Thermoregulatory responses of circum-pubertal children

Passive temperature lability of nine circumpubertal children [11.4 (1.2) years] was compared to that of nine young adult males [26.6 (5.2) years]. Each subject completed a 20-min period of exercise, followed immediately by post-exercise immersion in water at 28°C. The aim of the exercise protocol was to induce a steady rate of sweating (E _SW) while the postexercise immersion period induced cooling of the core region (tympanic temperature:T _ty). TheT _ty values (relative to rest, ΔT _ty) at which sweating abated and at which shivering commenced were defined as the thresholds for the cessation of sweating and onset of shivering, respectively. While there was no significant difference between the ΔT _ty sweating thresholds, the onset of shivering, as reflected in the oxygen uptake response, occurred at significantly higher (P < 0.05) ΔT _ty values in the children [mean (SD): −0.07 (0.07)°C] than in the adults [−0.22 (0.10)°C]. The slope of theE _SW/ΔT _ty relationship was found to be significantly lower in the children (z = −5.64;P < 0.05), while the slopes of the /ΔT _ty relationship were not significantly different (z = −0.84;P > 0.05). Skin blood perfusion was measured at the forehead (SkBP), and the slope of the SkBP/ΔT _ty relationship across the nullzone was significantly less in the children than in the adults (z = −2.13;P < 0.05) with the greatest reduction in perfusion occurring prior to the offset of sweating in the children. The subjective ratings of thermal comfort indicated that the children were more sensitive to changes in core temperature than the adults. It is concluded that maturation plays an important role in modifying thermoregulatory responses to deviations in core temperature. These results suggest that there may be differences in thermoregulatory “strategies” which are maturationally related.     

Gender differences in physiological reactions to thermal stress

Following an extensive anthropometric evaluation, thermoregulatory responses were studied in nine men and nine women who performed immersed exercise with post-exercise rest in 28°C water. During the post-exercise period esophageal temperature (T _es), oxygen consumption, heat flux and skin blood perfusion were monitored at 10s intervals, with average minute values used for calculations. The T _es (relative to restingT _es) at which sweating abated and shivering commenced were defined as the T _es thresholds for the cessation of sweating and onset of shivering, respectively. No significant gender differences were evident in the sweating and shivering threshold T _es values, or the magnitude of the null-zone. Usingz-tests for parallelism the rates of core cooling across the null-zone were not found to differ significantly between genders, nor were the slopes of the perfusion: T _es responses across the null-zone or the post-threshold shivering responses (ml·kg^–1·min^–1·°C^–1). The slope of the sweating response (measured from immersion until sweat cessation; g·m^–2·min^–1°C^–1) was, however, significantly lower in the female than in the male samples (z = 3.93;P < 0.01). Despite the gender-related dimorphic distribution of adipose tissue, both men and women lost equal proportions of their total heat flux from central and peripheral measurement sites. Performing a standardized regression using the rate of core cooling across the null-zone as the dependent variable and gender as a dummy variable, gender and adipose tissue mass were not found to be significant factors in determining the rate of core cooling, while mass ( = 1.73;P < 0.05) and muscle mass ( = 1.86;P < 0.05) did contribute significantly to the rate of core cooling. It was concluded that, except for the quantitative differences in the sweating response, men and women respond to deviations in core temperature in a similar manner, with mass and muscle mass modifying this response.     

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

Summary 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‡ C ambient air temperature (T_a) for 1 h · day⁻¹) under resting conditions. Before and after HA, the subjects' shivering responses were determined in a cold test (T_a+10 to 0‡ C). In series B, eight male subjects underwent mild exercise training (five consecutive days at a heart rate, HR, of 120b · min⁻¹) under T_a conditions individually adjusted (T_a + 15 to +5‡ 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<0.05) towards lower mean body temperatures ( ). The mean decrease in threshold was 0.36‡ C. “Sweatless training≓ resulted in a mean increase in work rate (at HR 120b · min⁻¹) and oxygen pulse of 13 and 8%, respectively. However, “sweatless training≓ did not change the threshold for shivering or sweating. Neither HA nor “sweatless training≓ changed the slopes of the relationships of shivering and sweating to . It is concluded that the previously reported lowering of shivering and sweating threshold 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. This study was supported by the Deutsche Forschungsgemeinschaft (Br 184/16-3)     

Thermoregulatory effects of three different types of head cooling in humans during a mild hyperthermia

Seven healthy young men participated in six trials with three different types of local cooling [cool air breathing (CAB), face skin cooling (FaC), and combined cooling (CoC)] in a warm environment for 90 min while either resting (operative temperature: T ₀ = 40°C, dew point temperature: T _dp = 15°C, air velocity: v _a = 0.3 m·s⁻¹) or exercising on a cycle ergometer with an external work load of 90 W (T ₀ = 36°C, T _dp = 15°C, v _a = 0.3 m·s⁻¹). Cool air (10°C) arrived at the entry point of the hood and/or the mask at a ventilation rate of 12 m · s⁻¹. Oesophageal temperature was not affected by any kind of cooling, while tympanic temperature was decreased at rest by both FaC and CoC [respectively −0.15 (0.06) and −0.09 (0.03)°C, P ≤ 0.05]. Mean skin temperature was decreased by FaC and CoC at rest [respectively −0.31 (0.07) and −0.27 (0.09)°C, P ≤ 0.05] and during exercise [respectively −0.64 (0.15) and −1.04 (0.22)°C, P ≤ 0.01]. CAB had no effect on skin temperatures. CoC and FaC reduced head skin temperature during both rest and work (P < 0.001) with no effect on the skin temperature of the rest of the body, except under CoC with exercise (P < 0.05). CAB did not influence local sweating. FaC, however, decreased the more profuse sweat rates (P ≤ 0.05) at rest, while CoC decreased all sweating rates at rest (P ≤ 0.05) and only the back, head and leg sweating rates during exercise (P ≤ 0.05). These results suggest that head skin cooling causes a reduction in heat strain, while CAB does not. This beneficial influence does not, however, appear to be the result of selective brain cooling. Tympanic temperature seems to be a good index of the core thermal inputs to the hypothalamic regulatory system, since variations in that parameter were associated with similarly directed variations in the sweating outputs. Accepted: 12 April 1999     

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.     

Shivering endurance and fatigue during cold water immersion in humans

An important component of survival time during cold exposure is shivering endurance. Nine male and three female healthy and fit subjects [mean (SD) age 24.8 (6.3) years, body mass 71.7 (13.2) kg, height 1.75 (0.10) m, body fat 22.7 (7.4)%] were immersed to the upper chest level in cold water for periods ranging from 105 to 388 min on two occasions to test a prediction of shivering endurance. The water was cooled from 20 to 8°C during the first 15 min of immersion and subsequently rewarmed (<20°C) to elicit a near constant submaximal shivering response. The data were divided according to moderate (M) and high (H) levels of shivering intensity. Respective mean total immersion times were 250 (75) and 199 (80) min (P=0.086) at different average shivering intensities of 61 (10) and 69 (8)% relative to maximal shivering (P<0.001). Blood plasma glucose concentration increased during the immersion [from 3.44 (0.54) pre- to 3.94 (0.60) mmol·l^–1 post-immersion (P=0.037)] and levels were higher during M (P=0.012). When compared to a model prediction of shivering endurance, shivering activity continued well beyond the predicted endurance times in 18 out of the 24 trials. The average rates of oxygen consumption over the entire immersion period were lower (P=0.002) during M [0.93 (0.20) l·min^–1] compared to H [1.05 (0.21) l·min^–1), and while these rates did not change during the last 90 min of immersion, there was an increase in fat oxidation. There were no trial differences in the average esophageal (T _es) and mean skin temperatures during the entire immersion period (36.0 and 18.0°C, respectively), yet T _es decreased (P=0.003) approximately 0.4°C during the last 90 min of immersion. When the shivering intensity was normalized to account for this decrease, a significant downward trend of approximately 17%·h^–1 in the normalized shivering intensity was found after the predicted end of shivering endurance. These results suggest that shivering drive, and not shivering intensity per se, decreased during the latter stages of the immersion. Underlying mechanisms such as fatigue and habituation for this diminishing cold sensitivity are discussed. Electronic Publication     

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

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

Time-of-day effect on nonthermal control of sweating response to maintained static exercise in humans

To investigate the influence of nonthermal factors in the time-of-day effect on the sweating response to maintained static exercise, eight healthy male subjects performed handgrip exercise at 20%, 35% and 50% maximal voluntary contraction (MVC) for 60 s at 0600 hours (morning) and at 1800 hours (evening). Oesophageal temperature (T _oes) before the experiment showed a diurnal rhythm [mean (SEM)] [36.3 (0.1) (morning) compared to 36.8 (0.1) °C (evening), P<0.01]. Experiments were conducted with subjects in a state of mild hyperthermia during which the mean skin temperature (T _sk) was kept constant at 35.5–36.5 °C using a water-perfused suit to activate sudomotor responses. The T _oes and mean T _sk remained stable during the pre-exercise, handgrip exercise and recovery periods. The response in sweating rate (ΔSR) on the chest and forearm to handgrip exercise increased significantly with increasing exercise intensity in both the morning and evening tests (P<0.05). The ΔSR on the palm did not change significantly with increasing exercise intensity in the morning test (P>0.1). During handgrip exercise at 50% MVC only, ΔSR on the chest, forearm and palm in the evening was significantly higher than in the morning (P<0.05). On the other hand, mean arterial blood pressure and the rating of perceived exertion during 50% MVC handgrip exercise were not significantly different between the morning and evening (P>0.1). These results indicate the presence of a time-of-day effect on nonthermal control of the sweating response to isometric handgrip exercise, and that this effect is dependent on exercise intensity. Electronic Publication     

Fat energy use and plasma lipid changes associated with exercise intensity and temperature

Summary The effect of 60 min of exercise at two intensities (50 and 60% ) and temperatures (0 and 22° C) on changes (A) in plasma lipids {triglycerides (TG), glycerol (GLY), total cholesterol (TC), and HDL-cholesterol (HDL-C)} was examined. Subjects were 10 men aged 27±7 years ( , % fat=12.2%±7.1%). and respiratory exchange ratio results indicated that total energy and fat energy use were similar at the two temperatures. Changes in plasma volume (%ΔPV) were different (P<0.05) at the two temperatures (22° C: −2.3% vs 0° C: 1.1%). Combining the data at each temperature revealed that the increases in concentrations were greater (P<0.05) at 22° C (ΔTG=0.22, ΔGLY=0.20, ΔTC=0.14, ΔHDL-C=0.05 mmol 1⁻¹) vs 0° C (ΔTG=0.10, ΔGLY=0.12, ΔTC=0.05, ΔHDL-C=0.02 mmol 1⁻¹). Combining the data for each intensity revealed that the increases in concentration were greater (P<0.05) at 60% for ΔTG and ΔHDL-C. The 60% C bout produced greater changes (P<0.05) than all other bouts for ΔTC and ΔHDL-C (0.21 and 0.08 mmol 1⁻¹, respectively). Only ΔTG and ΔGLY were greater at 22° C when adjusted for %ΔPV. These metabolic and plasma lipid results indicate that cold exposure does not act synergistically with exercise to further stimulate fat metabolism.     

Substrate utilisation during exercise and shivering

It is generally assumed that exercise and shivering are analogous processes with regard to substrate utilisation and that, as a consequence, exercise can be used as a model for shivering. In the present study, substrate utilisation during exercise and shivering at the same oxygen consumption (V˙O₂) were compared. Following an overnight fast, eight male subjects undertook a 2-h immersion in cold water, designed to evoke three different intensities of shivering. At least 1 week later they undertook a 2-h period of bicycle ergometry during which the exercise intensity was varied to match the V˙O₂ recorded during shivering. During both activities hepatic glucose output (HGO), the rate of glucose utilisation (Rd), blood glucose, plasma insulin, free fatty acid (FFA) and beta-hydroxybutyrate (B-HBA) concentrations were measured. The V˙O₂ measured during the different levels of shivering averaged 0.49 l · min⁻¹ (level 1: low), 0.6 l · min⁻¹ (level 2: low-moderate), and 0.9 l · min⁻¹ (level 3: moderate), and corresponded closely to the levels measured during exercise. HGO and Rd were greater (P < 0.05) during exercise than during shivering at the same V˙O₂ (9.5% and 14.7%, respectively). The average (SD) HGO during level 3 exercise was 3.0 (0.91) mg · kg⁻¹ . min⁻¹ compared to 2.76 (1.0) mg · kg⁻¹ . min⁻¹ during shivering. The values for Rd were 3.06 (0.98) mg · kg⁻¹ · min⁻¹ during level 3 exercise and 2.68 (0.82) mg · kg⁻¹ · min⁻¹ during shivering. Blood glucose levels did not differ between conditions, averaging 5.4 (0.3) mmol . l⁻¹ over all levels of shivering and 5.2 (0.3) mmol · l⁻¹ during exercise. Plasma FFA and B-HBA were higher (P < 0.01) during shivering than during corresponding exercise (12.3% and 33.3%, respectively). FFA averaged 0.61 (0.2) mmol · l⁻¹ over all levels of shivering and 0.47 (0.16) mmol · l⁻¹ during exercise. The figures for B-HBA were 0.44 (0.13) mmol · l⁻¹ during all levels of shivering and 0.32 (0.1) mmol · l⁻¹ during exercise. Plasma insulin was higher (P < 0.05) during level 2 and 3 shivering compared to corresponding exercise; at these levels the average value for plasma insulin was 95.9 (21.9) pmol · l⁻¹ during shivering and 80.6 (16.1) pmol · l⁻¹ during exercise. On the basis of the present findings it is concluded that, with regard to substrate utilisation, shivering and exercise of up to 2 h duration should not be regarded as analogous processes. Accepted: 12 February 1997     

Thermoregulation, pacing and fluid balance during mass participation distance running in a warm and humid environment

Deep body temperature (T _c), pacing strategy and fluid balance were investigated during a 21-km road race in a warm and humid environment. Thirty-one males (age 25.3 ± 3.2 years; maximal oxygen uptake 59.1 ± 4.2 ml kg⁻¹ min⁻¹) volunteered for this study. Continuous T _c responses were obtained in 25 runners. Research stations at approximately 3-km intervals permitted accurate assessment of split times and fluid intake. Environmental conditions averaged 26.4°C dry bulb temperature and 81% relative humidity. Peak T _c was 39.8 ± 0.5 (38.5–40.7) °C with 24 runners achieving T _c > 39.0°C, 17 runners ≥39.5°C, and 10 runners ≥40.0°C. In 12 runners attaining peak T _c ≥ 39.8°C, running speed did not differ significantly when T _c was below or above this threshold (208 ± 15 cf. 205 ± 24 m min⁻¹; P = 0.532). Running velocity was the main significant predictor variable of ∆T _c at 21 km (R ² = 0.42, P < 0.001) and was the main discriminating variable between hyperthermic (T _c ≥ 39.8°C) and normothermic runners (T _c < 39.8°C) up to 11.8 km. A reverse J-shaped pacing profile characterised by a marked reduction in running speed after 6.9 km and evidence of an end-spurt in 16 runners was observed. Variables relating to fluid balance were not associated with any T _c parameters or pacing. We conclude that hyperthermia, defined by a deep body temperature greater than 39.5°C, is common in trained individuals undertaking outdoor distance running in environmental heat, without evidence of fatigue or heat illness.     

Effect of water ingestion on cardiovascular and thermal responses to prolonged cycling and running in humans: a comparison

The aim of this investigation was to examine the effect of water ingestion on physiological responses to prolonged cycling (CYC) and running (RUN). A group of 11 men with mean (SEM) maximal oxygen uptake (V˙O_2max) 48.5 (1.8) ml·kg^–1·min^–1 on a cycle-ergometer and 52.1 (2.2) ml·kg^–1·min^–1 on a treadmill (P<0.01) exercised for 90 min on four occasions, twice on each ergometer, at 60% of mode specific V˙O_2max. No fluid was taken (D) in one trial on each ergometer, whereas 60% of fluid losses were replaced by drinking water in the other trial (W). In CYC, water ingestion attenuated the change in cardiac output ( ) and the reduction in stroke volume (ΔSV) [ΔSV: –22.7 (3.8) in D, –10.7 (2.9) ml·beat^–1 in W, P<0.01; : –1.9 (0.5) in D, –0.2 (0.4) l·min^–1 in W at 85 min, P<0.01], but did not affect rectal temperature [T _re at 90 min: 38.8 (0.1)°C in D, 38.7 (0.1)°C in W]. In contrast, fluid replacement reduced hyperthermia in RUN [T _re at 90 min: 39.6 (0.2) in D, 39.1 (0.2)°C in W, P<0.01], and this was linked with a higher skin blood flow [RUN-W 88.9 (8.5), RUN-D 70.7 (8.4)%, P<0.05]. The and ΔSV were also attenuated with water ingestion in this mode of exercise (P<0.05). It is concluded that water ingestion improves physiological function in both cycling and running, but that the underlying mechanism is different in the two modes of exercise. Electronic Publication     

Induction and decay of short-term heat acclimation

The purpose of this work was to investigate adaptation and decay from short-term (5-day) heat acclimation (STHA). Ten moderately trained males (mean ± SD age 28 ± 7 years; body mass 74.6 ± 4.4 kg; [(V)\dot]_{\textO 2\textpeak} \dot{V}_{{{\text{O}}_{ 2{\text{peak}}} }} 4.26 ± 0.37 l min⁻¹) underwent heat acclimation (Acc) for 90-min on 5-days consecutively (T _a = 39.5°C, 60% RH), under controlled hyperthermia (rectal temperature 38.5°C). Participants completed a heat stress test (HST) 1 week before acclimation (Acc), then on the 2nd and 8th day (1 week) following Acc (T _a = 35°C, 60% RH). Seven participants completed HSTs 2 and 3 weeks after Acc. HST consisted of 90-min cycling at 40% peak power output before an incremental performance test. Rectal temperature at rest (37.1 ± 0.4°C) was not lowered by Acc (95% CI −0.3 to 0.2°C), after 90-min exercise (38.6 ± 0.5°C) it reduced 0.3°C (−0.5 to −0.1°C) and remained at this level 1 week later (−0.5 to −0.1°C), but not two (0.1°C −0.4 to 0.5°C; n = 7) or 3 weeks. Similarly, heart rate after 90-min exercise (146 ± 21 b min⁻¹) was reduced (−13: −6 to −20 b min⁻¹) and remained at this level after 1 week (−13: −6 to −20 b min⁻¹) but not two (−9: 6 to −23 b min⁻¹; n = 7) or 3 weeks. Performance (746 s) increased 106 s: 59 to 152 s after Acc and remained higher after one (76 s: 31 to 122) but not two (15 s: −88 to 142 s; n = 7) or 3 weeks. Therefore, STHA (5-day) induced adaptations permitting increased heat loss and this persisted 1 week but not 2 weeks following Acc.     

Critical power of knee extension exercises does not depend upon maximal strength

A possible dependence of critical power (CP) and the Y-intercept of the work/exhaustion time relationship (Y _intercept) on maximal muscular strength of the same muscle group has been studied in nine endurance-trained subjects, seven gymnasts, and seven weight-lifters. CP was calculated as being equal to the slope of the linear relationship between exhaustion time and the work performed at exhaustion on a knee extension ergometer. Y _intercept was equal to the intercept between this relationship and the work axis. The muscular strength of the knee was evaluated by measuring the torques exerted on a Biodex knee isokinetic dynamometer at four angular velocities: 0° · s⁻¹ (T₀), 90° · s⁻¹ (T₉₀), 180° · s⁻¹ (T₁₈₀) and 240° · s⁻¹ (T₂₄₀). The results of the present study do not support the hypothesis that CP depends upon maximal strength. Indeed, CP was not correlated with T₀, T₉₀, T₁₈₀ or T₂₄₀ (|r| < 0.01). Y _intercept was significantly and positively correlated only with T₉₀. Accepted: 1 November 1999     

Control of sweating during the human menstrual cycle

Summary Thermoregulatory responses were studied in seven women during two separate experimental protocols in the follicular (F, days 4–7) phase and during the luteal (L, days 19–22) phase of the menstrual cycle. Continuous measurements of esophageal temperature (T _es), mean skin temperature ( ), oxygen uptake and forearm sweating ( ) were made during all experiments. Protocol I involved both passive heat exposure (3 h) and cycle exercise at ∼80% peak during which the environmental chamber was controlled atT _a=50.0° C, rh=14% (P _w=1.7 kPa). In protocol II subjects were tested during thirty-five minutes of exercise at ∼85% peak atT _a=35° C and rh=25% (P _w=1.4 kPa). The normal L increase in restingT _es (≈0.3° C) occurred in all seven subjects. was higher during L than F in all experiments conducted at 50° C. During exercise and passive heat exposure, theT _es threshold for sweating was higher in L, with no change in the thermosensitivity (slope) of toT _es between menstrual cycle phases. This rightward or upward shift inT _es threshold for initiation of sweating averaged 0.5° C for all experiments. The data indicate the luteal phase modulation in the control of sweating in healthy women is also apparent during severe exercise and/or heat stress.     

Mean skin temperature in warm humid climates

Summary Mean skin temperature was measured in 24 subjects during experiments in a climatic chamber. Three conditions of ambient temperature (T _a=25.6°, 28.9° and 32.2° C), and three of humidity (relative humidity = 50%, 70% and 90%) were studied. A relationship was established by a linear regression technique. It is valid in the 24°–34° C range, for air velocity =0.2 m·s⁻¹, clothing insulation =0.077° C·m²·w⁻¹ (0.5 clo), metabolic rate =64 w·m⁻² (1.1 met) and radiant temperature = air temperature. In these conditions =28.125+0.021P _w+0.210T _a (P _w: ambient water vapour pressure in mb). It shows a small humidity influence. The influences of sex, transition from one condition to the next, and air velocity were also studied. Measurements in Africa confirmed the small influence of humidity. Ethnic life-style differences indicated that a high precision in determination is difficult to achieve.     

Effects of temperature, ionic strength and pH on the function of skeletal muscle myosin from a eurythermal fish, Fundulus heteroclitus

The mummichog, Fundulus heteroclitus, is an intertidal fish that exhibits little change in swimming ability despite large and rapid variations in environmental parameters. We therefore tested the hypothesis that this nearly constant function is due to Fundulus myosin being intrinsically insensitive to changes of temperature, ionic strength and pH. In vitro motility assays were used to quantify the speed of unregulated actin filaments on myosin purified from F. heteroclitus glycolytic skeletal muscle. Filament speed was 2.07±0.17 μm s⁻¹ at 26°C, ionic strength (Γ/2) of 0.08 M Γ/2 and pH 7.4. Speed increased as temperature increased over the range of 5–36°C with an activation energy (E _a) of 94.0±7.0 kJ mol⁻¹) and an enthalpy (ΔH ^‡) of 91.5±7.0 kJ mol⁻¹ at 20°C. A linear relationship between temperature and ATPase activity was also obtained with actin-activated myosin Mg²⁺-ATPase assays over the temperature range 5–35°C with E _a=59.9±2.4 kJ mol⁻¹ and ΔH ^‡=57.4±2.4 kJ mol⁻¹ at 20°C. There was little or no effect of ionic strength on filament speed over the range 0.19 M Γ/2–0.54 M Γ/2. Speed increased significantly at lower ionic strengths and was 7.9-fold higher at 0.08 M Γ/2 than at 0.19 M Γ/2. Speed increased with pH with a 16-fold increase between pH 6.7 and 7.4. These results indicate that changes in physiological parameters that include temperature, pH and ionic strength affect the function of unregulated F. heteroclitus myosin, and thus other factors must be responsible for the mummichog’s swimming performance being comparatively insensitive to environmental variation.     

Relevance of individual characteristics for thermoregulation during exercise in a hot-dry environment

The aim of this study was to investigate the relevance of individual characteristics for thermoregulation during prolonged cycling in the heat. For this purpose, 28 subjects cycled for 60 min at 60% VO_2peak in a hot-dry environment (36 ± 1°C; 25 ± 2% relative humidity, airflow 2.5 m/s). Subjects had a wide range of body mass (99–43 kg), body surface area (2.2–1.4 m²), body fatness (28–5%) and aerobic fitness level (VO_2peak = 5.0–2.1 L/min). At rest and during exercise, rectal and mean skin temperatures were measured to calculate the increase in body temperature (ΔT _body) during the trial. Net metabolic heat production (M _NET) and potential heat loss (by means of evaporation, radiation and convection) were calculated. Although subjects exercised at the same relative intensity, ΔT _body presented high between-subjects variability (range from 0.44 to 1.65°C). ΔT _body correlated negatively with body mass (r = −0.49; P < 0.01), body surface area (r = −0.47; P < 0.01) and T_body at rest (r = −0.37; P < 0.05), but it did not significantly correlate with body fatness (r = 0.12; P > 0.05). ΔT _body positively correlated with the body surface area/mass ratio (r = 0.46; P < 0.01) and the difference between M _NET and potential heat loss (r = 0.56; P < 0.01). In conclusion, a large body size (mass and body surface area) is beneficial to reduce ΔT _body during cycling exercise in the heat. However, subjects with higher absolute heat production (more aerobically fit) accumulate more heat because heat production may exceed potential heat loss (uncompensability).     

Thermal exchanges during sleep in anhidrotic ectodermal dysplasia

Summary Anhidrotic ectodermal dysplasia is a congenital syndrome characterized by the absence of sweat glands. A sweating test was performed on such a patient and proved his inability to sweat. Thermal exchanges during night sleep were then measured in this patient and compared with data obtained from a healthy control subject. Ambient conditions were as follows: dry bulb temperature 32.2°C, relative humidity 30%–40%, wind speed 0.7 m·s⁻¹. Polysomnographic recordings showed normal sleep patterns in both subjects, but a “first night effect” in the patient. Rectal (T _re) and mean skin temperatures and loss of mass were monitored continuously throughout the 8-h sleep recording. Loss of mass averaged 34.1 g·h⁻¹ in the patient vs 78.1 g·h⁻¹ in the control subject. No relationship with sleep stages was observed in the patient, in contrast to the control subject who experienced a decrease in evaporation during rapid eye movement sleep. Body temperatures varied little in the patient, but decreased until the 6th h of sleep in the control subject. On two occasions there was a 0.3°C fall in theT _re of the patient during two slow wave sleep (SWS) phases, while and loss of mass did not change. As thermolytic processes had not varied on these two occasions, it was concluded that the fall inT _re indicated a concomitant decrease in metabolic heat production, in agreement with the assumption that SWS represented a state of energy conservation.     

Extracellular bicarbonate and non-bicarbonate buffering against lactic acid during and after exercise

Defense of extracellular pH constancy against lactic acidosis can be estimated from changes (Δ) in lactic acid ([La]), [HCO₃⁻], pH and PCO₂ in blood plasma because it is equilibrated with the interstitial fluid. These quantities were measured in earlobe blood during and after incremental bicycle exercise in 13 untrained (UT) and 21 endurance-trained (TR) males to find out if acute and chronic exercise influence the defense. During exercise the capacity of non-bicarbonate buffers (β_nbi = −Δ[La] · ΔpH⁻¹ − Δ[HCO₃⁻] · ΔpH⁻¹) available for the extracellular fluid (mainly hemoglobin, dissolved proteins and phosphates) amounted to 32 ± 2(SEM) and 20 ± 2 mmol l⁻¹ in UT and TR, respectively (P < 0.02). During recovery β_nbi decreased to 14 (UT) and 12 (TR) mmol l⁻¹ (both P < 0.001) corresponding to values previously found at rest by in vivo CO₂ titration. Bicarbonate buffering (β_bi) amounted to 44–48 mmol l⁻¹ during and after exercise. The large exercise β_nbi seems to be mainly caused by an increasing concentration of all buffers due to shrinking of the extracellular volume, exchange of small amounts of HCO₃⁻ or H⁺ with cells and delayed HCO₃⁻ equilibration between plasma and interstitial fluid. Increase of [HCO₃⁻] during titration by these mechanisms augments total β and thus the calculated β_nbi more than β_bi because it reduces ΔpH and Δ[HCO₃⁻] at constant Δ[La]. The smaller rise in exercise β_nbi in TR than UT may be caused by an increased extracellular volume and an improved exchange of La⁻, HCO₃⁻ and H⁺ between trained muscles and blood.