Baroreflex Control of Sympathetic Activity

Abstract

Various experimental models have been used to establish a link between plasma osmolality and sympathetic activity. However, in humans, it is difficult to experimentally isolate the contribution of plasma osmolality to the baroreflex control of sympathetic activity due to the varying changes in plasma volume (P V): with a dehydration model, there is a decrease in PV and an increase in osmolality; with a salt loading model, there is an increase in PV and an increase in osmolality. PURPOSE: To determine if plasma osmolality (for a given change in PV) alters baroreflex control of sympathetic activity; we hypothesized that baroreflex control of sympathetic activity would be greater during an osmotic stimulus compared to a volume-matched stimulus. METHODS: 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 (SBRS) was determined at 20 minutes of the HSI and 40 minutes of the ISO. Muscle sympathetic outflow (MSNA) was directly measured using the technique of peroneal microneurography. MSNA, osmolality, and blood pressure (Finometer; BP) were assessed. SBRS was determined by calculating the slope of the relationship between MSNA and diastolic BP during controlled breathing. A one-way repeated measures ANOVA and post-hoc tests (when appropriate) were used for statistical analysis (the alpha level was set at p<0.05). RESULTS: (mean ± SE): Plasma osmolality was greater during the HSI compared to ISO (HSI: 292±1, ISO: 289±1 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. SBRS was greater during the HSI compared to the ISO (HSI: −8.3±1.2vs. ISO: −4.0±0.8 au/beat/mmHg; p=0.018). CONCLUSIONS: Increased plasma osmolality enhances baroreflex control of sympathetic activity in humans. The interaction between plasma osmolality and sympathetic outflow may be an important neural mechanism contributing to cardiovascular control. Supported by NIH grant R15 HL074851–01