Days Of Heat Exposure Research Articles

Can our thermoregulatory system anticipate temperature exposure?

After daily heat exposure for approximately 5 h/day during a fixed time for more than 5 consecutive days, body core temperature of rats decreases during the period when they were previously exposed to heat. The fall in body temperature persists for a few days after terminating the timed daily heat exposure. Additionally, various thermoregulatory changes to resist heat are brought about, especially during the specific heat-exposure period. It is therefore hypothesized that the thermoregulatory system can memorize a time for heat exposure and that, in accordance with the memory, thermoregulatory responses for neutralizing heat stress are induced, even without actual heat exposure, around the period corresponding to that of the previous heat exposure.

Sweat distribution before and after repeated heat exposure.

We investigated the impact of short-term, moderate humidity heat acclimation upon sweat distribution. Eight males completed six daily heat exposures [cycling: ambient temperature 39.5 (0.2) degrees C, relative humidity 59.2 (0.8)%], during which auditory canal temperature (T(ac)) was maintained 1.4 degrees C above pre-exposure levels for 70 min by manipulating the work rate. On days 1 and 6, T(ac) and local sweat rates (m(sw): eight sites) were monitored. The pre-exposure, resting T(ac) and the T(ac) sweat threshold decreased from day 1 to day 6 [36.83 (0.05) degrees C vs 36.62 (0.05) degrees C, and 36.90 (0.05) degrees C vs 36.75 (0.05) degrees C, respectively; both P < 0.05]. However, the sweat-onset time, sweat sensitivity (delta m(sw)/deltaT(ac)) and established m(sw) were unaltered (P > 0.05). There was also no evidence of a post-acclimation redistribution in established m(sw) between the eight skin regions, though both the sweat sensitivity and established m(sw) for the forehead and hand were significantly greater than at the remaining sites (P < 0.05). It is concluded that the 5-day heat acclimation regimen provided only a minimal stimulus for sudomotor adaptation.

IS INTERMITTENT HEAT EXPOSURE AS EFFECTIVE AS DAILY HEAT EXPOSURE FOR HEAT ACCLIMATION? 563

In order to compare the effectiveness of a daily acclimation protocol to an intermittent acclimation protocol 14 competitive rowers were randomly assigned to either a consecutive acclimation (10 consecutive days) or intermittent acclimation group (10 sessions over three weeks). For every heat exposure, subjects in each group exercised for thirty minutes at 70% ˙VO2 max in an environmental chamber set at 38°C and 70% relative humidity. Acclimation state was monitored by measuring heart rate (HR), rectal and skin temperature (Trec and Tsk), ratings of perceived exertion (RPE) and whole body sweat rate (SR) during each heat exposure. Final exercise HR decreased by 5± 13 bpm and Trec decreased by 0.6 ± 0.7 °C with intermittent heat exposure but the rate of acclimation was significantly faster (p<0.05)with daily heat exposure (HR decreased by 13 ± 12 bpm and Trec decreased by 1.0 ± 0.1 °C). RPE also decreased with daily(Day 1 RPE=16 ± 2, Day 10 RPE = 11 ± 2) but did not change with intermittent heat exposure (Day 1 RPE=17 ± 2, Day 10 RPE = 16 ± 3). SR and Tsk showed no significant changes over time in either group. Although some acclimation occurs with intermittent heat exposure it appears that daily heat exposure is the most effective acclimation strategy.

Changes in hypothalamic temperature of rats after daily exposure to heat at a fixed time.

Rats were subjected to an ambient temperature (Ta) of 33 degrees C for ca. 5 h during the last half of the dark phase for 5, 14 or 28 consecutive days (heat-exposed rats, HE), while control rats were kept at a constant Ta of 24 degrees C. After the heat exposure schedule, the levels of hypothalamic temperature (T(hy)) as an index of body core temperature in the HE were significantly lower than those of the controls for 2-4 h in the last half of the dark phase. The low levels of T(hy) persisted during the specific period for 1, 3 and 6 days after the end of the 5-, 14- and 28-day heat exposure schedules, respectively. These results confirm that, in rats subjected to daily heat exposure for ca. 5 h at a fixed time per day, their T(hy) falls during the period when the rats were previously exposed to heat, and suggest that the duration of the specific T(hy) change observed after completing the heat exposure schedule depends on the length of the heat exposure schedule.

Development of arteriovenous anastomoses in the skin of the chicken and the influence of environmental temperature.

Posthatching differentiation of arteriovenous anastomoses (AVAs) and the effect of heat exposure (38 degrees C) and cold acclimation (2-3 degrees C) on AVA density were studied in naked skin areas (eyelids, comb, wattles) of chickens. The AVAs were identifiable in the newly hatched chick, but they were extremely simple. The number of cell layers in the wall of the AVAs increased from two at hatching to four to five in the 5-month-old chickens. The density of the AVAs increased significantly during posthatching maturation. Cold acclimation of the chickens increased the density of AVAs by a factor of 2.1-3.2. Daily heat exposure also increased the density of AVAs significantly in the eyelids but had no effect in the other skin areas. Heat and cold had only minor effect on the vascularity of the skin; the only significant change recorded was a slight increase in the number of blood vessels in the wattles of the cold-acclimated chickens.

Influence of repeated passive body heating on subsequent night sleep in humans.

Experiments were carried out on four healthy male subjects in two separate sessions: (a) A baseline period of two consecutive nights, one spent at thermoneutrality [operative temperature (To) = 30 degrees C, dew-point temperature (Tdp) = 7 degrees C, air velocity (Va) = 0.2 m.s-1] and the other in hot condition (To = 35 degrees C, Tdp = 7 degrees C, Va = 0.2 m.s-1). During the day, the subjects lived in their normal housing and were engaged in their usual activities. (b) An acclimation period of seven consecutive daily heat exposures from 1400 to 1700 hours (To = 44 degrees C, Tdp = 29 degrees C, Va = 0.3 m.s-1). During each night, the subjects slept in thermoneutral or in hot conditions. The sleep measurements were: EEG from two sites, EOG from both eyes, EMG and EKG. Esophageal and ten skin temperatures were recorded continuously during the night. In the nocturnal hot conditions, a sweat collection capsule recorded the sweat gland activity in the different sleep stages. Results showed that passive body heating had no significant effect on the sleep structure of subsequent nights at thermoneutrality. In contrast, during nights at To = 35 degrees C an effect of daily heat exposure was observed on sleep. During the 2nd night of the heat acclimation period, sleep was more restless and less efficient than during the baseline night. The rapid eye movement sleep duration was reduced, while the rate of transient activation phases observed in sleep stage 2 increased significantly. On the 7th night, stage 4 sleep increased (+68%) over values observed during the baseline night.(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid gland function during cross adaptation to heat and cold in man.

Plasma thyroxine (T4), triiodothyronine (T3) and thyrotropin (TSH) levels were monitored in 10 healthy euthyroid male subjects of the age group 20 to 30 years before and during heat and cold acclimatisation schedule in a sequential manner. The subjects were exposed to 45‡C DB and 30% relative humidity in a hot chamber for 2 hours daily for 8 consecutive days. Subsequently they were exposed to cold for 4 hours daily at 10‡C for 21 days. The mean plasma T4 and T3 concentration before exposure to heat were 7.87±0.82 ug/dl and 159.8±9.1 ng/dl respectively. A significant decrease in both T4 (p<0.05) and T3 (p<0.01) levels to mean values of 6.4±0.76Μg/dl and 129±7.9 ng/dl was recorded on day 4 of exposure to heat. Further significant decrease (p<0.05) over the preceding T3 levels was observed on day 8 of heat exposure. Plasma T4 and T3 on day 21 of cold exposure was not significantly different from the levels reckoned after last day of heat exposure but was significantly lower than the pre-exposure values. Throughout the thermal stress schedule there was no change in the TSH levels. These observations suggest that a decrease in thyroid hormone levels during exposure to heat might be an adaptive process which continues even during cold acclimatisation.

Secretory responses of plasma insulin, glucagon, cortisol and glucose to heat exposure in calves.

Secretory responses of plasma insulin, glucagon, cortical and glucose to heat exposure (33°C, 60% RH) were investigated using Holstein calves. Plasma gluagon concentration had a significant increase, but plasma insulin, cortisol and glucose had significant decreases within 24 h following heat exposure. Concerning subsequent responses to heat exposure, plasma glucagon level maintained increased concentrations for 8 days of heat exposure. Plasma insulin level increased on and after day 4 of heat exposure. However, plasma cortisol and glucose levels maintained declined levels throughout the heat days.

Effect of heat stress on acetate disappearance in goats.

Seven goats were used to study the effects of heat exposure (33°C, relative humidity 70%) for 4 days on the disappearance rate of injected acetate. Significant changes in several physiological responses were observed during heat exposure. Intravenous inj ection of acetate (1.25 mmole kg-1 of body weight) caused a marked increase in the peripheral acetate concentration at both 20°C and 33°C. At 20°C, plasma glucose increased and plasma free fatty acids decreased immediately after injection. On the 4th day of heat exposure, plasma glucose changed in a similar manner to that at 20°C, but plasma free fatty acids changed little. The fractional disappearance rate of blood acetate after injection was similar in the two environments. These results suggest that the potency of acetate utilization does not change during heat exposure.

Turnover and Oxidation Rates of Blood Glucose and Heat Production in Sheep Exposed to Heat

Five shorn sheep were used to study effects of heat exposure (30°C, relative humidity 70%) on metabolism of blood glucose and heat production. The isotope dilution method was applied for determination of metabolism of blood glucose in a thermoneutral environment (20°C, relative humidity 70%) and on the 4th and 10th days of heat exposure. Concentrations of free fatty acids and thyroxine of plasma decreased during heat exposure with no reduction in feed intake. At 20°C, glucose turnover rate, percentage of glucose oxidized, and contribution of exhaled carbon dioxide derived from glucose to the total carbon dioxide production were 6.2 mg/kg.75 per min, 34%, and 11%. Glucose turnover rate decreased significantly during heat exposure, but the proportion of glucose oxidized was unchanged. Heat production decreased almost parallel to the glucose turnover rate during heat exposure, resulting in an unchanged contribution of carbon dioxide derived from glucose to the total production of carbon dioxide. Blood glucose metabolism and heat production of sheep decrease, but the contribution of blood glucose to heat production remains unchanged during exposure to 30°C.

Potential secretory activity of salivary glands and electrolyte concentrations in the saliva of two rodent species during heat acclimatization

1. 1. The electrolyte concentrations and the potential secretory ability of the salivary glands were studied in Psammomys obesus (a diurnal desert species) and Rattus norvegicus during heat acclimatization. 2. 2. P. obesus had a larger potential secretory ability than the rat. A decrease in rate of secretion on days 2 and 5 was measured in P. obesus but not in the rat. 3. 3. After 2 days of heat exposure, the sodium and chloride concentration increased in R. norvegicus, whereas there was no change in the saliva of P. obesus. 4. 4. It was concluded that the desert species ( P. obesus) is able to conserve water, while in the laboratory rat, evaporative cooling has to take priority.

Recurrent heat exposure: enzymatic responses in resting and exercising men.

Heat acclimatization was induced in a group of healthy male test subjects by repetitive treadmill walking (5.6 km-h-1, 49 degrees/27 degrees C dry/wet bulb, 90 min-day-1, 7 days). A second group of men, paired for maximal O2 consumption and body weight, remained sedentary under identical environmental conditions. Total plasma protein increased significantly after 45 (P less than 0.05) and 90 (P less than 0.025) min of exercise on the first day of heat exposure, yet after 7 days no increments occurred. Even after heat acclimatization was achieved (day 7), plasma levels of creatine phosphokinase increased during the 90-min walk in the heat (time O vs. 90, P less than 0.025), as was also the case on day 1 (P less than 0.05). Levels of lactate dehydrogenase, glutamate-oxaloacetate transaminase, and glutamate-pyruvate transaminase were not significantly affected by exercise in the heat either before or after heat acclimatization. No correlations could be drawn between base-line enzyme levels and state of physical conditioning.

Seasonal difference in responses of body fluids to heat stress.

Ten male volunteers were subjected to heat stress in winter and in summer and changes in volume and composition of the body fluids were followed for about 24 hr afterwards.In summer, sodium and chloride concentration in serum and serum osmolality increased most just after sweating and returned to the control value within a day of heat exposure, while in winter they were highest three hours after sweating, and the effect of heat stress lasted until the following morning. Both in winter and in summer, the circulating blood volume tended to increase even when hemoconcentration was obvious. Plasma protein which was determined in summer experiments showed an increase after heat stress and the importance of plasma protein in maintaining the circulating blood volume during or after sweating and its contribution to so called voluntary dehydration are discussed.

The local training effect of secretory activity on the response of eccrine sweat glands.

1. The influence of repeatedly raising the body temperature by radiant heat to a level at which acclimatization to heat is normally acquired was investigated in two series of experiments, the first without the subjects sweating, the second with sweating.2. In a second investigation local sweat-gland activity was induced by drug injections on successive days without raising the body temperature.3. These experiments show that the increased sweating capacity characteristic of acclimatization to heat is a result of sweat-gland activity and does not appear to be induced by or to depend on an elevated body temperature.4. Secretory activity results in a loss of glycogen from sweat-gland cells on the first day of heat exposure but not after the glands have been ;trained' by acclimatization to heat.5. The state of acclimatization has no influence on the threshold concentration of acetylcholine required to elicit sweating when injected intradermally.

The Effect of Repeated Short Exposures to Heat on the Volume and Hydration of the Blood of the Calf

Summary 1. Three calves were individually exposed in a climatic room to a hot dry atmosphere (45° C. (113° F.) dry bulb temperature, 27.6° C. (81.7° F.) wet bulb temperature) for up to 5 hours each day over a period of 3 weeks (experimental period). Before and after this they spent 2 weeks in a neutral thermal environment (control and recovery periods). Three other calves were permanently kept as controls under normal atmospheric conditions. 2. During all three periods haematological determinations were made at 5-day intervals on all six animals. During each exposure to heat, rectal temperature, heart rate and respiration rate were recorded in the three experimental animals. 3. The results of the haematological determinations collected during the control period in all six calves are presented as “normal values” and compared with corresponding data in the literature. 4. Exposure to heat produced haematological changes which, although not of a statistically significant order, showed the following trends: on the first day of heat exposure there occurred haemoconcentration which was due mainly to an increase in the circulating cell volume and in a lesser degree to a loss of water from the plasma. In subsequent exposures haemoconcentration ceased and at the same time plasma volume increased. As the increase was greater in the plasma solids then in the plasma water, the concentration of the plasma solids remained slightly elevated throughout the experimental period. 5. With successive exposures to heat the experimental animals became acclimatised as shown by lower levels of rectal temperature, heart rate and respiration rate. 6. It is concluded that the haemoconcentration observed at the beginning of the period of heat exposures was a response to stress in general rather than to heat in particular, and that such fluid loss as did occur from the animals was insufficient to affect markedly their fluid exchange as reflected in the volume and hydration of the blood. 7. The results are discussed in the light of Selye's concept of the “general adaptation syndrome.”

Metabolic Acclimatization to Tropical Heat

Present enforced activity of large military forces in tropical heat gives sharp emphasis to our need for a clear understanding of the influences such heat exerts. Just what does acclimatization, through the weeks and months of exposure, mean in terms of mental and physiologic processes? It is indeed unfortunate that several recent reports from government-sponsored war research projects have failed to present the matter in a balanced manner, tending to give the impression that such adaptation involves nothing more than the needed vasomotor adjustment.Complete acclimatization to tropical heat has two quite distinct physiologic aspects. One of these is called into play during the first few days of heat exposure, while the other requires weeks for its appearance. When difficulty in heat dissipation is encountered by the unadapted subject, marked stress is placed upon the vasomotor mechanism for transport of waste heat from interior tissues to cutaneous surfaces.