Thermoregulation and body fluid in hot environment

Influence of acute heat mitigation strategies on core temperature, heart rate and aerobic performance in females: A systematic literature review

This review examined the effect of acute heat mitigation strategies on physiological strain and exercise performance in females exercising in the heat. Three databases were searched for original research with an acute heat mitigation (intervention) and control strategy in active females and reporting core temperature, heart rate and/or aerobic exercise performance/capacity with ≥ 24°C wet bulb globe temperature. Hedges' g effect sizes were calculated to evaluate outcomes. Thirteen studies (n = 118) were included. Most studies that applied an acute heat mitigation strategy to females did not reduce thermal (9/10) or cardiovascular (6/6) strain or improve exercise performance/capacity (8/10). The most effective strategies for attenuating thermal strain were pre-cooling with ice-slurry (effect size = -2.2 [95% CI, -3.2, -1.1]) and ice-vests (-1.9 [-2.7, -1.1]), and pre- and per-cooling with an ice-vest (-1.8 [-2.9, -0.7]). Only pre-cooling with an ice-vest improved running performance (-1.8 [-2.9, -0.7]; ~0.43 min) whilst sodium hyperhydration improved cycling capacity at 70% V O2peak (0.8 [0.0, 1.6]; ~20.1 min). There is currently limited research on acute heat mitigation strategies in females, so the evidence for the efficacy is scarce. Some studies show beneficial effects with ice-slurry, ice-vests and sodium hyperhydration, which can guide future research to support female exercise performance in the heat.

The median preoptic nucleus: front and centre for the regulation of body fluid, sodium, temperature, sleep and cardiovascular homeostasis

Located in the midline anterior wall of the third cerebral ventricle (i.e. the lamina terminalis), the median preoptic nucleus (MnPO) receives a unique set of afferent neural inputs from fore-, mid- and hindbrain. These afferent connections enable it to receive neural signals related to several important aspects of homeostasis. Included in these afferent projections are (i) neural inputs from two adjacent circumventricular organs, the subfornical organ and organum vasculosum laminae terminalis, that respond to hypertonicity, circulating angiotensin II or other humoural factors, (ii) signals from cutaneous warm and cold receptors that are relayed to MnPO, respectively, via different subnuclei in the lateral parabrachial nucleus and (iii) input from the medulla associated with baroreceptor and vagal afferents. These afferent signals reach appropriate neurones within the MnPO that enable relevant neural outputs, both excitatory and inhibitory, to be activated or inhibited. The efferent neural pathways that proceed from the MnPO terminate on (i) neuroendocrine cells in the hypothalamic supraoptic and paraventricular nuclei to regulate vasopressin release, while polysynaptic pathways from MnPO to cortical sites may drive thirst and water intake, (ii) thermoregulatory pathways to the dorsomedial hypothalamic nucleus and medullary raphé to regulate shivering, brown adipose tissue and skin vasoconstriction, (iii) parvocellular neurones in the hypothalamic paraventricular nucleus that drive autonomic pathways influencing cardiovascular function. As well, (iv) other efferent pathways from the MnPO to sites in the ventrolateral pre-optic nucleus, perifornical region of the lateral hypothalamic area and midbrain influence sleep mechanisms.

Divergent roles of plasma osmolality and the baroreflex on sweating and skin blood flow

Plasma hyperosmolality and baroreceptor unloading have been shown to independently influence the heat loss responses of sweating and cutaneous vasodilation. However, their combined effects remain unresolved. On four separate occasions, eight males were passively heated with a liquid-conditioned suit to 1.0°C above baseline core temperature during a resting isosmotic state (infusion of 0.9% NaCl saline) with (LBNP) and without (CON) application of lower-body negative pressure (−40 cmH2O) and during a hyperosmotic state (infusion of 3.0% NaCl saline) with (LBNP + HYP) and without (HYP) application of lower-body negative pressure. Forearm sweat rate (ventilated capsule) and skin blood flow (laser-Doppler), as well as core (esophageal) and mean skin temperatures, were measured continuously. Plasma osmolality increased by ∼10 mosmol/kgH2O during HYP and HYP + LBNP conditions, whereas it remained unchanged during CON and LBNP ( P ≤ 0.05). The change in mean body temperature (0.8 × core temperature + 0.2 × mean skin temperature) at the onset threshold for increases in cutaneous vascular conductance (CVC) was significantly greater during LBNP (0.56 ± 0.24°C) and HYP (0.69 ± 0.36°C) conditions compared with CON (0.28 ± 0.23°C, P ≤ 0.05). Additionally, the onset threshold for CVC during LBNP + HYP (0.88 ± 0.33°C) was significantly greater than CON and LBNP conditions ( P ≤ 0.05). In contrast, onset thresholds for sweating were not different during LBNP (0.50 ± 0.18°C) compared with CON (0.46 ± 0.26°C, P = 0.950) but were elevated ( P ≤ 0.05) similarly during HYP (0.91 ± 0.37°C) and LBNP + HYP (0.94 ± 0.40°C). Our findings show an additive effect of hyperosmolality and baroreceptor unloading on the onset threshold for increases in CVC during whole body heat stress. In contrast, the onset threshold for sweating during heat stress was only elevated by hyperosmolality with no effect of the baroreflex.

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.

Preexercise sodium loading aids fluid balance and endurance for women exercising in the heat

This study was conducted during the high-hormone phase of both natural and oral contraceptive pill (OCP)-mediated menstrual cycles to determine whether preexercise ingestion of a concentrated sodium beverage would increase plasma volume (PV), reduce physiological strain, and aid endurance of moderately trained women cycling in warm conditions. Thirteen trained cyclists [peak O2 uptake 52 ml·kg−1·min−1 (SD 2), age 26 yr (SD 6), weight 60.8 kg (SD 5)] who were oral contraceptive users ( n = 6) or not ( n = 7) completed this double-blind, crossover experiment. Cyclists ingested a concentrated-sodium (High Na+: 164 mmol Na+/l) or low-sodium (Low Na+: 10 mmol Na+/l) beverage (10 ml/kg) before cycling to exhaustion at 70% Peak O2 uptake in warm conditions (32°C, 50% relative humidity, air velocity 4.5 m/s). Beverage (∼628 ml) was ingested in seven portions across 60 min beginning 105 min before exercise, with no additional fluid given until the end of the trial. Trials were separated by one to two menstrual cycles. High Na+ increased PV (calculated from hematocrit and hemoglobin concentration) before exercise, whereas Low Na+ did not [−4.4 (SD 1.1) vs. −1.9% (SD 1.3); 95% confidence interval: for the difference 5.20, 6.92; P < 0.0001], and it involved greater time to exhaustion [98.8 (SD 25.6) vs. 78.7 (SD 24.6) min; 95% confidence interval: 13.3, 26.8; P < 0.0001]. Core temperature rose more quickly with Low Na+ [1.6°C/h (SD 0.2)] than High Na+ [1.2°C/h (SD 0.2); P = 0.04]. Plasma [AVP], [Na+] concentration, and osmolality, and urine volume, [Na+], and osmolality decreased with sodium loading ( P < 0.05) independent of pill usage. Thus preexercise ingestion of a concentrated sodium beverage increased PV, reduced thermoregulatory strain, and increased exercise capacity for women in the high-hormone phase of natural and oral contraceptive pill-mediated menstrual cycles, in warm conditions.

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.

Reduced thirst in old, thermally dehydrated rats

Water intake and blood parameters of young (7-month) and old (23-month) male Brown Norway rats were assessed following a period of thermal dehydration. Rats of both ages were randomly assigned to one of three groups: (1) Unheated-blood sample, (2) Heated-blood sample, and (3) Heated-water intake. The colonic temperature of heated rats was raised at the rate of 0.05 degrees C/min for 1 h using an infrared heat lamp. Water intake was then measured over the following 2 h. The heating protocol resulted in a similar level of dehydration in both young and old rats; however, plasma osmolality and sodium concentration increased to a significant extent only in the young rats. Old rats drank significantly less water at all time points during the 2 h following the heat stress. While neither group replaced the water lost as a result of the thermal dehydration, the young rats did rehydrate to a greater extent. These results suggest that the diminished level of rehydration in aged rats, following a thermal dehydration, is due to an attenuated rise in plasma osmolality.

Naltrexone attenuates plasma nitric oxide release following acute heat stress

Previous studies have shown that naltrexone attenuates morbidity and mortality in heat stress by inhibiting endogenous opioids. In this study, we hypothesized that naltrexone can decrease heat stress by attenuating nitric oxide release. Male Sprague-Dawley rats were pretreated with naltrexone or normal saline, and exposed to 45 degrees C for 25 min; controls were exposed to 25 degrees C. Colonic temperatures were recorded and plasma samples from an in-dwelling i.v. cannula were analyzed for nitrate/nitrite levels. Following heat stress, peak colonic temperature was significantly diminished (P < 0.05) in naltrexone-treated rats compared to saline-treated rats. Plasma nitrate/nitrite levels were significantly lower (P < 0.05) in naltrexone-treated rats compared to saline-treated rats. These findings suggest that naltrexone is able to attenuate the rise in plasma nitric oxide levels commonly observed after heat stress.

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

Heat acclimatization improves thermoregulatory responses to heat stress and decreases sweat sodium concentration ([Na(+)](sweat)). The reduced [Na(+)](sweat) results in a larger increase in plasma osmolality (P(osmol)) at a given amount of sweat output. The increase in P(osmol) inhibits thermoregulatory responses to increased body core temperature. Therefore, we hypothesized that the inhibitory effect of plasma hyperosmolality on the thermoregulatory responses to heat stress should be attenuated with the reduction of [Na(+)](sweat) due to heat acclimatization. Eleven subjects (9 male and 2 female) were passively heated by immersing their lower legs into water at 42 degrees C (room temperature 28 degrees C and relative humidity 30%) for 50 min following isotonic or hypertonic saline infusion. We determined the increase in the esophageal temperature (T(es)) required to elicit sweating and cutaneous vasodilation (CVD) (DeltaT(es) thresholds for sweating and CVD, respectively) in each condition and calculated the elevation of the T(es) thresholds per unit increase in P(osmol) as the osmotic inhibition of sweating and CVD. The osmotic shift in the DeltaT(es) thresholds for both sweating and CVD correlated linearly with [Na(+)](sweat) (r = 0.858 and r = 0.628, respectively). Thus subjects with a lower [Na(+)](sweat) showed a smaller osmotic elevation of the DeltaT(es) thresholds for sweating and CVD. These results suggest the possibility that heat acclimatization attenuates osmotic inhibition of thermoregulatory responses as well as reducing [Na(+)](sweat).