Changes in exercise and post-exercise core temperature under different clothing conditions

 This study evaluates the effect of different levels of insulation on esophageal (T _es) and rectal (T _re) temperature responses during and following moderate exercise. Seven subjects completed three 18-min bouts of treadmill exercise (75% VO_2max, 22°C ambient temperature) followed by 30 min of recovery wearing either: (1) jogging shoes, T-shirt and shorts (athletic clothing); (2) single-knit commercial coveralls worn over the athletic clothing (coveralls); or (3) a Canadian Armed Forces nuclear, bacteriological and chemical warfare protective overgarment with hood, worn over the athletic clothing (NBCW overgarment). T _es was similar at the start of exercise for each condition and baseline T _re was ∼0.4°C higher than T _es. The hourly equivalent rate of increase in T _es during the final 5 min of exercise was 1.8°C, 3.0°C and 4.2°C for athletic clothing, coveralls and NBCW overgarment respectively (P<0.05). End-exercise T _es was significantly different between conditions [37.7°C (SEM 0.1°C), 38.2°C (SEM 0.2°C and 38.5°C (SEM 0.2°C) for athletic clothing, coveralls and NBCW overgarment respectively)] (P<0.05). No comparable difference in the rate of temperature increase for T _re was demonstrated, except that end-exercise T _re for the NBCW overgarment condition was significantly greater (0.5°C) than that for the athletic clothing condition. There was a drop in T _es during the initial minutes of recovery to sustained plateaus which were significantly (P<0.05) elevated above pre-exercise resting values by 0.6°C, 0.8°C and 1.0°C, for athletic clothing, coveralls, and NBCW overgarment, respectively. Post-exercise T _re decreased very gradually from end-exercise values during the 30-min recovery. Only the NBCW overgarment condition T _re was significantly elevated (0.3°C) above the athletic clothing condition (P<0.05). In conclusion, T _es is far more sensitive in reflecting the heat stress of different levels of insulation during exercise and post-exercise than T _re. Physiological mechanisms are discussed as possible explanations for the differences in response. Received: 30 June 1998 / Accepted: 19 February 1999     

HSP70 expression in the CNS in response to exercise and heat stress in rats

We havepreviously documented the regional distribution of 70-kDa heat shockprotein (HSP70) in brains of rats made hyperthermic by brief exposureto high-powered microwaves (HPM; 2.06 GHz). We now compare HSP70expression induced by HPM exposure to that induced by exertionaland/or environmental heat stress. Rats were chronicallyimplanted with a temperature probe guide in the hypothalamic region ofthe brain (T_br). After recovery,the following treatment groups were examined: HPM; sham exposed;treadmill exercise at room temperature (24°C; Ex-1); treadmillexercise in a warm environment (34°C; Ex-2); and sedentary groups(Sed-1 and Sed-2), in which ambient temperature was adjusted so thatthe T_br mimicked the T_br in the corresponding exercisegroups. Significant HSP70 expression occurred only in the hyperthermic(Ex-2, Sed-2, and HPM) groups. The pattern of HSP70 expression wassimilar among Ex-2 and Sed-2 rats but differed from that in HPM rats.We conclude that 1) the pattern ofHSP70 expression differs between HPM and nonmicrowave heating, and2) exercise alone was not sufficientto induce central HSP70 expression.

     

Spontaneously reduced body temperature and gasping ability as a mechanism of extreme tolerance to asphyxia in neonatal rats

The aim of the investigation was to verify our hypothesis that extreme tolerance of newborn rodents to anoxia is determined by their ability to maintain reduced body temperature and to keep on gasping.Newborn Wistar rats were used. In separate experiments we checked (1) effect of extreme thermal conditions on rectal temperature (T_re) of the newborns in their nests; (2) effect of ambient temperature (T_a) on oxygen consumption; (3) effects of controlled changes in T_re on thermoregulatory and respiratory responses to anoxia and on anoxia tolerance.In their nests rat pups controlled T_re at 32–36 °C while the T_re–T_a difference changed within a range of 1–20 °C. The lowest oxygen consumption of ∼24 ml O₂ kg⁻¹ min⁻¹ was recorded at T_a of 32 °C. Pups, exposed to anoxia at their normal T_re of 33 °C, were able to decrease T_re by another 1.7 °C and they kept on extremely slow and quiescent gasping for scheduled 25 min. In contrast, rats at T_re of 37 °C and 39 °C reached a critical phase of accelerated and shallow gasping after 14.95±0.40 min and 9.25±0.30 min, respectively.In conclusion, reduced T_re and unique gasping ability make newborn rats extremely tolerant to asphyxia.     

THERMOREGULATORY EFFECT IN HUMANS OF SUPPRESSED ENDOGENOUS MELATONIN BY PRE-SLEEP BRIGHT-LIGHT EXPOSURE IN A COLD ENVIRONMENT

This study investigated the physiological function of suppressed melatonin through thermoregulation in a cold environment. Interactions between thermoregulation directly affected by exposure to a cold environment and indirectly affected by endogenous melatonin suppression by bright-light exposure were examined. Ten male subjects were exposed to two different illumination intensities (30 and 5000 lux) for 4.5?h, and two different ambient temperatures (15 and 27°C) for 2?h before sleep under dark and thermoneutral conditions. Salivary melatonin level was suppressed by bright light (p?<?0.001), although the ambient temperature condition had no significant effect on melatonin. During sleep, significant effects of pre-sleep exposure to a cold ambient temperature (p?<?0.001) and bright light (p?<?0.01) on rectal temperature (T_re) were observed. Pre-sleep, bright-light exposure led to an attenuated fall in T_re during sleep. Moreover, T_re dropped more precipitously after cold exposure than thermoneutral conditions (cold: ?0.54?±?0.07°C/h; thermoneutral: ?0.16?±?0.03°C/h; p?<?0.001). Pre-sleep, bright-light exposure delayed the nadir time of T_re under thermoneutral conditions (p?<?0.05), while cold exposure masked the circadian rhythm with a precipitous decrease in T_re. A significant correlation between the T_re nadir and melatonin level (r?=??0.774, p?<?0.05) indicated that inter-individual differences with higher melatonin levels lead to a reduction in T_re after cold exposure. These results suggest that suppressed endogenous melatonin inhibits the downregulation of the body temperature set-point during sleep. (Author correspondence: ishibasi@design.kyushu-u.ac.jp)     

Relevance of individual characteristics for human heat stress response is dependent on exercise intensity and climate type

Multiple heterogeneous groups of subjects (both sexes and a wide range of maximal oxygen uptake V˙O_{2 max} , body mass, body surface area (A _D),% body fat, and A _D/mass coefficient) exercised on a cycle ergometer at a relative (%V˙O_2max, REL) or an absolute (60 W) exercise intensity in a cool (CO 21°C, 50% relative humidity), warm humid (WH 35°C, 80%) and a hot dry (HD 45°C, 20%) environment. Rectal temperature (T _re) responses were analysed for the influence of the individual's characteristics, environment and exercise intensity. Exposures consisted of 30-min rest, followed by 60-min exercise. The T _re was negatively correlated with mass in all conditions. Body mass acted as a passive heat sink in all the conditions tested. While negatively correlated with V˙O_{2 max} and V˙O_{2 max} per kilogram body mass in most climates, T _re was positively correlated with V˙O_{2 max} and V˙O_{2 max} per kilogram body mass in the WH/REL condition. Thus, when evaporative heat loss was limited as in WH, the higher heat production of the fitter subjects in the REL trials determined T _re and not the greater efficiency for heat loss associated with high V˙O_{2 max} . Body fatness significantly affected T _re only in the CO condition, where, with low skin blood flows (measured as increases in forearm blood flow), the insulative effect of fat was pronounced. In the warmer environments, high skin blood flows offset the resistance offered by peripheral adipose tissue. Contrary to other studies, T _re was positively correlated with A _D/mass coefficient for all conditions tested. For both exercise types used, being big (a high heat loss area and heat capacity) was apparently more beneficial from a heat strain standpoint than having a favourable A _D/mass coefficient (high in small subjects). The total amount of variance in T _re responses which could be attributed to individual characteristics was dependent on the climate and the type of exercise. Though substantial for absolute exercise intensities (52%–58%) the variance explained in T _re differed markedly for relative intensities: 72% for the WH climate with its limited evaporative capacity, and only 10%–26% for the HD and CO climates. The results showed that individual characteristics play a significant role in determining the responses of body core temperature in all conditions tested, but their contribution was low for relative exercise intensities when evaporative heat loss was not restricted. This study demonstrated that effects of individual characteristics on human responses to heat stress cannot be interpreted without taking into consideration both the heat transfer properties of the environment and the metabolic heat production resulting from the exercise type and intensity chosen. Their impact varies substantially among conditions. Accepted: 4 July 1997     

Rectal and esophageal temperatures during upper- and lower-body exercise

This study investigated the question: is core temperature measurement influenced by whether exercise involves predominantly upper- or lower-body musculature? Healthy men were allocated to three groups: treadmill ergometry (T) n=4, cycle ergometry (C) n=6 and arm crank ergometry (AC) n=5. Subjects underwent an incremental exercise test to exhaustion on an exercise-specific ergometer to determine maximum/peak oxygen consumption (V˙O_2max). One week later subjects exercised for 36 min on the same ergometer at approximately 65% V˙O_2max while temperatures at the rectum (T _re) and esophagus (T _es) were simultaneously measured. The V˙O_2max (l · min⁻¹) for groups T [4.76 (0.50)] and C [4.35 (0.30)] was significantly higher than that for the AC group [2.61 (0.24)]. At rest, T _re was significantly higher than T _es in all groups (P<0.05). At the end of submaximal exercise in the C group, T _re [38.32 (0.11)°C] was significantly higher than T _es [38.02 (0.12)°C, P<0.05]. No significant differences between T _re and T _es at the end of exercise were noted for AC and T groups. The temperature difference (T _diff) between T _re and T _es was dissimilar at rest in the three groups; however, by the end of exercise T _diff was approximately 0.2°C for each of the groups, suggesting that at the end of steady-state exercise T _re can validly be used to estimate core temperature. Accepted: 3 November 1997     

The effect of body temperature on the hunting response of the middle finger skin temperature

The relationship between body temperature and the hunting response (intermittent supply of warm blood to cold exposed extremities) was quantified for nine subjects by immersing one hand in 8°C water while their body was either warm, cool or comfortable. Core and skin temperatures were manipulated by exposing the subjects to different ambient temperatures (30, 22, or 15°C), by adjusting their clothing insulation (moderate, light, or none), and by drinking beverages at different temperatures (43, 37 and 0°C). The middle finger temperature (T _fi) response was recorded, together with ear canal (T _ear), rectal (T _re), and mean skin temperature (Tˉ _sk). The induced mean T _ear changes were −0.34 (0.08) and +0.29 (0.03)°C following consumption of the cold and hot beverage, respectively. Tˉ _sk ranged from 26.7 to 34.5°C during the tests. In the warm environment after a hot drink, the initial finger temperature (T _fi,base) was 35.3 (0.4)°C, the minimum finger temperature during immersion (T _fi,min) was 11.3 (0.5)°C, and 2.6 (0.4) hunting waves occurred in the 30-min immersion period. In the neutral condition (thermoneutral room and beverage) T _fi,base was 32.1 (1.0)°C, T _fi,min was 9.6 (0.3)°C, and 1.6 (0.2) waves occurred. In the cold environment after a cold drink, these values were 19.3 (0.9)°C, 8.7 (0.2)°C, and 0.8 (0.2) waves, respectively. A colder body induced a decrease in the magnitude and frequency of the hunting response. The total heat transferred from the hand to the water, as estimated by the area under the middle finger temperature curve, was also dependent upon the induced increase or decrease in T _ear and Tˉ _sk. We conclude that the characteristics of the hunting temperature response curve of the finger are in part determined by core temperature and Tˉ _sk. Both T _fi,min and the maximal finger temperature during immersion were higher when the core temperature was elevated; Tˉ _sk seemed to be an important determinant of the onset time of the cold-induced vasodilation response. Accepted: 29 April 1997     

Hypothalamic Temperature of Rats Subjected to Treadmill Running in a Cold Environment

Different strategies for cooling the body prior to or during physical exercise have been shown to improve prolonged performance. Because of ethical and methodological issues, no studies conducted in humans have evaluated the changes in brain temperature promoted by cooling strategies. Therefore, our first aim sought to measure the hypothalamic temperature (T_hyp) of rats subjected to treadmill running in a cold environment. Moreover, evidence suggests that T_hyp and abdominal temperature (T_abd) are regulated by different physiological mechanisms. Thus, this study also investigated the dynamics of exercise-induced changes in T_hyp and T_abd at two ambient temperatures: 25°C (temperate environment) and 12°C (cold). Adult male Wistar rats were used in these experiments. The rats were implanted with a guide cannula in the hypothalamus and a temperature sensor in the abdominal cavity. After recovery from this surgery, the rats were familiarized with running on a treadmill and were then subjected to the two experimental trials: constant-speed running (20 m/min) at 12°C and 25°C. Both T_hyp and T_abd increased during exercise at 25°C. In contrast, T_hyp and T_abd remained unchanged during fatiguing exercise at 12°C. The temperature differential (i.e., T_hyp - T_abd) increased during the initial min of running at 25°C and thereafter decreased toward pre-exercise values. Interestingly, external cooling prevented this early increase in the temperature differential from the 2^nd to the 8^th min of running. In addition, the time until volitional fatigue was higher during the constant exercise at 12°C compared with 25°C. Together, our results indicate that T_hyp and T_abd are regulated by different mechanisms in running rats and that external cooling affected the relationship between both temperature indexes observed during exercise without environmental thermal stress. Our data also suggest that attenuated hypothalamic hyperthermia may contribute to improved performance in cold environments.     

Irreversible changes in barley leaf chlorophyll fluorescence detected by the fluorescence temperature curve in a linear heating/cooling regime

The chlorophyll fluorescence (F) temperature curves in a linear time-temperature heating/cooling regime were used to study heat-induced irreversible F changes in primary green leaves of spring barley (Hordeum vulgare L. cv. Akcent). The leaf segments were heated in a stirred water bath at heating rates of 0.0083, 0.0166, 0.0333, and 0.0500 °C s⁻¹ from room temperature up to maximal temperature T _m and then linearly cooled to 35 °C at the same rate. The F intensity was measured by a pulse-modulated technique. The results support the existence of the two critical temperatures of irreversible F changes postulated earlier, at 45–48 and 53–55 °C. The critical temperatures are slightly dependent on the heating rate. Two types of parameters were used to characterize the irreversibility of the F changes: the coefficient of irreversibility μ defined as the ratio of F intensity at 35 °C at the starting/ending parts of the cycle and the slopes of tangents of linear parts of the F temperature curve. The dependence of μ on T _m revealed a maximum, which moved from 54 to 61 °C with the increasing heating/cooling rate v from 0.0083 to 0.0500 °C s⁻¹, showing two basic phases of the irreversible changes. The Arrhenius and Eyring approaches were applied to calculate the activation energies of the initial increase in μ. The values varied between 30 and 50 kJ mol⁻¹ and decreased slightly with the increasing heating rate.     

Body temperature in the mouse,hamster, and rat exposed to radiofrequency radiation: An interspecies comparison

• 1.1.|Colonic temperatures of BALB/c and CBA/J mice, golden hamsters, and Sprague-Dawley rats were taken immediately after exposure for 90 min to radiofrequency (RF) radiation.
• 2.2.|Exposures were made in 2450 MHz (mouse and hamster) or 600 MHz (rat) waveguide exposure systems while the dose rate, specific absorption rate (SAR), was continuously recorded. Experiments were performed on naive, unrestrained animals at ambient temperatures (T_a) of 20 and 30°C.
• 3.3.|Body mass and T_a) were found to be significant factors in influencing the threshold SAR for the elevation of colonic temperature. The threshold SARs at T_a's of 20 and 30°C were respectively: 27.5 and 12.1 W/kg for the BALB/c mouse; 40.7 and 8.5 W/kg for the CBA/J mouse; 8.7 and 0.61 W/kg for the golden hamster; and 1.58 and 0.4 W/kg for the Sprague-Dawley rat.
• 4.4.|The relationship between threshold SAR or SAR for a 1.0°C elevation in colonic temperature vs body mass were linearly and inversely related on a double logarithmic plot. The results of this study suggest that the thermoregulatory sensitivity to RF radiation in these rodent species is heavily dependent on body mass and T_a.

     

The effect of low- and high-power microwave irradiation on in vitro grown <Emphasis Type="Italic">Sequoia</Emphasis> plants and their recovery after cryostorage

Two distinct microwave power levels and techniques have been studied in two cases: low-power microwave (LPM) irradiation on in vitro Sequoia plants and high-power microwave (HPM) exposure on recovery rates of cryostored (?196°C) Sequoia shoot apices. Experimental variants for LPM exposure included: (a) in vitro plants grown in regular conditions (at 24 ± 1°C during a 16-h light photoperiod with a light intensity of 39.06 μEm^?2 s^?1 photosynthetically active radiation), (b) in vitro plants grown in the anechoic chamber with controlled environment without microwave irradiation, and (c) in vitro plants grown in the anechoic chamber with LPM irradiation for various times (5, 15, 30, 40 days). In comparison to control plants, significant differences in shoot multiplication and growth parameters (length of shoots and roots) were observed after 40 days of LPM exposure. An opposite effect was achieved regarding the content of total soluble proteins, which decreased with increasing exposure time to LPM. HPM irradiation was tested as a novel rewarming method following storage in liquid nitrogen. To our knowledge, this is the first report using this type of rewarming method. Although, shoot tips subjected to HPM exposure showed 28% recovery following cryostorage compared to 44% for shoot tips rewarmed in liquid medium at 22 ± 1 °C, we consider that the method represent a basis and can be further improved. The results lead to the overall conclusion that LPM had a stimulating effect on growth and multiplication of in vitro Sequoia plants, while the HPM used for rewarming of cryopreserved apices was not effective to achieve high rates of regrowth after liquid nitrogen exposure.     

Thermal status of wet-suited divers using closed circuit O2 apparatus in sea water of 17–18.5°C

A wet suit may not provide adequate thermal protection when diving in moderately cold water (17–18°C), and any resultant mild hypothermia may impair performance during prolonged diving. We studied heat exchange during a dive to a depth of 5 m in sea water (17–18.5°C) in divers wearing a full wet suit and using closed-circuit oxygen breathing apparatus. Eight fin swimmers dived for 3.1 h and six underwater scooter (UWS) divers propelled themselves through the water for 3.7 h. The measurements taken throughout the dive were the oxygen pressure in the cylinder and skin and rectal temperatures (T _re). Each subject also completed a cold score questionnaire. The T _re decreased continuously in all subjects. Oxygen consumption in the fin divers (1.40 l · min⁻¹) was higher than that of the UWS divers (1.05 l · min⁻¹). The mean total insulation was 0.087°C · m² · W⁻¹ in both groups. Mean body insulation was 37% of the total insulation (suit insulation was 63%). The reduction in T _re over the 1st hour was related to subcutaneous fat thickness. There was a correlation between cold score and T _re at the end of 1 h, but not after that. A full wet suit does not appear to provide adequate thermal protection when diving in moderately cold water. Accepted: 21 January 1997     

The influence of exposure to cold on the incidence of gastric ulcers after reserpine in rats

Groups of female rats (n=20) exposed from 0 to 13 days to T_a's from 8.0° to 29.0°C were given 2.5 mg/kg reserpine i.p.; T_re and gastric ulcers (GU) were recorded 24 h afterwards. At exposure temperatures below 21.0°C there was a highly significant positive correlation between T_a and T_re (r=0.85) and a negative correlation between T_a and GU (r=–0.92). The GU rate after reserpine was not affected by temperatures above 21°C up to 29°C. Below 16.5°C a difference of the reserpine response was found between rats with less and more than 3 days acclimation to a given T_a. In rats with less than 3 days acclimation the mean T_re after reserpine was 1.0°C lower and the mean GU rate was 1.7 ulcers/rat higher than in rats with more than 3 days acclimation. The correlation of T_re with GU rate revealed that the mean number of GU increased with decreasing T_re · T_re and GU were negatively correlated in both series of experiments: r= –0.92 for non-acclimated rats and r= –0.95 for >3d acclimated rats. Cold acclimation of rats for 8 days at 13.0 °C or 13 days at 10.0 °C did not significantly affect T_re and the GU rate if the rats were taken to 21.0°C after reserpine administration. The results show that with and without cold-acclimation the extent of hypothermia in rats after a standard reserpine dose depends on the prevailing ambient temperature below the comfort range and the GU rate depends on the extent of the hypothermia.     

Ice slurry ingestion before and during exercise inhibit the increase in core and deep-forehead temperatures in the second half of the exercise in a hot environment

Many studies have reported that pre-exercise ice slurry ingestion improves exercise performance; however, it may increase the risk of developing heat stroke. Some studies have suggested that pre-exercise ice slurry ingestion accelerates the core temperature increase that occurs during exercise. Therefore, this study aimed to investigate whether the ingestion of ice slurry before and during exercise can inhibit this acceleration. Moreover, we measured the deep-forehead temperature (T_{deep head}) to determine whether ice slurry ingestion before and during exercise can maintain this reduction in brain temperature. Eleven male participants at room temperature (24 °C, 50% relative humidity [RH]) ingested 7.5 g/kg of ice slurry or a thermoneutral sports drink within 30 min. They then exercised for approximately 60 min at 50% of the maximal oxygen uptake in a hot environment (34 °C, 50% RH) while ingesting 1.25 g/kg of ice slurry or a thermoneutral sports drink every 10 min. Rectal temperature (T_re), T_{deep head}, forehead skin temperature, mean skin temperature, heart rate, nude body mass, and urine specific gravity were measured as physiological indices. The rating of perceived exertion, thermal sensation, and thermal comfort were measured at 5-min intervals throughout the experiment. The T_re and T_{deep head} during the second half of the exercise session were significantly reduced after ingestion of the ice slurry before and during exercise (p < 0.05). In addition, the rate of increase in T_re and T_{deep head} slowed during the second half of the exercise session after the ingestion of the ice slurry before and during exercise (p < 0.05). These results indicate that the increases in T_re and T_{deep head}, reflecting brain temperature in the second half of the exercise session, were significantly inhibited by ice slurry ingestion before and during exercise.     

Exercise time to fatigue and the critical limiting temperature: effect of hydration

The purpose of this study was to investigate the effect of active pre-warming combined with three regimens of fluid ingestion: (1) fluid replacement equal to sweat rate (FF), (2) fluid replacement equal to half the sweat rate (HF), and (3) no fluid replacement (NF). Eight males cycled to voluntary fatigue at 70% of peak power output (PPO) in 31.3±0.4°C, 63.3±1.2% relative humidity in a randomised fashion in either of FF, HF or NF conditions. For each trial the time to fatigue test was preceded by 2×20 min active pre-warming periods where subjects also cycled at 70% PPO. Subjects commenced each exercise period with identical rectal temperatures (T_re). The rate of increase in T_re for each condition during the first 20 min of active pre-warming was not different. However, the rate of increase in T_re was significantly reduced in the second active pre-warming period for all fluid conditions but no differences between conditions were noted. During the fatigue test, the rate of increase in T_re for FF was 0.29°C h⁻¹ and 0.58°C h⁻¹ for HF but were not significantly different. The rate of increase in T_re for the NF trial was 0.92°C h⁻¹ and was significantly higher compared to the FF trial. Overall mean skin temperatures and mean body temperatures were higher for NF compared to FF and HF. The rate of heat storage during the fatigue test was similar for FF (80.1±11.7 W m⁻²) and HF (73.0±13.7 W m⁻²) conditions but increased to 155.8±31.2 W m⁻² (P<0.05) in the NF trial. The results indicate that fluid ingestion equal to sweat rate has no added benefit over fluid ingestion equal to half the sweat rate in determining time to fatigue over 40 min of sub-maximal exercise in warm humid conditions. Fluid restriction accelerates the rate of increase in T_re after 40 min of exercise, thereby reducing the time to fatigue. The data support the model that anticipation of impending thermal limits reduces efferent command to working skeletal muscle ensuring cellular preservation.     

Heat acclimation does not modify autonomic responses to core cooling and the skin thermal comfort zone

Exercise heat acclimation (HA) is known to magnify the sweating response by virtue of a lower threshold as well as increased gain and maximal capacity of sweating. However, HA has been shown to potentiate the shivering response in a cold-air environment. We investigated whether HA would alter heat loss and heat production responses during water immersion. Twelve healthy male participants underwent a 10-day HA protocol comprising daily 90-min controlled-hyperthermia (target rectal temperature, T_re 38.5 °C) exercise sessions. Preceding and following HA, the participants performed a maximal exercise test in thermoneutral conditions (ambient temperature 23 °C, relative humidity 50%) and were, following exercise, immersed in 28 °C water for 60 min. Thermal comfort zone (TCZ) was also assessed with participants regulating the temperature of a water-perfused suit during heating and cooling. Baseline pre-immersion T_re was similar pre- and post-HA (pre: 38.33 ± 0.33 °C vs post: 38.12 ± 0.36 °C, p = 0.092). The T_re cooling rate was identical pre-to post-HA (−0.03 ± 0.01 °C·min⁻¹, p = 0.31), as was the vasomotor response reflected in the forearm-fingertip temperature difference. Shivering thresholds (p = 0.43) and gains (p = 0.61) were not affected by HA. TCZ was established at similar temperatures, with the magnitude in regulated water temperature being 7.6 (16.3) °C pre-HA and 5.1 (24.7) °C post-HA (p = 0.65). The present findings suggest that heat production and heat loss responses during whole body cooling as well as the skin thermal comfort zone remained unaltered by a controlled-hyperthermia HA protocol.     

Dissecting seed dormancy and germination in Aquilegia barbaricina,through thermal kinetics of embryo growth

• Threshold‐based thermal time models provide insight into the physiological switch from the dormant to the non‐dormant germinating seed.
• This approach was used to quantify the different growth responses of the embryo of seeds purported to have morphophysiological dormancy (MPD) through the complex phases of dormancy release and germination. Aquilegia barbaricina seeds were incubated at constant temperatures (10–25 °C) and 25/10 °C, without pre‐treatment, after warm+cold stratification (W+C) and GA₃ treatment. Embryo growth was assessed and the time of testa and endosperm rupture scored. Base temperatures (T_b) and thermal times for 50% (θ₅₀) of embryo growth and seed germination were calculated.
• W+C enabled slow embryo growth. W+C and GA₃ promoted rapid embryo growth and subsequent radicle emergence. The embryo internal growth base temperature (T_be) was ca. 5 °C for W+C and GA₃‐treated seeds. GA₃ treatment also resulted in similar T_b estimates for radicle emergence. The thermal times for embryo growth (θ_e50) and germination (θ_g50) were four‐ to six‐fold longer in the presence of GA₃ compared to W+C.
• A. barbaricina is characterised by a multi‐step seed germination. The slow embryo growth during W+C reflects continuation of the maternal programme of development, whilst the thermal kinetics of both embryo and radicle growth after the removal of physiological dormancy are distinctly different. The effects of W+C on the multiphasic germination response in MPD seeds are only partially mimicked by 250 mg·l^?1 GA₃. The thermal time approach could be a valid tool to model thermal kinetics of embryo growth and radicle protrusion.

     

Daily activity and body temperature

Body temperature varies between 36 and 39° C in states ranging from sleep to high levels of sustained exercise, but it is not known whether this continuum of body temperature is related to a continuum of activity. Calorimetric studies of sedentary days were undertaken with four levels of food intake, men doing mild sustained exercise, and men and women walking and cycling vigorously. Steady states of metabolism were followed by slow exponential changes to steady states of heat loss (Q), followed in turn by changes in rectal temperature (T _re). Regression analysis showed a continuous, curvilinear relationship between Q andT _re from the low end of the activity spectrum (50 W) to progressively higher levels of exercise (600 W). These related continua of activity and body temperature appear to be the result of heat regulation.     

Heat defense control in an experimental heat disorder

Both whole-body heat exposure and intraperitoneal heating (IPH) result in a body temperature (T _b) fall that occurs once heating is abated (”hyperthermia- induced hypothermia”). This phenomenon involves a decrease in the threshold T _b (T _b-thresh) for activation of metabolic heat production (cold defense). Whether the T _b-thresh for ear skin vasodilation (heat defense) also changes during hyperthermia-induced hypothermia remains unknown. In experiment 1, we applied IPH to guinea pigs by perfusing water through a preimplanted intraperitoneal thermode and delivered the total heat load of either approximately 1.5 kJ (”short” IPH; perfusion duration: 14 min) or approximately 3.0 kJ (”long” IPH; 40 min). Short IPH caused skin vasodilation and a 1.1°C rise in T _b; no hypothermia occurred when IPH ceased. Long IPH caused vasodilation and hyperthermia of a comparable magnitude (1.4°C) that were followed by a T _b fall to 1.9°C below the preheating value. In experiment 2, the T _b-thresh for skin vasodilation was measured twice: at the beginning of long IPH and at the nadir of the post-IPH hypothermia. The two T _b-thresh values were 39.0 (SEM 0.1)°C and 39.2 (SEM 0.2)°C respectively. In the controls, the T _b-thresh was measured at the beginning and after short IPH; both control values were 39.0 (SEM 0.2)°C. We conclude that the hyperthermia- induced hypothermia, although previously shown to be coupled with a decrease in the T _b-thresh for cold defense, occurs without any substantial change in the T _b-thresh for heat defense. We speculate that postheating thermoregulatory disorders are associated with threshold dissociation, thus representing the poikilothermic (wide dead-band) type of T _b control. Received: 20 August 1999 / Revised: 18 November 1999 / Accepted: 24 November 1999