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)     

Effect of food restriction on cold adaptability of rats

To determine the role of the nutritional state in nonshivering thermogenesis during cold adaptation, cold adaptability was compared between cold-adapted (5 degrees C for 4-5 weeks) rats fed ad libitum and cold-adapted rats pair fed with warm controls having the same food intake. Cold-adapted pair-fed rats suffered a significant loss in body weight during cold exposure. However, brown adipose tissue (BAT) in both cold-adapted ad libitum fed and cold-adapted pair-fed rats was enlarged to the same extent as compared with that in control rats. Fat-free dry matter in BAT also increased in cold-adapted ad libitum fed and cold-adapted pair-fed rats to the same extent. Cold tolerance as assessed by the change in the colonic temperature at -5 degrees C was improved relative to control rats and was the same for cold-adapted ad libitum fed and cold-adapted pair-fed rats. Nonshivering thermogenesis as estimated by the noradrenaline-induced increase in oxygen consumption was significantly greater in the cold-exposed rats and there was no significant difference between cold-adapted ad libitum fed and cold-adapted pair-fed rats. These results suggest that an improved cold tolerance by means of nonshivering thermogenesis in brown adipose tissue is closely related to the low temperature itself but not the increased food intake which occurred in the cold.     

Reduction in metabolic heat production during exposure to radio-frequency radiation in the rat

The purpose of this study was to assess the ability of the rat to reduce metabolic rate when exposed to deep-penetrating radio-frequency (RF) radiation. Male Sprague-Dawley rats were maintained at an ambient temperature (Ta) of 10 degrees C and exposed to 600-MHz radiation while metabolic rate (MR) was measured by indirect calorimetry. RF radiation exposures were made in a waveguide-type system that permitted the continuous control of specific absorption rate (SAR). SAR's of 2-5 W/kg led to significant reductions in MR when averaged from 30 to 60 min after the initiation of RF radiation exposure. The total decrease in MR during RF radiation exposure accounted for approximately 37% of the total RF heat load. Exposure of another group of rats to the same SAR's at a Ta of 10 degrees C resulted in a significant elevation in colonic temperature. Thus, despite the decrease in MR, heat gain still exceeded heat loss during RF radiation exposure, with a resultant elevation in deep body temperature. In conclusion, in a cold environment the rat exposed to RF radiation decreases its MR. However, the response time and efficiency of the response is not adequate to prevent an increase in body temperature.     

Effects on fetal and maternal body temperatures of exposure of pregnant ewes to heat, cold, and exercise.

We exposed Dorper-cross ewes at approximately 120-135 days of gestation to a hot (40 degrees C, 60% relative humidity) and a cold (4 degrees C, 90% relative humidity) environment and to treadmill exercise (2.1 km/h, 5 degrees gradient) and measured fetal lamb and ewe body temperatures using previously implanted abdominal radiotelemeters. When ewes were exposed to 2 h of heat or 30 min of exercise, body temperature rose less in the fetus than in the mother, such that the difference between fetal and maternal body temperature, on average 0.6 degrees C before the thermal stress, fell significantly by 0.54 +/- 0.06 degrees C (SE, n = 8) during heat exposure and by 0.21 +/- 0.08 degrees C (n = 7) during exercise. During 6 h of maternal exposure to cold, temperature fell significantly less in the fetus than in the ewe, and the difference between fetal and maternal body temperature rose to 1.16 +/- 0.26 degrees C (n = 9). Thermoregulatory strategies used by the pregnant ewe for thermoregulation during heat or cold exposure appear to protect the fetus from changes in its thermal environment.     

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.     

Influence of increased catecholamine levels in blood plasma during cold-adaptation and intramuscular infusion on thresholds of thermoregulatory reactions in guinea-pigs

Summary Catecholamines and some of their metabolites were determined in urine and blood plasma of guinea-pigs before, during and after acclimation to a cold or warm environment. During adaptation to 5°C the amounts of noradrenaline in plasma and 24-h urine samples continuously increased up to 600% compared with values obtained at an ambient temperature of 22°C. Higher levels of dihydroxyphenylglycol and 3-methoxy-4-hydroxyphenylglycol further indicated an increased turnover of noradrenaline during cold adaptation. Acclimation to an ambient temperature of 28°C reduced the peripheral release of noradrenaline in comparison to the release observed at 22°C. Cold-induced increases in metabolic rate and electrical muscle activity both occur at a considerably lower mean body temperature in cold-than in warm-adapted guinea-pigs. The shift of thermoregulatory cold defence reactions to a lower mean body temperature could also be observed in warm-adapted animals after intramuscular infusion of noradrenaline in amounts comparable to those released during cold adaptation.It is concluded that high peripheral sympathetic activity directly or indirectly inhibits noradrenergic neurons in the lower brain stem that modulate the thermoregulatory control system by means of their afferents to the hypothalamus. As a consequence of this peripheral influence the thermoregulatory set point is shifted to a lower mean body temperature.Abbreviations A adrenaline - CA cold adapted - CNS central nervous system - DHPG dihydroxyphenylglycol - EMA electrical muscle activity - HPLC high performance liquid chromatography - 5-HT 5-hydroxytryptamine (serotonine) - MHPG 3-methoxy-4-hydroxypheyylglycol - MR metabolic rate - NA noradrenaline - T _b mean body temperature - WA warm adapted     

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.     

Pancreatic enzyme spectrum in chronic stress]

In acute experiments on white male rats the enzyme spectrum of pancreas homogenate was studied in the process of 30-day adaptation after 3-hour exposure to the muscular loading (compulsory swimming in the water at a temperature of 32 +/- 1 degree C), the heat (overheating at 40-41 degrees C) and the cold (cooling at -3-4 degrees C). It was shown that the initial periods of adaptation to the factors mentioned (2d-12th day) were characterized by a considerable decrease in the activity of all the enzymes under study. With subsequent adaptation of the animal body to these stresses the enzyme activity level recovered to the initial value (18-24th day) and remained more or less stable up to the end of the experiment (30th day). It is suggested that the alteration of the pancreatic enzyme spectrum proceeds with the participation of the hypothalamic-hypophyseal-adrenal systems as a general adaptation syndrome.     

Factors affecting cold-induced hypertension in rats

A 3- to 4-week exposure of rats to a cold environment (5 +/- 2 degrees C) induces hypertension, including elevation of systolic, diastolic, and mean blood pressures and cardiac (left ventricular) hypertrophy. The studies described here were designed to investigate some factors affecting both the magnitude and the time course for development of cold-induced hypertension. The objective of the first study was to determine whether there was an ambient temperature at which the cold-induced elevation of blood pressure did not occur. The objective of the second experiment was to determine whether body weight at the time of exposure to cold affected the magnitude and time course for development of hypertension. To assess the first objective, male rats were housed in a chamber whose temperature was maintained at 5 +/- 2 degrees C while others were housed in an identical chamber at 9 +/- 2 degrees C. After 7 days of exposure to cold, the rats exposed to the colder temperature had a significant elevation of blood pressure (140 +/- 2 mm Hg) compared with the group maintained at 9 degrees C (122 +/- 3 mm Hg). The rats exposed to 9 degrees C had no significant elevation of systolic blood pressure at either 27 or 40 days after initiation of exposure to cold. At the latter time, the temperature in the second chamber was reduced to 5 +/- 2 degrees C. By the 25th day of exposure to this ambient temperature, the rats had a significant increase in systolic blood pressure above their levels at 9 degrees C. Thus, there appears to be a threshold ambient temperature for elevation of blood pressure during exposure to cold. That temperature appears to lie somewhere between 5 and 9 degrees C. The second objective was assessed by placing rats varying in weight from approximately 250 to 430 g in air at 5 degrees C. There was a highly significant direct relationship (r = 0.96) between body weight at the time of introduction to cold and the number of days required to increase systolic blood pressure by 10 mm Hg above pre-cold exposure level. The third objective was to make an initial assessment of potential differences among strains of rats with respect to development of cold-induced hypertension. To this end, rats of the Fischer 344 strain were used. Systolic blood pressures of these rats also increased during chronic exposure to cold.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effect of cold air inhalation on core temperature in exercising subjects under heat stress

Whether increasing respiratory heat loss (RHL) during exercise under heat stress can contain elevation of rectal temperature (Tre) was examined. Eight men cycled twice at 45-50% their maximum work rate until exhaustion at ambient temperature and relative humidity of 38 degrees C and 90-95%, respectively. They inspired either cold (3.6 degrees C) or ambient air in random sequence. When subjects breathed cold air during 23 min of exercise, a ninefold increase in RHL was observed vs. similar work during hot air inhalation (32.81 vs. 3.46 W). Respiratory frequency (f) and rate of rise in Tre decreased significantly (P less than or equal to 0.004 and P less than or equal to 0.002, respectively). The rise in skin temperature in each inhalant gas condition was accompanied by a parallel almost equal increase in core temperature above basal (delta Tre) for equivalent gains in skin temperature. The increase in tidal volume and decreased f in the cold condition allowed more effective physical conditioning of cold inspirate gas in the upper airways and aided RHL. Cold air inhalation also produced a significant (P less than or equal to 0.05) decrease in heart rate vs. hot air inhalation in the final stages of exercise. Insignificant changes in O2 consumption and total body fluid loss were found. These data show that cold air inhalation during exercise diminishes elevation of Tre and suggest that both the intensity and duration of work can thus be extended. The importance of the physical exchange of heat energy and any physiological mechanisms induced by the cold inspirate in producing the changes is undetermined.     

Effects of passive heat adaptation and moderate sweatless conditioning on responses to cold and heat

Two series of experiments were performed in physically untrained subjects. In series A (heat adaptation, HA), seven male subjects were adapted to dry heat (five consecutive days at 55 degrees C ambient air temperature (Ta) for 1 h X day-1) under resting conditions. Before and after HA, the subjects' shivering responses were determined in a cold test (Ta + 10 to 0 degrees C). In series B, eight male subjects underwent mild exercise training (five consecutive days at a heart rate, HR, of 120 b X min-1) under Ta conditions individually adjusted (Ta + 15 to +5 degrees C) to prevent both sweating and cold sensations. Before and after "sweatless training", the subjects were subjected to a combined cold and heat test. During HA the thresholds for shivering, cutaneous vasodilatation (thumb and forearm) and sweating were shifted significantly (p less than 0.05) towards lower mean body temperatures (Tb). The mean decrease in threshold Tb was 0.36 degrees C. "Sweatless training" resulted in a mean increase in work rate (at HR 120 b X min-1) and oxygen pulse of 13 and 8%, respectively. However, "sweatless training" did not change the threshold Tb for shivering or sweating. Neither HA nor "sweatless training" changed the slopes of the relationships of shivering and sweating to Tb. It is concluded that the previously reported lowering of shivering and sweating threshold Tb in long-distance runners is not due to an increased fitness level, but is essentially identical with HA. The decreased shivering threshold following HA is interpreted as "cross adaptation" produced by the stressors cold and heat.     

Electrophysiological characteristics of cardiac function and the intensity of protein synthesis in cardiomyocytes during the arousal of susliks from hibernation

It has been demonstrated that during winter hibernation (body temperature 2-4 degrees C), the heart rate in ground squirrels is equal to 100 10-12 beats/min. At the initial stage of the arousal, while body temperature remains still low (9-10 degrees C), the heart rate may increase up to 160-200 beats/min. At this stage, practically all electrophysiological parameters of the heart correspond to those in active animals. These results may indicate the ability of "cold" heart in arousing ground squirrels to operate as a normothermic organ and reveal certain role of the heart in body warming. Significant increase of the intensity of protein synthesis in cardiomyocytes together with periodic changes in protein composition of their membranes were found during arousal which may account for regulation of the level of metabolism in cells and for adaptation of the latter to different temperatures.     

Interaction between exercise training and cold acclimation in rats

Five groups of 10 rats were used. Group A included sedentary rats kept at 24 degrees C, group B exercised-trained rats and group C rats exposed at -15 degrees C for 2 h every day and kept at 24 degrees C for the remaining time. These 3 groups were kept on this regimen for 10 weeks. In addition group D was acclimated to cold (2 h.d-1 at -15 degrees C) for 6 weeks and subsequently deacclimated at 24 degrees C for 4 weeks. Group E was also acclimated to cold for 6 weeks and during the deacclimation, at 24 degrees C period which lasted 4 weeks, the animals were exercised 2 h per day. Following the 10 week experimental period all animals were sacrificed and DNA and protein content of the IBAT as well as its total mass were measured. The results show significant increases in the cold adapted group. Exercise training which had no effect on brown adipose tissue IBAT at room temperature, caused an accelerated reduction in weight, DNA and protein content of the BAT in rats previously acclimated to cold. In spite of this, the thermogenic response to noradrenaline was significantly enhanced in the group which exercised during the deacclimation period. It is suggested that tissues other than IBAT may explain this enhanced heat production capacity.     

Effects of cold stress and exercise on fat loss in females

A Latin Square design has been used to test the responses of 24 relatively fit young women to 200 minute bouts of exercise performed over 5 day trials under each of three different ambient conditions: 15 degrees C (warm-warm; (WW)); -20 degrees C while inhaling, from a facemask, air heated to 18 degrees C (cold-warm; (CW)); and -20 degrees C (cold-cold; (CC)). In both of the cold environments, special clothing and boots were provided (insulation 0.47 degree C X watt-1 X m-2 and 0.62 degree C X watt-1 X m-2; (4 and 3 CLO units)). All three trials led to a small (0.6-0.7 degree C) rise of rectal temperature, but in the two cold environments mean body temperatures fell by over 1.0 degree C. A large increase of serum ketones occurred under all conditions, and the exercise respiratory quotient suggested some increase of fat utilization, WW (0.85) through CW (0.84) to CC (0.83). A fat loss of about 0.5 kg over the five days was confirmed by hydrostatic weighing and measurement of skinfold thicknesses. This was much less than the change previously observed in men, and moreover, it seemed to be independent of ambient conditions. Possible reasons why cold did not increase fat loss in these women include: a lower relative intensity of exercise; a greater stability of fat stores in women; avoidance of caffeine; a possible translocation of subcutaneous fat to deep fat depots; and a greater desire to "lose weight" irrespective of environmental conditions.     

The effect of temperature acclimation on NADP-dependent dehydrogenase activity in the carp liver and various mechanisms of its regulation

Acclimation of carp both to the temperature fall (from 20 to 5 degrees C) and rise (from 20 to 30 degrees C) induces an increase in activity of cytoplasmic liver NADPH-generating enzymes--glucose-6-phosphate dehydrogenase (G6PDG) and malic-enzyme (ME) 6-phosphogluconate dehydrogenase (6PGDG) and NADP-isocitrate dehydrogenase (NADP-IDG) activities are unchanged. Actinomycin D does not prevent cold activation of G6PDG but blocks activation of ME. "Warm" G6PDG has minimal Km value for glucose-6-phosphate and "warm" ME has minimal Km value for glucose-6-phosphate and "warm" ME has minimal Km value for malate at 25 degrees C "Cold" G6PDG and ME have the warmest Km values at 5 degrees C. Isozyme composition of cytoplasmic G6PDG (2 bands with Rf 0.16 and 0.20) does not change within the limits of 5-30 degrees C. The prolactin action on G6PDG and ME is similar to the effect of cold acclimation (activity increases Km value decreases, isozyme pattern (for G6PDG) remains unchanged). It is supposed that activation of G6PDG and ME during cold adaptation may be a result of the prolactin action on substrate-binding ability without changes in the enzyme biosynthesis and isozyme pattern.     

Effects of cold exposure on calcitonin secretion in the young rat

The effects of cold exposure on calcitonin (CT) secretion were evaluated in young rats. Acute cold exposure (5 h to 5 degrees C) induced a rise in plasma CT concentrations and a decrease in thyroidal CT stores without change in total and ionized plasma calcium levels. The cold activation of sympathico-adrenomedullary axis and the inhibition of CT response to cold after beta-antagonist treatment might suggest that endogenous catecholamines can enhance CT secretion in young rats. Cold adaptation (3 weeks to 5 degrees C) induced a fall in plasma calcium concentration and a rise in thyroidal CT stores without change in plasma CT levels. The high plasma glucocorticoid levels which are known to occur during chronic cold exposure could be involved in the rise of thyroidal CT content in cold adapted rats.     

Body temperature regulation in the rat

In loosely-restrained adult conscious rats exposed to stepwise changes in ambient temperature (T(a)) from 25 to 5 degrees C or from 20 to 35 degrees C, we have recorded body and tail temperatures, metabolic rate (VO(2)), shivering and ventilation (V). It was found that VO(2) and V vary with T(a) and show a nadir for a T(a) of 30 degrees C whereas shivering starts at 20 degrees C and increases progressively with cold exposure. T(tail) follows changes in T(a) whereas T(body) decreases slightly in cold and increases markedly in warm exposure. These results suggest that the control of T(body) interacts with the control of breathing in order to increase VO(2) during cold exposure and to facilitate evaporative respiratory heat dissipation during warm exposure.     

Comparison of heat and cold stress to assess thermoregulatory dysfunction in hypothyroid rats

How borderline impairment of thyroid function can affect thermoregulation is an important issue because of the antithyroidal properties of a many environmental toxicants. This study compared the efficacy of heat and cold stress to identify thermoregulatory deficits in rats subjected to borderline and overt hypothyroidism via subchronic exposure to propylthiouracil (PTU). After 3 wk of exposure to PTU in the drinking water (0, 2.5, 5, 10, and 25 mg/l), rats were subjected to a heat stress challenge (34 degrees C for 2.5 h). After one more week of PTU treatment, the same rats were subjected to a cold stress challenge (7 degrees C for 2.5 h). Core temperature (T(c)) was monitored by radiotelemetry. Baseline T(c) during the light phase was reduced by treatment with 25 mg/l PTU. The rate of rise and overall increase in T(c) during heat stress was attenuated by PTU doses of 10 and 25 mg/l. Cold stress resulted in a 1.0 degrees C increase in T(c) regardless of PTU treatment. The rate of rise in T(c) during the cold stress challenge was similar in all PTU treatment groups. There was a dose-related decrease in serum thyroxine (T(4)) at PTU doses >/=5 mg/l. Serum triiodothyronine (T(3)) was reduced at PTU doses of 5 and 25 mg/l. Serum thyroid-stimulating hormone (TSH) was marginally elevated by PTU treatment. Overall, heat stress was more effective than cold stress for detecting a thermoregulatory deficit in borderline (i.e., 10 mg/l PTU) and overtly hypothyroid rats (i.e., 25 mg/l PTU). A significant thermoregulatory deficit is manifested with a 78% decrease in serum T(4). A thermoregulatory deficit is more correlated with a reduction in serum T(4) compared with T(3). Serum levels of TSH are unrelated to thermoregulatory response to heat and cold stress.     

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.     

Depression of sublingual temperature by cold saliva.

Sublingual and oesophageal temperatures were compared at various air temperatures in 16 subjects. In warm air (25-44 degrees C) sublingual temperatures stabilized within plus or minus 0-45 degrees C of oesophageal temperatures, but in air at room temperature (18-24 degrees C) they were sometimes as much as 1-1 degrees C below and in cold air (5-10 degrees C) as much as 4-4 degrees C below oesophageal readings. The sublingual-oesophageal temperature difference in cold air was greatly reduced by keeping the face warm, but it was not reduced in two patients breathing through tracheostomies and thereby eliminating cold air flow from the nose and pharynx. Parotid saliva temperature was low and saliva flow high during exposure, and cold saliva seemed to be mainly responsible for the erratic depression of sublingual temperature in the cold. These results indicate hazards in the casual use of sublingual temperatures, and indicate that external heat may have to be supplied to enable them to give reliable clinical assessments of body temperature.