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1 Department of Health, Nutrition, and Exercise Sciences, University of Delaware, Newark, Delaware, United States; Cardiovascular Research, Christiana Care Health Systems, Inc., Newark, Delaware, United States
2 Department of Health, Nutrition, and Exercise Sciences, University of Delaware, Newark, Delaware, United States
3 School of Nursing, University of Delaware, Newark, Delaware, United States
* To whom correspondence should be addressed. E-mail: wbf{at}udel.edu.
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-minute intravenous hypertonic saline infusion (HSI; 3% NaCl) on one day and during a 60-minute 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 per minute; during the ISO infusion burst frequency initially declined from 14.7 ± 2.5 to 12.0 ± 2.1 bursts per minute. Plasma norepinephrine concentration was greater at the end of the HSI compared to 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.
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