Human plasma inflammatory response during 5 days of exercise training in the heat

The aim of the present investigation was to examine the response of 80 plasma inflammatory analytes during five days of exercise in a hot (38 °C, 40% relative humidity) environment. 15 male participants (25±4yrs, 54±6 ml kg⁻¹ min⁻¹ VO₂ max), with no heat exposure within the previous 3 weeks, were asked to cycle in a hot environment at 70% of their VO₂ max workload until their terminal temperature was obtained, for 5 consecutive days. Terminal temperature was determined as the core temperature at volitional exhaustion or a core temperature of 39.5 °C, whichever came first. Blood samples were collected pre- and post-exercise on day 1 and day 5. Pre-trial urine specific gravity and body weight was not different on day 1 and day 5. Exercise time and heart rate at terminal temperature did not change during the five days. Of the 52 plasma analytes that increased in concentration on day 1, only 30 demonstrated increased concentrations at terminal temperature on day 5. Resting concentrations of 18, both pro- (IL-12p40, IL-15) and anti-inflammatory (IL-1ra, IL-10, IL-13) analytes were elevated on day 5 compared to day 1. We conclude that individuals completing consecutive days of exercise in the heat, but not definitively attaining heat acclimation, have increased resting levels of many inflammatory analytes associated with heat illness, but also demonstrate a reduced inflammatory response to a subsequent bout of exercise in the heat.     

Effects of heat acclimatisation on work tolerance and thermoregulation in trained tropical natives

This study aimed to investigate the effects of heat acclimatisation on thermoregulatory responses and work tolerance in trained individuals residing in the tropics. Eighteen male trained soldiers, who are native to a warm and humid climate, performed a total of four heat stress tests donning the Skeletal Battle Order (SBO, 20.5 kg) and Full Battle Order (FBO, 24.7 kg) before (PRE) and after (POST) a 10-day heat acclimatisation programme. The trials were conducted in an environmental chamber (dry bulb temperature: 32 °C, relative humidity: 70%, solar radiation: 400 W/m²). Excluding the data sets of which participants fully completed the heat stress tests (210 min) before and after heat acclimatisation, work tolerance was improved from 173±30 to 201±18 min (∼21%, p<0.05, n=9) following heat acclimatisation. Following heat acclimatisation, chest skin temperature during exercise was lowered in SBO (PRE=36.7±0.3 vs. POST=36.5±0.3 °C, p<0.01) and FBO (PRE=36.8±0.4 vs. POST=36.6±0.3 °C, p<0.01). Ratings of perceived exertion were decreased with SBO and FBO (PRE=11±2; POST=10±2; p<0.05) after heat acclimatisation. Heat acclimatisation had no effects on baseline body core temperature, heart rate and sweat rate across trials (p>0.05). A heat acclimatisation programme improves work tolerance with minimal effects on thermoregulation in trained tropical natives.     

Duration tenacity: A method for assessing acclimatory capacity of the Antarctic limpet, Nacella concinna

The limpet, Nacella concinna, collected from the Antarctic Peninsula (67°S), was incubated at − 0.3 °C and 2.9 °C for 9 months to test if the previously reported absence of acclimation capacity in Antarctic marine ectotherms could be due to the extended time it takes for them to adjust their physiology to a new stable state. Acclimation was tested through acute measurements of upper lethal limit and a modified measure of tenacity, that tested muscle capacity by measuring the length of time that N. concinna were able to remain attached to the substratum at different temperatures. Both measures acclimated in response to incubation to the higher temperature. Lethal limits were elevated in N. concinna incubated at 2.9 °C (8.1 ± 0.3 °C) compared to those incubated at − 0.3 °C (6.9 ± 0.4 °C). 2.9 °C incubated N. concinna also had a maximum tenacity at 2.1 °C, a higher temperature than the maximum tenacity of those incubated at − 0.3 °C, which occurred at − 1.0 °C. This study is the first to show that the Antarctic limpet can acclimate its physiology, but that it requires a greater period of time for acclimation to occur than previous studies have allowed for.     

Time-course for attainment and reversal of acclimation to constant temperature in two Ceratitis species

Acclimation in the thermal tolerance range of insects occurs when they are exposed to novel temperatures in the laboratory. In contrast to the large number of studies that have tested for the ability of insects to acclimate, relatively few have sought to determine the time-course for attainment and reversal of thermal acclimation. In this study the time required for the Mediterranean fruit fly, Ceratitis capitata Wiedemann, and the Natal fruit fly, Ceratitis rosa Karsch, to acclimate to a range of constant temperatures was tested by determining the chill-coma recovery time and heat knock-down time of flies that had been exposed to novel benign temperatures for different durations. The time required for reversal of acclimation for both Ceratitis species was also determined after flies had been returned to the control temperature. Acclimation to 31 °C for only one day significantly improved the heat knock-down time of C. capitata, but also led to slower recovery from chill-coma. Heat knock-down time indicated that acclimation was achieved after only one day in C. rosa, but it took three days for C. rosa to exhibit a significant acclimation response to a novel temperature of 33 °C when measured using chill-coma recovery time. Reversal of acclimation after return to initial temperature conditions was achieved after only one day in both C. capitata and C. rosa. Adult C. capitata held at 31.5 °C initially exhibited improved heat knock-down times but after 9 days the heat knock-down time of these flies had declined to levels not significantly different from that of control flies held at the baseline temperature of 24 °C. In both Ceratitis species, heat knock-down time declined with age whereas chill-coma recovery time increased with age, indicating an increased susceptibility to high and low temperatures, respectively.     

Thermopreference,tolerance and metabolic rate of early stages juvenile Octopus maya acclimated to different temperatures

Thermopreference, tolerance and oxygen consumption rates of early juveniles Octopus maya (O. maya; weight range 0.38–0.78 g) were determined after acclimating the octopuses to temperatures (18, 22, 26, and 30 °C) for 20 days. The results indicated a direct relationship between preferred temperature (PT) and acclimated temperature, the PT was 23.4 °C. Critical Thermal Maxima, (CTMax; 31.8±1.2, 32.7±0.9, 34.8±1.4 and 36.5±1.0) and Critical Thermal Minima, (CTMin; 11.6±0.2, 12.8±0.6, 13.7±1.0, 19.00±0.9) increased significantly (P<0.05) with increasing acclimation temperatures. The endpoint for CTMax was ink release and for CTMin was tentacles curled, respectively. A thermal tolerance polygon over the range of 18–30 °C resulted in a calculated area of 210.0 °C². The oxygen consumption rate increased significantly α=0.05 with increasing acclimation temperatures between 18 and 30 °C. Maximum and minimum temperature quotients (Q₁₀) were observed between 26–30 °C and 22–26 °C as 3.03 and 1.71, respectively. These results suggest that O. maya has an increased capability for adapting to moderate temperatures, and suggest increased culture potential in subtropical regions southeast of México.     

Shivering modulation in humans: Effects of rapid changes in environmental temperature

During cold exposure, increase in heat production is produced via the activation of shivering thermogenesis and nonshivering thermogenesis, the former being the main contributor to compensatory heat production in non-acclimatized humans. In rats, it has been demonstrated that shivering thermogenesis is modulated solely by skin thermoreceptors but this modulation has yet to be investigated in humans. The aim of this study was to determine if cold-induced shivering in humans can be modulated by cutaneous thermoreceptors in conditions where increases in heat loss can be adequately compensated by increases in thermogenic rate. Using a liquid-conditioned suit, six non-acclimatized men were exposed to cold (6 °C) for four 30 min periods, each of them separated by 15 min of heat exposure (33 °C). Core temperature remained stable throughout exposures whereas skin temperatures significantly decreased by 12% in average during the sequential cold/heat exposures compared to baseline (p<0.0001). Shivering intensity and metabolic rate increased significantly during 6 °C exposures (3.3±0.7% MVC, 0.40±0.0 L O₂/min, respectively) and were significantly reduced during 33 °C exposure (0.5±0.1% MVC, 0.25±0.0 L O₂/min; p<0.005 for both). Most importantly, shivering could be quickly and strongly inhibited during 33 °C exposure although skin temperature often remained below baseline values. In conclusion, under compensatory conditions, cutaneous thermoreceptors appear to be a major modulator of the shivering response in humans and seem to react rapidly to changes in the microclimate right next to the skin and to skin temperature.     

Ambient temperature modulates the magnitude of LPS-induced fevers in Pekin ducks

The consequences of variations in environmental temperature on innate immune responses in birds are by and large not known. We investigated the influence of ambient temperature on the febrile response in female Pekin ducks (Anas platyrhynchos). Ducks, implanted with temperature data loggers to measure body temperature, were injected with lipopolysaccharide (100 μg kg⁻¹) to evoke febrile responses and kept at ambient temperatures higher, within, and lower than their thermoneutral zone (n=10), and in conditions that simulated one day of a heat wave (n=6). Compared to the febrile response at thermoneutrality, at low temperatures, febrile responses were significantly attenuated; fevers reached lower magnitudes (from basal body temperature of 41.2±0.3 °C to a peak of 42.0±0.3 °C). In contrast, at high ambient temperatures, ducks rapidly developed significantly enhanced fevers, which reached markedly higher febrile peaks (from basal body temperature of 41.6 °C to a peak of 44.0 °C in a simulated heat wave when ambient temperature reached 40 °C). These results indicate that ambient temperature affects the febrile response in female Pekin ducks. Our findings reveal a key difference in febrile mediation between ducks and mammals, and have implications for avian survival because high environmental temperatures during febrile mediation could lead to febrile responses becoming physiologically deleterious.     

The effect of thermopreference on circadian thermoregulation in sprague-dawley and fisher 344 rats

Interstrain differences in thermoregulation of rats are important in biomedical research because subtleties in thermoregulatory sensitivities may greatly affect data collected. Little is known regarding how individual rodent strains differentially utilize behavioral thermal preference to regulate core temperature (Tc). Sprague-Dawley (SD) and Fischer 344 (F344) rats are known to have differences in thermoregulation including heat tolerance and are useful models to study interstrain differences in thermoregulation. Adult male SD and F344 rats of similar body size were implanted with radiotelemetry thermoprobes (DSI) to measure Tc and MA and housed in either a longitudinal temperature gradient with an ambient temperature (Ta) range of ∼15–40 °C to measure selected Ta (STa) or control environment maintained at a Ta of 23 °C. When continuously monitored for 48 h, Tc and MA increased at night, while STa decreased, according to their normal circadian cycle in both strains. SD rats were more active than F344 rats throughout the circadian cycle (SD gradient: day=12.9±1.2 m/h, night=32.1±2.4 m/h; F344 gradient: day=4.1±0.6 m/h, night=16.8±1.8 m/h; p<0.05 interstrain and circadian effects). The STa of each strain was greater during the daytime (SD: 26.4±0.2 °C; F344: 27.8±0.3 °C) than at night (SD: 24.7±0.3 °C; F344: 25.7±0.3 °C) confirming past studies that thermopreference during the day and night is greater than standard room temperature (∼23 °C). Correlations between MA and Tc suggest that MA has a greater effect on Tc in the F344 but not the SD strain when housed in a temperature gradient. There were significant strain differences in Tc depending on whether rats were housed in a temperature gradient. That is, the control F344 rats had a lower Tc during the transition from dark to light compared to rats housed in a gradient. Tc of the SD strain was unaffected by housing in the gradient. Rats are typically housed at a standard room temperature of 23 °C. However, the results demonstrate that when given the opportunity to behaviorally thermoregulate in a temperature gradient, the F344 strain selects a warmer environment that affects the regulation of Tc. This may be important in the experimenters' choice of ambient temperatures to house and study rats and other rodents.     

Effects of acclimation temperature on thermal tolerance, locomotion performance and respiratory metabolism in Acheta domesticus L. (Orthoptera: Gryllidae)

The effects of acclimation temperature on insect thermal performance curves are generally poorly understood but significant for understanding responses to future climate variation and the evolution of these reaction norms. Here, in Acheta domesticus, we examine the physiological effects of 7-9 days acclimation to temperatures 4 °C above and below optimum growth temperature of 29 °C (i.e. 25, 29, 33 °C) for traits of resistance to thermal extremes, temperature-dependence of locomotion performance (jumping distance and running speed) and temperature-dependence of respiratory metabolism. We also examine the effects of acclimation on mitochondrial cytochrome c oxidase (CCO) enzyme activity. Chill coma recovery time (CRRT) was significantly reduced from 38 to 13 min with acclimation at 33-25 °C, respectively. Heat knockdown resistance was less responsive than CCRT to acclimation, with no significant effects of acclimation detected for heat knockdown times (25 °C: 18.25, 29 °C: 18.07, 33 °C: 25.5 min). Thermal optima for running speed were higher (39.4-40.6 °C) than those for jumping performance (25.6-30.9 °C). Acclimation temperature affected jumping distance but not running speed (general linear model, p = 0.0075) although maximum performance (U_MAX) and optimum temperature (T_OPT) of the performance curves showed small or insignificant effects of acclimation temperature. However, these effects were sensitive to the method of analysis since analyses of T_OPT, U_MAX and the temperature breadth (T_BR) derived from non-linear curve-fitting approaches produced high inter-individual variation within acclimation groups and reduced variation between acclimation groups. Standard metabolic rate (SMR) was positively related to body mass and test temperature. Acclimation temperature significantly influenced the slope of the SMR-temperature reaction norms, whereas no variation in the intercept was found. The CCO enzyme activity remained unaffected by thermal acclimation. Finally, high temperature acclimation resulted in significant increases in mortality (60-70% at 33 °C vs. 20-30% at 25 and 29 °C). These results suggest that although A. domesticus may be able to cope with low temperature extremes to some degree through phenotypic plasticity, population declines with warmer mean temperatures of only a few degrees are likely owing to the limited plasticity of their performance curves.     

Effects of acclimation and diapause on the thermal tolerance of the two-spotted spider mite Tetranychus urticae

The two-spotted spider mite, Tetranychus urticae, is a worldwide pest species that overwinters as diapausing females. Cold hardening is presumed to start during diapause development to ensure the successful overwintering of this species. To address this hypothesis, we compared cold tolerance between non-diapausing and diapausing females. We measured supercooling point (SCP) and survival to acute cold stress by exposing the mites at a range of sub-zero temperatures (from −4 to −28 °C for 2 h). The mean SCPs of non-diapausing and diapausing females were −19.6±0.5 and −24.7±0.3 °C respectively, and freezing killed the mites. Diapausing females were significantly more cold tolerant than non-diapausing ones, with LT₅₀ of −19.7 and −13.3 °C, respectively. Further, we also examined the effects of cold acclimation (10 d at 0 or 5 °C) in non-diapausing and diapausing females. Our findings indicated that diapause decreased SCP significantly, while cold acclimation had no effect on the SCP except for non-diapausing females that were acclimated at 5 °C. Acclimation at 5 °C enhanced survival to acute cold stress in diapausing and non-diapausing females, with LT₅₀ of −22.0 and −17.1 °C, respectively. Altogether, our results indicate that T. urticae is a chill tolerant species, and that diapause and cold acclimation elevate cold hardiness in this species.     

The combined influence of sub-optimal temperature and salinity on the in vitro viability of Perkinsus marinus, a protistan parasite of the eastern oyster Crassostrea virginica

Perkinsus marinus is a major cause of mortality in eastern oysters along the Gulf of Mexico and Atlantic coasts. It is also well documented that temperature and salinity are the primary environmental factors affecting P. marinus viability and proliferation. However, little is known about the effects of combined sub-optimal temperatures and salinities on P. marinus viability. This in vitro study examined those effects by acclimating P. marinus at three salinities (7, 15, 25 ppt) to 10 °C to represent the lowest temperatures generally reached in the Gulf of Mexico, and to 2 °C to represent the lowest temperatures reached along the mid-Atlantic coasts and by measuring changes in cell viability and density on days 1, 30, 60 and 90 following acclimation. Cell viability and density were also measured in 7 ppt cultures acclimated to each temperature and then transferred to 3.5 ppt. The largest decreases in cell viability occurred only with combined low temperature and salinity, indicating that there is clearly a synergistic effect. The largest decreases in cell viability occurred only with both low temperature and salinity after 30 days (3.5 ppt, 2 °C: 0% viability), 60 days (3.5 ppt, 10 °C: 0% viability) and 90 days (7 ppt, 2 °C: 0.6 ± 0.7%; 7 ppt, 10 °C: 0.2 ± 0.2%).     

Blood volume measurement: The comparison of pulse dye densitometry and Dill and Costill's methods

In many clinical situations, it is crucial to determine circulating blood volume (BV) easily and to repeat this measurement. The Dye DensitoGram Analyzer® (DDG, Nihon Kohden Corp) measures semi-automatically BV, using an injection of IndoCyanine Green (ICG, 10 mg), and avoiding intermittent blood samples. The DDG was used during a 90-day microgravity simulation by Head-Down-Tilt bed rest (HDT) to measure BV and compared with the calculation of the plasma volume (PV) variations according to Dill and Costill's formula (DC). Seventeen healthy volunteers were included: 8 control subjects (Co) and 9 subjects submitted to a resistive exercise counter-measure (CM). Measurements were performed, one day before HDT, on days 3 and 90 of HDT and on day 9 after HDT. A double measurement of the BV was performed to assess the repeatability of this method. On the last day of HDT a significant decrease (p < 0.05) in the PV was noted with the DDG (Co: − 12.3 ± 5.7%, CM: − 9.0 ± 5.3%) and DC; (Co: − 4.7 ± 1.8%, CM: − 6.8 ± 2.5%). A good repeatability of the technique was shown with a low intrasubjects coefficient of variation (4.95 ± 0.95%) and an acceptable intersubjects coefficient of variation (15.30 ± 1.13%). No correlation was noted between DDG and DC (r² = 0.27). The DDG gives a good repeatability, not affected by the microgravity exposure. Thanks to its capacity to measure accurately the BV within 7-10 min, this device presents major advantages for clinical use and research purpose.     

Microscopic evaluation of vesicles shed by rat erythrocytes at elevated temperatures

It is hypothesized that the elevation of the temperature of the blood during heat stress may cause an increase of the shedding of erythrocyte membrane vesicles. Therefore, the increase of vesicle numbers following heat stress may be indicative of and proportional to the level heat stress. In order to test this hypothesis, erythrocytes and the vesicles shed by erythrocytes were collected from rat blood and analyzed after the elevation of body temperature by exposure to external heat. The images of erythrocytes and vesicles were analyzed by a custom light microscopy system with spatial resolution of better than 90 nm. The samples were observed in an aqueous environment and required no freezing, dehydration, staining, shadowing, marking or any other manipulation. The elevation of temperature from 36.7±0.3 to 40.3±0.4 °C resulted in significant increase of the concentration of vesicles in blood. At a temperature of 37 °C, mean vesicle concentrations and diameters found in rat blood were (1.4±0.2)×10⁶ vesicles/μL and 0.436±0.03 μm, respectively. The concentration of free vesicles increased after exposure to heat to (3.8±0.3)×10⁶ vesicles/μL. It was estimated that 80% of all vesicles found in rat blood are smaller than 0.45 μm. The increase in the number of vesicle associated with elevated temperatures may be indicative of the heat stress level and serve as diagnostic test of erythrocyte stability and heat resistance.     

Rectal,telemetry pill and tympanic membrane thermometry during exercise heat stress

We compared the accuracy of an ingestible telemetry pill method of core temperature (T_c) measurement and an infrared tympanic membrane thermometer to values from a rectal thermistor during exercise-induced heat stress. Ten well-trained subjects completed four exercise trials consisting of 40 min constant-load exercise at 63% of maximum work rate followed by a 16.1 km time trial at 30 °C and 70% relative humidity. Temperature at rest was not different between the three methods of T_c measurement (T_re: 37.2±0.3 °C; T_p: 37.2±0.2 °C; T_ty: 37.1±0.3 °C; P=0.40$P=0.40$). Temperature rose continuously during the exercise period (ΔT_re: 2.2±0.5 °C; ΔT_p: 2.2±0.5 °C; ΔT_ty: 1.9±0.5 ±°C and there were no differences between T_re and T_p measurements at any time throughout exercise (P=0.32$P=0.32$). While there were no differences between T_re and T_ty after 10 min (P=0.11$P=0.11$) and 20 min (P=0.06$P=0.06$) of exercise, T_ty was lower than T_re after 30 min of exercise (P<0.01$P<0.01$) and remained significantly lower throughout the remainder of the exercise period. These results demonstrate that the telemetry pill system provides a valid measurement of trunk temperature during rest and exercise-induced thermal strain. T_ty was significantly lower than T_re when temperature exceeded 37.5 °C. However, whether these differences are due to selective brain cooling or imperfections in the tympanic membrane thermometer methodology remains to be determined.     

Thermoregulatory behavior and oxygen consumption of Octopus mimus paralarvae: The effect of age

Octopus mimus is an important cephalopod species in the coastal zone of Peru and Chile that is exposed to temperature variations from time to time due to El Niño/Southern Oscillation (ENSO) episodes when surface temperatures can reach 24 °C, 6 °C above typical temperatures in their habitat. The relationships between temperature and food availability are important factors that determine the recruitment of juveniles into the O. mimus population. The present study was to evaluate the relationship between thermoregulatory behavior and the age of paralarvae (summer population) to determine whether changes in this behavior occur during internal yolk consumption, making larvae more vulnerable to environmental temperature change. Oxygen consumption of paralarvae when 1–4 d old was determined to establish if respiration could be used to monitor the physiological changes that occur during yolk consumption. Horizontal thermal selection (17–30 °C), critical thermal maxima (CTMax), minima (CTMin), and oxygen consumption experiments were conducted with fasting paralarvae 1–4 d old at 20 °C. Preferred temperatures were dependent on the age of O. mimus paralarvae. One day old paralarvae selected a temperature 1.1 °C (23·4 °C) higher than 2 – 4 d old paralarvae (22·3 °C). The CTMax of paralarvae increased with age with values of 31·9±1.1 °C in 1-d-olds and 33·4±0.3 to 4-d-olds. CTMin also changed with age with low values in 2-d-old paralarvae (9.1±1·3 °C) and 11·9±0·9 °C in 4-d-old animals. The temperature tolerance range of paralarvae was age-dependent (TTD=difference between CTMax and CTMin) with higher values in 2 and 3 d old paralarvae (25–26 °C) as compared to 1 d old (23·1 °C) and 4 d old animals (22.7 °C). Oxygen consumption was not affected by the age of paralarvae, suggesting that mechanisms exist that compensate their metabloism until at least 4 d of age. The temperature tolerance range of a planktonic paralarvae of octopus species is presented for the first time. This range was dependent on the age of paralarvae, and so rendered the paralarvae more vunerable to a combination of high temperature and food deprivation during first days of life. Results in the present study provide evidence that O. mimus could be under ecological pressure if a climate change causes increased or decreased temperatures into their distribution range.     

Heat acclimation causes some alterations in serum enzymes activities in streptozotocin-diabetic rats

The present study investigated the effect of long-term heat acclimation and experimental diabetes on serum activity of transaminases (AST, ALT), ALP, LDH and elastase complex, as well as blood glucose and HbA1c level in Wistar rats. The heat acclimation model was established with the artificial heat chamber (35±1 °C and 30-40% humidity) for a period of 28 days, while the control groups were held on 20±2 °C. Experimental diabetes was induced by single streptozotocine (STZ) injection (55 mg/kg bodyweight) The changes caused by insulin treatment (2 IU/100 g body weight, 14 days, twice daily) in both thermal groups were also investigated.STZ-diabetes leads to significant increase in blood glucose and HbA1c level, AST, ALT and ALP activities in both thermal groups (normothermic and heat acclimated), decrease in LDH activity in normothermic animals and increase in heat-acclimated ones. Treatment with insulin restores the blood glucose, HbA1c and enzymes activities regardless of the previous thermal exposure.Prolonged acclimation of control animals to elevated ambient temperature resulted in significant decrease in blood glucose level, AST, ALT, ALP and LDH activities and non-significant changes in HbA1c. Compared to diabetic rats from room temperature, heat-acclimated diabetic ones have significantly higher blood glucose, AST, ALP and LDH activity, lower HbA1c concentration and no significant changes in ALT. Most of the changes observed in heat-acclimated insulin-treated diabetic rats did not significantly differ compared to those from room temperature.The overall two-way ANOVA analyses showed that diabetic state causes significant changes in the blood glucose, HbA1c, AST, ALT and ALP activity, while heat acclimation causes significant changes only in HbA1c level and AST activity. Both of the factors (diabetic state and heat acclimation) have significant common effects on AST, ALP and LDH activity.     

Evaluation of a temperate environment test to predict heat tolerance

A temperate environment heat tolerance test (HTT) was formerly reported (Shvartz et al. 1977b) to distinguish heat acclimatized humans from former heat stroke patients. The purpose of this investigation was to evaluate the ability of HTT to measure acute individual changes in the HR and Tre responses of normal subjects, induced by classical heat acclimation procedures, thereby assessing the utility and sensitivity of HTT as a heat tolerance screening procedure. On day 1, 14 healthy males performed HTT (23.2 +/- 0.5 degrees C db, 14.9 +/- 0.5 degrees C wb) by bench stepping (30 cm high, 27 steps x min-1) for 15 min at 67 +/- 3% VO2max. On days 2-9, all subjects underwent heat acclimation (41.2 +/- 0.3 degrees C db, 28.4 +/- 0.3 degrees C wb) via treadmill exercise. Heat acclimation trials (identical on days 2 and 9) resulted in significant decreases in HR (170 +/- 3 vs 144 +/- 5 beats x min-1), Tre (39.21 +/- 0.09 vs 38.56 +/- 0.17 degrees C), and ratings of perceived exertion; plasma volume expanded 5.2 +/- 1.7%. On day 10, subjects repeated HTT; day 1 vs day 10 HR were statistically similar (143 +/- 6 vs 137 +/- 6 beats x min-1, p greater than 0.05) but Tre decreased significantly (37.7 +/- 0.1 vs 37.5 +/- 0.1 degrees C, p less than 0.05). Group mean HTT composite score (day 1 vs day 10) was unchanged (63 +/- 5 vs 72 +/- 6, p greater than 0.05), and individual composite scores indicated that HTT did not accurately measure HR and Tre trends at 41.2 +/- degrees C in 6 out of 14 subjects.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relationship between change in core temperature and change in cortisol and TNFα during exercise

The combined thermal load created by exercise and a hot environment is associated with an exaggerated core temperature response. The elevated core temperature is believed to increase the total stress of the exercise. Increased stress during exercise has been associated with increased levels of cortisol. The association of cortisol with increased inflammatory responses following exercise in the heat is equivocal. Thus, the purpose of the current investigation was to explore the relationship between increases in rectal temperature (T_re) and TNFα and cortisol. To induce T_re changes, 8 male subjects (mean±SD, age=23.6±2 yr, VO₂max=52.8±3.7 mL/kg/min, BMI=24.2±1.9) participated in two 40 min trials of cycle ergometry at 65% of VO₂peak immersed to chest level in cool (25 °C) and warm (38.5 °C) water. T_re was monitored throughout each trial, with blood samples taken immediately pre and post of each trial. Neither cortisol nor TNFα changed significantly during exercise in the cool water; however, in the warm trial, both cortisol and TNFα significantly increased (p<0.004). Concordance correlations (R_c) between Δ cortisol and Δ TNFα indicated a strong but non-significant correlation (R_c=0.833, p=0.135). In conclusion, changes in core temperature may be impacting the relationship between exercise induced changes in cortisol and TNFα. Therefore, acute moderate-intensity exercise (40 min or less) in warm water impacts the stress and inflammatory response. Understanding this is important because exercise load may need to be adjusted in warm and hot environments to avoid the negative effects of elevated stress and inflammation response.     

A non-invasive device to continuously determine heat strain in humans

Heat stroke remains a very dangerous, potentially lethal illness in humans. The Physiological Strain Index (PSI), originally based on heart rate and rectal temperature recordings in humans, describes heat strain in quantitative terms. The objective of our study was to establish whether the rectal temperature recordings serving to determine the PSI could be replaced by a non-invasive skin temperature sensor combined with a heat flux sensor (Double Sensor) attached to the inside of a helmet. We assumed (i) that the difference between the recordings by the device under test and the rectal temperature should be less than ±1.0 °C for ±2 S.D. at 10, 25, and 40 °C ambient temperature, and (ii) that the temperature predictions based on the Double Sensor temperature should differ by less than 1 PSI score from the calculations based on recordings of the rectal temperature. Twenty male subjects participated in the study. Rectal, nasopharyngeal, and skin temperatures, heat flux, and cardiovascular data were collected continuously during different experimental setups at ambient temperatures of 10, 25, and 40 °C. Depending on the protocols, the exercise intensities varied from 25% to 55% of the individual VO_2max. A comparison of the recordings obtained from the device under test with those of the rectal temperature revealed that (i) the recordings of the Double Sensor differed by −0.16 to 0.1 °C from the mean rectal temperature, (ii) the concordance correlation coefficients (CCC) during all work and rest periods rose with rising ambient temperatures (all work periods: 10 °C: 0.49; 25 °C: 0.69; 40 °C: 0.75; all rest periods: 10 °C: 0.39; 25 °C: 0.81; 40 °C: 0.74), and that (iii) the Double Sensor in the helmet showed that during all rest periods and in all ambient conditions, the temperature dropped much more quickly than what was recorded when taking the rectal temperature (p<0.01). When we compared the PSI values based on the rectal temperature recordings to those determined by the Double Sensor, it was found that (i) the PSI based on the Double Sensor recordings differed by −0.27 to 0.17 from the mean PSI established by rectal temperature recordings. Furthermore, the CCC for the PSI rose during all work periods (10 °C: 0.81, 25 °C: 0.93, 40 °C: 0.87) and rest periods (10 °C: 0.68; 25 °C: 0.93; 40 °C: 0.79). In conclusion, under warm/hot environmental conditions the device under test provided a reliable method of assessing the PSI in operational environments to improve physiological situational awareness and safety in action. However, there are some limitations that reduce the device's performance in cold environments; these need to be investigated further.