessed in rats after 48 h of WD and compared with normally hydrated control rats (CON). In a parallel randomized-crossover human experiment (n 13 healthy young adults), sympathetic (microneurography) and pressor (photoplethysmography) responses to static exercise were compared between 16-h WD and CON conditions. In rats, plasma [Na] was significantly higher in WD versus CON [136 2 vs. 144 2 (SD) mM, P 0.01], but sciatic afferent nerve stimulation produced similar increases in renal SNA [5 Hz, 174 34 vs. 169 49% (SD), n 6 – 8] and mean ABP [5 Hz, 21 6 vs. 18 7 (SD mmHg, n 6 – 8]. RVLM injection of L-glutamate also produced similar increases in SNA and ABP in WD versus CON rats. In humans, WD increased serum [Na] [140.6 2.1 vs. 142.1 1.9 mM (SD), P 0.02] but did not augment sympathetic [muscle SNA: change from baseline () 6 7 vs. 5 7 (SD) bursts/min, P 0.83] or mean ABP [ 12 5 vs. 11 8 (SD) mmHg, P 0.73; WD vs. CON for all results] responses during the final minute of exercise. These findings suggest that despite eliciting relative hypernatremia, WD does not augment sympathetic or pressor responses to sciatic afferent stimulation in rats or to static exercise in humans.