Relationship of osmotic inhibition in thermoregulatory responses and sweat sodium concentration in humans

The effect of temporal adaptation to different temperatures and osmolarities on heat response of TRPV4 in cultured cells

Transient receptor potential vanilloid 4 (TRPV4) channel is a polymodal receptor activated by moderate heat and hypoosmolarity. TRPV4 expressed in the skin area contributes to several skin functions as a barrier to maintain internal body physiology and a transporter of external stimuli. The skin condition such as skin temperature and osmolarity varies with internal and external changes, and may influence the activity of TRPV4 contributing to skin physiology, thermal sensation, and thermoregulation. However, the combination effect of skin conditions such as temperature and osmolarity on the activity of TRPV4 has not been examined. In the current study, we investigated the effect of temporal adaptation (5-10 min) to different temperature (25-35 °C) and osmolarity (250-350 mOsm) conditions on the heat response (until 40 °C) of human TRPV4 in cultured cells using Ca²⁺ imaging. The temperature to activate TRPV4 increased with elevation of the adaptation temperature, and decreased with the adaptation to hypoosmolarity in the range of 25-35 °C. In addition, the heat response was inhibited with the adaptation to hyperosmolarity in the range of 25-35 °C. Thus, we demonstrated that the activation temperature of TRPV4 varied with the temporal sensory adaptation to different temperature and osmolarity conditions. These findings may contribute to gaining better understanding of the variation in several TRPV4-mediated skin functions.

Impact of 3-day high and low dietary sodium intake on sodium status in response to exertional-heat stress: a double-blind randomized control trial

PURPOSE

To determine the impact of altering dietary sodium intake for 3 days preceding exercise on sweat sodium concentration [Na⁺], and cardiovascular and thermoregulatory variables.

METHODS

Fifteen male endurance athletes (runners n = 8, cyclists n = 7) consumed a low (LNa, 15 mg kg^-1 day^-1) or high (HNa, 100 mg kg^-1 day^-1) sodium diet, or their usual free-living diet [UDiet, 46 (37-56) mg kg^-1 day^-1] for 3 days in a double-blind, randomized cross-over design, collecting excreted urine (UNa) and refraining from exercise. On day 4, they completed 2 h running at 55% [Formula: see text]O_2max or cycling at 55% maximum aerobic power in T_amb 35 °C. Pre- and post-exercise blood samples were collected, and sweat from five sites using absorbent patches along the exercise protocol.

RESULTS

UNa on days 2-3 pre-exercise [mean (95% CI) LNa 16 (12-19) mg kg^-1 day^-1, UDiet 46 (37-56) mg kg^-1 day^-1, HNa 79 (72-85) mg kg^-1 day^-1; p < 0.001] and pre-exercise aldosterone [LNa 240 (193-286) mg kg^-1 day^-1, UDiet 170 (116-224) mg kg^-1 day^-1, HNa 141 (111-171) mg kg^-1 day^-1; p = 0.001] reflected sodium intake as expected. Pre-exercise total body water was greater following HNa compared to LNa (p < 0.05), but not UDiet. Estimated whole-body sweat [Na⁺] following UDiet was 10-11% higher than LNa and 10-12% lower than HNa (p < 0.001), and correlated with pre-exercise aldosterone (1st h r =  - 0.568, 2nd h r =  - 0.675; p < 0.01). Rectal temperature rose more quickly in LNa vs HNa (40-70 min; p < 0.05), but was similar at the conclusion of exercise, and no significant differences in heart rate or perceived exertion were observed.

CONCLUSIONS

Three day altered sodium intake influenced urinary sodium excretion and sweat [Na⁺], and the rise in rectal temperature, but had no effect on perceived exertion during moderate-intensity exercise in hot ambient conditions.

Integrating Competing Demands of Osmoregulatory and Thermoregulatory Homeostasis

Mammals are characterized by a stable core body temperature. When maintenance of core temperature is challenged by ambient or internal heat loads, mammals increase blood flow to the skin, sweat and/or pant, or salivate. These thermoregulatory responses enable evaporative cooling at moist surfaces to dissipate body heat. If water losses incurred during evaporative cooling are not replaced, body fluid homeostasis is challenged. This article reviews the way mammals balance thermoregulation and osmoregulation.

Heat Acclimation Decay and Re-Induction: A Systematic Review and Meta-Analysis

Background

Although the acquisition of heat acclimation (HA) is well-documented, less is known about HA decay (HAD) and heat re-acclimation (HRA). The available literature suggests 1 day of HA is lost following 2 days of HAD. Understanding this relationship has the potential to impact upon the manner in which athletes prepare for major competitions, as a HA regimen may be disruptive during final preparations (i.e., taper).

Objective

The aim of this systematic review and meta-analysis was to determine the rate of HAD and HRA in three of the main physiological adaptations occurring during HA: heart rate (HR), core temperature (T_c), and sweat rate (SR).

Data Sources

Data for this systematic review were retrieved from Scopus and critical review of the cited references.

Study Selection

Studies were included when they met the following criteria: HA, HAD, and HRA (when available) were quantified in terms of exposure and duration. HA had to be for at least 5 days and HAD for at least 7 days for longitudinal studies. HR, T_c, or SR had to be monitored in human participants.

Study Appraisal

The level of bias in each study was assessed using the McMaster critical review form. Multiple linear regression techniques were used to determine the dependency of HAD in HR, T_c, and SR from the number of HA and HAD days, daily HA exposure duration, and intensity.

Results

Twelve studies met the criteria and were systematically reviewed. HAD was quantified as a percentage change relative to HA (0% = HA, 100% = unacclimated state). Adaptations in end-exercise HR decreased by 2.3% (P < 0.001) for every day of HAD. For end-exercise T_c, the daily decrease was 2.6% (P < 0.01). The adaptations in T_c during the HA period were more sustainable when the daily heat exposure duration was increased and heat exposure intensity decreased. The decay in SR was not related to the number of decay days. However, protracted HA-regimens seem to induce longer-lasting adaptations in SR. High heat exposure intensities during HA seem to evoke more sustained adaptations in SR than lower heat stress. Only eight studies investigated HRA. HRA was 8–12 times faster than HAD at inducing adaptations in HR and T_c, but no differences could be established for SR.

Limitations

The available studies lacked standardization in the protocols for HA and HAD.

Conclusions

HAD and HRA differ considerably between physiological systems. Five or more HA days are sufficient to cause adaptations in HR and T_c; however, extending the daily heat exposure duration enhances T_c adaptations. For every decay day, ~ 2.5% of the adaptations in HR and T_c are lost. For SR, longer HA periods are related to better adaptations. High heat exposure intensities seem beneficial for adaptations in SR, but not in T_c. HRA induces adaptations in HR and T_c at a faster rate than HA. HRA may thus provide a practical and less disruptive means of maintaining and optimizing HA prior to competition.

Heat acclimatization blunts copeptin responses to hypertonicity from dehydrating exercise in humans

Acclimatization favors greater extracellular tonicity from lower sweat sodium, yet hyperosmolality may impair thermoregulation during heat stress. Enhanced secretion or action of vasopressin could mitigate this through increased free water retention. Aims were to determine responses of the vasopressin surrogate copeptin to dehydrating exercise and investigate its relationships with tonicity during short and long-term acclimatization. Twenty-three participants completed a structured exercise programme following arrival from a temperate to a hot climate. A Heat Tolerance Test (HTT) was conducted on Day-2, 6, 9 and 23, consisting of 60-min block-stepping at 50% VO2 peak, with no fluid intake. Resting sweat [Na+ ] was measured by iontophoresis. Changes in body mass (sweat loss), core temperature, heart rate, osmolality (serum and urine) and copeptin and aldosterone (plasma) were measured with each Test. From Day 2 to Day 23, sweat [Na+ ] decreased significantly (adjusted P < 0.05) and core temperature and heart rate fell. Over the same interval, HTT-associated excursions were increased for serum osmolality (5 [-1, 9] vs. 9 [5, 12] mosm·kg-1 ), did not differ for copeptin (9.6 [6.0, 15.0] vs. 7.9 [4.3, 14.7] pmol·L-1 ) and were reduced for aldosterone (602 [415, 946] vs. 347 [263, 537] pmol·L-1 ). Urine osmolality was unchanging and related consistently to copeptin at end-exercise, whereas the association between copeptin and serum osmolality was right-shifted (P = 0.0109) with acclimatization. Unchanging urine:serum osmolality argued against increased renal action of vasopressin. In conclusion, where exercise in the heat is performed without fluid replacement, heat acclimatization does not appear to enhance AVP-mediated free water retention in humans.

Impact of sodium citrate ingestion during recovery after dehydrating exercise on rehydration and subsequent 40-km cycling time-trial performance in the heat

The purpose of this study was to assess the impact of sodium citrate (CIT) ingestion (600 mg·kg^-1) during recovery from dehydrating cycling exercise (DE) on subsequent 40-km cycling performance in a warm environment (32 °C). Twenty male nonheat-acclimated endurance athletes exercised in the heat until 4% body mass (BM) loss occurred. After 16 h recovery with consumption of water ad libitum and prescribed diet (evening meal 20 kcal·kg^-1, breakfast 12 kcal·kg^-1) supplemented in a double-blind, randomized, crossover manner with CIT or placebo (PLC), they performed 40-km time-trial (TT) on a cycle ergometer in a warm environment. During recovery greater increases in BM and plasma volume (PV) concomitant with greater water intake and retention occurred in the CIT trial compared with the PLC trial (p < 0.0001). During TT there was greater water intake and smaller BM loss in the CIT trial than in the PLC trial (p < 0.05) with no between-trial differences (p > 0.05) in sweat loss, PV decrement, ratings of perceived exertion, or TT time (CIT 68.10 ± 3.28 min, PLC 68.11 ± 2.87 min). At the end of TT blood lactate concentration was higher (7.58 ± 2.44 mmol·L^-1 vs 5.58 ± 1.32 mmol·L^-1; p = 0.0002) and rectal temperature lower (39.54 ± 0.50 °C vs 39.65 ± 0.52 °C; p = 0.033) in the CIT trial than in the PLC trial. Compared with pre-DE time point, PV had decreased to a lower level in the PLC trial than in the CIT trial (p = 0.0001). In conclusion, CIT enhances rehydration after exercise-induced dehydration but has no impact on subsequent 40-km cycling TT performance in a warm uncompensable environment.

PERDA ELETROLÍTICA DE CÁLCIO, MAGNÉSIO E FERRO NO SUOR DURANTE CORRIDA EM ESTEIRA

RESUMO Introdução: O suor e sua consequente evaporação são fundamentais para manutenção da temperatura corporal durante o exercício. Objetivo: Avaliar a perda de cálcio (Ca++), magnésio (Mg++) e ferro (Fe++) no suor de corredores e de indivíduos ativos. Métodos: Foram avaliados 15 atletas corredores de fundo {VO2máx = 68 ± 5,4 ml(kg.min)-1} e 15 indivíduos ativos não atletas {VO2máx = 50,3 ± 6,3 ml(kg.min)-1}, com média de idade, respectivamente, de 25,3 ± 2,4 e 23,1 ± 4,3 anos. Ambos os grupos se exercitaram por 80 minutos em esteira, com intensidade de 75% a 85% da frequência cardíaca de reserva, e ingeriram 3 ml de água/kg de peso corporal a cada 15 minutos. As condições ambientais da prova foram 21,9 ± 1,5 °C e 89,2 ± 5,6% de umidade relativa para os atletas e 21,8 ± 1,6 °C e 93,2 ± 3,5% de UR para os ativos. As amostras de suor foram coletadas em intervalos regulares de 20 minutos nas regiões do peito, torácica e lombar das costas, para posterior análise dos minerais Ca++, Mg++ e Fe++ por espectrofotômetro de absorção atômica. Resultados: Não foram registradas diferenças significativas para os minerais em função do nível de condicionamento. Observou-se tendência à diminuição na concentração do Mg++ e Fe++ do suor ao longo do exercício. Conclusão: Nas condições ambientais e de exercício estudadas, o condicionamento não interfere na perda de Ca++, Mg++ e Fe++.

Control of the Cutaneous Circulation by the Central Nervous System

The central nervous system (CNS), via its control of sympathetic outflow, regulates blood flow to the acral cutaneous beds (containing arteriovenous anastomoses) as part of the homeostatic thermoregulatory process, as part of the febrile response, and as part of cognitive-emotional processes associated with purposeful interactions with the external environment, including those initiated by salient or threatening events (we go pale with fright). Inputs to the CNS for the thermoregulatory process include cutaneous sensory neurons, and neurons in the preoptic area sensitive to the temperature of the blood in the internal carotid artery. Inputs for cognitive-emotional control from the exteroceptive sense organs (touch, vision, sound, smell, etc.) are integrated in forebrain centers including the amygdala. Psychoactive drugs have major effects on the acral cutaneous circulation. Interoceptors, chemoreceptors more than baroreceptors, also influence cutaneous sympathetic outflow. A major advance has been the discovery of a lower brainstem control center in the rostral medullary raphé, regulating outflow to both brown adipose tissue (BAT) and to the acral cutaneous beds. Neurons in the medullary raphé, via their descending axonal projections, increase the discharge of spinal sympathetic preganglionic neurons controlling the cutaneous vasculature, utilizing glutamate, and serotonin as neurotransmitters. Present evidence suggests that both thermoregulatory and cognitive-emotional control of the cutaneous beds from preoptic, hypothalamic, and forebrain centers is channeled via the medullary raphé. Future studies will no doubt further unravel the details of neurotransmitter pathways connecting these rostral control centers with the medullary raphé, and those operative within the raphé itself. © 2016 American Physiological Society. Compr Physiol 6:1161-1197, 2016.

Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports

Exercise heat acclimation induces physiological adaptations that improve thermoregulation, attenuate physiological strain, reduce the risk of serious heat illness, and improve aerobic performance in warm-hot environments and potentially in temperate environments. The adaptations include improved sweating, improved skin blood flow, lowered body temperatures, reduced cardiovascular strain, improved fluid balance, altered metabolism, and enhanced cellular protection. The magnitudes of adaptations are determined by the intensity, duration, frequency, and number of heat exposures, as well as the environmental conditions (i.e., dry or humid heat). Evidence is emerging that controlled hyperthermia regimens where a target core temperature is maintained, enable more rapid and complete adaptations relative to the traditional constant work rate exercise heat acclimation regimens. Furthermore, inducing heat acclimation outdoors in a natural field setting may provide more specific adaptations based on direct exposure to the exact environmental and exercise conditions to be encountered during competition. This review initially examines the physiological adaptations associated with heat acclimation induction regimens, and subsequently emphasizes their application to competitive athletes and sports.

Osmolality and respiratory regulation in humans: respiratory compensation for hyperchloremic metabolic acidosis is absent after infusion of hypertonic saline in healthy volunteers

BACKGROUND

Several animal studies show that changes in plasma osmolality may influence ventilation. Respiratory depression caused by increased plasma osmolality is interpreted as inhibition of water-dependent thermoregulation because conservation of body fluid predominates at the cost of increased core temperature. Respiratory alkalosis, on the other hand, is associated with a decrease in plasma osmolality and strong ion difference (SID) during human pregnancy. We investigated the hypothesis that osmolality would influence ventilation, so that increased osmolality will decrease ventilation and decreased osmolality will stimulate ventilation in both men and women.

METHODS

Our study participants were healthy volunteers of both sexes (ASA physical status I). Ten men (mean 28 years; range 20-40) and 9 women (mean 33 years; range 22-43) were included. All women participated in both the follicular and luteal phases of the menstrual cycle. Hyperosmolality was induced by IV infusion of hypertonic saline 3%, and hypoosmolality by drinking tap water. Arterial blood samples were collected for analysis of electrolytes, osmolality, and blood gases. Sensitivity to CO2 was determined by rebreathing tests performed before and after the fluid-loading procedures.

RESULTS

Infusion of hypertonic saline caused hyperchloremic metabolic acidosis with decreased SID in all subjects. Analysis of pooled data showed absence of respiratory compensation. Baseline arterial PCO2 (PaCO2) mean (SD) 37.8 (2.9) mm Hg remained unaltered, with lowest PaCO2 37.8 (2.9) mm Hg after 100 minutes, P = 0.70, causing a decrease in pH from mean (SD) 7.42 (0.02) to 7.38 (0.02), P < 0.001. Metabolic acidosis was also observed during water loading. Pooled results show that PaCO2 decreased from 38.2 (3.3) mm Hg at baseline to 35.7 (2.8) mm Hg after 80 minutes of drinking water, P = 0.002, and pH remained unaltered: pH 7.43 (0.02) at baseline to pH 7.42 (0.02), P = 0.14, mean difference (confidence interval) = pH -0.007 (-0.017 to 0.003).

CONCLUSIONS

Our results indicate that osmolality has an influence on ventilation. Respiratory compensation for hyperchloremic metabolic acidosis was suppressed during hyperosmolality. Water loading caused a decrease in plasma osmolality and metabolic acidosis, and although the decrease in SID was smaller compared with salt loading, the expected respiratory compensation was observed. Ventilation was also stimulated in men, therefore independently of progesterone levels. We propose that the influence of osmolality on ventilation consists mainly as depression in conditions of hyperosmolality and that this depression is absent during hypoosmolality.

Circadian modulation of osmoregulated firing in rat supraoptic nucleus neurones

The antidiuretic hormone vasopressin (VP) promotes water reabsorption from the kidney and levels of circulating VP are normally related linearly to plasma osmolality, aiming to maintain the latter close to a predetermined set point. Interestingly, VP levels rise also in the absence of an increase in osmolality during late sleep in various mammals, including rats and humans. This circadian rhythm is functionally important because the absence of a late night VP surge results in polyuria and disrupts sleep in humans. Previous work has indicated that the VP surge may be caused by facilitation of the central processes mediating the osmotic control of VP release, and the mechanism by which this occurs was recently studied in angled slices of rat hypothalamus that preserve intact network interactions between the suprachiasmatic nucleus (SCN; the biological clock), the organum vasculosum lamina terminalis (OVLT; the central osmosensory nucleus) and the supraoptic nucleus (SON; which contains VP-releasing neurohypophysial neurones). These studies confirmed that the electrical activity of SCN clock neurones is higher during the middle sleep period (MSP) than during the late sleep period (LSP). Moreover, they revealed that the excitation of SON neurones caused by hyperosmotic stimulation of the OVLT was greater during the LSP than during the MSP. Activation of clock neurones by repetitive electrical stimulation, or by injection of glutamate into the SCN, caused a presynaptic inhibition of glutamatergic synapses made between the axon terminals of OVLT neurones and SON neurones. Consistent with this effect, activation of clock neurones with glutamate also reduced the excitation of SON neurones caused by hyperosmotic stimulation of the OVLT. These results suggest that clock neurones in the SCN can mediate an increase in VP release through a disinhibition of excitatory synapses between the OVLT and the SON during the LSP.

Understanding hypernatremia

Understanding hypernatremia is at times difficult for many clinicians. However, hypernatremia can often be deciphered easily with some basic understanding of water and sodium balance. Here, the basic pathophysiological abnormalities underlying the development of sodium disorders are reviewed, and case examples are given. Hypernatremia often arises in the hospital, especially in the intensive care units due to the combination of (1) not being able to drink water; (2) inability to concentrate the urine (most often from having kidney failure); (3) osmotic diuresis from having high serum urea concentrations, and (4) large urine or stool outputs.

Arginine vasopressin, fluid balance and exercise: is exercise-associated hyponatraemia a disorder of arginine vasopressin secretion?

The ability of the human body to regulate plasma osmolality (POsm) within a very narrow and well defined physiological range underscores the vital importance of preserving water and sodium balance at rest and during exercise. The principle endocrine regulator of whole body fluid homeostasis is the posterior pituitary hormone, arginine vasopressin (AVP). Inappropriate AVP secretion may perpetuate either slow or rapid violation of these biological boundaries, thereby promoting pathophysiology, morbidity and occasional mortality. In the resting state, AVP secretion is primarily regulated by changes in POsm (osmotic regulation). The osmotic regulation of AVP secretion during exercise, however, may possibly be enhanced or overridden by many potential non-osmotic factors concurrently stimulated during physical activity, particularly during competition. The prevalence of these highly volatile non-osmotic AVP stimuli during strenuous or prolonged physical activity may reflect a teleological mechanism to promote water conservation during exercise. However, non-osmotic AVP secretion, combined with high fluid availability plus sustained fluid intake (exceeding fluid output), has been hypothesized to lead to an increase in both the incidence and related deaths from exercise-associated hyponatraemia (EAH) in lay and military populations. Inappropriately, high plasma AVP concentrations ([AVP](p)) associated with low blood sodium concentrations facilitate fluid retention and sodium loss, thereby possibly reconciling both the water intoxication and sodium loss theories of hyponatraemia that are currently under debate. Therefore, given the potential for a variety of exercise-induced non-osmotic stimuli for AVP secretion, hydration strategies must be flexible, individualized and open to change during competitive events to prevent the occurrence of rare, but life-threatening, EAH. This review focuses on the potential osmotic and non-osmotic stimuli to AVP secretion that may affect fluid homeostasis during physical activity. Recent laboratory and field data support: (i) stimulatory effects of exercise intensity and duration on [AVP](p); (ii) possible relationships between changes in POsm with changes in both sweat and urinary osmolality; (iii) alterations in the AVP osmoregulatory set-point by sex steroid hormones; (iv) differences in [AVP](p) in trained versus untrained athletes; and (v) potential inter-relationships between AVP and classical (aldosterone, atrial natriuretic peptide) and non-classical (oxytocin, interleukin-6) endocrine mediators. The review concludes with a hypothesis on how sustained fluid intakes beyond the capacity for fluid loss might possibly facilitate the development of hyponatraemia if exercise-induced non-osmotic stimuli override 'normal' osmotic suppression of AVP when hypo-osmolality exists.

Plasma hyperosmolality elevates the internal temperature threshold for active thermoregulatory vasodilation during heat stress in humans

Plasma hyperosmolality delays the response in skin blood flow to heat stress by elevating the internal temperature threshold for cutaneous vasodilation. This elevation could be because of a delayed onset of cutaneous active vasodilation and/or to persistent cutaneous active vasoconstriction. Seven healthy men were infused with either hypertonic (3% NaCl) or isotonic (0.9% NaCl) saline and passively heated by immersing their lower legs in 42 degrees C water for 60 min (room temperature, 28 degrees C; relative humidity, 40%). Skin blood flow was monitored via laser-Doppler flowmetry at sites pretreated with bretylium tosylate (BT) to block sympathetic vasoconstriction selectively and at adjacent control sites. Plasma osmolality was increased by approximately 13 mosmol/kgH(2)O following hypertonic saline infusion and was unchanged following isotonic saline infusion. The esophageal temperature (T(es)) threshold for cutaneous vasodilation at untreated sites was significantly elevated in the hyperosmotic state (37.73 +/- 0.11 degrees C) relative to the isosmotic state (36.63 +/- 0.12 degrees C, P < 0.001). A similar elevation of the T(es) threshold for cutaneous vasodilation was observed between osmotic conditions at the BT-treated sites (37.74 +/- 0.18 vs. 36.67 +/- 0.07 degrees C, P < 0.001) as well as sweating. These results suggest that the hyperosmotically induced elevation of the internal temperature threshold for cutaneous vasodilation is due primarily to an elevation in the internal temperature threshold for the onset of active vasodilation, and not to an enhancement of vasoconstrictor activity.

Drinking-induced thermoregulatory panting in rehydrated sheep: influences of oropharyngeal/esophageal signals, core temperature, and thirst satiety

Dehydrated mammals conserve body water by reducing thermoregulatory evaporative cooling responses e.g., panting and sweating. Increased core temperature (Tc) may result. Following rehydration and correction of fluid deficits, panting and sweating commence. We investigated the role of oropharyngeal/esophageal, postabsorptive and thermal signals in the panting response, and reduced Tc that occurs when unshorn sheep drink water following water deprivation for 2 days (ambient temperature 20°C). Ingestion of water (at body temperature) resulted in increased respiratory rate (panting) and reduced Tc within 4 min that persisted for at least 90 min. Initially, a similar panting response and reduced Tc occurred following rehydration by drinking isotonic saline solution, but panting was not sustained after 20 min, and Tc began to rise again. Rehydration by intraruminal administration of water, without any drinking, resulted in delayed panting and fall in Tc. Intraruminal infusion of saline was ineffective. Rehydration by drinking cool water (20°C) resulted in a rapid fall in Tc without increased panting. Shorn sheep had lower basal Tc that did not increase during 2 days of water deprivation, and they did not pant on rehydration by drinking water. Our results indicate that signals from the oropharyngeal and/or esophageal region associated with the act of drinking play a crucial role in the initial 20–30 min of the panting response to rehydration. Postabsorptive factors most likely reduced plasma tonicity and cause continued panting and further reduction in Tc. Tc also influences rehydration-induced panting. It occurs only if sheep incur a heat load during bodily dehydration.

Acute changes in arginine vasopressin, sweat, urine and serum sodium concentrations in exercising humans: does a coordinated homeostatic relationship exist?

The parallel response of sweat rate and urine production to changes in plasma osmolality and volume support a role for arginine vasopressin (AVP) as the main endocrine regulator of both excretions. A maximal test to exhaustion and a steady-state run on a motorised treadmill were both completed by 10 moderately trained runners, 1 week apart. Sweat, urine and serum sodium concentrations ([Na+]) were evaluated in association with the plasma concentrations of cytokines, neurohypophyseal and natriuretic peptides, and adrenal steroid hormones. When data from both the high-intensity and steady-state runs were combined, significant linear correlations were noted between: sweat [Na+] versus postexercise urine [Na+] (r=0.80; p<0.001), postexercise serum [Na+] versus both postexercise urine [Na+] (r=0.56; p<0.05) and sweat [Na+] (r=0.64; p<0.01) and postexercise urine [Na+] versus postexercise plasma arginine vasopressin concentration ([AVP](P)) (r=0.48; p<0.05). A significant positive correlation was noted between postexercise [AVP](P) and sweat [Na+] during the steady-state condition only (r=0.66; p<0.05). These correlations suggest that changes in serum [Na+] during exercise may evoke corresponding changes in sweat and urine [Na+], which are likely regulated coordinately by changes in [AVP](P) to preserve body fluid homeostasis.

Na+ secretion rate increases proportionally more than the Na+ reabsorption rate with increases in sweat rate

The purpose of this study was to measure the in vivo Na(+) secretion and Na(+) reabsorption rates of the human eccrine sweat gland with increases in sweat rate. Such data should help to elucidate the physiological mechanism responsible for the previously reported linear relationship between increases in sweat rate and Na(+) concentration in sweat. On 5 days, each subject (n = 10) completed a 30-min exercise bout in an environmental chamber set at 35 degrees C and 40% relative humidity. The intensity for the five exercise bouts in the heat was set to approximate 50, 60, 70, 80, and 90% of age-predicted maximum heart rate. Forearm sweat samples and capillary blood samples were collected during each of the five 30-min exercise bouts. The sweat and blood samples were analyzed for Na(+) concentration in sweat and serum, which were used to calculate the rate of Na(+) secretion and Na(+) reabsorption. The mean correlation between sweat rate and Na(+) concentration in sweat was found to be r = 0.73. Within the sweat rate range of the present study, both Na(+) secretion rate and Na(+) reabsorption rate increased linearly; however, the Na(+) secretion rate increased almost twice as fast (slope = 141 vs. 80). Thus the rate at which Na(+) escaped reabsorption increased with increases in sweat rate and was significantly (P < 0.05) correlated to the Na(+) concentration in sweat (mean r = 0.90). Such results strongly suggest that the physiological mechanism responsible for the previously reported linear increase in Na(+) concentration in sweat seen with increases in sweat rate is that the Na(+) secretion rate increases proportionally more than the Na(+) reabsorption rate.

Central mechanisms of osmosensation and systemic osmoregulation

Systemic osmoregulation is a vital process whereby changes in plasma osmolality, detected by osmoreceptors, modulate ingestive behaviour, sympathetic outflow and renal function to stabilize the tonicity and volume of the extracellular fluid. Furthermore, changes in the central processing of osmosensory signals are likely to affect the hydro-mineral balance and other related aspects of homeostasis, including thermoregulation and cardiovascular balance. Surprisingly little is known about how the brain orchestrates these responses. Here, recent advances in our understanding of the molecular, cellular and network mechanisms that mediate the central control of osmotic homeostasis in mammals are reviewed.

Central osmoregulatory influences on thermoregulation

1. Many mammals maintain a constant core body temperature in the face of a heat load by using evaporative cooling responses, such as sweating, panting and spreading of saliva. These cooling mechanisms incur a body fluid deficit if the fluid lost as sweat, saliva or respiratory moisture is not replaced by the ingestion of water; body fluid hypertonicity and hypovolaemia result. 2. Evidence in several mammals shows that, as they become dehydrated, evaporative cooling mechanisms such as sweating and panting are inhibited so that further fluid loss from the body is reduced. As a result, core temperature in the dehydrated animal is maintained at a higher than normal level. 3. Increasing the osmotic pressure of plasma has an inhibitory effect on panting and sweating in mammals. It has been proposed that osmoreceptors mediate these inhibitory influences of plasma hypertonicity on sweating and panting. 4. The suppression of panting in dehydrated sheep is mediated by cerebral osmoreceptors that are probably located in the lamina terminalis. We speculate that osmoreceptors in the lamina terminalis may also influence thermoregulatory sweating. 5. When dehydrated animals drink water, sweating and panting resume rapidly before water has been absorbed from the gut. It is likely that the act of drinking initiates a reflex that can override the osmoreceptor inhibition of panting, resulting in core temperature falling back quickly to a normal level.

Sodium ion concentration vs. sweat rate relationship in humans

The purpose of this study was to determine the effect of active heat acclimation on the sweat osmolality and sweat sodium ion concentration vs. sweat rate relationship in humans. Eight healthy male volunteers completed 10 days of exercise in the heat. The mean exercising heart rate and core temperature were significantly decreased ( P < 0.05) by 18 beats/min and 0.6°C, respectively, following heat acclimation. Furthermore, sweat osmolality and the sweat sodium ion concentration vs. sweat rate relationships were shifted to the right. Specifically, the slopes of the relationships were not affected by heat acclimation. Rather, heat acclimation significantly reduced the y-intercepts of the sweat osmolality and sweat sodium relationships with sweat rate by 28 mosmol/kgH2O and 15 mmol/l, respectively. Thus there was a significantly lower sweat sodium ion concentration for a given sweat rate following heat acclimation. These results suggest that heat acclimation increases the sodium ion reabsorption capacity of the human eccrine sweat gland.

Sodium Loading Aids Fluid Balance and Reduces Physiological Strain of Trained Men Exercising in the Heat

PURPOSE

This study was conducted to determine whether preexercise ingestion of a highly concentrated sodium beverage would increase plasma volume (PV) and reduce the physiological strain of moderately trained males running in the heat.

METHODS

Eight endurance-trained (.VO2max: 58 mL.kg(-1).min(-1) (SD 5); 36 yr (SD 11)) runners completed this double-blind, crossover experiment. Runners ingested a high-sodium (High Na+: 164 mmol Na+.L(-1)) or low-sodium (Low Na+: 10 mmol Na+.L(-1)) beverage (10 mL.kg(-1)) before running to exhaustion at 70% .VO2max in warm conditions (32 degrees C, 50% RH, V(a) approximately equal to 1.5 m.s(-1)). Beverages (approximately 757 mL) were ingested in seven portions across 60 min beginning 105 min before exercise. Trials were separated by 1-3 wk. Heart rate and core and skin temperatures were measured throughout exercise. Urine and venous blood were sampled before and after drinking and exercise.

RESULTS

High Na+ increased PV before exercise (4.5% (SD 3.7)), calculated from Hct and [Hb]), whereas Low Na+ did not (0.0% (SD 0.5); P = 0.04), and involved greater time to exercise termination in the six who stopped because of an ethical end point (core temperature 39.5 degrees C: 57.9 min (SD 6) vs 46.4 min (SD 4); P = 0.04) and those who were exhausted (96.1 min (SD 22) vs 75.3 min (SD 21); P = 0.03; High Na+ vs Low Na+, respectively). At equivalent times before exercise termination, High Na+ also resulted in lower core temperature (38.9 vs 39.3 degrees C; P = 0.00) and perceived exertion (P = 0.01) and a tendency for lower heart rate (164 vs 174 bpm; P = 0.08).

CONCLUSIONS

Preexercise ingestion of a high-sodium beverage increased plasma volume before exercise and involved less thermoregulatory and perceived strain during exercise and increased exercise capacity in warm conditions.

Heat illness during working and preventive considerations from body fluid homeostasis

The purposes of this review are to show pathophysiological mechanisms for heat illness during working in a hot environment and accordingly provide some preventive considerations from a viewpoint of body fluid homeostasis. The incidence of the heat illness is closely associated with body temperature regulation, which is much affected by body fluid state in humans. Heat generated by contracting muscles during working increases body temperature, which, in a feedback manner, drives heat-dissipation mechanisms of skin blood flow and sweating to prevent a rise in body temperature. However, the impairment of heat-dissipation mechanisms caused by hard work in hot, humid, and dehydrated conditions accelerates the increase in body temperature, and, if not properly treated, leads to heat illness. First, we overviewed thermoregulation during working (exercising) in a hot environment, describe the effects of dehydration on skin blood flow and sweating, and then explained how they contributes to the progression toward heat illness. Second, we described the advantageous effects of blood volume expansion after heat acclimatization on temperature regulation during exercise as well as those of restitution from dehydration by supplementation of carbohydrate-electrolyte solution. Finally, we described that the deteriorated thermoregulation in the elderly is closely associated with the impaired body fluid regulation and that blood volume expansion by exercise training with protein supplementation improves thermoregulation.

Transient cutaneous vasodilatation and hypotension after drinking in dehydrated and exercising men

We examined whether oropharyngeal stimulation by drinking released the dehydration-induced suppression of cutaneous vasodilatation and decreased mean arterial pressure (MAP) in exercising subjects, and assessed the effects of hypovolaemia or hyperosmolality alone on these responses. Seven young males underwent four hydration conditions. These were two normal plasma volume (PV) trials: normal plasma osmolality (P(osmol), control trial) and hyperosmolality (DeltaP(osmol) = +11 mosmol (kg H(2)O)(-1)); and two low PV trials: isosmolality (DeltaPV = -310 ml) and hyperosmolality (DeltaPV = -345 ml; DeltaP(osmol) = +9 mosmol (kg H(2)O)(-1)), attained by combined treatment with furosemide (frusemide), hypertonic saline and/or 24 h water restriction. In each trial, the subjects exercised at 60% peak aerobic power for approximately 50 min at 30 degrees C atmospheric temperature and 50% relative humidity. When oesophageal temperature (T(oes)) reached a plateau after approximately 30 min of exercise, the subjects drank 200 ml water at 37.5 degrees C within a minute. Before drinking, forearm vascular conductance (FVC), calculated as forearm blood flow divided by MAP, was lowered by 20-40% in hypovolaemia, hyperosmolality, or both, compared with that in the control trial, despite increased T(oes). After drinking, FVC increased by approximately 20% compared with that before drinking (P < 0.05) in both hyperosmotic trials, but it was greater in normovolaemia than in hypovolaemia (P < 0.05). However, no increases occurred in either isosmotic trial. MAP fell by 4-8 mmHg in both hyperosmotic trials (P < 0.05) after drinking, but more rapidly in normovolaemia than in hypovolaemia. PV and P(osmol) did not change during this period. Thus, oropharyngeal stimulation by drinking released the dehydration-induced suppression of cutaneous vasodilatation and reduced MAP during exercise, and this was accelerated when PV was restored.

Plasma hyperosmolality augments peripheral vascular response to baroreceptor unloading during heat stress

The aim of this study was to elucidate the interactive effect of central hypovolemia and plasma hyperosmolality on regulation of peripheral vascular response and AVP secretion during heat stress. Seven male subjects were infused with either isotonic (0.9%; NOSM) or hypertonic (3.0%; HOSM) NaCl solution and then heated by perfusing 42 degrees C (heat stress; HT) or 34.5 degrees C water (normothermia; NT) through water perfusion suits. Sixty minutes later, subjects were exposed to progressive lower body negative pressure (LBNP) to -40 mmHg. Plasma osmolality (P(osmol)) increased by approximately 11 mosmol/kgH(2)O in HOSM conditions. The increase in esophageal temperature before LBNP was much larger in HT-HOSM (0.90 +/- 0.09 degrees C) than in HT-NOSM (0.30 +/- 0.07 degrees C) (P < 0.01) because of osmotic inhibition of thermoregulation. During LBNP, mean arterial pressure was well maintained, and changes in thoracic impedance and stroke volume were similar in all conditions. Forearm vascular conductance (FVC) before application of LBNP was higher in HT than in NT conditions (P < 0.001) and was not influenced by P(osmol) within the thermal conditions. The reduction in FVC at -40 mmHg in HT-HOSM (-9.99 +/- 0.96 units; 58.8 +/- 4.1%) was significantly larger than in HT-NOSM (-6.02 +/- 1.23 units; 44.7 +/- 8.1%) (P < 0.05), whereas the FVC response was not different between NT-NOSM and NT-HOSM. Plasma AVP response to LBNP did not interact with P(osmol) in either NT or HT conditions. These data indicate that there apparently exists an interactive effect of P(osmol) and central hypovolemia on the peripheral vascular response during heat stress, or peripheral vasodilated conditions, but not in normothermia.

Acute hypoosmolality attenuates the suppression of cutaneous vasodilation with increased exercise intensity

We examined the hypothesis that elevation of the body core temperature threshold for forearm skin vasodilation (TH(FVC)) with increased exercise intensity is partially caused by concomitantly increased plasma osmolality (P(osmol)). Eight young male subjects, wearing a body suit perfused with warm water to maintain the mean skin temperature at 34 +/- 1 degree C (ranges), performed 20-min cycle-ergometer exercise at 30% peak aerobic power (VO2(peak)) under isoosmotic conditions (C), and at 65% VO2(peak) under isoosmotic (H(EX)I(OS)) and hypoosmotic (H(EX)L(OS)) conditions. In H(EX)L(OS), hypoosmolality was attained by hypotonic saline infusion with DDAVP, a V2 agonist, before exercise. P(osmol) (mosmol/kg H2O) increased after the start of exercise in both H(EX) trials (P < 0.01) but not in C. The average P(osmol) at 5 and 10 min in H(EX)I(OS) was higher than in C (P < 0.01), whereas that in H(EX)L(OS) was lower than in H(EX)I(OS) (P < 0.01). The change in TH(FVC) was proportional to that in P(osmol) in every subject for three trials. The change in TH(FVC) per unit change in P(osmol) (deltaTH(FVC)/deltaP(osmol), degrees C x mosmol(-1) x kg H2O(-1)) was 0.064 +/- 0.012 when exercise intensity increased from C to H(EX)I(OS), similar to 0.086 +/- 0.020 when P(osmol) decreased from H(EX)I(OS) to H(EX)L(OS) (P > 0.1). Moreover, there were no significant differences in plasma volume, heart rate, mean arterial pressure, and plasma lactate concentration around TH(FVC) between H(EX)I(OS) and H(EX)L(OS) (P > 0.1). Thus the increase in TH(FVC) due to increased exercise intensity was at least partially explained by the concomitantly increased P(osmol).

Ten-day endurance training attenuates the hyperosmotic suppression of cutaneous vasodilation during exercise but not sweating

It is well known that hyperosmolality suppresses thermoregulatory responses and that plasma osmolality (P(osmol)) increases with exercise intensity. We examined whether the decreased esophageal temperature thresholds for cutaneous vasodilation (TH(FVC)) and sweating (TH(SR)) after 10-day endurance training (ET) are caused by either attenuated increase in P(osmol) at a given exercise intensity or blunted sensitivity of hyperosmotic suppression. Nine young male volunteers exercised on a cycle ergometer at 60% peak oxygen consumption rate (V(O2 peak)) for 1 h/day for 10 days at 30 degrees C. Before and after ET, thermoregulatory responses were measured during 20-min exercise at pretraining 70% V(O2 peak) in the same environment as during ET under isoosmotic or hyperosmotic conditions. Hyperosmolality by approximately 10 mosmol/kgH2O was attained by acute hypertonic saline infusion. After ET, V(O2 peak) and blood volume (BV) both increased by approximately 4% (P < 0.05), followed by a decrease in TH(FVC) (P < 0.05) but not by that in TH(SR). Although there was no significant decrease in P(osmol) at the thresholds after ET, the sensitivity of increase in TH(FVC) at a given increase in P(osmol) [deltaTH(FVC)/deltaP(osmol), degrees C x (mosmol/kgH2O)(-1)], determined by hypertonic infusion, was reduced to 0.021 +/- 0.005 from 0.039 +/- 0.004 before ET (P < 0.05). The individual reductions in deltaTH(FVC)/deltaP(osmol) after ET were highly correlated with their increases in BV around TH(FVC) (r = -0.89, P < 0.005). In contrast, there was no alteration in the sensitivity of the hyperosmotic suppression of sweating after ET. Thus the downward shift of TH(FVC) after ET was partially explained by the blunted sensitivity to hyperosmolality, which occurred in proportion to the increase in BV.

Exercise-Associated Hyponatraemia

In 1958, Edelman and colleagues empirically showed plasma sodium concentration ([Na+]p) to be primarily a function of the sum of exchangeable sodium and potassium (E) divided by total body water (TBW). Based on Edelman's equation, Nguyen and Kurtz derived an equation to show how [Na+]p changes as a function of TBW, change in TBW (DeltaTBW), and change in the sum of exchangeable sodium and potassium (DeltaE). Using the Nguyen-Kurtz equation, the present study examines the sensitivity of [Na+]p to these parameters: [Na+]p is very sensitive to DeltaTBW and moderately sensitive to DeltaE, and is modulated by TBW. For example, for a person with 50 L TBW, a net increase of 1L water lowers [Na+]p by 3.2 mEq/L, but for a person with 25 L TBW it lowers [Na+]p by 6.3 mEq/L (assuming initial [Na+]p is 140 mEq/L). In each case, a loss of 159 mEq of sodium plus potassium (roughly equivalent to 1.5 teaspoons of table salt) would be required to produce the same effect as the net increase of 1 L water. The present review demonstrates why fluid overload predominates over electrolyte loss in the aetiology of exercise-associated hyponatraemia (EAH), and why the excretion of electrolyte-dilute urine is highly effective in correcting EAH (nonetheless, loss of sodium and potassium is significant in long events in warm weather). Sports drinks will, if overconsumed, result in hyponatraemia. Administration of a sports drink to an athlete with fluid overload hyponatraemia further lowers [Na+]p and increases fluid overload. Administration of either a sports drink or normal (0.9%) saline increases fluid overload.

Attenuated thermoregulatory sweating and cutaneous vasodilation after 14-day bed rest in humans

We investigated the effect of head-down bed rest (HDBR) for 14 days on thermoregulatory sweating and cutaneous vasodilation in humans. Fluid intake was ad libitum during HDBR. We induced whole body heating by increasing skin temperature for 1 h with a water-perfused blanket through which hot water (42 degrees C) was circulated. The experimental room was air-conditioned (27 degrees C, 30-40% relative humidity). We measured skin blood flow (chest and forearm), skin temperatures (chest, upper arm, forearm, thigh, and calf), and tympanic temperature. We also measured sweat rate by the ventilated capsule method in which the skin area for measurement was drained by dry air conditioned at 27 degrees C under similar skin temperatures in both trials. We calculated cutaneous vascular conductance (CVC) from the ratio of skin blood flow to mean blood pressure. From tympanic temperature-sweat rate and -CVC relationships, we assessed the threshold temperature and sensitivity as the slope response of variables to a given change in tympanic temperature. HDBR increased the threshold temperature for sweating by 0.31 degrees C at the chest and 0.32 degrees C at the forearm, whereas it reduced sensitivity by 40% at the chest and 31% at the forearm. HDBR increased the threshold temperature for cutaneous vasodilation, whereas it decreased sensitivity. HDBR reduced plasma volume by 11%, whereas it did not change plasma osmolarity. The increase in the threshold temperature for sweating correlated with that for cutaneous vasodilation. In conclusion, HDBR attenuated thermoregulatory sweating and cutaneous vasodilation by increasing the threshold temperature and decreasing sensitivity. HDBR increased the threshold temperature for sweating and cutaneous vasodilation by similar magnitudes, whereas it decreased their sensitivity by different magnitudes.