The effect of antidiuretic hormone on human sweating

The effect of 8-day oral taurine supplementation on thermoregulation during low-intensity exercise at fixed heat production in hot conditions of incremental humidity

PURPOSE

To determine the effect of taurine supplementation on sweating and core temperature responses, including the transition from compensable to uncompensable heat stress, during prolonged low-intensity exercise of a fixed-heat production (~ 200W/m2) in hot conditions (37.5 °C), at both fixed and incremental vapour-pressure.

METHODS

Fifteen females (n = 3) and males (n = 12; 27 ± 5 years, 78 ± 9 kg, V ˙ O2max 50.3 ± 7.8 mL/kg/min), completed a treadmill walking protocol (~ 200W/m2 heat production [Ḣprod]) in the heat (37.5 ± 0.1 °C) at fixed-(16-mmHg) and ramped-humidity (∆1.5-mmHg/5-min) following 1 week of oral taurine supplementation (50 mg/kg/bm) or placebo, in a double-blind, randomised, cross-over design. Participants were assessed for whole-body sweat loss (WBSL), local sweat rate (LSR), sweat gland activation (SGA), core temperature (Tcore), breakpoint of compensability (Pcrit) and calorimetric heat transfer components. Plasma volume and plasma taurine concentrations were established through pre- and post-trial blood samples.

RESULTS

Taurine supplementation increased WBSL by 26.6% and 5.1% (p = 0.035), LSR by 15.5% and 7.8% (p = 0.013), SGA (1 × 1 cm) by 32.2% and 29.9% (p < 0.001) and SGA (3 × 3 cm) by 22.1% and 17.1% (p = 0.015) during the fixed- and ramped-humidity exercise periods, respectively. Evaporative heat loss was enhanced by 27% (p = 0.010), heat-storage reduced by 72% (p = 0.024) and Pcrit was greater in taurine vs placebo (25.0-mmHg vs 21.7-mmHg; p = 0.002).

CONCLUSION

Taurine supplementation increased sweating responses during fixed Ḣprod in hot conditions, prior to substantial heat strain and before the breakpoint of compensability, demonstrating improved thermoregulatory capacity. The enhanced evaporative cooling and reduced heat-storage delayed the subsequent upward inflection in Tcore-represented by a greater Pcrit-and offers a potential dietary supplementation strategy to support thermoregulation.

The effect of extracellular hyperosmolality on sweat rate during metaboreflex activation in passively heated young men

Metaboreflex activation augments sweating during mild-to-moderate hyperthermia in euhydrated (isosmotic isovolemic) individuals. Recent work indicates that extracellular hyperosmolality may augment metaboreflex-mediated elevations in sympathetic nervous activity. Our primary objective was therefore to test the hypothesis that extracellular hyperosmolality would exacerbate metaboreflex-mediated increases in sweat rate. On two separate occasions, 12 young men (mean (SD): 25 (5) years) received a 90-min intravenous infusion of either 0.9% saline (isosmotic condition, ISO) or 3.0% saline (hyperosmotic condition, HYP), resulting in a post-infusion serum osmolality of 290 (3) and 301 (7) mOsm/kg, respectively. A whole-body water perfusion suit was then used to increase esophageal temperature by 0.8°C above resting. Participants then performed a metaboreflex activation protocol consisting of 90 s isometric handgrip exercise (40% of their pre-determined maximum voluntary contraction), followed by 150 s of brachial occlusion (trapping produced metabolites within the limb). Metaboreflex-induced sweating was quantified as the change in global sweat rate (from pre-isometric handgrip exercise to brachial occlusion), estimated as the surface area-weighted average of local sweat rate on the abdomen, axilla, chest, bicep, quadriceps, and calf, measured using ventilated capsules (3.8 cm²). We also explored whether this response differed between body regions. The change in global sweat rate due to metaboreflex activation was significantly greater in HYP compared to ISO (0.03 mg/min/cm² [95% confidence interval: 0.00, 0.06]; p=0.047), but was not modulated by body region (site*condition interaction: p=0.679). These findings indicate that extracellular hyperosmolality augments metaboreflex-induced increases in global sweat rate, with no evidence for region-specific differences.

Ageing attenuates the effect of extracellular hyperosmolality on whole-body heat exchange during exercise-heat stress

KEY POINTS

In humans, hypohydration attenuates sweat secretion and attenuates whole-body heat loss, probably to mitigate further fluid losses and thereby support blood pressure regulation. Recently, however, we demonstrated that the hypohydration-mediated reduction in net whole-body heat exchange (evaporative heat loss - dry heat gain) was blunted in middle-aged compared to younger men during moderate exercise in dry heat; albeit, the underpinning mechanisms could not be determined. Here we evaluated the hypothesis that those findings stemmed from a diminished influence of extracellular hyperosmolality on net whole-body heat exchange in middle-aged-to-older compared to young men. Consistent with that hypothesis, extracellular hyperosmolality induced by an intravenous infusion of hypertonic saline (3% NaCl) reduced net heat exchange and augmented rectal temperature to a greater extent in the young compared to middle-aged-to-older men. Thus, age-related differences in the influence of hypohydration on thermoregulatory function appear to be due to blunted sensitivity to hyperosmolality with ageing.

ABSTRACT

We recently demonstrated that sweating-induced hypohydration attenuated whole-body heat dissipation to a greater extent in young compared to middle-aged men during exercise-heat stress. Here, we evaluated whether this divergent response stemmed from an attenuated influence of extracellular hyperosmolality on heat exchange with ageing. To achieve this, ten young (mean (SD): 25 (5) years) and ten middle-aged-to-older (61 (5) years) men completed two trials involving a 90-min intravenous infusion of isosmotic saline (0.9% NaCl; ISO) or hyperosmotic saline (3.0% NaCl; HYP) followed by 60 min of cycling at a fixed metabolic heat production of 250 W/m² (∼50% peak aerobic power) in dry heat (40°C, ∼17% relative humidity). Whole-body net heat exchange (evaporative heat loss - dry heat gain) was measured via direct calorimetry. Rectal temperature was monitored continuously. Heat exchange was attenuated in HYP compared to ISO in the young (233 (20) vs. 251 (17) W/m² ; P = 0.002) but not older group (229 (16) vs. 227 (20) W/m² ; P = 0.621). Further, heat exchange was lower in the middle-aged-to-older vs. young men in ISO (P = 0.034) but not in HYP (P = 0.623). Similarly, end-exercise rectal temperature was greater in HYP relative to ISO in the young (38.3 (0.4)°C vs. 37.9 (0.3)°C; P = 0.015) but not the middle-aged-to-older men (38.3 (0.3)°C vs. 38.2 (0.2)°C; P = 0.652). Compared to the young, rectal temperature was greater in the middle-aged-to-older during ISO (P = 0.035) whereas no between-group difference was observed in HYP (P = 0.746). Our findings indicate that ageing blunts the effect of extracellular hyperosmolality on thermoregulatory function during exercise-heat stress.

Physiology of sweat gland function: The roles of sweating and sweat composition in human health

The purpose of this comprehensive review is to: 1) review the physiology of sweat gland function and mechanisms determining the amount and composition of sweat excreted onto the skin surface; 2) provide an overview of the well-established thermoregulatory functions and adaptive responses of the sweat gland; and 3) discuss the state of evidence for potential non-thermoregulatory roles of sweat in the maintenance and/or perturbation of human health. The role of sweating to eliminate waste products and toxicants seems to be minor compared with other avenues of excretion via the kidneys and gastrointestinal tract; as eccrine glands do not adapt to increase excretion rates either via concentrating sweat or increasing overall sweating rate. Studies suggesting a larger role of sweat glands in clearing waste products or toxicants from the body may be an artifact of methodological issues rather than evidence for selective transport. Furthermore, unlike the renal system, it seems that sweat glands do not conserve water loss or concentrate sweat fluid through vasopressin-mediated water reabsorption. Individuals with high NaCl concentrations in sweat (e.g. cystic fibrosis) have an increased risk of NaCl imbalances during prolonged periods of heavy sweating; however, sweat-induced deficiencies appear to be of minimal risk for trace minerals and vitamins. Additional research is needed to elucidate the potential role of eccrine sweating in skin hydration and microbial defense. Finally, the utility of sweat composition as a biomarker for human physiology is currently limited; as more research is needed to determine potential relations between sweat and blood solute concentrations.

Sweat tests in patients with diabetes insipidus

Increased sweat chloride concentrations have been reported in patients with nephrogenic (NDI) but not central diabetes insipidus (CDI). To determine whether false-positive sweat tests also occur in CDI, six subjects with CDI had plasma electrolytes, osmolatities, and sweat tests performed before and after water deprivation. All subjects were hyperosmolar after dehydration (287 +/- 2.0 and 296 +/- 3.2 mosm/kg, plasma osmolality before and after dehydration, respectively; p = 0.02). Sweat chloride concentrations before and after dehydration were not different, and no positive sweat tests were observed. Sweat chloride concentrations after dehydration did not correlate with the degree of dehydration as assessed by either plasma sodium or plasma osmolality. We conclude that increased concentrations of chloride in sweat are not typically found in dehydrated subjects with CDI, although they have been reported in subjects with the nephrogenic form.

Efficacy of mexiletine in the medium-term treatment of ventricular arrhythmias. A randomized, double-blind, crossover trial against placebo in ambulatory patients

A double-blind, crossover study was designed to compare the safety and efficacy of mexiletine with that of placebo in reducing premature ventricular complexes (PVC) in ambulatory patients and to find out the dose which gives a good therapeutic response with a minimal incidence of side-effects. Twenty-six patients, who had on average 427.9 PVCs/hour, were admitted to the study. The doses given were designed to reduce the frequency of PVCs by 50% or more from the baseline value. Two out of the twenty-six patients stopped treatment because of major side-effects. In the remaining twenty-four patients the 3 weeks of treatment with mexiletine significantly reduced the rate of PVCs by comparison with placebo (-63.8% versus +7.5%). In the nineteen responders (per cent reduction of PVCs over 50%) the dose of mexiletine was 600 mg daily (200 mg every 8 hours). In the non-responders plasma levels of mexiletine proved to be in the therapeutic range, not in any way different from responders. The most frequent side-effects were digestive difficulties (fifteen patients taking mexiletine and six taking placebo). These results show that mexiletine is an effective anti-arrhythmic drug in the management of ventricular arrhythmias occurring in ambulatory patients. In the majority of patients mexiletine was found to be effective even at the lowest dose studied of 600 mg/day.

Vasopressin and the regulation of evaporative water loss and body temperature in the cat

The role of vasopressin as a possible mediator of the inhibition of evaporative water loss (EWL) in dehydrated, heat-stressed cats has been examined by intravenous (i.v.) and intracerebroventricular (i.c.v.) injections of arginine vasopressin (AVP). In normally hydrated cats exposed to an ambient temperature (Ta) of 38 degrees C, neither EWL nor body temperature (Tb), measured in the hypothalamus, was significantly altered by i.v. AVP infusion. Measurements of plasma osmolality (pOsm), pAVP and cerebrospinal fluid AVP (csfAVP) were made in normally hydrated cats at Tas of 25 and 38 degrees C and after dehydration for 1-4 days at these temperatures. The relationship between pOsm and pAVP can be described equally well by either a linear model or a log-linear model (r = 0.81 for both models). The pOsm-csfAVP relationship is best described by a log-linear model (r = 0.80). A possible role for intracranially released AVP in body temperature regulation and control of EWL was examined by injecting various doses of AVP into the lateral ventricles of normally hydrated cats. No effect of AVP injection on Tb was observed at either a Ta of 23 degrees C or 38 degrees C. EWL was also unaffected by i.c.v. AVP administration at a Ta of 38 degrees C. To confirm further that intracranial AVP is not responsible for elevation of Tb and reduction of EWL during dehydration and heat-stress, specific antiserum to AVP was injected into the ventricles of dehydrated animals at a Ta of 38 degrees C. No significant effect on either Tb or EWL was measured subsequent to antiserum infusion. These negative findings indicate that AVP does not suppress EWL by either a peripheral or a central action and is therefore not responsible for lowered EWL and elevated Tb seen in dehydrated heat-stressed cats.

The effect of mexiletine on the electrophysical properties of the intact human heart

The effect of an intravenous dose of mexiletine, a new anti-arrhythmic agent, on the electrophysiological properties of the intact human heart was studied in ten subjects with no evidence of cardiovascular disease. Mexiletine produced no significant changes in the sinus node recovery time, the corrected sinus node recovery time or in the atrial, atrio-ventricular and ventricular effective refractory periods. In addition mexiletine had no significant effect on proximal atrioventricular conduction. The drug did, however, prolong atrio-ventricular conduction distal to the bundle of His in all ten subjects by an average of 6 msec. This change was statistically significant (P less than 0.01). The drug is recommended to be used with caution in patients with conduction disturbances.

A single-dose study of mexiletine (Kö 1173)

Serum concentrations of mexiletine after a single dose were determined in 8 adult Caucasian males with complex partial seizures who were continuing to receive other antiepileptic drugs. Two patients each received a single dose of either 100, 200, 300, or 400 mg mexiletine. Serum concentrations were determined by two gas chromatographic methods. Serum concentrations ranged up to 795 ng/ml. Peak concentrations occurred 1 to 3 hr after administration of the drug and were significantly different between the 100- and 300-mg, 100- and 400-mg, 200- and 300-mg, and 200- and 400-mg doses. Differences between the other doses were not significant. Serum concentrations declined monoexponentially. Half-life ranged from 2.7 to 7.2 hr. Numerous papers have appeared in the European literature on the use of mexiletine to treat cardiac arrhythmias. Preliminary studies in the United States suggest the use of mexiletine as an adjunct for therapy of epilepsy.