Effect of voluntary dehydration on thermoregulatory responses to heat in men and women

The effects of dehydration prior to heat exposure on sweating and body temperature were tested in 8 men and 8 women, dehydration being 1.3 and 1.0% of body weight, respectively. The subjects were exposed to 40 degrees C for 60 min. Compared with controls (C), in the dehydrated men (D) there was a longer delay in the onset of sweating (C, 7.8, D, 11.6 min, p less than 0.05), a lower total sweat loss (C, 153, D, 127 g X m-2 X h-1, p less than 0.001), and a greater increase in Tre (C, 0.31, D, 0.43 degree C, p less than 0.002). In women, dehydration did not influence the control time course of sweating significantly, nor were these significant body temperature increases during heat exposure. Delay in the onset of sweating in women (C, 18.1, D, 18.7 min) was generally longer than in men (C, 7.8, D, 11.6 min), [F(1,14) = 7.41, p less than 0.05]. A significant correlation was found between the inertia time of sweating and delta Tre in both control and dehydration conditions in the men (r = 0.81, p less than 0.01). The rectal temperature increases in men were also related to the inertia time of electrical skin resistance (r = 0.83, p less than 0.01). It is concluded that dehydration affects sweating and body temperature in men more severely than in women.     

Importance of dynamics of sweating in men during exercise

Influence of dynamics of sweating on rectal temperature increase was tested in 3 groups of men performing cycle exercise with intensity of 65, 90 and 120 W, respectively, in 22 degrees C chamber temperature and 30% of relative air humidity. During exercise at 65 and 90 W the subjects wore suits while exercising with intensity of 120 W they wore only shorts. The dynamics of sweating was described by delay in onset of sweating and time constant of the reaction. Wearing caused significant increase in skin humidity and decreased evaporative rate of sweating. Sweat rate during steady state was related to the metabolic rate in naked (r = 0.89, p less than 0.002) as well as in wearing subjects (r = 0.93, p less than 0.01). Delay in onset of sweating was, in average, 5 min with a time constant of 7 min. Both factors showed a tendency to be shorter with increasing work intensity. Mean increase in rectal temperature was proportional to the intensity of exercise although the individual delta Tre correlated well with the dynamics of sweating in naked (r = 0.83, p less than 0.01) and wearing subjects (r = 0.84, p less than 0.01). Since delta Tre was smaller in subjects with shorter inertia time of sweating in response to beginning of exercise at the same intensity it is concluded that the dynamics of sweating can play an important role in limiting body temperature increase in working men.     

Efficiency of thermoregulatory system in man under endogenous and exogenous heat loads

The aim of the present work was to estimate the dynamics and efficiency (eta sw) of sweating, and thermoregulatory index (TI) defined as a ratio of heat loaded the body to the heat removed to the environment. In the first part of this work 22 men exercised with an intensity of 50% VO2 max. in 22 degrees C, 16 men were exposed to 40 degrees C at rest, and 9 men exercised at the level of 50% VO2 max. at 30 degrees C. In the second part, 8 men and 8 women were exposed to 40 degrees C before and after dehydration (1% of body mass, approximately), 8 men exercised at 23 degrees C before and after hyperhydration (35 ml/kg of body mass) and 22 men exercised before and after 3 months of endurance training. Body heat balance, rectal (Tre), tympanic (Tty) and mean skin (Tsk) temperatures were measured in all subjects. TI was greater during simultaneous (0.84) than during separate endo- (0.76, p less than 0.01) or exogenous (0.67, p less than 0.001) heat loads. The respective values of eta sw were 0.82; 0.57 (p less than 0.001) and 0.78 (p less than 0.001). No difference in TI was found between men and women. Dynamics of sweating was greater in men but efficiency of sweating was greater in women. Dehydration before heat exposure decreased both dynamics of sweating and TI but it increased eta sw in men. As a result Tre was greater in dehydrated (0.45 degrees C) than in normally hydrated men (0.31 degrees C, p less than 0.002). Dehydration did not affect the measured variables in women. Hyperhydration of exercising men caused an increase in TI from 0.72 to 0.82 (p less than 0.05) and in eta sw from 0.57 to 0.81 (p less than 0.01). In men exercising after endurance training the onset of sweating was shortened from 4.0 to 0.9 min (p less than 0.002). TI increased from 0.76 to 0.89 (p less than 0.001), eta sw increased from 0.57 to 0.74 (p less than 0.02) whereas Tty was lower (1.10 and 0.58 degrees C, p less than 0.001, respectively). It is concluded that dynamics and efficiency of sweating, as well as the thermoregulatory index depend on the type of heat load. Men and women tolerate dry heat equally well. Dehydration changes thermoregulatory function in men but not in women. Hyperhydration before exercise and particularly endurance training increase tolerance of endogenous heat.(ABSTRACT TRUNCATED AT 400 WORDS)     

Influence of menstrual cycle on shivering, skin blood flow, and sweating responses measured at night

In 10 women, external cold and heat exposures were performed both in the middle of luteal phase (L) and in the early follicular phase (F) of the menstrual cycle. Serum progesterone concentrations in L and F averaged 46.0 and 0.9 nmol X l-1, respectively. The experiments took place between 3:00 and 4:30 A.M., when the L-F core temperature difference is maximal. At neutral ambient temperature, esophageal (Tes), tympanic (Tty), rectal (Tre), and mean skin (Tsk) temperatures averaged 0.59 degrees C higher in L than in F. The thresholds for shivering, chest sweating, and cutaneous vasodilation (heat clearance technique) at the thumb and forearm were increased in L by an average of 0.47 degrees C, related to mean body temperature [Tb(es) = 0.87Tes + 0.13 Tsk] and to Tes, Tty, Tre, or Tsk. The above-threshold chest sweat rate and cutaneous heat clearances at the thumb and forearm were also enhanced in L, when related to Tb(es) or time. The metabolic rate, arm blood flow, and heart rate at thermoneutral conditions were increased in L by 5.0%, 1.1 ml X 100 ml-1 X min-1, and 4.6 beats X min-1, respectively. The concomitant increase in threshold temperatures for all autonomic thermoregulatory responses in L supports the concept of a resetting of the set point underlying the basal body temperature elevation in L. The effects of the increased threshold temperatures are counteracted by enhanced heat loss responses.     

Influence of beta-adrenergic blockade on the control of sweating in humans

To evaluate the role of beta-adrenergic receptors in the control of human sweating, we studied six subjects during 40 min of cycle-ergometer exercise (60% maximal O2 consumption) at 22 degrees C 2 h after oral administration of placebo or nonselective beta-blockade (BB, 80 mg propranolol). Internal temperature (esophageal temperature, Tes), mean skin temperature (Tsk), local chest temperature (Tch), and local chest sweat rate (msw) were continuously recorded. The control of sweating was best described by the slope of the linear relationship between msw and Tes and the threshold Tes for the onset of sweating. The slope of the msw-Tes relationship decreased 27% (P less than 0.01), from 1.80 to 1.30 mg X cm-2 X min-1 X degree C-1 during BB. The Tes threshold for sweating (36.8 degrees C) was not altered as the result of BB. These data suggest that BB modified the control of sweating via some peripheral interaction. Since Tsk was significantly (P less than 0.05) reduced during BB exercise, from a control value of 32.8 to 32.2 degrees C, we evaluated the influence of the reduction in local skin temperature (Tsk) in the altered control of sweating. Reductions in Tch accounted for only 45% of the decrease in the slope of the msw-Tes relationship during BB. Since evaporative heat loss requirement during exercise with BB, as estimated from the energy balance equation, was also reduced 18%, compared with control exercise, we concluded that during BB the reduction in sweating at any Tes is the consequence of both a decrease in local Tsk and a direct effect on sweat gland.     

Comparison in men of physiological responses to exercise of increasing intensity at low and moderate ambient temperatures

In six male subjects the sweating thresholds, heart rate (fc), as well as the metabolic responses to exercise of different intensities [40%, 60% and 80% maximal oxygen uptake (VO2max)], were compared at ambient temperatures (Ta) of 5 degrees C (LT) and 24 degrees C (MT). Each period of exercise was preceded by a rest period at the same temperature. In LT experiments, the subjects rested until shivering occurred and in MT experiments the rest period was made to be of exactly equivalent length. Oxygen uptake (VO2) at the end of each rest period was higher in LT than MT (P less than 0.05). During 20-min exercise at 40% VO2max performed in the cold no sweating was recorded, while at higher exercise intensities sweating occurred at similar rectal temperatures (Tre) but at lower mean skin (Tsk) and mean body temperatures (Tb) in LT than MT experiments (P less than 0.001). The exercise induced VO2 increase was greater only at the end of the light (40% VO2max) exercise in the cold in comparison with MT (P less than 0.001). Both fc and blood lactate concentration [1a]b were lower at the end of LT than MT for moderate (60% VO2max) and heavy (80% VO2max) exercises. It was concluded that the sweating threshold during exercise in the cold environment had shifted towards lower Tb and Tsk. It was also found that subjects exposed to cold possessed a potentially greater ability to exercise at moderate and high intensities than those at 24 degrees C since the increases in Tre, fc and [1a]b were lower at the lower Ta.     

Control of heat-induced cutaneous vasodilatation in relation to age

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 (approximately 40% VO2max) in a hot environment (37 degrees C, 60% rh). Rectal temperature (Tre), mean skin temperature (Tsk), arm blood flow (ABF, strain gauge plethysmography), and cardiac output (Q, CO2 rebreathing) were measured to examine age-related differences in heat-induced vasodilatation. Tre and Tsk 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 Q for the same exercise intensity. The slope of the ABF-Tre relationship was attenuated in the older subjects (9.3 +/- 1.3 vs 17.9 +/- 3.3 ml.100 ml-1.min-1.degrees C-1, p less than 0.05), but the Tre threshold for vasodilatation was about 37.0 degrees 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 VO2max, and may reflect an age-related change in thermoregulatory cardiovascular function.     

Effect of age on heat-activated sweat gland density and flow during exercise in dry heat

Physiological responses of eight postmenopausal older women (age 52-62 yr) and eight younger women (age 20-30 yr) were compared during moderate intensity exercise in a hot dry environment (48 degrees C dry bulb, 25 degrees C wet bulb). The age groups were matched on the basis of maximal O2 consumption (VO2max), body surface area, and body fatness. After heat acclimation the women walked at 40% VO2max for up to 2 h in the hot dry environment while heart rate (HR), rectal temperature (Tre), mean skin temperature (Tsk), whole-body sweating rate (Msw), and local sweating rates (msw; forearm, chest, and scapula) were measured. Additionally, the density of heat-activated sweat glands (HASG) was determined and average sweat gland flow (SGF) was calculated for the scapular area. Although no differences between age groups were found in HR response (when analyzed as percent of maximal HR) or Tsk, the older women had a significantly higher Tre throughout the heat-exercise session. The greater heat storage of the older women may be explained by their significantly lower Msw and msw. There were no differences between the younger and older women in the density of HASG after 30 min; therefore, the lower msw reflects a diminished output per HASG rather than a decrease in the number of sweat glands recruited. The diminished thermoregulatory ability of the older women, unrelated to differences in VO2max, appears to reflect either 1) a diminished response of the sweat glands to central and/or peripheral stimuli, or 2) an age-related structural alteration in the eccrine glands or surrounding skin cells.     

Regional differences in the sweating responses of older and younger men.

Ten older (60-71 yr) and nine younger (20-25 yr) active healthy men were exposed to passive heating [by placing the lower legs and feet in a 43 degrees C water bath for 60 min while sitting in a warm (35 degrees C, 45% relative humidity) chamber] in summer and winter. The increase in rectal temperature (Tre) was significantly (P less than 0.05) greater, and mean skin temperature and forearm blood flow were lower, for the older men in both seasons. Total sweating rate was lower in the older men, but significantly (P less than 0.05) so only in the summer. The Tre threshold for sweating was unaffected by either age or site (back vs. thigh). The local sweating rate (msw) on the thigh was significantly lower (P less than 0.05) for the older men throughout the exposure, whereas there were no significant age-related differences for the average or peak values of back msw, although lesser sweating on the back occurred during the first 30 min of exposure. The decreased msw on the thigh was due to a lower sweat output per heat-activated sweat gland rather than from recruitment of fewer glands. It was concluded that regional differences exist in the age-related decrement in sweat gland function. Furthermore, these findings suggest that aging leads to a decreased ability to maintain body temperature with passive heating of the extremities, which may be attributed in part to decreased regional sweat gland function.     

Hypohydration and exercise: effects of heat acclimation, gender, and environment

This study examined the effects of heat acclimation and subject gender on treadmill exercise in comfortable (20 degrees C, 40% rh), hot-dry (49 degrees C, 20% rh), and hot-wet (35 degrees C, 79% rh) environments while subjects were hypo- or euhydrated. Six male and six female subjects, matched for maximal aerobic power and percent body fat, completed two exercise tests in each environment both before and after a 10-day heat acclimation program. One exercise test was completed during euhydration and one during hypohydration (-5.0% from baseline body weight). In general, no significant (P greater than 0.05) differences were noted between men and women at the completion of exercise for rectal temperature (Tre), mean skin temperature (Tsk), or heat rate (HR) during any of the experimental conditions. Hypohydration generally increased Tre and HR values and decreased sweat rate values while not altering Tsk values. In the hypohydration experiments, heat acclimation significantly reduced Tre (0.19 degrees C) and HR (13 beats X min-1) values in the comfortable environment, but only HR values were reduced in hot-dry (21 beats X min-1) and hot-wet (21 beats X min-1) environments. The present findings indicated that men and women respond in a physiologically similar manner to hypohydration during exercise. They also indicated that for hypohydrated subjects heat acclimation decreased thermoregulatory and cardiovascular strain in a comfortable environment, but only cardiovascular strain decreased in hot environments.     

Functional differentiation of thermoregulation, with particular references to seasonal variation of whole body sweat rate of exercising man

We investigated the seasonal variation of sweating response during exercise. Four adult healthy men repeated a moderate bicycle exercise (60 watts) in a climatic chamber of an ambient temperature of 30 degrees C (relative humidity, 45%) in winter, spring, summer, and fall. In summer, sweat rate immediately increased as soon as the exercise started, whereas in winter in a few minutes. The mean sweat rate during exercise was significantly different between winter and summer. The transient reduction of the Tsk was observed at the beginning of the exercise in winter. The Tsk decreased in proportion to increasing of sweat rate in each season. Significantly negative correlations were found between sweat rate and the rate of change of Tsk during exercise in each season. The slope and intercept of regression line were significantly different between winter and summer. The index of sweating was made available for the relative value, changing rate against annual mean value of total sweat loss (delta SR, %). The relative value rather than the absolute value (i.e., expressed as g.m-2.h-1) corrected well with skin temperature. It is suggested that the present results may reflect adapted changes in the thermoregulatory mechanisms to seasonal acclimatization. Moreover, the fall in skin temperature during exercise may be not due to increased evaporative cooling, but may be the result of vasoconstriction probably caused by non-thermal factors.     

Comparison of thermal responses between rest and leg exercise in water

This study examined both the thermal and metabolic responses of individuals in cool (30 degrees C, n = 9) and cold (18 degrees C, n = 7; 20 degrees C, n = 2) water. Male volunteers were immersed up to the neck for 1 h during both seated rest (R) and leg exercise (LE). In 30 degrees C water, metabolic rate (M) remained unchanged over time during both R (115 W, 60 min) and LE (528 W, 60 min). Mean skin temperature (Tsk) declined (P less than 0.05) over 1 h during R, while Tsk was unchanged during LE. Rectal (Tre) and esophageal (Tes) temperatures decreased (P less than 0.05) during R (delta Tre, -0.5 degrees C; delta Tes, -0.3 degrees C) and increased (P less than 0.05) during LE (delta Tre, 0.4 degrees C; Tsk, 0.4 degrees C). M, Tsk, Tre, and Tes were higher (P less than 0.05) during LE compared with R. In cool water, all regional heat flows (leg, chest, and arm) were generally greater (P less than 0.05) during LE than R. In cold water, M increased (P less than 0.05) over 1 h during R but remained unchanged during LE. Tre decreased (P less than 0.05) during R (delta Tre, -0.8 degrees C) but was unchanged during LE. Tes declined (P less than 0.05) during R (delta Tes, -0.4 degrees C) but increased (P less than 0.05) during LE (delta Tes, 0.2 degrees C). M, Tre, and Tes were higher (P less than 0.05), whereas Tsk was not different during LE compared with R at 60 min.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in thermal insulation during underwater exercise in Korean female wet-suit divers

The present work was undertaken to examine the effect of wet suits on the pattern of heat exchange during immersion in cold water. Four Korean women divers wearing wet suits were immersed to the neck in water of critical temperature (Tcw) while resting for 3 h or exercising (2-3 met on a bicycle ergometer) for 2 h. During immersion both rectal (Tre) and skin temperatures and O2 consumption (VO2) were measured, from which heat production (M = 4.83 VO2), skin heat loss (Hsk = 0.92 M +/- heat store change based on delta Tre), and thermal insulation were calculated. The average Tcw of the subjects with wet suits was 16.5 +/- 1.2 degrees C (SE), which was 12.3 degrees C lower than that of the same subjects with swim suits (28.8 +/- 0.4 degrees C). During the 3rd h of immersion, Tre and mean skin temperatures (Tsk) averaged 37.3 +/- 0.1 and 28.0 +/- 0.5 degrees C, and skin heat loss per unit surface area 42.3 +/- 2.66 kcal X m-2 X h. The calculated body insulation [Ibody = Tre - Tsk/Hsk] and the total shell insulation [Itotal = (Tre - TW)/Hsk] were 0.23 +/- 0.02 and 0.5 +/- 0.04 degrees C X kcal-1 X m2 X h, respectively. During immersion exercise, both Itotal and Ibody declined exponentially as the exercise intensity increased. Surprisingly, the insulation due to wet suit (Isuit = Itotal - Ibody) also decreased with exercise intensity, from 0.28 degrees C X kcal-1 X m2 X h at rest to 0.12 degrees C X kcal-1 X m2 X h at exercise levels of 2-3 met.(ABSTRACT TRUNCATED AT 250 WORDS)     

Exercise training and 3-day head down bed rest deconditioning: exercise thermoregulation.

Bed rest (BR) deconditioning causes excessive increase of exercise core body tempera-ture, while aerobic training improves exercise thermoregulation. The study was designed to determine whether 3 days of 6 degrees head-down bed rest (HDBR) affects body temperature and sweating dynamics during exercise and, if so, whether endurance training before HDBR modifies these responses. Twelve healthy men (20.7+/-0.9 yrs, VO2max: 46+/-4 ml x kg(-1) x min(-1) ) underwent HDBR twice: before and after 6 weeks of endurance training. Before and after HDBR, the subjects performed 45 min sitting cycle exercise at the same workload equal to 60% of VO2max determined before training. During exercise the VO2, HR, tympanic (Ttymp) and skin (Tsk) temperatures were recorded; sweating dynamics was assayed from a ventilated capsule on chest. Training increased VO2max by 12.1% (p<0.001). Resting Ttymp increased only after first HDBR (by 0.22 +/- 0.08 degrees C, p<0.05), while exercise equilibrium levels of Ttymp were increased (p<0.05) by 0.21 +/- 0.07 and 0.26 +/- 0.08 degrees C after first and second HDBR, respectively. Exercise mean Tsk tended to be lower after both HDBR periods. Total sweat loss and time-course of sweating responses were similar in all exercise tests. The sweating threshold related to Ttymp was elevated (p<0.05) only after first HDBR. In conclusion: six-week training regimen prevents HDBR-induced elevation of core temperature (Ttymp) at rest but not during ex-ercise. The post-HDBR increases of Ttymp without changes in sweating rate and the tendency for lower Tsk suggest an early (<3d) influence of BR on skin blood flow.     

Water loss distribution in exercising men under hot conditions

Dynamics of sweating and water loss distribution were studied in 7 exercising men under thermoneutral conditions (Ta, 25 degrees C; Tw, 24 degrees C and RH, 54%) and during moderate heat exposure (Ta, 30 degrees C; Tw, 30 degrees C; RH, 54%). The subjects performed bicycle exercise at intensity of 50% V O2 max. Dynamics of sweating was greater after heat exposure (delay in onset of sweating 3.6 and 1.4 min, p less than 0.05; time constant 10.1 and 7.3 min, p less than 0.02). The dynamics of sweating was related to the net body heat load (r = -0.80, p less than 0.001). Sweat evaporation from the skin (Esk) was significantly higher in heat exposed exercising subjects while dripping sweat (mdrip) did not differ significantly. Water loss distribution in relation to total water loss during control exercise was as follows: (Ediff + Eres) 14.8% (Esk) 59.6%; and (mdrip) 25.6%. During exercise under heat exposure (Ediff + Eres) was 12.1%; (Esk) was 67.5%; and (mdrip) was 20.4%. It is concluded that moderate heat exposure accelerate sweating reaction but does not change significantly water loss distribution in exercising subjects. Dripping sweat seems to be an attribute of sweating not only in hot humid conditions but also under temperate temperature and air humidity.     

Thermoregulation during exercise in relation to sex and age

The thermoregulatory responses to 1 h exercise of 14 male (age range 18--65 year) and 7 female (age range 18--46 year) athletes and 4 (3 male and 1 female) non-athletic subjects have been investigated in a moderate environment (Tdb = 21 degrees C, Twb = 15 degrees C and rh less than 50%) and analysed in relation to age, sex, and maximum aerobic power output (VO2max). The maximal sweat loss (Msw max) under the given conditions was closely related (r = + 0.90) to VO2max and for a given relative work load (%VO2max), rectal (Tre) and mean skin (Tsk) temperatures was the same in all subjects. Sweat loss (Msw) was linearly related to total heat production (H) and to peripheral tissue heat conductance (K) and if expressed in relative terms (%Mswmax) was linearly related to Tre. For a given Tre relative sweat rate was identical in the groups studied. From these results it would seem that during exercise Tre rises to meet the requirements of heat dissipation by establishing a thermal gradient from core to skin and stimulating sweating in proportion to maximal capacity of the system. Thus provided the thermal responses to work were standardised using the appropriate physiological variables, there was no evidence to be found for differences in thermoregulatory function which could be ascribed to sex or age.     

Thermal adjustment to cold-water exposure in resting men and women

Thermoregulatory responses were studied in 10 men and 8 women at rest in air and during 1-h immersion in water at 20, 24, and 28 degrees C. For men of high body fat (27.6%), rectal temperature (Tre) and oxygen consumption (VO2) were maintained at air values at all water temperatures (Tw). For men of average (16.8%) and low (9.2%) fat the change in Tre (delta Tre) was inversely related to body fat at all Tw with VO2 increasing to 1.07 l X min-1 for a -1.6 degrees C delta Tre for lean men. For women of average (25.2%) and low (18.5%) fat Tre decreased steadily during immersion at all Tw. The greatest changes occurred at 20 degrees C with little differences in delta Tre and VO2 noted between these groups of women. In comparison with males of similar percent fat, Tre dropped to a greater extent (P less than 0.05) in females at 20 and 24 degrees C. Stated somewhat differently, lean women with twice the percentage of fat have similar delta Tre as lean men at all Tw. For delta Tre greater than -1.0 degree C men showed significantly greater (P less than 0.05) thermogenesis compared with women. The differences in thermoregulation between men and women during cold stress at rest may be due partly to the sensitivity of the thermogenic response as well as the significant differences in lean body weight and surface area-to-mass ratio between the sexes.     

Mechanisms underlying the age-related decrement in the human sweating response

To examine the mechanisms underlying the age-related decrement in the ability to sweat, seven older (64-76 years) and seven younger (20-24 years) men participated in a 60-min sweating test. The test consisted of placing the subject's lower legs in a water bath at 42 degrees C while sitting in a controlled environment of 35 degrees C ambient temperature and 45% relative humidity. The rectal (Trc) and skin temperatures, local sweating rates (m(sw): on the forehead, chest, back, forearm and thigh) and the frequency of sweat expulsion (f(sw)) were measured during the test. No group difference was observed in the mean body temperature (Tb) throughout the passive heating, although the older men had a higher Tre and a lower mean skin temperature during the last half of the 60-min test. There were no group differences in the Tb threshold for sweating, although the time to the onset of sweating tended to be longer for the older men regardless of body site. The m(sw) increased gradually for approximately 35 min after the start of heat exposure in the older men and for 30 min in the younger men and then reached a steady state. During the first half of the test, the older men had a significantly lower m(sw) at all sites. During the last half of the test, only m(sw) on the thigh was significantly lower in the older men than in the younger men. There was no group difference in the slope of f(sw) versus Tb (an indicator of the change in the central sudomotor response to thermal input). The slope of m(sw) versus f(sw) (an indicator of the change in peripheral activity in response to central sudomotor changes) was significantly lower on the thigh in the older men, but there were no differences for the other sites. These results suggest that in older men the lower thigh m(sw) observed during the last half of the heat test was possibly due to age-related modifications of peripheral mechanisms involving the sweat glands and surrounding tissues. It was not due to a change in the central drive to sudomotor function. Furthermore, the sluggish m(sw) responses in the older men appear to have been related to age-related modifications of the sensitivity of thermoreceptors in various body regions to thermal stimuli. They may also involve lower sweat glands' sensitivity to cholinergic stimulus or sluggish vasodilatation, and do not reflect age-related changes in the central drive.     

Endogenous hormones subtly alter women's response to heat stress

The thermoregulatory responses of menstruant women to exercise in dry heat (dry-bulb temperature/wet-bulb temperature = 48/25 degrees C) were evaluated at three times during the menstrual cycle: menstrual flow (MF), 3-5 days during midcycle including ovulation (OV), and in the middle of the luteal phase (LU). Serum concentrations of estradiol-17 beta (E2), progesterone (Pg), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were measured by radioimmunoassay, and these values were used to determine the dates of OV (peak LH and FSH) and LU (peak postovulatory Pg). After heat acclimation, subjects received heat stress tests (HST) consisting of a 2-h cycle-ergometer exercise at 30% of maximal O2 consumption in the heat. Rectal (Tre) and mean skin (Tsk) temperatures, heart rate (HR), and sweat rate on the chest and thigh were recorded continuously. Total sweat loss (Msw), as indicated by weight loss, was recorded every 20 min, and equivalent water replacement was given. Steady-state exercise metabolic rate (M) was measured at 45 and 110 min. Seven of eight subjects had ovulatory cycles during experimental months. At rest, Tre was lowest at OV and significantly higher at LU. During steady-state exercise both Tre and Tsk were lowest at OV and significantly higher at LU. There were no differences between phases in Msw, sweat rate on the chest and thigh or M. Despite higher Tre and Tsk at LU, all subjects were able to complete the 2-h of exercise.(ABSTRACT TRUNCATED AT 250 WORDS)     

Thermal and metabolic responses to cold by men and by eumenorrheic and amenorrheic women

Previous work has suggested that men (M) are more sensitive to cold stress than women. There have also been observations that suggest that amenorrheic women (AW) are less thermally responsive than eumenorrheic women (EW). We investigated the hypothesis that M, EW, and AW would have different responses to cold stress. The subjects (6/group) were tested four times: twice at rest for 60 min (5 and 22 degrees C) and twice in a progressive exercise test (5 and 22 degrees C). At rest at 22 degrees C AW had a lower O2 uptake (VO2) than M and lower rectal (Tre) and finger temperatures than EW. At rest at 5 degrees C both AW and EW had lower skin temperature (Tsk) than M, but there were no group differences in peripheral Tsk sites. M increased VO2 after 10 min and EW after 20 min of cold stress; however, AW did not increase metabolism until 60 min. In the two exercise tests Tre increased in proportion to relative work load; in the 5 degrees C test there was little evidence that exercise increased Tsk sites above rest levels. Few of the metabolic or thermal differences could be accounted for by body fatness, body surface area (BSA), or BSA/kg. The data support the hypothesis that M, EW, and AW have different responses to cold stress.