Interactions of plasma osmolality with arterial and central venous pressures in control of sympathetic activity and heart rate in humans

Abstract

. Interactions of plasma osmolality with arterial and central venous pressures in control of sympathetic activity and heart rate in humans. Am J Physiol Heart Circ Physiol 289: H2456 -H2460, 2005. First published September 30, 2005 doi:10.1152/ajpheart.00601.2005.-Plasma osmolality alters control of sympathetic activity and heart rate in animal models; however, it is unknown whether physiological increases in plasma osmolality have such influences in humans and what effect concurrent changes in central venous and/or arterial pressures may have. We tested whether physiological increases in plasma osmolality (similar to those during exercise dehydration) alter control of muscle sympathetic nerve activity (MSNA) and heart rate (HR) in humans. We studied 17 healthy young adults (7 women, 10 men) at baseline and during arterial pressure (AP) transients induced by sequential injections of nitroprusside and phenylephrine, under three conditions: control (C), after 1 ml/kg intravenous hypertonic saline (HT1), and after 2 ml/kg hypertonic saline (HT2). We continuously measured HR, AP, central venous pressure (CVP; peripherally inserted central catheter) and MSNA (peroneal microneurography) in all conditions. Plasma osmolality increased from 287 Ϯ 1 mosmol/kg in C to 290 Ϯ 1 mosmol/kg in HT1 (P Ͻ 0.05) but did not increase further in HT2 (291 Ϯ 1 mosmol/kg; P Ͼ 0.05 vs. C). Mean AP and CVP were similar between C and HT1, but both increased slightly in HT2. HR increased slightly but significantly during both HT1 and HT2 vs. C (P Ͻ 0.05). Sensitivity of baroreflex control of MSNA was significantly increased vs. C in HT1 [Ϫ7.59 Ϯ 0.97 (HT1) vs. Ϫ5.85 Ϯ 0.63 (C) arbitrary units (au) ⅐ beat Ϫ1 ⅐ mmHg Ϫ1 ; P Ͻ 0.01] but was not different in HT2 (Ϫ6.55 Ϯ 0.94 au ⅐ beat Ϫ1 ⅐ mmHg Ϫ1 ). We conclude that physiological changes in plasma osmolality significantly alter control of MSNA and HR in humans, and that this influence can be modified by CVP and AP. baroreflex; sympathetic nervous system; hydration; plasma volume CHANGES IN HYDRATION ELICIT important alterations in neural mechanisms controlling arterial pressure (AP) via the baroreflex (5, 6, 17). Influences of increased plasma osmolality, which can accompany decreases in volume seen with certain types of dehydration (such as with prolonged exercise), may also interact with changes in volume to alter autonomic mechanisms that have important implications for control of blood pressure in these conditions.Because increased plasma osmolality often accompanies dehydration, physiological responses to hyperosmolality are generally those that would tend to defend plasma volume and AP. In animal models, these have been shown to include sympathoexcitation (2, 7) and decreased urine output (20). Furthermore, hyperosmolality has been shown to alter arterial baroreflex control of both heart rate (HR) and sympathetic activity in rats (1, 7). Such influences of osmolality on baroreflex control mechanisms may also involve osmolality-induced increases in circulating volume-regulatory hormones ...

Neuromechanical Features of the Cardiac Baroreflex After Exercise

“…15,17,26 Furthermore, the timing of our protocol also allows direct comparison with previous studies that have used the modified Oxford method and the same time protocol with respect to exercise intervention. 3,30 As opposed to neck-collar suction/pressure devices that preclude access to the neck, the use of vasoactive compounds to perturb BP allows imaging of the carotid artery. With the advent of vascular ultrasound technologies that enable highresolution quantification of instantaneous carotid artery diameter, it is possible to assess the mechanical transduction of pressure and neural transduction of stretch as individual components of the integrated cardiac baroreflex response.…”

Baroreflex control of muscle sympathetic nerve activity: a nonpharmacological measure of baroreflex sensitivity

“…All records used in this study were retrospectively analyzed from baseline and modified Oxford trials from other studies completed in our laboratory (6,7). The investigations from which the data were obtained had ethical approval from the Institutional Review Board of the Mayo Clinic.…”

Mechanistic bases for differences in passive absorption

“…Prior to administration of the gavage solution containing test molecules, rats and pigeons were gavaged twice (2.5·ml each for rats, 3.0·ml each for pigeons) with an isosmotic solution (rats: 290·mmol·kg -1 , pigeons: 350·mmol·kg -1 ) containing 30·mmol·l -1 NaCl, 10·mmol·l -1 D-glucose and mannitol [either 280·mmol·l -1 for pigeons or 230·mmol·l -1 for rats, to be isosmotic with avian and mammalian plasma, respectively (Charkoudian et al, 2005;Goldstein and Zahedi, 1990)]. Animals were then gavaged with an isosmotic solution containing 30·mmol·l…”

“…Syringes were weighed before and after dosing animals to determine the actual dose administered. Osmotic pressures of solutions were measured (Wescor VAPRO 5520, Logan, UT, USA) prior to administration and averaged 320±7·mmol·kg -1 for rats and 365±3·mmol·kg -1 for pigeons to be isosmotic with mammalian and avian plasma, respectively (Charkoudian et al, 2005;Goldstein and Zahedi, 1990).…”