title: Influence of plasma osmolality on baroreflex control of sympathetic activity.
abstract: 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 with an isoosmotic 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 x kg(-1) x min(-1). To isolate the effect of osmolality, comparisons between HSI and ISO conditions were retrospectively matched based on hematocrit; therefore, baroreflex control of sympathetic 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: 39.1 +/- 1% and 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 with the ISO (HSI: -8.3 +/- 1.2 arbitrary units x beat(-1) x mmHg(-1) vs. ISO: -4.0 +/- 0.8 arbitrary units x beat(-1) x mmHg(-1), P = 0.01). In conclusion, when controlling for intravascular volume, increased plasma osmolality enhances baroreflex control of sympathetic activity in humans.
authors.affiliation: Department of Health, Nutrition, College of Health Sciences, University of Delaware, Delaware 19716, USA.
authors.forename: Megan M
authors.initials: MM
authors.lastname: Wenner
authors.forename: William C
authors.initials: WC
authors.lastname: Rose
authors.forename: Erin P
authors.initials: EP
authors.lastname: Delaney
authors.forename: Michael E
authors.initials: ME
authors.lastname: Stillabower
authors.forename: William B
authors.initials: WB
authors.lastname: Farquhar
mesh_terms: D000328:Adult; D017704:Baroreflex; D001794:Blood Pressure; D001810:Blood Volume; D002712:Chlorides / Q000097:blood; D005260:Female; D006339:Heart Rate; D006400:Hematocrit; D006801:Humans; D007262:Infusions, Intravenous; D007552:Isotonic Solutions; D008297:Male; D018482:Muscle, Skeletal / Q000294:innervation*; D009994:Osmolar Concentration; D011188:Potassium / Q000097:blood; D012189:Retrospective Studies; D012462:Saline Solution, Hypertonic / Q000008:administration & dosage* / Q000737:chemistry; D012964:Sodium / Q000097:blood; D012965:Sodium Chloride / Q000008:administration & dosage* / Q000737:chemistry; D013564:Sympathetic Nervous System / Q000502:physiology*; D013997:Time Factors
chemical_list: D002712:Chlorides; D007552:Isotonic Solutions; D012462:Saline Solution, Hypertonic; D012965:Sodium Chloride; D012964:Sodium; D011188:Potassium