Thermosensitivity of preoptic neurones in a hibernator at high and low ambient temperatures

Summary The effect of ambient temperature on the thermosensitivity of preoptic neurones was studied in euthermic golden hamsters. At skin temperatures (T_sk) of 20°C, preoptic units were still responsive to hypothalamic temperatures (T_hy) below 10°C, while at T_sk=36°C these neurones became inactive at T_hy=15°C on the average. These studies suggest that thermoreceptive preoptic neurones, influenced by a high activity of cutaneous cold-receptors, are capable of sensing core temperatures even in deep hibernation.Supported by the Deutsche Forschungsgemeinschaft, Wu 63/2     

Thermosensitivity of preoptic neurones in a hibernator (golden hamster) and a non-hibernator (guinea pig)

Summary Thermosensitivity of preoptic units was studied at hypothalamic temperatures (T _hy) ranging from 8–43°C in golden hamsters in a non-hibernating state as well as in guinea pigs. In golden hamsters 2 types of thermoresponsive preoptic neurones were found: 1. Neurones sensitive toT _hy ranging from 10–42°C with an exponential characteristic and very high spontaneous firing rates (29–59 imp/s) atT _hy 36–37°C. 2. Neurones with a bell-shaped temperature-firing rate characteristic, a negative temperature coefficient atT _hy 40–30°C, a maximal activity atT _hy 20–30°C and a positive temperature coefficient (+0.8 to +4 imp/s·°C) even atT _hy close to 10°C. In guinea pigs thermoresponsive preoptic units became inactive or insensitive to thermal stimulation as soon asT _hy fell below 30°C. These results suggest that in hibernators central nervous structures involved in temperature regulation are adapted to maintain their function over the wide range of core temperatures which occur during the different phases of hibernation.Supported by the Deutsche Forschungsgemeinschaft, Project B1. SFB 122.     

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.     

Skin AVA and capillary dilatation and constriction induced by local skin heating

In conscious sheep, total femoral blood flow and flow through arteriovenous anastomoses (AVAs) and capillaries (CAP) in skin of the hindleg were measured employing electromagnetic and radioactive microsphere techniques. Core temperature (T _c) was manipulated using intravascular heat exchangers and hindleg skin temperature (T _sk) was manipulated by immersion in temperature controlled water. WithT _c set 1°C above normal, AVA flow was highest at the lowestT _sk tested (34°C); AVAs progressively constricted asT _sk was increased from 34 to 40–41°C, then dilated again asT _sk reached the highest levels tested (42–44°C). Skin CAP flow was not altered byT _sk of 34 to 42°C but was increased at aT _sk of 44°C. Therefore total skin blood flow followed essentially the same pattern as AVA flow; total femoral flow also followed this pattern. WhenT _c was set 0.5°C below normal, AVA flow was low at all levels ofT _sk. It is concluded thatT _c plays a dominant role in control of skin blood flow, however, onceT _c is at a level requiring increased heat loss,T _sk exerts an extremely potent influence on the nature and magnitude of changes in skin blood flow. The pattern of flow changes appears to reflect principally a negative feedback mechanism aimed at maintainingT _sk at approximately 40°C; this may contrast with mechanisms associated with sweating and/or active vasodilation in other species.     

Appropriate and inappropriate hypothalamic cold thermosensitivity in Willow Ptarmigan

Hypothalamic thermosensitivity has been investigated in conscious Willow Ptarmigan (Lagopus lagopus lagopus) provided with chronically implanted hypothalamic perfusion thermodes. The birds were exposed to either cold (T_a-10°C) or warm (T_a+ 25°C) ambient conditions while hypothalamic temperature (T_hy) was clamped for periods of 20 min at different set levels between 28°C and 43°C. The responses of the animals to hypothalamic thermal stimulation were classified by comparing them with those normally found in mammals. At T_a– 10°C hypothalamic heating inhibited ongoing shivering, causing a fall in body-core temperature (T_c) (appropriate mammalian-like response). Strong levels of hypothalamic cooling (T_hy < 34.0°C) also caused a fall in T_c due to inhibition of shivering (inappropriate mammalian-like response). However, weaker levels of hypothalamic cooling (T_hy 34–36°C), facilitated ongoing shivering, resulting in small increases in T_c (appropriate mammalian-like response). At T_a+25°C hypothalamic heating facilitated ongoing panting while weak (T_hy 38°C) levels of hypothalamic cooling inhibited ongoing panting (both mammalian-like responses). The observation of a weak mammalian-like cold hypothalamic thermosensitivity in Willow Ptarmigan indicates that these birds possess some specific cold thermosensors in the hypothalamic region. This finding suggests that hypothalamic temperature dependence in birds and mammals is fundamentally similar.     

Vasomotor response of the human face: laser-Doppler measurements during mild hypo- and hyperthermia

The skin of the face is reputed not to vasoconstrict in response to cold stress because the face skin temperature remains steady during hypothermia. The purpose of the present work was to measure the vasomotor response of the human face to whole-body hypothermia, and to compare it with hyperthermia. Six male subjects were immersed in cold and in warm water to obtain the two conditions. Skin blood flow, evaporation, and skin temperature (T_sk) were recorded in three loci of the face, the forehead, the infra orbital area, and the cheek. Tympanic (T_ty) and oesophageal (T_oes) temperatures were also recorded during the different thermal states. Normothermic measurements served as control. Blood flow was recorded with a laser-Doppler flowmeter, evaporation measured with an evaporimeter. Face T_sk remained stable between normo-, hypo-, and hyperthermia. Facial blood flow, however, did not follow the same pattern. The facial blood flow remained at minimal vasoconstricted level when the subjects' condition was changed from normo- to hypothermia. When the condition changed from hypo- to hyperthermia a 3 to 9–fold increase in the blood flow was recorded. From these results it was concluded that a vasoconstriction seems to be the general vasomotor state in the face during normothermia.     

Thermoregulation during entrance into hibernation

Summary The hypothalamic temperature (T_hy) and the rate of oxygen consumption of golden-mantled ground squirrels were continuously measured as they entered hibernation. T_hy was manipulated with chronically implanted, water-perfused thermodes. A threshold T_hy for eliciting an increase in metabolic heat production was demonstrable at all times during entry. During smooth entries this threshold T_hy showed a progressive decline so that it was below actual T_hy at all times. These results are interpreted to mean that the normal mammalian central nervous regulator of body temperature is functional throughout the entrance into hibernation, and it can be reset to any level over the 35°C range of body temperatures experienced by the hibernator.     

Serotoninergic mechanisms of beta-endorphin-induced hypothermia in rats

The effects of intraventricular administration of beta-endorphin on thermoregulatory responses of unanesthetized rats to different ambient temperatures (T _a ) of 8, 22 and 30°C were assessed. Administration of beta-endorphin produced a fall in rectal temperature at bothT _a 8 and 22°C. The hypothermia in response to beta-endorphin was brought about by both cutaneous vasodilation (as indicated by an increase in both the tail and the foot skin temperatures) and decreases in metabolic heat production. However, atT _a 30°C, administration of beta-endorphin produced no change in rectal temperature or other thermoregulatory responses. Furthermore, the hypothermic effect induced by beta-endorphin was greatly attenuated by either the depletion of brain serotonin levels (with 5,6-dihydroxytryptamine andp-chlorophenylanine) or the blockade of opiate receptors (with naloxone). The data indicate that beta-endorphin leads to hypothermia in rats by increasing sensible heat loss and decreasing metabolic heat production, probably via the release of endogenous serotonin within brain.     

Effects of repeated carbon dioxide-rich water bathing on core temperature,cutaneous blood flow and thermal sensation

We examined the effects of repeated artificial CO₂ (1,000 ppm) bathing on tympanic temperature (T _ty), cutaneous blood flow, and thermal sensation in six healthy males. Each subject was immersed in CO₂-rich water at a temperature of 34°C up to the level of the diaphragm for 20 min. The CO₂-rich water was prepared using a multi-layered composite hollow-fiber membrane. The CO₂ bathing was performed consecutively for 5 days. As a control study, subjects bathed in fresh water at 34°C under the same conditions. T _ty was significantly lowered during CO₂ bathing (P<0.05). Cutaneous blood flow in the immersed skin (right forearm) was significantly increased during CO₂ bathing compared with that during fresh-water bathing (P<0.05), whereas cutaneous blood flow in the non-immersed skin (chest) was not different between CO₂ and fresh-water bathing. Subjects reported a "warm" sensation during the CO₂ bathing, whereas they reported a "neutral" sensation during the fresh-water bathing. The effects of the repeated CO₂ bathing were not obvious for core temperature and cutaneous blood flow, but the thermal sensation score during the CO₂ bathing was reduced sequentially by repeated CO₂ bathing (P<0.05). These thermal effects of CO₂ bathing could be ascribed largely to the direct action of CO₂ on vascular smooth muscles and to the activity of thermoreceptors in the skin. Serial CO₂ bathing may influence the activity of thermoreceptors in the skin. Electronic Publication     

Regional differences in peripheral vasoconstriction of prepubertal boys

To elucidate the mechanisms that underlie the greater decline of skin temperature on the limbs in prepubertal boys as compared to young men, we compared cutaneous vascular conductance (CVC) of the boys and men in response to a reduced ambient temperature (T _a). The boys had a greater surface area-to-mass ratio (A _D/mass) and a lower mean skinfold thickness on the trunk but not on the limbs compared to the men. As T _a decreased from 30 to 17°C over 60 min, the skin temperature (T _sl) on the limbs (as represented by forearm, finger and thigh) decreased significantly more in the boys than in the men; while T _sl on the trunk (chest, back and abdomen) and forehead decreased to the same extent. The CVC decreased at all body sites in all subjects, but regional difference existed in age-related alterations in CVC responses despite the similar rectal and mean body temperatures of the groups. The decline in the finger CVC was greater for the boys than for the men, suggesting that greater vasoconstriction and greater A _D/mass on the fingers may have contributed to the lower finger T _sl of the boys. However, thigh CVC in the boys was similar to that in the men over the 60-minute exposure, indicating that the lower thigh T _sl of the boys may be the result of greater heat loss owing to the greater A _D/mass on the limbs of the boys (but not to greater vasoconstriction or subcutaneous fat). The CVC on the chest and back was greater in the boys over the cold exposure, suggesting that similar T _sl on the chest and back of the boys and men may result from greater cooling owing to the larger A _D/mass being offset by combination of less vasoconstriction and more conductive heat transfer in the presence of less subcutaneous fat. These results suggest that the age-related difference in T _sl in response to mild cold stress may not directly reflect that in cutaneous vasoconstriction alone owing to the differences in anthropometric characteristics (such as greater A _D/mass and lower subcutaneous fat on trunk) between boys and men.     

Blood flow through the ophthalmic veins during exercise in humans

Summary The blood from the face flows into the intracranium through the ophthalmic veins when human subjects become hyperthermic. To investigate a possible mechanism underlying this change in direction of flow, five young men were subjected to either passive body warming or exercise on a cycle ergometer, in a climatic chamber whose air temperature and relative humidity were 28°C and 40%. Tympanic (T _ty) and oesophageal temperatures, forehead sweat rate (m _sw), skin blood flow. (Q _sk) and blood flow through the ophthalmic vein (Q _ov) were measured, and the mean skin (T _sk) and mean body (T _b) temperatures were computed. Passive body warming was induced by a box-shaped body warming unit enclosing all but the subject's head. Exercise was performed either at an intensity of 60% maximal oxygen consumption or with the intensity increasing in increments. During both tests, m _sw and Q _sk started to increase shortly after the imposition of the heat load. The Q _ov began to change with the venous blood flowing from the face into the intracranium and a complete reversal in the direction of Q _ov (from the face to the intracranium). came significantly later than the increases in Q _sw and Q _sk. The T _ty at the time of flow reversal was the same in both tests. The T _sk (and hence T _b) at flow reversal was, however, significantly higher during passive body warming than during exercise. The mechanism for switching the direction of Q _ov appeared to have been triggered by a high temperature in the brain, and not by thermal input from the periphery of the body. In a febrile subject who volunteered for this study, the direction of Q _ov was consistently inwards even when sitting quietly. From these results, we suggest that there are elements within the brain that control the mechanisms for switching the direction of venous flow through the emissary veins to keep the brain cool during hyperthermia.     

Hand and finger skin temperatures in convective and contact cold exposure

The present study aimed at investigating the spatial variability of skin temperature (T _sk) measured at various points on the hand during convective and cold contact exposure. A group of 8 subjects participated in a study of convective cooling of the hand (60 min) and 20 subjects to contact cooling of the finger pad (5 min). Experiments were carried out in a small climatic chamber into which the hand was inserted. For convective cold exposure,T _sk was measured at seven points on the palmar surface of the fingers of the left hand, one on the palmar surface and one on the dorsal surface of the hand. The air temperature inside the mini-chamber was 0, 4, 10 and 16°C. With the contact cold exposure, the subjects touched at constant pressures an aluminium cube cooled to temperatures of –7, 0 and 7°C in the same mini-chamber. ContactT _sk was measured on the finger pad of the index finger of the left hand. TheT _sk of the proximal phalanx of the index finger (on both palm and back sides), and of the middle phalanx of the little finger was also measured. The variation ofT _sk between the proximal and the distal phalanx of the index finger was between 1.5 to 10°C during the convective cold exposure to an air temperature of 0°C. Considerable gradients persisted between the hand and fingers (from 2 to 17°C at 0°C air temperature) and between the phalanges of the finger (from 0.5 to 11.4°C at 0°C air temperature). The onset of cold induced vasodilatation (CIVD) on different fingers varied from about 5 to 15 min and it did not always appear in every finger. For contact cold exposure, whenT _sk on the contact skin cooled down to nearly 0°C, the temperature at the area close to the contact skin could still be 30°C. Some cases of CIVD were observed in the contact skin area, but not on other measuring points of the same finger. These results indicated that local thermal stimuli were the main determinents of CIVD. Representative hand skin temperature may require five or more measuring points. Our results strongly emphasised a need to consider the large spatial and individual variations in the prediction and modelling of extremity cooling.     

Temperature changes of the hypothalamus and body core in ducks feeding in cold water

Summary Hypothalamic (T _hy), spinal (T _sc) and colonic (T _c) temperatures were measured in Pekin ducks spontaneously dabbling for food in cold water (5° C). In agreement with observations in the pigeon and the fowlT _hy was found to be consistently lower by about 0.5° C than the other core temperatures. The drop ofT _hy during dipping head and neck into the cold water was not substantially greater than that ofT _sc, while both changed more thanT _c. The measurements do not support the assumption that the hypothalamic region in the duck is exposed to substantially greater temperature fluctuations than other thermosensitive parts of the body core.     

Seasonal changes in circadian rhythms of body temperatures in humans living in a dry tropical climate

Summary Seven volunteers (3 females and 4 males; 3 Caucasians and 4 Africans) participated in two 24 h sessions during the cool dry (CD) and the hot dry (HD) seasons of the sahelian tropical climate. Body temperatures were taken on portable cassette recorders for 24 h. Rectal (T _re) and mean skin (¯T _sk) temperatures decreased in the HD compared to the CD conditions, meeting one of the criteria for adaptation to heat. No ethnic differences in thermal responses were found. Males and females differed in their body temperature rhythms and in their reactions to heat. Body temperatures were higher in females than in males. Males reacted to heat with a decrease in T _re, without change in the T _re-¯T _sk gradient. Females showed a decrease in both T _re and ¯T _sk, more marked for ¯T _sk, with an increase in the T _re-¯T _sk gradient. It was concluded that males showed seasonal acclimatization to heat via a decrease in metabolism confirmed by a decrease in plasma levels of thyroid stimulating hormone (TSH) in the HD condition. Females showed a mixed metabolic and thermolytic type of acclimatization, with an absence of variation in plasma TSH levels. In conclusion, the steady rise in temperature between the CD and HD conditions was sufficient to trigger an acclimatization to heat similar in Caucasian and African subjects, although exposure to the external climate differed widely.     

Regulation of local sweating in sleep-deprived exercising humans

Summary Thermoregulatory sweating [total body (m _sw,b), chest (m _sw,c) and thigh (m _sw,t) sweating], body temperatures [oesophageal (T _oes) and mean skin temperature (T _sk)] and heart rate were investigated in five sleep-deprived subjects (kept awake for 27 h) while exercising on a cycle (45 min at approximately 50% maximal oxygen consumption) in moderate heat (T _air andT _wall at 35° C. Them _sw,c andm _sw,t were measured under local thermal clamp (T _sk,1), set at 35.5° C. After sleep deprivation, neither the levels of body temperatures (T _oes,T _sk) nor the levels ofm _{sw, b},m _{sw, c} orm _{sw, t} differed from control at rest or during exercise steady state. During the transient phase of exercise (whenT _sk andT _sk,1 were unvarying), them _{sw, c} andm _{sw, t} changes were positively correlated with those ofT _oes. The slopes of them _{sw, c} versusT _oes, orm _{sw, t} versusT _oes relationships remained unchanged between control and sleep-loss experiments. Thus the slopes of the local sweating versusT _oes, relationships (m _{sw, c} andm _{sw, t} sweating data pooled which reached 1.05 (SEM 0.14) mg·cm^–2·min^–1°C^–1 and 1.14 (SEM 0.18) mg·cm^–2·min^–1·°C^–1 before and after sleep deprivation) respectively did not differ. However, in our experiment, sleep deprivation significantly increased theT _oes threshold for the onset of bothm _{sw, c} andm _{sw, t} (+0.3° C,P<0.001). From our investigations it would seem that the delayed core temperature for sweating onset in sleep-deprived humans, while exercising moderately in the heat, is likely to have been due to alterations occurring at the central level.     

Comparison in men of physiological responses to exercise of increasing intensity at low and moderate ambient temperatures

Summary In six male subjects the sweating thresholds, heart rate (f _c, as well as the metabolic responses to exercise of different intensities [40%, 60% and 80% maximal oxygen uptake (VO_2max)], were compared at ambient temperatures (T _a) of 5° C (LT) and 24° C (MT). Each period of exercise was preceded by a rest period at the same temperature. In LT experiments, the subjects rested until shivering occurred and in MT experiments the rest period was made to be of exactly equivalent length. Oxygen uptake (VO₂) at the end of each rest period was higher in LT than MT (P< 0.05). During 20-min exercise at 40%VO_2max performed in the cold no sweating was recorded, while at higher exercise intensities sweating occurred at similar rectal temperatures (T _re) but at lower mean skin (T _sk) and mean body temperatures (T _b) in LT than MT experiments (P<0.001). The exercise inducedVO₂ increase was greater only at the end of the light (40%VO_2max) exercise in the cold in comparison with MT (P<0.001). Bothf _c and blood lactate concentration [la]_b were lower at the end of LT than MT for moderate (60%VO_2max) and heavy (80%VO_2max) exercises. It was concluded that the sweating threshold during exercise in the cold environment had shifted towards lower (T _b) andT _sk. It was also found that subjects exposed to cold possessed a potentially greater ability to exercise at moderate and high intensities than those at 24° C since the increases inT _re,f _c and [la]_b were lower at the lowerT _a.     

Control of shivering and heart rate in incubating bantam hens upon sudden exposure to cold eggs

To distinguish shivering released by cooling of the brood patch from shivering released by low core temperature, incubating bantam hens were exposed to water perfused eggs. Responses to a period of egg cooling were compared to the recovery period after egg temperature had been returned to 40 ^oC, but cloacal temperature (T_b) still was low. At an ambient temperature (T_a) of 23 ^oC and exposure of the hens to between two and eight eggs cooled to 10–35 ^oC (series 1), electromyographic (EMG) activity of musculus iliotibialis increased rapidly with an occasional overshoot, and was higher during egg cooling than during recovery. This hysteresis in EMG activity and T_b was weakly correlated to egg temperature and clutch size. Heart rate (HR) showed an almost parallel increased to shivering except that a maximum HR was reached at high shivering intensities. These responses were also present at a T_a of 37 ^oC when the hens were slightly hyperthermic before exposure to eight eggs at 20 ^oC (Series 2). At the highest starting T_bs EMG activity increased linearly after a drop in T_b. Shivering in m. pectoralis showed a lower threshold T_b and lower activity than m. iliotibialis during egg cooling, and immediately ceased at the end of egg cooling. Total body thermosensitivity estimated from the recovery periods at low and high T_a was –9.7 and –6.4 W kg^-1oC^-1, respectively. It is concluded that shivering in incubating birds warming cold eggs probably is stimulated both by peripheral and central thermoreceptors. The peripheral component shows phasic properties typical for skin receptors.     

Prediction of the average skin temperature in warm and hot environments

The prediction of the mean skin temperature used for the Required Sweat Rate index was criticised for not being valid in conditions with high radiation and high humidity. Based on a large database provided by 9 institutes, 1999 data points obtained using steady-state conditions, from 1399 experiments and involving 377 male subjects, were used for the development of a new prediction model. The observed mean skin temperatures ranged from 30.7 °C to 38.6 °C. Experimental conditions included air temperatures (T _a) between 20 and 55 °C, mean radiant temperatures (T _r) up to 145 °C, partial vapour pressures (P _a) from 0.2 to 5.3 kPa, air velocities (v _a) between 0.1 and 2 m/s, and metabolic rates (M) from 102 to 620 W. Rectal temperature (T _re) was included in the models to increase the accuracy of prediction. Separate models were derived for nude (clothing insulation, I_cl, ≤0.2 clo, where 1 clo=0.155 m² · °C · W⁻¹, which is equivalent to the thermal insulation of clothing necessary to maintain a resting subject in comfort in a normally ventilated room, air movement=10 cm/s, at a temperature of 21 °C and a humidity of less than 50%) and clothed (0.6 ≤ I_cl ≤ 1.0 clo) subjects using a multiple linear regression technique with re-sampling (non-parametric bootstrap). The following expressions were obtained for nude and clothed subjects, respectively: T _sk=7.19 + 0.064T _a + 0.061T _r + 0.198P _a− 0.348v _a + 0.616T _re and T _sk=12.17 + 0.020T _a + 0.044T _r + 0.194P _a − 0.253v _a + 0.0029M + 0.513T _re. For the nude and clothed subjects, 83.3% and 81.8%, respectively, of the predicted skin temperatures were within the range of ±1 °C of the observed skin temperatures. It is concluded that the proposed models for the prediction of the mean skin temperature are valid for a wide range of warm and hot ambient conditions in steady-state conditions, including those of high radiation and high humidity. Accepted: 7 February 2000     

Residual analysis in the determination of factors affecting the estimates of body heat storage in clothed subjects

Body heat storage can be estimated by calorimetry (from heat gains and losses) or by thermometry [from changes (Δ) in mean body temperature (T _b) calculated as a weighted combination of rectal (T _re) and mean skin temperatures (T _sk)]. If an invariant weighting factor ofT _re andT _sk were to be used (for instance, ΔT _b = 0.8 · ΔT _re + 0.2 · ΔT _sk under hot conditions), body heat storage could be over- or underestimated substantially relative to calorimetry, depending on whether the subject was wearing light or protective clothing. This study investigated whether discrepancies between calorimetry and thermometry arise from methodological errors in the calorimetric estimate of heat storage, from inappropriate weightings in the thermometric estimate, or from both. Residuals of calorimetry versus thermometric estimates were plotted against individual variables in the standard heat balance equation, applying various weighting factors toT _re andT _sk. Whether light or protective clothing was worn, the calorimetric approach generally gave appropriate estimates of heat exchange components and thus heat storage. One exception was in estimating latent heat loss from sweat evaporation. If sweat evaporation exceeded 650 g·h⁻¹ when wearing normal clothing, evaporative heat loss was overestimated and thus body heat storage was underestimated. Nevertheless, if data beyond this ceiling were excluded from the analyses, the standard 4:1 weighting matched calorimetric heat storage estimates quite well. When wearing protective clothing, the same 4:1 weighting approximated calorimetric heat storage with errors of less than approximately 10%, but only if environmental conditions allowed a subject to exercise for more than 90 min. The best thermometric estimates of heat storage were provided by using two sets of relative weightings, based upon the individual's metabolic heat production ( in kilojoules per metre squared per hour): {4 − [( )· ] 2}:1 for an initial, thermoneutral environment and {4 + [( ) · ] · 5}: 1 for a final, hot environment; the optimal value of lay between 450 and 500 kJ m⁻² · h⁻¹. We concluded that the accuracy of thermometric estimates of heat storage can be improved by modifying weighting factors ofT _re andT _sk according to the environment, type of clothing, and metabolic rate.     

Local sweating responses during recovery sleep after sleep deprivation in humans

Changes in the central control of sweating were investigated in five sleep-deprived subjects (kept awake for 40 h) during their recovery sleep under warm ambient conditions [operative temperature (T _o) was either 35 or 38° C]. Oesophageal (T _oes) and mean skin (T _sk) temperatures, chest sweat rate (m _sw,ch), and concomitant electro-encephalographic data were recorded. Throughout the night at 35 or 38° C T _o, m _sw,ch changes were measured at a constant local chest skin temperature (T _ch) of 35.5° C. The results showed that body temperatures (T _oes and T _sk) of sleep-deprived subjects were influenced by thermal and hypnogogic conditions. The m _sw,ch levels correlated positively with T _oes in the subjects studied during sleep stage 1–2 (light sleep: LS), sleep stage 3–4 (slow wave sleep: SWS) and rapid eye movement (REM) sleep. Contrary to what has been reported in normal sleep, firstly, the T _oes threshold for sweating onset differed between REM sleep and both LS and SWS, and, secondly, the slopes of the m _sw,ch versus T _oes relationships were unchanged between REM and non-REM (i.e. LS or SWS) sleep. The changes observed after sleep deprivation were hypothesized to be due to alterations in the functioning of the central nervous system controller.