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Sympathetic neural responses to increased osmolality in humans

¹Department of Health, Nutrition, and Exercise Sciences, and ²School of Nursing, University of Delaware; and ³Christiana Care Health System, Inc., Cardiovascular Research, Newark, Delaware

Submitted 22 February 2006 ; accepted in final form 7 June 2006

The purpose of this study was to examine the relationship between osmolality and efferent sympathetic outflow in humans. We hypothesized that increased plasma osmolality would be associated with increases in directly measured sympathetic outflow. Muscle sympathetic outflow was successfully recorded in eight healthy subjects during a 60-min intravenous hypertonic saline infusion (HSI; 3% NaCl) on one day and during a 60-min intravenous isotonic saline (ISO) infusion (0.9% NaCl) on a different day. The HSI provides an osmotic and volume stimulus, whereas the ISO infusion provides a volume-only stimulus. Muscle sympathetic nerve activity was quantified using the technique of peroneal microneurography. Plasma osmolality increased during the HSI but not during the ISO infusion (ANOVA, P < 0.05). Sympathetic outflow differed between the trials (ANOVA, P < 0.05); during the HSI burst, frequency initially increased from 14.6 ± 2.5 to 18.1 ± 1.9 bursts/min; during the ISO infusion, burst frequency initially declined from 14.7 ± 2.5 to 12.0 ± 2.1 bursts/min. Plasma norepinephrine concentration was greater at the end of the HSI compared with the end of the ISO infusion (HSI: 297 ± 64 vs. ISO: 202 ± 49 pg/ml; ANOVA, P < 0.05). We conclude that HSI-induced increases in plasma osmolality are associated with increases in sympathetic activity in humans.

blood pressure; salt sensitivity; hypertension

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