Influence of plasma osmolality on baroreflex control of sympathetic activity

Abstract

Physiol 293: H2313–H2319, 2007. First published July 20, 2007; doi:10.1152/ajpheart.01383.2006.—The purpose of this study was to determine if plasma osmolality alters baroreflex control of sympa-thetic 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 with an isoos-motic stimulus when intravascular volume expansion was matched. Seven healthy subjects (25 2 yr) completed two 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 kg1 min1. To isolate the effect of osmolality, comparisons between HSI and ISO conditions were retrospectively matched based on hematocrit; therefore, baroreflex control of sympa-thetic outflow was determined at 20 min of a HSI and 40 min of an ISO. Muscle sympathetic outflow (MSNA) was directly measured using the technique of peroneal microneurography; osmolality and blood pressure (Finometer) were assessed. The baroreflex control of sympathetic outflow was estimated by calculating the slope of the relationship between MSNA and diastolic blood pressure during controlled breathing. Plasma osmolality was greater during the HSI compared with the ISO (HSI: 292 0.9 mosmol/kg and ISO: 289 0.8 mosmol/kg, P 0.05). Hematocrits were matched (HSI: 39.