Predicting sweat loss response to exercise,environment and clothing

Summary Metabolic heat production (M), clothing heat transfer characteristics, and the environment dictate a required evaporative cooling (E_req) from the body to maintain thermal balance. However, the maximal evaporative capacity (E_max) is dictated by vapor transfer properties of the clothing and environment. Relationships between metabolic load, environmental conditions, clothing and sweat loss were studied in 34 heat-acclimatized males categorized into four groups (eight, eight, eight, and ten subjects) and exposed to various environmental conditions (ambient temperature, 20–54 C, and relative humidity, 10–90%), three levels of metabolic rate (resting; walking 1.34 m·s^–1, level; or walking 1.34 m·s^–1, 5% grade) while wearing various clothing ensembles (shorts and T-shirts, fatigues, fatigues plus overgarment, or sweat suit). Individual groups were not exposed to all combinations. Exposures lasted 120 min: either 10 min rest — 50 min exercise — 10 min rest — 50 min exercise, or 120 min at rest. Physiological measurements included heart rate, rectal temperature, mean skin temperature, energy expenditure and sweat loss (m_sw). E_max and E_req were calculated from environmental conditions, metabolism, clothing insulation and permeability. The ratio E_req/m_sw was found to correlate with E_max and not with M. The predictive equation for sweat loss was: m_sw=18.7×E_req×(E_max)^–0.455 within the limits 50req<360; W·m^–2 and 20max<525; W·m^–2. This formula predicts sweat loss for specific work loads, climates and clothing ensembles.     

Effects of training and acclimation on heat tolerance in exercising men wearing protective clothing

This study examined the effectiveness of endurance training and heat acclimation in reducing the physiological strain imposed by exercising in the heat while wearing protective clothing. Seven young men underwent 8 weeks of physical training [60–80% maximal aerobic power (VO_2max) for 30–45 min · day^–1, 3–4 days · week^–1 at < 25° C] followed by 6 days of heat acclimation (45–55% VO_2max for 60 min · day^–1 at 40° C, 30% relative humidity). Nine other young men underwent corresponding periods of control observation and heat acclimation. Before and after each treatment, subjects completed a treadmill walk (4.8 km · h^–1, 2% grade) in a climatic chamber (40° C, 30% relative humidity), wearing in turn normal combat clothing or clothing protecting against nuclear, biological, and chemical (NBC) agents. Criteria for halting this test were: (1) a rectal temperature (T _re) of 39.3° C; (2) a heart rate (f _c) 95% of the subject's observed maximum, maintained for 3 min; (3) unwillingness of the subject to continue; (4) the elapse of 120 min. The training regimen increased mean VO_2max by 16% and mean plasma volume by 8%. When tested in normal combat clothing, the rates of increase in T _re and f _c were slower after training. However, when wearing NBC protective clothing, the only significant change induced by training was a higher mean skin temperature (T _sk) in the early part of the test. Heat acclimation increased the mean plasma volume of untrained subjects by 8%, but their VO_2max remained unchanged. When tested in normal combat clothing, acclimation decreased their mean values of T _re, T _sk, f _c, and metabolic rate. When wearing NBC protective clothing, the only significant decrease after acclimation was in overall T _re. In trained subjects, heat acclimation induced no further improvement in any physiological variable when wearing normal combat clothing, but reduced overall T _re and T _sk when wearing NBC protective clothing. Training- or acclimation-induced increases of sweat secretion (an average increment of 0.14–0.23 kg · h^–1) were not accompanied by any statistically significant increase in sweat evaporation when wearing NBC protective clothing. Moreover, tolerance times were unchanged in either normal combat (116–120 min) or NBC protective clothing (47–52 min). We conclude that neither endurance training nor heat acclimation do much to improve exercise tolerance when wearing NBC protective clothing in hot environments, because any added sweat secretion decreases blood volume and increases discomfort without augmenting body cooling.     

Regulation of local sweating in sleep-deprived exercising humans

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

Control of heat-induced cutaneous vasodilatation in relation to age

Summary Well matched unacclimatised older (age 55–68, 4 women, 2 men) and younger (age 19–30, 4 women, 2 men) subjects performed 75 min cycle exercise (40% ) in a hot environment (37°C, 60% rh). Rectal temperature (T _re), mean skin temperature (¯T _sk), arm blood flow (ABF, strain gauge plethysmography), and cardiac output (Q, CO₂ rebreathing) were measured to examine age-related differences in heat-induced vasodilatation.T _re and¯T _sk rose to the same extent in each group during the exposure. There was no significant intergroup difference in sweat rate (older: 332±43 ml · m^–2 · h^–1, younger: 435±49 ml · m^–2 · h^–1; mean±SEM). However, the older subjects responded to exercise in the heat with a lower ABF response which could be attributed to a lower for the same exercise intensity. The slope of the ABF-T _re relationship was attenuated in the older subjects (9.3±1.3 vs 17.9±3.3 ml · 100 ml^–1 · min^–1 · °C^–1,p <0.05), but theT _re threshold for vasodilatation was about 37.0°C for both groups. These results suggest an altered control of skin vasodilatation during exercise in the heat in older individuals. This attenuated ABF response appears to be unrelated to , and may reflect an age-related change in thermoregulatory cardiovascular function.     

Rewarming of feet by lower and upper body exercise

Summary The capacity of different types of exercise to rewarm the body, especially the feet, was studied. Six healthy male subjects wearing winter clothing (2.4 clo, 0.37° C·m²·W^–1) were exposed on three occasions to –15° C for 120 min. For the first 60 min the subjects were cooled while sitting motionless and for the latter 60 min they were submitted to cycle ergometer exercise (CE), arm ergometer exercise (AE) or step exercise (ST). The rate of work in CE (about 350 W) served as a reference value for AE and ST. The cooling resulted in an average 1.7 (SEM 0.03) °C decrease in mean body temperature (T _b) corresponding to a 425 (SEM 9) kJ heat debt. The ST increased most effectively mean skin, rectal and lower body skin temperatures as well as dry heat loss. The ST increased T _b by 0.83 (SEM 0.16) °C, CE by 0.10 (SEM 0.11) °C and AE by only 0.07 (SEM 0.12) °C. At the end of the exercise the foot temperature was approximately 6°C higher in ST than in CE. The superior rewarming by ST was apparently due to its low mechanical efficiency. Because the increase in T _b could not explain all the changes in foot temperatures, increased circulation and metabolism of the feet would also appear to have been involved.     

Efficiency of sweat evaporation in unacclimatized man working in a hot humid environment

Summary A method is devised where evaporation and dripping sweat rates can be continuously determined during work. 6 unacclimatized men performed work on a bicycle ergometer at 3 different workloads and in 3 humidities. Ambient temperatures were always equal to mean skin temperatures, thus eliminating all sensible heat transfer. Evaporation rates ranged between 6.8 and 11.2 g · min^–1. Rates of dripping sweat ranged from a mean of 2.2 to 10.4 g · min^–1. One subject dripped 20.3 g · min^–1 in condition H3 (70% RH, 100 W). The fully wet skin in condition H3 corresponded to an evaporative heat transfer coefficient of 99 W · m^–2 kPa. Efficiency of sweating, defined as the ratio between secreted and evaporated sweat, ranged from 87 (50% RH, 50 W) to 51% (70% RH, 100 W). Corresponding values of wettedness were 0.56 and 1.0. Efficiency fell to 51% for fully wet skin (H3), and in some subjects the efficiency values were remarkably low. One subject displayed an efficiency of 31% in condition H3. The reduction in efficiency at a given level of wettedness was higher than previously reported.     

Effects of moisture absorption by clothing on thermal responses during intermittent exercise at 24°C

The effects of two kinds of clothing with different properties with respect to moisture absorption on thermophysiological responses and pulse rate were studied during intermittent exercise at an ambient temperature (T _a) of 24°C. The two kinds of clothing ensemble tested were cotton T-shirt with short sleeves and cotton long-sleeved working dress with full-length trousers (C), and polyester T-shirt with short sleeves and polyester long-sleeved working dress with full-length trousers (P), the thermal resistances of which were nearly equal. Five women aged 21–32 years, served as subjects. The environmental conditions were 24°CT _a, 50% relative humidity and 0.14m·s^–1 air velocity. The subjects, wearing either C or P, exercised for 10-min on a cycle ergometer at an intensity of 30% maximal oxygen uptake and then 10-min rest. This sequence was repeated four times. Rectal and skin temperatures at several sites, local sweating rate, pulse rate and clothing microclimates were continuously compared between C and P throughout the experiment. The major findings were firstly, rectal temperature rose significantly higher in P; secondly, pulse rate was higher in P both during exercise and rest; thirdly, clothing surface temperature on the back rose highly significantly during the fourth exercise period and then fell significantly during the fourth rest period in C; and fourthly, four out of five subjects felt wetter in P during the latter half of the experiment. These results are discussed from the viewpoint that the reduced thermal insulation due to the absorption of moisture in C accelerated dry heat loss, resulting in an inhibition of the increases in core temperature and pulse rate.     

Effect of pyridostigmine on the exercise-heat response of man

Summary The effect of pyridostigmine on thermoregulatory responses was evaluated during exercise and heat stress. Eight heat acclimated, young adult male subjects received four doses of pyridostigmine (30 mg) or identical placebo tablets every 8 h, in a double blind, randomized, cross-over trial. A 30.3%, SD 4.6% inhibition of the circulating cholinesterase (ChE) activity was induced in the pyridostigmine-treated group. The subjects were exposed to 170-min exercise and heat-stress (dry bulb temperature, 33° C; relative humidity 60%) consisting of 60 min in a sitting position and two bouts of 50-min walking (1.39 m · s^–1, 5% gradient) which were separated by 10-min rest periods. No differences were found between treatments in the physiological responses and heat balance parameters at the end of exposure: heart rate (f _c) was 141 beats · min^–1, SD 16 and 150 beats · min^–1, SD 12, rectal temperature (T _re) was 38.5°C, SD 0.4° and 38.6°C, SD 0.3°, heat storage was 60 W · m^–2, SD 16 and 59 W · m^–2, SD 15 and sweat rate was 678 g · h^–1, SD 184 and 661 g · h^–1, SD 133, in the pyridostigmine and placebo treatments, respectively. The changes in T _re and f _c over the heat-exercise period were parallel in both study and control groups. Pyridostigmine caused a slight slowing of f _c (5 beats·min^–1) which was consistent throughout the entire exposure (P<0.001) but was of no clinical significance. The overall change in f_c was similar for both groups. We have concluded that pyridostigmine administration, in a dose sufficient to induce a moderate degree of ChE inhibition, does not significantly affect performance of exercise in the heat.     

Core temperature thresholds for hyperpnea during passive hyperthermia in humans

Humans have higher ventilation when they are hyperthermic but it is not known whether core temperature thresholds for ventilation exist, nor has a physiological rationale been presented for this response. To examine this question, ventilation was studied in relation to core temperatures in humans rendered hyperthermic in a warm bath. Seven subjects [mean (SE), 23.3 (1.4) years] wearing only shorts and a thick felt hat with ear flaps were immersed to the neck in a bath at 41 (0.5)°C for 25 min. Tympanic (T _ty), esophageal (T _es), thigh skin and forehead skin temperatures, heart rate, inspired minute ventilation (V _I at body temperature and pressure, saturated), ventilation frequency and oxygen consumption (VO₂ at standard temperature and pressure, dry) were recorded at 30-s intervals. At immersion V _I briefly increased to 18.6 (3.0)l·min^–1 returned to about the pre-immersion value,, and significantly increased to 19.3 (3.0) l·min^–1 by the end of immersion. VO₂ increased significantly from the pre-immersion value of 0.27 l·min^–1 to 0.67 l·min^–1 by the first 0.5 min of immersion, but then returned to its pre-immersion value. T _ty increased to 38.7 (0.2)°C and T _es increased to 39.0 (0.2)°C by the end of immersion. Core temperature thresholds for increases in V _I were evident at 38.1°C when expressed against T _ty and at 38.5°C when expressed against T _es. The results indicated that during body warming core temperature thresholds for V _I are reached and subsequently a hyperpnea was evident, despite VO₂ remaining at a resting value. This hyperpnea is seen as a thermoregulatory response likely to participate in selective brain cooling.     

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.     

Is exhaustive training adequate preparation for endurance performance?

Summary Our purpose was to test the significance of exhaustive training in aerobic or endurance capacity. The extent of adaptations to endurance training was evaluated by assessing the increase in physical performance capability and oxidative markers in the organs of rats trained by various exercise programs. Rats were trained by treadmill running 5 days · week^–1 at 30 m · min^–1 for 8 weeks by one of three protocols:T ₁ — 60 min · day^–1;T ₂ — 120 min · day^–1; andT ₃ — 120 min · day^–1 (3 days · week^–1) and to exhaustion (2 days · week^–1). GroupsT ₂ andT ₃ ran for longer thanT ₁ in an endurance exercise test (P<0.05), in which the animals ran at 30 m · min^–1 to exhaustion; no difference was observed between groupsT ₂ andT ₃. All 3 trained groups showed a similar increase (20–27%) in the fast-twitch oxidative-glycolytic (FOG) fibers with a concomitant decrease in the fast-twitch glycolytic (FG) fiber population in gastrocnemius (p<0.05). The capillary supply in gastrocnemius increased with the duration of exercise (p<0.05): no difference was found between groupsT ₂ andT ₃. Likewise, no distinction was seen between groupsT ₂ andT ₃ in the increase in succinate dehydrogenase activity in gastrocnemius and the heart. These results suggest that the maximal adaptive response to endurance training does not require daily exhaustive exercise.     

Capsaicin fails to produce disturbances of autonomic heat and cold defence in an avian species (Anas platyrhynchos)

Capsaicin was intravenously administered to adult domestic ducks of 1.8–2.6 kg body weight, with a cumulative dose of 1.0 g/kg body weight given in 4–6 single infusions at intervals of 2–3 days. There were no acute, nociceptive or hypothermic effects, as typically seen in mammals. Before and after capsaicin treatment respiratory evaporative heat loss (REHL, w·kg^–1), breathing frequency (BF, min^–1) and metabolic heat production (M, w·kg^–1) were determined in a warm environment (35–38°C) as a function of core temperature, measured in the esophagus (T _es), which was altered by graded heat extraction with a colonic thermode. Capsaicin treatment reduced the rate at which REHL increased with increasing BF, however, this was compensated by a steeper increase of BF with risingT _es so that the relationship betweenT _es and REHL remained unchanged. TheT _es threshold for activation of M was increased by 0.3°C and the slope reduced by 27% after capsaicin, but identical maximum M values were attained before and after capsaicin at identical degrees of hypothermia. Skin temperature measurements revealed no influence of capsaicin on the thresholdT _es values for skin vasoconstriction. It is concluded that capsaicin fails to exert effects in birds on those afferents and central neurons which are involved in thermo- and nociception, in contrast to mammals in which these perceptive functions become severely impaired.     

Exercise thermoregulation after 6 h of chair rest, 6° head-down bed-rest,and water immersion deconditioning in men

The purpose was to investigate the mechanism for the excessive exercise hyperthermia following deconditioning (reduction of physical fitness). Rectal (T _re) and mean skin ( ) temperatures and thermoregulatory responses were measured in six men [mean (SD) age, 32 (6) years; mass, 78.26 (5.80) kg; surface area, 1.95 (0.11)m²; maximum oxygen uptake ( ), 48 (6) ml·min^–1·kg^–1; whilst supine in air at dry bulb temperature 23.2 (0.6)°C, relative humidity 31.1 (11.1)% and air speed 5.6 (0.1) m·min^–1] during 70 min of leg cycle exercise [51 (4)% ] in ambulatory control (AC), or following 6 h of chair rest (CR), 6° head-down bed rest (BR), and 20° (WI20) and 80° (WI80) foot-down water immersion [water temperature, 35.0 (0.1)°C]. Compared with the AC exercise T _re [mean (SD) 0.77 (0.13)°C], T _re after CR was 0.83 (0.08)°C (NS), after BR 0.92 (0.13)°C (^*P<0.05), after WI80 0.96 (0.13)°C^*, and after WI20 1.03 (0.09)°C^*. All responded similarly to exercise: they decreased (NS) by 0.5–0.7°C in minutes 4–8 and equilibrated at +0.1 to +0.5°C at 60–70. Skin heat conductance was not different among the five conditions (range = 147–159 kJ·m^–2·h^–1·°C^–1). Results from an intercorrelation matrix suggested that total body sweat rate was more closely related toT _re at 70 min (T _re70) than limb sweat rate or blood flow. Only 36% of the variability inT _re70 could be accounted for by total sweating, and less than 10% from total body dehydration. It would appear that multiple factors are involved which may include change in sensitivity of thermo- and osmoreceptors.     

Thermal tolerance following artificially induced polycythaemia

Polycythaemia has been shown to improve physical performance, possibly due to increased arterial oxygen transport. Enhanced thermoregulatory function may also accompany this manipulation, since a greater proportion of the cardiac output becomes available for heat dissipation. We further examined this possibility in five trained men, who participated in three-phase heat stress trials (20 min rest, 20 min cycling at 30% peak power W_peak and 20 min at 45% W_peak at 38.3 (SEM 0.7)°C [relative humidity 41.4 (SEM 2.9)%]. Trials were performed during normocythaemia (control) and polycythaemia, obtained by reinfusion of autologous red blood cells and resulting in significant elevation of arterial oxygen transport. During the polycythaemic trials, the subjects demonstrated diminished thermal strain, as evidenced by a significant reduction in cardiac frequency (f _c: 12 beats · min^–1 lower throughout the test;P < 0.05), and reduced auditory canal temperatures (T _ae) during the latter 20-min phase (P < 0.05). Forearm sweat onset was more rapid (363.0 compared to 1083.0 s;P < 0.05), and forearm sweat rate (. _msw) sensitivity was elevated from 1.80 to 2.91 · mg · cm^–2 · min^–1 · °C^–1 (P < 0.05). Foreheadm _sw was depressed during the final 20 min, while forearmm _sw was greater during all test phases, averaging 0.94 and 1.20 mg · cm^–2 · min^–1, respectively, over the 60 min. Skin blood flows for the upper back, upper arm and forearm were reduced (P < 0.05). Polycythaemia enhanced thermoregulation, through an elevation in forearm sweat sensitivity and.m _sw, but not via increased cutaneous blood flow. These modifications occurred simultaneously with decreases inf _c andT _ae, resulting in greater thermal tolerance.     

Modelling fire-fighter responses to exercise and asymmetric infrared radiation using a dynamic multi-mode model of human physiology and results from the Sweating Agile thermal Manikin

Abstract In this study, predicted dynamic physiological responses are compared with wear trials results for firefighter suits: impermeable (A), semi-permeable (B) and permeable (C), and underwear. Wear trials consisted of three rest phases and two moderate work phases, with a frontal infrared (IR) radiation exposure of 500 W/m² for the last 15 min of each work phase. Simulations were performed by detailed modelling of the experimental boundary conditions, including the inhomogeneous IR radiation combined with clothing properties for still and walking conditions measured using the Sweating Agile thermal Manikin. Accounting for the effect of sweat gland activity suppression with increased skin wettedness, the predicted total moisture loss was insignificantly different (P<0.05) from the wear trial value for suits B and C but was 37% too high for suit A. Predicted evolution of core, mean skin and local skin temperatures agreed well with the wear trial results for all clothing. Root mean square deviations ranged from 0.11°C to 0.26°C for core temperatures and from 0.28°C to 0.38°C for mean skin temperatures, which where typically lower than the experimental error. Transient thermodynamic processes occurring within suit A may account for the delayed/reduced fall in core temperature following exercise.     

Determination of body heat storage in clothing: calorimetry versus thermometry

Two methods of estimating body heat storage were compared under differing conditions of clothing, training, and acclimation to heat. Six male subjects underwent 8 weeks of physical training [60–80% of maximal aerobic power ( ) for 30–45 min · day–, 3–4 days · week^–1 at < 25 °C dry bulb (db)] followed by 6 consecutive days of heat acclimation (45–55% for 60 min · day^–1 at 40°C db, 30% relative humidity)]. Nine other male subjects underwent corresponding periods of control observation followed by heat acclimation. Before and after each treatment, subjects walked continuously on a treadmill (1.34 m · s^–1, 2% grade) in a climatic chamber (40°C db, 30% relative humidity) for an average of 118 min (range 92–120 min) when wearing normal light combat clothing and for an average of 50 min (range 32–68 min) when wearing protective clothing resistant to nuclear, biological, and chemical agents. The heat storage was determined calorimetrically (by the balance of heat gains and losses) and thermometrically [by the conventional equations, using one or two set(s) of relative weightings for the rectal temperature (T _re) to mean skin temperature _sk of 4:1 and 4:1, 2:1 and 4:1, or 2:1 and 9:1 in thermoneutral and hot environments, respectively]. _sk was calculated from 12-site measurements, weighted according to the regional distribution of body surface area and the first eigenvectors of principal component analysis. There were only minor differences (< 5%) between the heat storage values calculated by given weighting factors forT _re and _sk, whether the individual coefficients were derived from estimates of regional surface area or principal component methodologies. When wearing normal clothing, no significant differences were found between the two estimates of heat storage (calorimetry vs thermometry with an invariant relative weighting of 4:1) in any experimental condition, with one specific exception: when wearing protective clothing, thermometry underestimated the heat storage by 24–31%. This underestimation was attenuated by using two sets of relative weightings of 2: 1 and 4: 1 or 2: 1 and 9: 1. The results suggest that when subjects wearing protective clothing are transferred from thermoneutral to hot environments, the accuracy of thermometric estimates of heat storage can be improved by using two sets of weighting factors forT _re and _sk     

Pathways of phosphate transport in chick jejunum

The effect of vitamin D₃ on intestinal phosphate (P_i) absorption was studied in everted sacs prepared from jejunum of either vitamin D-deficient (–D) or vitamin D-replete (+D) chicks. Vitamin D₃ stimulates the maximal velocity (V _max) of a mucosal active P_i transport mechanism from 125 to 314 nmol·min^–1·g^–1 tissue.K _m of this process remains virtually unchanged (–D: 0.15 mmol·l^–1; + D: 0.18 mmol·l^–1).Active P_i entry into the epithelium depends on extracellular Na⁺. Reduction of buffer Na⁺ reducesV _max in the + D group to 182 nmol·min^–1·g^–1 tissue but has no significant effect in the –D animals (V _max=105 nmol·min^–1·g^–1 tissue). In this group, the predominant effect of Na⁺ substitution is a shift ofK _m to 1.13 mmol·l^–1, whileK _m in the +D group is changed only to 0.53 mmol·l^–1.Transeptithelial P_i transport in the + D group involves the mucosal phosphate pump and hence an intracellular pathway, proceeding at a rate of 48 nmol·min^–1·g^–1 tissue. This is in contrast to –D P_i transfer (8 nmol·l^–1·g^–1 tissue) which is by a diffusional, Na⁺-insensitive, and presumably paracellular pathway.Transepithelial calcium transport (–D: 3.3 nmol·min^–1·g^–1; + D: 7.6 nmol·min^–1·g^–1 tissue) does not require the presence of extracellular Na⁺ and apparently involves pathways different from those of the P_i absorptive system.Presented in part at the Annual Meeting of the Austrian Biochemical Society, Salzburg, September 1978     

The topography of eccrine sweating in humans during exercise

The purpose of this study was to investigate the distribution of steady-state sweating rates (m _sw), during stressful exercise and heat exposures. Six men completed 42-min trials: 2-min rest and 40-min cycling at 40% peak power in 36.6° C (relative humidity 46.0%). The m _sw, was monitored using ventilated capsules at the forehead, and at three additional sites. Repeat trials allowed monitoring from eleven skin surfaces. Auditory canal temperature (T_ac) and 11 skin temperatures were measured. After normalising m _sw to the forehead response within subjects, differences in T _ac and onset time thresholds, and transient and steady-state m _sw were examined. The pooled, lower torso m _sw onset [mean 45.5 (SEM 42.0) s] preceded that of the head [mean 126.5 (SEM 34.8) s, P<0.05], but was not significantly different from the legs [mean 66.6 (SEM 25.7) s], upper torso [mean 80.2 (SEM 36.8) s] or arms [mean 108.6 (SEM 31.2) s]. Transient m _sw did not differ among regions (P=0.16). Mean, steady-state forehead m _sw [3.20 (SEM 0.51) mg · cm^–2 · min^–1]was not significantly greater than the scapula, forearm, hand, stomach and lower back m _sw (in descending order), but was greater than the chest [1.6 (SEM 0.2)], upperarm [1.6 (SEM 0.2)], calf [1.5 (SEM 0.3)] and thigh m _sw [1.0 (SEM 0.2), P<0.05 for all comparisons]. The results did not support the caudal-to-rostral sweat onset evident during supine, resting heat stress. Equivalent T _ac sweat thresholds existed between sites, while steady-state m _sw topography varied among subjects and was not dominated by central regions.     

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.     

Thermal responses to light, moderate and heavy daily outdoor work in cold weather

Working in the cold is part of the normal routine in outdoor occupations in winter in the subarctic regions, but there are few data available on occupational exposure to cold during outdoor work. In the present study, thermal responses were measured in winter in Finland during 23 working days among young, healthy men working in heavy, moderate and light daily outdoor jobs. During the measurements ambient temperature ranged from + 3 to – 27°C, air velocity from 0.2 to 4.3 m · s^–1, and the subjects wore normal winter clothing. The skin temperatures measured often indicated disturbed performance, discomfort and a risk of adverse health effects, especially during the very cold days (ambient temperature less than – 15°C) in the light work. The most common problems were cooling of the extremities and the face and cool or cold sensations. The temperatures on the distal parts of the upper extremities correlated significantly with the heaviness of the work (r = 0.51, P = 0.014). The core temperature remained at the safety level in each case. Apart from clothing, an appropriate work load proved to be an effective way of keeping up the temperature of the extremities in cold work, and that should be taken into account when outdoor work is being planned.