Submitted on December 20, 2006
Accepted on July 11, 2007
The influence of plasma osmolality on baroreflex control of sympathetic activity
Megan Michelle Wenner1, William C Rose2, Erin Paul Delaney1, Michael E Stillabower3, and William B. Farquhar4*
1 Health, Nutrition, and Exercise Sciences, University of Delaware, Newark, Delaware, United States
2 Nursing, University of Delaware, Newark, Delaware, United States
3 Cardiovascular Research, Christiana Care Health System, Inc., Newark, Delaware, United States
4 Department of Health, Nutrition, and Exercise Sciences, University of Delaware, Newark, Delaware, United States; Cardiovascular Research, Christiana Care Health System, Inc., Newark, Delaware, United States
* To whom correspondence should be addressed. E-mail: wbf{at}udel.edu.
The purpose of this study was to determine if plasma osmolality alters baroreflex control of sympathetic activity when controlling for a change in intravascular volume; we hypothesized that baroreflex control of sympathetic activity would be greater during a hyperosmotic stimulus compared to an isoosmotic stimulus when intravascular volume expansion was matched. Seven healthy subjects (25±2yrs) completed 2 intravenous infusions: a hypertonic saline infusion (HSI: 3% NaCl) and on a separate occasion, an isotonic saline infusion (ISO: 0.9% NaCl), both at a rate of 0.15 ml/kg/min. In order to isolate the effect of osmolality, comparisons between the HSI and ISO conditions were retrospectively matched based on hematocrit (Hct); therefore, baroreflex control of sympathetic outflow was determined at 20 minutes of a HSI and 40 minutes of an ISO. Muscle sympathetic outflow (MSNA) was directly measured using the technique of peroneal microneurography; osmolality and blood pressure (Finometer; BP) were assessed. The baroreflex control of sympathetic outflow was estimated by calculating the slope of the relationship between MSNA and diastolic BP during controlled breathing. Plasma osmolality was greater during the HSI compared to ISO (HSI: 292±0.9, ISO: 289±0.8 mOsmols/kg; p
0.05). Hcts were matched (HSI: 39.1±1, ISO: 39.1±1%; p
0.40); thus we were successful in isolating osmolality. The baroreflex control of sympathetic outflow was greater during the HSI compared to the ISO (HSI: -8.3±1.2 vs. ISO: -4.0±0.8 AU/Beat/mmHg; p=0.01). In conclusion, when controlling for intravascular volume, increased plasma osmolality enhances baroreflex control of sympathetic activity in humans.
