The influence of acclimatization on stress hormone concentration in serum during heat stress

This study was aimed to examine the influence of acclimatization on the change of concentration of stress hormones in men’s serum exposed to heat stress during physical training. The study included a total of 40 men, aged 19–21 years, divided randomly into four groups: CTRL group: control, exposed to the Exercise Tolerance Testing in comfortable conditions; O group: exposed to Exercise Tolerance Testing in a warm environment; P group: exposed to passive acclimation to heat for 10 days, followed by Exercise Tolerance Testing in a warm environment; A group: exposed to active acclimation to heat for 10 days, followed by Exercise Tolerance Testing in a warm environment. All participants were tested for thermoregulation and acclimatization, skin and tympanic temperature, heart rate (HR), hormonal status and sweating. The mean skin temperature was the lowest in the control group of subjects exposed to physical exertion under comfortable conditions, and at each point of measurement it was statistically significantly different from that of the other study groups (p?<?0.001). Sweating intensity was statistically significantly the lowest in the CTRL group (0.32?±?0.04 l/m²/h; p?<?0.001), compared to all other groups. Cortisol was significantly altered in O group (632.2?±?92.3; 467.2?±?89.7), testosterone levels were significantly altered in P (19.2?±?9.3; 16.4?±?7.3) and in A groups (22.1?±?12.4; 14.9?±?9.9), while prolactin was changed in O (392.1?±?51.3; 181.4?±?42.3), P (595.1?±?191.1; 191.2?±?52.5), and A group (407.4?±?189.3; 173.4?±?43.9) after the experimental period. The impact of acclimatization on hormonal indicators emphasizes its importance in the response of the endocrine system of soldiers to perform military activities in warm climates.

     

The importance of aerobic fitness in determining tolerance to uncompensable heat stress

When protective clothing is worn that restricts evaporative heat loss, it is not valid to assume that the higher sweat rates associated with improvements in aerobic fitness will increase heat tolerance. An initial study compared thermoregulatory and cardiovascular responses to both compensable and uncompensable heat stress before and after 8 weeks of endurance training in previously sedentary males. Despite a 15% improvement in VO2peak, and lower heart rates and rectal temperature (T(re)) responses while wearing combat clothing, no changes were noted when subjects wore a protective clothing ensemble. Tolerance times were unchanged at approximately 50 min. A subsequent short-term training model that used daily 1-h exercise sessions for 2 weeks also failed to show any benefit when the protective clothing was worn in the heat. Cross-sectional comparisons between groups of high and low aerobic fitness, however, have revealed that a high aerobic fitness is associated with extended tolerance time when the protective clothing is worn. The longer tolerance time is a function of both a lower starting T(re) and a higher T(re) tolerated at exhaustion. Improvements in cardiovascular function with long-term training may allow higher core temperatures to be reached prior to exhaustion. Conversely, elevations in core temperature that occur with normal training sessions may familiarize the more fit subjects to the discomforts of exercise in the heat. Other factors such as differences in body fatness may account for a faster increase in tissue temperature at a given metabolic rate for less fit individuals.     

The effects of Bacillus subtilis on Caenorhabditis elegans fitness after heat stress

Microbes can provide their hosts with protection from biotic and abiotic factors. While many studies have examined how certain bacteria can increase host lifespan, fewer studies have examined how host reproduction can be altered. The nematode Caenorhabditis elegans has been a particularly useful model system to examine how bacteria affect the fitness of their hosts under different contexts. Here, we examine how the bacterium Bacillus subtilis, compared to the standard C. elegans lab diet, Escherichia coli, affects C. elegans survival and reproduction after experiencing a period of intense heat stress. We find that under standard conditions, nematodes reared on B. subtilis produce fewer offspring than when reared on E. coli.However, despite greater mortality rates on B. subtilis after heat shock, young adult nematodes produced more offspring after heat shock when fed B. subtilis compared to E. coli. Because offspring production is necessary for host population growth and evolution, the reproductive advantage conferred by B. subtilis supersedes the survival advantage of E. coli. Furthermore, we found that nematodes must be reared on B. subtilis (particularly at the early stages of development) and not merely be exposed to the bacterium during heat shock, to obtain the reproductive benefits provided by B. subtilis. Taken together, our findings lend insight into the importance of environmental context and interaction timing in shaping the protective benefits conferred by a microbe toward its host.     

The factor structure of self-reported physical stress reactions

A self-report questionnaire tapping 98 frequently reported physical stress reactions was subjected to factor analysis. The instrument was given to 1,210 subjects (593 males, 520 females, and 97 who failed to indicate their gender). The final factor solution identified 18 interpretable factors for the combined sample. Some of the more notable findings are: (a) Gastric Distress (Factor 1) is by far the most prominent factor and accounts for .491 of the total variance; (b) the factor solutions for males and females are highly similar, although Cardiorespiratory Activity (Factor 2) is defined by a greater diversity of noncardiac reactions for females than for males; and (c) six independent striated muscle tension factors can be identified for the entire sample. Results are discussed in terms of a behavioral conceptualization of psychosomatic symptoms. Clinical implications are noted.     

Influence of aerobic fitness and body fatness on tolerance to uncompensable heat stress.

This study examined the independent and combined importance of aerobic fitness and body fatness on physiological tolerance and exercise time during weight-bearing exercise while wearing a semipermeable protective ensemble. Twenty-four men and women were matched for aerobic fitness and body fatness in one of four groups (4 men and 2 women in each group). Aerobic fitness was expressed per kilogram of lean body mass (LBM) to eliminate the influence of body fatness on the expression of fitness. Subjects were defined as trained (T; regularly active with a peak aerobic power of 65 ml x kg LBM(-1) x min(-1)) or untrained (UT; sedentary with a peak aerobic power of 53 ml x kg LBM(-1) x min(-1)) with high (High; 20%) or low (Low; 11%) body fatness. Subjects exercised until exhaustion or until rectal temperature reached 39.5 degrees C or heart rate reached 95% of maximum. Exercise times were significantly greater in T(Low) (116 +/- 6.5 min) compared with their matched sedentary (UT(Low); 70 +/- 3.6 min) or fatness (T(High); 82 +/- 3.9 min) counterparts, indicating an advantage for both a high aerobic fitness and low body fatness. However, similar effects were not evident between T(High) and UT(High) (74 +/- 4.1 min) or between the UT groups (UT(Low) and UT(High)). The major advantage attributed to a higher aerobic fitness was the ability to tolerate a higher core temperature at exhaustion (the difference being as great as 0.9 degrees C), whereas both body fatness and rate of heat storage affected the exercise time as independent factors.     

Effects of exposure to short-term heat stress on fitness components in Drosophila melanogaster

Effects of thermal stress on survival and reproductive success in ten recently collected isofemale lines of Drosophila melanogaster were compared for flies treated as follows: always held at 25° C, placed in an incubator set at 37° C for 120 min, or exposed to 40° C in an incubator for 90 min, with or without previous exposure to 37° C. Short-term exposure to the higher temperature greatly reduced adult survival, the mating frequency of males and females, and female fecundity, which was measured as offspring produced over ten days. Male fertility, measured as the progeny produced by a female mated once, differed little among treatments. Previous exposure to a high, but non-lethal, temperature before exposure to the higher one, improved survival of males and females, and improved offspring production of females. Genetic variation was present among lines for offspring production, but genetic variation for survival was not significant, and genotype by environment interactions for fitness components of females were small. These results indicated low genetic variation in thermal resistance in the studied population, such that a threshold for temperature stress probably exists, above which local extinction is more likely than the evolution of resistance.     

Heat acclimation, aerobic fitness, and hydration effects on tolerance during uncompensable heat stress

The purpose ofthe present study was to determine the separate and combined effects ofaerobic fitness, short-term heat acclimation, and hypohydration ontolerance during light exercise while wearing nuclear, biological, andchemical protective clothing in the heat (40°C, 30% relativehumidity). Men who were moderately fit [(MF); <50ml · kg¹ · min¹maximal O₂ consumption;n = 7] and highly fit[(HF); >55ml · kg¹ · min¹maximal O₂ consumption;n = 8] were tested while theywere euhydrated or hypohydrated by ~2.5% of body mass throughexercise and fluid restriction the day preceding the trials. Tests wereconducted before and after 2 wk of daily heat acclimation (1-htreadmill exercise at 40°C, 30% relative humidity, while wearingthe nuclear, biological, and chemical protective clothing). Heatacclimation increased sweat rate and decreased skin temperature andrectal temperature (T_re) in HF subjects but had no effecton tolerance time (TT). MF subjects increased sweat rate but did notalter heart rate, Tre, or TT. In both MF and HF groups, hypohydration significantly increased T_re and heart rate and decreasedthe respiratory exchange ratio and the TT regardless of acclimationstate. Overall, the rate of rise of skin temperature was less, whileT_re, the rate of rise of T_re, and the TTwere greater in HF than in MF subjects. It was concluded thatexercise-heat tolerance in this uncompensable heat-stress environmentis not influenced by short-term heat acclimation but is significantlyimproved by long-term aerobic fitness.