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

Rats were subjected to an ambient temperature (T_a) of 33°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 T_a of 24°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.     

Regulation of TSH secretion in rats chronically exposed to heat (34°C)

Previous studies have shown that in heat exposed rats, a decreased plasma T4 concentration was associated with a normal biologically active TSH concentration. This study was designed to clarify this apparent discrepancy in the regulation of TSH secretion. In experimental rats (34°C for 25 days) and controls (25°C), plasma total T4 was 3.2 vs. 5.7×10^–8 mol/l. (P< 0.01), plasma total T3: 2.4 vs. 5.7×10^–10 mol/l. (P<0.01) and plasma TSH: bioassay 0.34 vs. 0.29 mU/ml (ns), radioimmunoassay: 1.04 vs. 0.87 g RP1/ml (ns), After TRH, plasma TSH increased identically in the two groups. In heat-exposed rats, the dialysable fraction of T4 and T3 were increased: 0.032 vs. 0.020% (P<0.05) and 0.102 vs. 0.086% (P<0.05), respectively; accordingly, free T4 concentration was normal and that of free T3 was low; total plasma proteins were slightly increased.It is concluded that in heat-exposed rats: (1) plasma thyroid hormone binding activity was decreased as shown by the association: decreased plasma total T4-elevated free T4 fraction. The normality of the free T4 concentration accounted for the normal plasma TSH. (2) the combination of normal plasma TSH, normal plasma free T4, low plasma free T3 concentrations would suggest that T4 is predominantly involved in the regulation of TSH secretion.Part of this work was presented at the Vth International Congress of Endocrinology, Hamburg, July 1976.     

Somatostatin: a hypothalamic transmitter for thermoregulation in rats

The changes in both the thermoregulatory responses and brain somatostatin (SS) levels produced by ambient temperature (T _a) changes were assessed in rats after they had been equilibrated to each of theT _a for a period of about 90 min. Cold exposure, in addition to elevating hypothalamic SS-levels, led to increased metabolism and cutaneous vasoconstriction atT _a=8° C. In contrast, heat exposure, in addition to lowering hypothalamic SS-levels, resulted in decreased metabolism and cutaneous vasodilation atT _a=30° C. Rats were chronically implanted with a hypothalamic cannula to allow intrahypothalamic injection of SS on the conscious rats. Direct administration of SS (0.1–0.3 g) into the preoptic anterior hypothalamic area caused a dose-related rise in colon temperature at threeT _a tested. The SS-induced hyperthermia was produced by increased metabolism atT _a=8° C, whereas atT _a=30° C, it was caused by cutaneous vasoconstriction. AtT _a=22° C, the hyperthermia was caused by increased metabolism and cutaneous vasoconstriction. Systemic administration of cysteamine, in addition to lowering hypothalamic SS-levels, produced a dose-related fall in colon temperature atT _a of 8°C and 22°C. The hypothermia induced by cysteamine was produced by decreased metabolism atT _a=8° C, whereas atT _a=22° C, it was caused by both decreased metabolism and cutaneous vasodilation. The data indicate that the hypothalamic SS-levels mediate normal body temperature responses in rats.     

Does feeding regime affect physiologic and thermal responses during exposure to 8, 20, and 27° C?

Summary When the loss of body heat is accelerated by exposure to low environmental temperatures, additional substrates must be oxidized to provide energy to sustain temperature homeostasis. Therefore, the present investigation examined the relation between feeding regime [pre-experimental carbohydrate feeding (FED) vs a fast (FAST)], during 120 min of exposure to 8, 20, and 27° C in well-nourished men. The following were examined: tissue insulation (I; °C · m² · W^–1), rectal temperature (T _re; °C), and oxygen consumption ( O₂; ml · kg^–1 · min^–1). O₂, T _re, and I revealed no significant differences between treatments (FED vs FAST) at any temperature. At 27° C, I was less (P < 0.05) than at 20 and 8° C, and decreased (P < 0.05) as exposure time increased. At 8° C, O₂was higher (P < 0.5) than at 20 or 27°C, and O₂increased as time increased (P < 0.05). T _re decreased (P < 0.05) as time increased for all conditions. Respiratory exchange ratio (R) differed (P < 0.05) between treatments (FED vs FAST), temperature (8 vs 20° C), and across time. Values for R suggests that carbohydrate accounted for 56% and 33% of caloric utilization during the FED vs FAST conditions, respectively. At 8 vs 20° C, R represented 54% vs 30% of cabohydrate utilization. Across time, R demonstrated that in both conditions (FED vs FAST) there was a decreased reliance on carbohydrate utilization for energy provision. From these data it appears that while substrate utilization differed between dietary treatment and across time this did not differentially affect O₂or T _re during protracted exposure to 8, 20, and 27° C. The higher R in the 8° C condition for both dietary treatments demonstrates that carbohydrate utilization is increased in shivering cold-exposed humans. However, the reduction in R across time suggests that fat oxidation is also involved in metabolic heat production and core temperature maintenance during shivering in the cold.     

The thermogenic effect of a carbohydrate feeding during exposure to 8,12 and 27° C

The increased metabolic heat production in humans exposed to cold stress results from an increased oxidation of both carbohydrate and fat to provide energy to sustain temperature homeostasis. Research suggests that dietary manipulations may enhance metabolic heat production, thereby delaying hypothermia. Therefore, the present investigation examined the thermogenic effect of a sequential timed feeding regime of either a carbohydrate (CHO) or a placebo beverage (PL) before and again midway through 120 min of exposure to 8, 12 and 27° C in well-nourished men. The following were examined: tissue insulation (I), rectal temperature (T _re), mean skin temperature , metabolism (M), time-weighted heat production and respiratory exchange ratio (R).T _re, T _re, M, M,I and time-weighted heat production revealed no significant differences between treatment (PL vs CHO) at any temperature (8, 12 and 27° C). However,T _re decreased (P < 0.05) as time increased at 8, 12 and 27° C while M increased (P < 0.05) andI decreased (P < 0.05) at 8 and 12° C. At 8 and 27° C,R differed (P < 0.05) between the PL and CHO treatments. In addition, at 8 and 12° C,R increased (P < 0.05) across time reflecting the feeding. From these data it appears that while substrate utilization differed between dietary treatment (8 and 27° C) and across time, this feeding regime did not differentially affectT,T _re, andI during 120 min of exposure to 8, 12 and 27° C.     

Evaluation of a temperate-environment test of heat tolerance in prior heatstroke patients and controls

Summary It has been reported that scores from a temperate-environment step test describe the heat-tolerance status of prior heatstroke patients (HP). This investigation evaluated the ability of this temperate-environment heat-tolerance test (HTT) to indicate altered heart rate (HR) and rectal temperature (T _re )responses of HP, after 7 days of heat acclimation. On day 1, ten male HP (61 ± 7 days post-heatstroke) and five control subjects (C) bench-stepped (0.30 m high, 27 steps · min^–1) for 15 min (25.8° C dry bulb, 16.2° C wet bulb). On days 2–8, subjects underwent heat acclimation (40.1° C dry bulb, 23.8°C wet bulb; treadmill, 90 min · day^–1). Heat acclimation resulted in significant decreases in final HR (152±5 vs 130±3 beats·min^–1 P<0.025) and finalT _re (38.62±0.11 vs 38.13±0.07°C,p < 0.01) in HP. One HP but no C was defined heat intolerant, exhibiting inability to adapt to daily exercise in the heat. On day 9, HP repeated HTT, exactly as performed on day 1; mean group HTT scores did not change (day 1=39±6; day 9=48±6,P>0.05). All physical characteristics and physiological responses of HP (days 1, 2, 7, 9) were statistically similar (P>0.05) to those of C. In contrast to heat-acclimation data, HTT scores (score 30) indicated that four HP were heat intolerant on day 1 and two HP were heat intolerant on day 9. It was concluded that HTT was not a substitute for lengthier tests of heat tolerance conducted in hot environments, because HTT scores (at 25.8°C did not reflect HR andT _re responses (at 40.1° C) in 33% of heat-acclimated (e.g., heat-tolerant) HP. In addition, HTT scores did not validly discriminate between heat tolerant and heat-intolerant HP.     

The M. omohyoideus of the mouse as a convenient mammalian muscle preparation

Summary Muscles from cats, rats, guinea pigs and mice have been investigated as preparations for visualizing mammalian neuromuscular junctions with the aid of Nomarski interference optics. The M. omohyoideus of the mouse was found to be most convenient. Electrophysiological investigations showed that an endplate is normally surrounded by a population of perijunctional receptors. For junctional receptors in the endplate, a Hill coefficient ofn _H=2.6 for acetylcholine was determined at 38°C, decreasing to a value of 2.3 at room temperature. For both perijunctional and extrajunctional receptors (the latter occurring after denervation), the coefficientn _H was 1.9. Noise analysis revealed a channel conductance which changed abruptly from 22.4±1.0 pS (10–23°C) to 45.6±0.9 pS (34–39°C) in a very small temperature range around 25.5°C. The mean channel lifetime was 0.3 ms at 39°C and 1.0 ms at 23°C.Supported by the Deutsche Forschungsgemeinschaft, SFB 38, N.     

Influence of the menstrual cycle on the sweating response measured by direct calorimetry in women exposed to warm environmental conditions

Summary The whole body sweating response was measured at rest in eight women during the follicular (F) and the luteal (L) phases of the menstrual cycle. Subjects were exposed for 30-min to neutral (N) environmental conditions [ambient temperature (T _a) 28°C] and then for 90-min to warm (W) environmental conditions (Ta, 35°C) in a direct calorimeter. At the end of the N exposure, tympanic temperature (T _ty) was 0.18 (SEM 0.06)°C higher in the L than in the F phase (P<0.05), whereas mean skin temperature ( ) was unchanged. During W exposure, the time to the onset of sweating as well as the concomitant increase in body heat content were similar in both phases. At the onset of sweating, the tympanic threshold temperature (T _{ty, thresh}) was higher in the L phase [37.18 (SEM 0.08)°C] than in the F phase [36.95 (SEM 0.07)°C;P<0.01]. The magnitude of the shift inT _ty, thresh [0.23 (SEM 0.07)°C] was similar to the L-F difference inT _ty observed at the end of the N exposure. The mean skin threshold temperature was not statistically different between the two phases. The slope of the relationship between sweating rate andT _ty was similar in F and L. It was concluded that the internal set point temperature of resting women exposed to warm environmental conditions shifted to a higher value during the L phase compared to the F phase of the menstrual cycle; and that the magnitude of the shift corresponded to the difference in internal temperature observed in neutral environmental conditions between the two phases.     

Brain and core temperatures and peripheral vasomotion during sleep and wakefulness at various ambient temperatures in the rat

Changes in brain, core and tail skin temperatures (T _br, T _c and T _t) associated with transitions in the arousal states were recorded in rats throughout the 24-h diurnal cycle at 10 °C, 21 °C and 29 °C. Falling asleep was accompanied by decreases in both T _br and T _c and vasodilation at 10 C and 21 °C. At 29 °C, tail vessels were permanently dilated, and further dilation was not found on sleep onset. T _br and T _c, however, continued to decrease during non-rapid-eye-movement sleep (NREMS); these changes are likely to result from reductions in heat production and increased conductive heat loss. The changes in T _br, T _c and T _t on awakening mirrored those on falling asleep. It is suggested that the suppression of sleep in the cold and the enhancement of NREMS in the heat promote thermoregulation. Rapid-eye-movement sleep (REMS) was associated with sharp rises in T _br. The rise in T _br was the largest in the cold and was attenuated at 29 °C. T _c decreased and T _t increased in the cold, whereas T _c tended to increase and T _t to decrease in the heat. The paradoxical peripheral vasomotion during REMS supports previous suggestions on severe thermoregulatory impairment during REMS in other species.     

Body temperature and adrenal function in heat-exposed hypothalamic disconnected rats

Summary The effects of hypothalamic disconnection on body temperature and hypothalamo-pituitary-adrenal activity following acute and repeated exposures to heat were studied. Intact male rats, or animals with complete posterior or anterior hypothalamic disconnection, were exposed to a temperature of 36°C and a relative humidity of 35–45%. In the complete posterior and anterior hypothalamic disconnected rats the basalT _re was higher than that of the intact rats; the rise inT _re following heat exposure was lower in the operated rats than in the intact animals. All the experimental animals, except for those with anterior hypothalamic disconnection, showed a significant inhibition of corticosterone release on exposure to heat for 30 min, but no inhibition was observed in any of the disconnected rats when they were exposed to heat for 120 min. These results suggest that the main stimulus for ACTH release, during the first 30 min of heat exposure, is mediated by a neural input through the posterior hypothalamus and this is followed by a neural and/or humoral mechanism which enables the animals to increase their corticosterone secretion.     

Thermoregulatory responses of young and older men to cold exposure

Summary Nine young (20–25 years) and ten older (60–71 years) men, matched for body fatness and surface area :mass ratio, underwent cold tests in summer and winter. The cold tests consisted of a 60-min exposure, wearing only swimming trunks, to an air temperature of 17°C (both seasons) and 12°C (winter only). Rectal (T _re) and mean skin ( _sk) temperatures, metabolic heat production (M), systolic (BP_S) and diastolic (BP_d) blood pressures and heart rate (f _c) were measured. During the equilibrium period (28°C air temperature) there were no age-related differences inT _re, _sk, BP_S, BP_d, orf _c regardless of season, although M of the older men was significantly lower (P<0.003). The decrease inT _re and _sk (due to the marked decrease in six of the older men) and the increase in BPS and BPd were significantly greater (P<0.004) for the older men during all the cold exposures. The rate of increase inM was significantly greater (P<0.01) for the older group when exposed to 12°C in winter and 17°C in summer (due to the marked increase in four of the older men). This trend was not apparent during the 17°C exposure in winter. There was no age-related difference in f_c during the exposures. Significant decreases inT _re and _sk and increases inM, BP_S and BP_d during the 12°C exposure were observed for the older group (P< 0.003) compared to their responses during the 17°C exposure in winter. In contrast,T _re,M, BP_S in the young group were not affected as much by the colder environment. It was concluded that older men have more variable responses and some appear more or less responsive to mild and moderate cold air than young men.     

Effects of brief leg cooling after moderate exercise on cardiorespiratory responses to subsequent exercise in the heat

We investigated the effects of brief leg cooling after moderate exercise on the cardiorespiratory responses to subsequent exercise in the heat. Following 40 min of ergometer cycling [65% peak oxygen uptake (O_2peak)] at 35°C (Ex. 1), seven male subjects [21.9 (1.1) years of age; 170.9 (1.9) cm height; 66.0 (2.0) kg body mass; 46.7 (2.0) ml kg^–1 min^–1 O_2peak] immersed their legs in 35°C (control condition, CONT) or 20°C (cooling condition, COOL) water for 5 min and then repeated the cycling (as before, but for 10 min) (Ex. 2). Just before Ex. 2, esophageal temperature (T _es) was lower in COOL than in CONT [36.9 (0.2) vs 37.5 (0.1)°C] (P<0.01), as also were both mean skin temperature [33.9 (0.2) vs 35.2 (0.2)°C] (P<0.01), and heart rate (HR) [93.2 (6.0) vs 102.7 (4.9) beats min^–1] (P<0.05). During Ex. 2, no differences between CONT and COOL were observed in oxygen uptake, arterial blood pressure, blood lactate concentration, or ratings of perceived exertion; however, T _es, skin temperature, and HR were lower in COOL than in CONT. Further, during the first 5 min of Ex. 2, minute ventilation was significantly lower in COOL than in CONT [50.3 (2.0) vs 53.4 (2.6) l min^–1] (P<0.01). These results suggest that brief leg cooling during the recovery period may be effective at reducing thermal and cardiorespiratory strain during subsequent exercise in the heat.     

Thermoregulation during heat exposure of young children compared to their mothers

The study was conducted to investigate the thermoregulation of young children compared to that of adults. A group of 19 children (ages 9 months-4.5 years), with only 3 children aged 3 years or above, and 16 adults first rested in a thermoneutral room (air temperature 25°C relative humidity 50%, air velocity 0.2 m·s^–1). They were then exposed to a hot room (air temperature 35°C, relative humidity 70%, air velocity 0.3 m·s^–) next door for 30 min, and then returned to the thermoneutral room where they stayed for a further 30 min. The rectal temperature (T _re), skin temperatures (T _sk) at seven sites, heart rate (HR), total sweat rate ( ), local sweat rate ( ) and the Na⁺ concentration of the sweat were measured. There was no significant difference inT _re between the children and their mothers in the rest phase. However, theT _re of the children increased as soon as they entered the hot room and was significantly higher than during the control period, and than that of the mothers during heat exposure. MeanT _sk, forehead, abdomen and instepT _sk were significantly higher in the children during both the thermoneutral and heat exposure. The was significantly higher and Na⁺ concentrations in the sweat on the back and upperarm were significantly lower for the children during the heat exposure. They had a greater body surface area-to-mass ratio than the mothers by 64%, which indicated that they had advantages for thermal regulation. However, the sweating andT _sk responses of the children were not enough to prevent a rise in body temperature. These results would suggest that the young children had the disadvantage of heating up easily due to their smaller body sizes and there may be maturation-related differences in thermoregulation during the heat exposure between young children and mothers.     

Influence of ambient temperature on calorigenic action of thyroid hormones in young mice

In 5- and 7-day-old HAICR mice the calorigenic action of thyroxine (T₄) and triiodothyronine (T₃) was investigated at different ambient temperatures. After treating animals with 5 g T₄ or 1 g T₃ per g body weight and day, their oxygen consumption was compared with that of litter mate controls at 23°, 27°, 31° and 35°C. The hormone action was highly dependent on ambient temperature, 1 g T₃ being most effective. A thyroid hormone effect could never be observed at 31°C, the animals' nest temperature. By application of propranolol, which almost selectively blocks nonshivering thermogenesis (NST), we could show that T₃ increases minimal oxygen consumption (MOC) as well as the capacity for NST. In addition, it could be shown that also oxygen consumption of liver tissue of 5-day-old mice is elevated by preceding treatment with thyroid hormones. The ambient temperature effects are discussed in relation to the thermoregulatory situation of the mice during the early postnatal period.     

Effect of photoperiod and acclimation temperature on nonshivering thermogenesis and GDP-binding of brown fat mitochondria in the Djungarian hamsterPhodopus s. sungorus

Acclimation to short photoperiod at 23° C constantT _a causedP. sungorus to improve their NST capacity from 752 to 1,082 mW. Chronic cold exposure in short photoperiod further enhanced the NST capacity, reaching a maximum level of 1,573 mW at –5° C acclimation temperature. Improvements in NST capacity were always accompanied by an increase in brown fat mitochondrial mass and GDP-binding of brown fat mitochondria, in proportion with the cold load applied during temperature acclimation (23°, 15°, 5°, –5° C). Brown fat mitochondrial protein increased from 7.41 mg (23° CT _a, long photoperiod) through 21.6 mg (23° CT _a, short photoperiod) and 81.6 mg (–5° CT _a, short photoperiod). This 10-fold increase was accompanied by a 35-fold increase in GDP-binding (2.0, 7.3 and 71.6 nmol GDP bound, respectively), demonstrating that the increase in capacity for uncoupled respiration in brown fat is of primary significance for thermogenic acclimation to cold as well as to short photoperiod.Abbreviations Cox cytochrome c oxidase - GDP guanosine-5-diphosphate - NST nonshivering thermogenesis - K-Tes potassium salt of N-tris-(hydroxymethyl)-methyl-2-amino-ethane-sulfonic-acid - T _a ambient temperatures - U units of enzyme activity (mol min^–1 at 25° C     

General and local cold adaptation after a ski journey in a severe arctic environment

Summary It is hypothesized that some of the variability in the conclusions of several human cold adaptation studies could be explained if not only were the changes in core and shell temperatures taken into account, before and after cold adaptation, but also the absolute temperatures and metabolic rate in both thermally neutral environments and in the cold. Such an approach was used in a group of volunteers before and after a ski journey (3 weeks at –20 to –30°C) across Greenland. Eight subjects were submitted to cold tests (T _db = 1°C, r.h. = 40%, wind speed = 0.8 m · s^–1) for 2 hours. Thermoregulatory changes were also monitored in a neutral environment (T _db = 30°C). In the neutral environment, the arctic journey increased metabolic rate (11.2%; P<0.05) and mean skin temperature [ : 33.5 (SEM 0.2)°C vs 32.9 (SEM 0.2)° C, P<0.05]. During the cold test, the arctic journey was associated with a lower final rectal temperature [36.8 (SEM 0.2)°C vs 37.3 (SEM 0.2)° C,P<0.01], a lower final [20.7 (SEM 0.4)° C vs 21.2 (SEM 0.3)° C,P<0.011 with no change in metabolic heat production. These observations are indicative of an hypothermic insulative isometabolic general cold adaptation, which was associated with a local cold adaptation of the extremities, as shown by warmer foot temperatures [12.3 (SEM 0.9)°C vs 9.8 (SEM 0.9)°C,P<0.001].     

Control of heat-induced cutaneous vasodilatation in relation to age

Summary Well matched unacclimatised older (age 55–68, 4 women, 2 men) and younger (age 19–30, 4 women, 2 men) subjects performed 75 min cycle exercise (40% ) in a hot environment (37°C, 60% rh). Rectal temperature (T _re), mean skin temperature (¯T _sk), arm blood flow (ABF, strain gauge plethysmography), and cardiac output (Q, CO₂ rebreathing) were measured to examine age-related differences in heat-induced vasodilatation.T _re and¯T _sk rose to the same extent in each group during the exposure. There was no significant intergroup difference in sweat rate (older: 332±43 ml · m^–2 · h^–1, younger: 435±49 ml · m^–2 · h^–1; mean±SEM). However, the older subjects responded to exercise in the heat with a lower ABF response which could be attributed to a lower for the same exercise intensity. The slope of the ABF-T _re relationship was attenuated in the older subjects (9.3±1.3 vs 17.9±3.3 ml · 100 ml^–1 · min^–1 · °C^–1,p <0.05), but theT _re threshold for vasodilatation was about 37.0°C for both groups. These results suggest an altered control of skin vasodilatation during exercise in the heat in older individuals. This attenuated ABF response appears to be unrelated to , and may reflect an age-related change in thermoregulatory cardiovascular function.     

Differential acute-phase responses in febrile and cold- and heat-exposed rabbits

Endotoxin (LPS) characteristically provokes both fever and the acute-phase reaction (APR). The present studies were undertaken to determine whether these two responses are mutually dependent or controlled separately. Rabbits were injected with LPS (2 g/kg, IV) or exposed to heat (37±1° C) such that the patterns and magnitudes of the body temperature (T_co) rises (1.6±0.5° C) were similar. While the plasma levels of Fe and Zn decreased, and those of Cu and N-acetylneuraminic acid (NANA) increased significantly after LPS-induced fever, no such changes occurred after heat-induced hyperthermia. In a subsequent study, different rabbits were exposed to cold (3±1° C), first furred, then sheared. While furred, they thermoregulated by cutaneous vasoconstriction, whereas, while shaved, they additionally increased their metabolic rates. However, in neither case were the plasma levels of Fe, Zn, Cu, or NANA different in comparison to their pre-cooling values (T_a=22±1° C). It would seem, therefore, that the acute-phase blood responses to LPS (and ultimately, to endogenous pyrogen [EP]) probably are not mediated merely by the rise in the T_co or by the concomitantly induced vascular and/or thermogenic effectors of fever; but rather, the APR appears to be a regulated, separate response to LPS/EP.     

Comparison of two cool vests on heat-strain reduction while wearing a firefighting ensemble

This study evaluated the effectiveness of a six-pack versus a four-pack cool vest in reducing heat strain in men dressed in firefighting ensemble, while resting and exercising in a warm/humid environment [34.4°C (day bulb), 28.9°C (wet bulb)]. Male volunteers (n = 12) were monitored for rectal temperature (T _re), mean skin temperature (T _sk), heart rate, and energy expenditure during three test trials: control (no cool vest), four-pack vest, and six-pack vest. The cool vests were worn under the firefighting ensemble and over Navy dungarees. The protocol consisted of two cycles of 30 min seated rest and 30 min walking on a motorized treadmill (1.12 m · s^–1, 0% grade). Tolerance time for the control trial (93 min) was significantly less than both vest trials (120 min). Throughout heat exposure, energy expenditure varied during rest and exercise, but no differences existed among all trials (P > 0.05). During the first 60 min of heat exposure, physiological responses were similar for the four-pack and six-pack vests. However, during the second 60 min of heat exposure the six-pack vest had a greater impact on reducing heat strain than the four-pack vest. PeakT _e andT _sk at the end of heat exposure for 6-pack vest [mean (SD) 38.0(0.3)°C and 36.8(0.7)°C] were significantly lower compared to four-pack [38.6 (0.4)°C and 38.1(0.5)°C] and controls [38.9(0.5)°C and 38.4(0.5)°C]. Our findings suggest that the six-pack vest is more effective than the four-pack vest at reducing heat strain and improves performance of personnel wearing a firefighting ensemble.     

The influences of thermal stress on serum biochemical parameters of Iranian fat-tailed sheep and their correlation with triiodothyronine (T3), thyroxine (T4) and cortisol concentrations

In order to study the influences of thermal stress on serum biochemical parameters of Iranian fat-tailed sheep and their correlation with triiodothyronine (T₃), thyroxine (T₄) and cortisol concentrations, an experiment was conducted using 45 clinically healthy Iranian fat-tailed sheep. These animals were divided into three experimental groups: group I, 15 sheep at 4 °C, group II 15 sheep at 21 °C and group III 15 sheep at 40 °C. Blood samples were taken from the jugular vein of all animals after 7 days exposure to the appropriate environmental temperature. The concentrations of total protein, glucose, cholesterol, total lipid, calcium, inorganic phosphorus, magnesium, creatine kinase, lactate dehydrogenase, triiodothyronine and thyroxine in cold stress conditions were higher than in heat stress; in contrast, the activities of aspartate aminotransferase and alanine aminotransferase were higher in heat stress conditions than in cold (p = <0.05). Although there were no significant differences in the concentrations of blood urea nitrogen, triglyceride, alkaline phosphatase and cortisol at either heat stress or cold stress, the first three assays were significantly lower than in animals kept at optimum temperatures, with cortisol having significantly higher values. These results revealed that very hot and very cold conditions had a profound effect on serum biochemical parameters.Abbreviations ALT alanine aminotransferase - AST aspartate aminotransferase - BUN blood urea nitrogen - LD lactate dehydrogenase - CK creatine kinase - ALP alkaline phosphatase