Physical fitness and thermoregulatory reactions in a cold environment in men

The relationship between the physical fitness level (maximal O2 consumption, VO2max) and thermoregulatory reactions was studied in 17 adult males submitted to an acute cold exposure. Standard cold tests were performed in nude subjects, lying for 2 h in a climatic chamber at three ambient air temperatures (10, 5, and 1 degrees C). The level of physical fitness conditioned the intensity of thermoregulatory reactions to cold. For all subjects, there was a direct relationship between physical fitness and 1) metabolic heat production, 2) level of mean skin temperature (Tsk), 3) level of skin conductance, and 4) level of Tsk at the onset of shivering. The predominance of thermogenic or insulative reactions depended on the intensity of the cold stress: insulative reactions were preferential at 10 degrees C, or even at 5 degrees C, whereas colder ambient temperature (1 degree C) triggered metabolic heat production abilities, which were closely related to the subject's physical fitness level. Fit subjects have more efficient thermoregulatory abilities against cold stress than unfit subjects, certainly because of an improved sensitivity of the thermoregulatory system.     

Influence of thermal and nutritional acclimatization on body temperatures and metabolic rate

1. An investigation of the influence of previous thermal and nutritional experience on body temperatures and metabolic rate has been carried out with growing piglets. Littermates were kept, from shortly after birth, at either 10 or 35 degrees C and fed either a high (H) or a low (L) energy intake. At 8 weeks of age the animals were exposed to a series of environmental temperatures of 10, 20, 27 and 35 degrees C for 1.5 hr and their rates of oxygen consumption were determined over the last 45 min. At the end of the session body temperatures were measured. 2. Rectal temperatures measured 24 hr after the start of the last meal were higher at each test temperature in piglets which had been living at 35 degrees C than in those at 10 degrees C. Also, rectal temperatures were higher in those on the H intake for animals which had been living in either the hot or the cold environment. 3. Skin temperature on the back was similar in all groups at any given test temperature although there was a tendency for those on an H intake to have the higher temperatures. Skin temperatures of the legs and ears were higher in the 10H and 10L groups than in the 35H or 35L groups at all the test environmental temperatures; energy intake had little effect. 4. Metabolic rate was greater for the animals on the H than the L intake, for those which had been living at either 10 or 35 degrees C at all the test environmental temperatures. The analysis did not reveal any significant difference related to the overall effect of living temperature, which was independent of energy intake. 5. At thermal neutrality (27 degrees C) there was a significant interaction, between energy intake and normal living temperature, on metabolic rate. Living temperature was found to modify the effect of intake: the difference between the two intakes was greater in those from the cold environment than from the hot.     

Deep interscapular temperature and thermogenesis of brown fat during sleep

Deep interscapular temperature measured just below the brown fat lobes was studied in rats during sleep at two ambient temperatures (24 degrees C and 4 degrees C) before and after adaptation (9 days) to cold (4 degrees C). The results show that in the cold ambient deep interscapular temperature decreases during desynchronized sleep independently of adaptation. Such change in temperature is probably the result of the depression in sympathetic vasoconstrictor influences on heat exchangers producing blood and brown fat cooling in sequence during this stage of sleep.     

Thermoregulatory responses of the immature rat following repeated postnatal exposures to 2,450-MHz microwaves

This study was designed to determine the changes that occur in the thermoregulatory ability of the immature rat repeatedly exposed to low-level microwave radiation. Beginning at 6-7 days of age, previously untreated rats were exposed to 2,450-MHz continuous-wave microwaves at a power density of 5 mW/cm2 for 10 days (4 h/day). Microwave and sham (control) exposures were conducted at ambient temperatures (Ta) which represent different levels of cold stress for the immature rat (ie, "exposure" Ta = 20 and 30 degrees C). Physiological tests were conducted at 5-6 and 16-17 days of age, in the absence of microwaves, to determine pre- and postexposure responses, respectively. Measurements of metabolic rate, colonic temperature, and tail skin temperature were made at "test" Ta = 25.0, 30.0, 32.5, and 35.0 degrees C. Mean growth rates were lower for rats exposed to Ta = 20 degrees C than for those exposed to Ta = 30 degrees C, but microwave exposure exerted no effect at either exposure Ta. Metabolic rates and body temperatures of all exposure groups were similar to values for untreated animals at test Ta of 32.5 degrees C and 35.0 degrees C. Colonic temperatures of rats repeatedly exposed to sham or microwave conditions at exposure Ta = 20 degrees C or to sham conditions at exposure Ta = 30 degrees C were approximately 1 degrees C below the level for untreated animals at test Ta of 25.0 degrees C and 30.0 degrees C. However, when the exposure Ta was warmer, rats exhibited a higher colonic temperature at these cold test Ta, indicating that the effectiveness of low-level microwave treatment to alter thermoregulatory responses depends on the magnitude of the cold stress.     

Fetal and maternal body temperatures measured by radiotelemetry in near-term sheep during thermal stress.

Using implanted radiotelemeters, we have measured amniotic temperature and fetal lamb and pregnant ewe body temperatures continuously over the last 34 days of gestation and during conditions of thermal stress. Body temperature of the fetus was approximately 0.6 degrees C higher than that of the mother, and the fetomaternal temperature difference remained constant over the last 25 days of gestation, until the immediate prepartum period, when it rose. During exposure to mild heat stress (35 degrees C dry-bulb temperature, 24 degrees C wet-bulb temperature), ewe and fetal body temperatures rose, but fetal temperature rose at a slower rate. Thus the fetomaternal temperature gradient fell significantly in the initial exposure period. In an environment of 4 degrees C, body temperature of the pregnant ewes fell, but the fetomaternal gradient did not change significantly. During maternal fever, heat loss from the fetus was compromised; body temperature of the fetus rose more than that of the mother, and the fetomaternal temperature gradient rose significantly. We suggest that mild heat or cold exposure in pregnant animals constitutes little risk of fetal thermal stress. During maternal fever, however, the fetus may be at risk of thermal injury.     

Cortex senses environmental temperature earlier than the hypothalamus in awake rats

Simultaneous and direct recording of temperature from the body, hypothalamus, and cortex in animals exposed to acute thermal challenges lack evidence. This study was conducted to assess the usual concept, that brain temperature is rather stable when animals are exposed to different ambient temperatures. In this study, we report the characteristic changes in the body, hypothalamic, and cortical temperature, when the rats were acutely exposed to cold (6 °C) and hot (36 °C) ambient temperature. The results of our study show that the body temperature is robustly regulated while hypothalamic and cortical temperatures vary on challenges to ambient cold (6 °C) and warm (36 °C) exposure in awake rats. The onset of response was observed quickest in the cortex, indicating that the cortical thermal sensing may relay intracranial thermal input to the hypothalamus for the regulation of body temperature within narrow limits. The present findings contradict earlier evidence, which stated that the brain does not participate in thermal sensing.     

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

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 degrees C) across Greenland. Eight subjects were submitted to cold tests (Tdb = 1 degree C, r.h. = 40%, wind speed = 0.8 m.s-1) for 2 hours. Thermoregulatory changes were also monitored in a neutral environment (Tdb = 30 degrees C). In the neutral environment, the arctic journey increased metabolic rate (11.2%; P less than 0.05) and mean skin temperature [Tsk: 33.5 (SEM 0.2) degrees C vs 32.9 (SEM 0.2) degrees C, P less than 0.05]. During the cold test, the arctic journey was associated with a lower final rectal temperature [36.8 (SEM 0.2) degrees C vs 37.3 (SEM 0.2) degrees C, P less than 0.01], a lower final Tsk [20.7 (SEM 0.4) degrees C vs 21.2 (SEM 0.3) degrees C, P less than 0.01] 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) degrees C vs 9.8 (SEM 0.9) degrees C, P less than 0.001].     

Changes in blood glucose, blood and muscle lactate,and plasma lactic dehydrogenase levels in the pigeon on acute exposure to cold.

Adult pigeons were subjected to acute cold exposure (-25 degrees C; 30 min) after which the levels of blood glucose, blood and muscle lactate and plasma lactic dehydrogenase were measured. Partially defeathered (dorsum and pectoral regions) birds, following exposure to cold, showed marked reduction in blood glucose and blood and muscle (pectoralis) lactate. Fully plumed birds, in contrast, showed no significant reduction in body temperature or blood glucose and only moderately reduced lactate levels indicating the effectiveness of the insulative feather coat in maintaining thermal and metabolic homeostasis. The partially-defeathered pigeons exposed to cold showed a two-to-three-fold increase in plasma lactic dehydrogenase activity, which may reflect a molecular adaptation in their calorigenic response to cold.     

Behavioral thermoregulation in Hemigrapsus nudus,the amphibious purple shore crab

The thermoregulatory behavior of Hemigrapsus nudus, the amphibious purple shore crab, was examined in both aquatic and aerial environments. Crabs warmed and cooled more rapidly in water than in air. Acclimation in water of 16 degrees C (summer temperatures) raised the critical thermal maximum temperature (CTMax); acclimation in water of 10 degrees C (winter temperatures) lowered the critical thermal minimum temperature (CTMin). The changes occurred in both water and air. However, these survival regimes did not reflect the thermal preferences of the animals. In water, the thermal preference of crabs acclimated to 16 degrees C was 14.6 degrees C, and they avoided water warmer than 25.5 degrees C. These values were significantly lower than those of the crabs acclimated to 10 degrees C; these animals demonstrated temperature preferences for water that was 17 degrees C, and they avoided water that was warmer than 26.9 degrees C. This temperature preference was also exhibited in air, where 10 degrees C acclimated crabs exited from under rocks at a temperature that was 3.2 degrees C higher than that at which the 16 degrees C acclimated animals responded. This behavioral pattern was possibly due to a decreased thermal tolerance of 16 degrees C acclimated crabs, related with the molting process. H. nudus was better able to survive prolonged exposure to cold temperatures than to warm temperatures, and there was a trend towards lower exit temperatures with the lower acclimation (10 degrees C) temperature. Using a complex series of behaviors, the crabs were able to precisely control body temperature independent of the medium, by shuttling between air and water. The time spent in either air or water was influenced more strongly by the temperature than by the medium. In the field, this species may experience ranges in temperatures of up to 20 degrees C; however, it is able to utilize thermal microhabitats underneath rocks to maintain its body temperature within fairly narrow limits.     

Energy metabolism and body temperature of barn owls fasting in the cold

Energetic adaptation to fasting in the cold has been investigated in a nocturnal raptor, the barn owl (Tyto alba), during winter. Metabolic rate and body temperature (Tb) were monitored in captive birds, (1) after acute exposure to different ambient temperatures (Ta), and (2) during a prolonged fast in the cold (4 degrees C), to take into account the three characteristic phases of body fuel utilization that occur during a long-term but reversible fast. In postabsorptive birds, metabolic rate in the thermoneutral zone was 4. 1+/-0.1 W kg-1 and increased linearly below a lower critical temperature of 23 degrees C. Metabolic rate was 70% above basal at +4 degrees C Ta. Wet thermal conductance was 0.22 W kg-1 degrees C-1. During fasting in the cold, the mass-specific resting metabolic rate decreased by 16% during the first day (phase I) and remained constant thereafter. The amplitude of the daily rhythm in Tb was only moderately increased during phase II, with a slight lowering (0. 6 degrees C) in minimal diurnal Tb, but rose markedly in phase III with a larger drop (1.4 degrees C) in minimal diurnal Tb. Refeeding the birds ended phase III and reversed the observed changes. These results indicate that diurnal hypothermia may be used in long-term fasting barn owls and could be triggered by a threshold of body lipid depletion, according to the shift from lipid to protein fuel metabolism occurring at the phase II/phase III transition. The high cost of regulatory thermogenesis and the limited use of hypothermia during fasting may contribute to the high mortality of barn owls during winter.     

The effect of repeated mild cold water immersions on the adaptation of the vasomotor responses

There are several types of cold adaptation based on the alteration of thermoregulatory response. It has been thought that the temperature of repeated cold exposures during the adaptation period is one of the factors affecting the type of cold adaptation developed. This study tested the hypothesis that repeated mild cold immersions would induce an insulative cold adaptation but would not alter the metabolic response. Seven healthy male participants were immersed to their xiphoid process level repeatedly in 26°C water for 60 min, 3 days every week, for 4 weeks. During the first and last exposure of this cold acclimation period, the participants underwent body immersion tests measuring their thermoregulatory responses to cold. Separately, they conducted finger immersion into 5°C water for 30 min to assess their cold-induced vasodilation (CIVD) response before and after cold acclimation. During the immersion to xiphoid process, participants showed significantly lower mean skin temperature and skin blood flow in the forearm post-acclimation, while no adaptation was observed in the metabolic response. Additionally, blunted CIVD responses were observed after cold acclimation. From these results, it was considered that the participants showed an insulative-type of cold acclimation after the repeated mild cold immersions. The major finding of this study was the acceptance of the hypothesis that repeated mild cold immersion was sufficient to induce insulative cold adaptation but did not alter the metabolic response. It is suggested that the adaptation in the thermoregulatory response is specific to the response which is repeatedly stimulated during the adaptation process.     

Hypothalamic temperature and thermogenesis of brown fat during desynchronized sleep in rats

The relationship between hypothalamic temperature and deep interscapular temperature measured just below the brown fat lobes has been studied during desynchronized sleep at two ambient temperatures (24 degrees C and 4 degrees C) before and after adaptation (9 days) to cold (4 degrees C). The results show that the increase in hypothalamic temperature during this stage of sleep occurs independently of a transfer of heat from interscapular brown fat.     

Heat debt as an index for cold adaptation in men

Several types of cold adaptation in men have been described in the literature (metabolic, insulative, hypothermic). The aim of this study is to show that the decrease of heat debt can be considered as a new index for cold adaptation. Ten male subjects were acclimated by water immersions (temperature 10-15 degrees C, 4 immersions/wk over 2 mo). Thermoregulatory responses before and after acclimation were tested by a standard cold test in a climatic chamber for 2 h at rest [dry bulb temperature (Tdb): 10 degrees C; relative humidity (rh): 25%]. After adaptation, four thermoregulatory modifications were observed: an increase in the delay for the onset of shivering (32.7 +/- 7.99 instead of 14.1 +/- 5.25 min); a decrease of body temperature levels for the onset of shivering [rectal temperature (Tre): 37.06 +/- 0.08 instead of 37.31 +/- 0.06 degrees C; mean skin temperature (Tsk): 24.83 +/- 0.56 instead of 26.86 +/- 0.46 degrees C; mean body temperature (Tb): 33.03 +/- 0.20 instead of 34.16 +/- 0.37 degrees C); a lower level of body temperatures in thermoneutrality (Tre = 37.16 +/- 0.08 instead of 37.39 +/- 0.06 degrees C; Tsk = 31.29 +/- 0.21 instead of 32.01 +/- 0.22 degrees C; Tb = 35.92 +/- 0.08 instead of 36.22 +/- 0.05 degrees C); a decrease of heat debt calculated from the difference between heat gains and heat losses (5.66 +/- 0.08 instead of 8.33 +/- 0.38 kJ/kg). The different types of cold adaptation observed are related to the physical characteristics of the subjects (percent body fat content) and the level of physical fitness (VO2max).(ABSTRACT TRUNCATED AT 250 WORDS)     

An infrared thermographic study of surface temperature in the euthermic woodchuck (Marmota monax).

Surface temperatures were measured in euthermic woodchucks (Marmota monax) using infrared thermography across a range of ambient temperatures from -10 degrees C to 32 degrees C. The woodchuck keeps surface temperature of the peripalpebral region uniformly high, while head and body surfaces change proportionally with ambient temperature. When ambient temperature was below 0 degrees C, all surface temperatures increased which prevents freezing. At no point did the animals appear to be unable to regulate heat exchange. This species appears to be especially well adapted to the higher temperatures it encounters in its range. Vasomotion in the feet and to a lesser extent in the pinnae was used to regulate heat loss. At ambient temperature of 32 degrees C, mean temperatures of nose surfaces were 0.2 degrees C and 0.3 degrees C less than ambient temperature suggesting a type of counter current cooling mechanism may be present.     

Mechanism of action of physical antipyresis in the rat

To study the mechanism of action of physical antipyresis, core temperature was measured in two groups of rats in which heat loss was increased by cold exposure and by cooling an inferior cava heat exchanger, respectively, both before and after infection with Salmonella enteritidis. Cold exposure did not influence core temperature. On the other hand, cooling the heat exchanger caused a fall in core temperature of approximately 0.7 degree C, to 37 degrees C in normothermia and to 38.5 degrees C 24 h after the infection. These lower core temperatures were then regulated against any further increase in heat loss until the thermoregulatory metabolic capacity of the animals was exhausted and a hypothermia developed. It is concluded that in infectious fever the threshold temperature of shivering increases as much as core temperature. Furthermore it is suggested that physical antipyresis, such as sponging with tepid water, induces a moderate but regulated fall in temperature to about the threshold of shivering and that its efficacy may increase with ambient temperature.     

The structure and insulation properties of the reindeer fur

The structure of the fur of the reindeer (6 adults, 4 calves) was studied with light and scanning electron microscopy and skin and rectal temperatures were measured in 216 living animals at varying ambient temperatures (-28 to +15 degrees C) and also on excised skin samples in the laboratory (temperature range -20 to +20 degrees C, wind 0 or 10 m/sec, 5 different directions). Guard hair count and length varied according to the site of excision and were on average 2000/cm2 and 12 mm on the foreleg, 1000/cm2 and 30 mm on the abdomen and 1700/cm2 and 30 mm on the back. The corresponding counts in the calves were higher but the hairs were shorter. The rectal temperatures ranged from 38 to 40 degrees C independently of the ambient temperature. The dependence of the skin temperature on the ambient temperature was complex in living animals. The dependence was strongest in the legs. The skin temperature of the excised samples depended rather linearly on the ambient temperature. It is concluded that the reindeer can maintain its body temperature also in severe cold although the extremities show characteristics of heterothermia.     

Brown fat thermogenesis in cold-acclimated rats is not abolished by the suppression of thyroid function

The effects of long-term cold exposure on brown adipose tissue (BAT) thermogenesis in hypothyroid rats have been examined. Thyroid ablation was performed in normal rats after 2 mo of exposure to 4 degrees C, when BAT hypertrophy and thermogenic activity were maximal. After ablation, hypothyroid and normal controls remained in the cold for 2 additional months. At the end of the 4-mo cold exposure, all untreated hypothyroid rats were alive, had normal body temperature, and had gained an average 12.8% more weight than normal controls. Long-term cold exposure of hypothyroid rats markedly increased BAT weight, mitochondrial proteins, uncoupling protein (UCP)-1, mRNA for UCP-1, and oxygen consumption to levels similar to those seen in cold-exposed normal rats. The results indicate that thyroid hormones are required for increased thermogenic capacity to occur as an adaptation to long-term cold exposure. However, cold adaptation can be maintained in the absence of thyroid hormone.     

Effect of different environmental temperatures on the serotonin concentration and turnover in the brain stem of a hibernator.

The serotonin (5-HT) and 5-hydroxyindoleacttic acid (5-HIAA) levels and 5-HT turnover were studies in the brain stem of warm- (+30 degrees C) and cold- (+6 degrees C) acclimated golden hamsters, exposed for 3 hours to temperatures of +6 degrees C, +30 degrees C and +37 degrees C, respectively. In war-acclimated hamsters kept under conditions the 5-HT level in the brain did not change significantly during the year. The 5-HIAA level was slightly higher in the winter. The 5-HT turnover varied within limits of 0.071 to 0.180 mug/g/hour-1. Three hours' exposure of warm-acclimated golden hamsters to cold (6 degrees C) increased the concentrations of 5-HT and 5-HIAA and the 5-HT turnover in the brain. After long-term adaptation to cold (6 degrees C) the 5-HT level, and the 5-HT turnover returned to the original level. Three hours' exposure of golden hamsters to higher environmental temperatures (warm-acclimated individuals to 37 degrees C and cold-acclimated individuals to 30 degrees C) also increased the 5-HT turnover. The concentrations of 5-HT and 5-HIAA increased in cold-acclimated golden hamsters exposed to 30 degrees C and was not changed in warm-acclimated ones, exposed to 37 degrees C. Although the elevated temperatures induce greater changes in serotonin metabolism than lowered temperatures, the serotonin pathways in the brain do not seem to be affected by short-term temperature changes specifically. The findings are rather indicative that changes in 5-HT turnover may be the primary reaction to stressful conditions.     

Fatty acid composition and extreme temperature tolerance following exposure to fluctuating temperatures in a soil arthropod

Ectotherms commonly adjust their lipid composition to ambient temperature to counteract detrimental thermal effects on lipid fluidity. However, the extent of lipid remodeling and the associated fitness consequences under continuous temperature fluctuations are not well-described. The objective of this study was to investigate the effect of repeated temperature fluctuations on fatty acid composition and thermal tolerance. We exposed the springtail Orchesella cincta to two constant temperatures of 5 and 20 °C, and a continuously fluctuating treatment between 5 and 20 °C every 2 days. Fatty acid composition differed significantly between constant low and high temperatures. As expected, animals were most cold tolerant in the low temperature treatment, while heat tolerance was highest under high temperature. Under fluctuating temperatures, fatty acid composition changed with temperature initially, but later in the experiment fatty acid composition stabilized and closely resembled that found under constant warm temperatures. Consistent with this, heat tolerance in the fluctuating temperature treatment was comparable to the constant warm treatment. Cold tolerance in the fluctuating temperature treatment was intermediate compared to animals acclimated to constant cold or warmth, despite the fact that fatty acid composition was adjusted to warm conditions. This unexpected finding suggests that in animals acclimated to fluctuating temperatures an additional underlying mechanism is involved in the cold shock response. Other aspects of homeoviscous adaptation may protect animals during extreme cold. This paper forms a next step to fully understand the functioning of ectotherms in more thermally variable environments.     

Effect of ambient temperatures ranging from cold to heat on thermoregulation in conscious MK801-treated rats

The glutamate NMDA receptor has been suggested to be involved in thermoregulation. To further analyse its role, the thermoregulatory responses of rats treated with 0.5 mg.kg-1 of dizocilpine (MK801) were compared with those of control rats treated only with the same volume of saline during a 180-min exposure at one of the six different ambient temperatures, ranging from cold to heat. Colonic temperature (Tco) and tail skin temperature (Ttail) were measured throughout using Cu-Ct thermocouples. In the cold (2.4 and 12.3 degrees C), Tco decreased either sharply (MK801) or progressively (saline), reaching the same final value (2.4 degrees C) or a lower value in the MK801-treated rats (12.3 degrees C). At the same time, Ttail decreased in both groups. In the cool environment (20.7 degrees C), Tco and Ttail decreased in both groups, with lower final values in MK801-treated rats. At thermoneutrality (28.8 degrees C), the MK801-induced hyperthermia remained steady, while Ttail increased in both groups. In the heat (34.6 and 36.2 degrees C), Tco and Ttail increased in both groups, with higher final values in MK801-treated rats. Moreover, at 36.2 degrees C, only MK801-treated rats exhibited heatstroke. It is thus suggested that MK801-induced inhibition of NMDA receptors impairs thermoregulation, especially in the heat.