The purpose of this investigation was to examine the effects of aging and aerobic fitness on exercise- and methylcholine-induced sweating responses during heat acclimation. Five younger [Y group-age: 23 +/- 1 (SEM) years; maximal oxygen consumption (VO2max): 47 +/- 3 ml.kg-1.min-1], four highly fit older (HO group- 63 +/- 3 years; 48 +/- 4 ml.kg-1.min-1) and five normally fit older men (NO group -67 +/- 3 years; 30 +/- 1 ml.kg-1.min-1) who were matched for height, body mass and percentage fat, were heat acclimated by daily cycle exercise (approximately 35% VO2max for 90 min) in a hot (43 degrees C, 30% RH) environment for 8 days. The heat acclimation regimen increased performance time, lowered final rectal temperature (Tre) and percentage maximal heart rate (%HRmax), improved thermal comfort and decreased sweat sodium concentration similarly in all groups. Although total body sweating rates (Msw) during acclimation were significantly greater in the Y and HO groups than in the NO group (P < 0.01) (because of the lower absolute workload in the NO group), the Msw did not change in all groups with the acclimation sessions. Neither were local sweating rates (msw) on chest, back, forearm and thigh changed in all groups by the acclimation. The HO group presented greater forearm msw (30-90 min) values and the Y group had greater back and thigh msw (early in exercise) values, compared to the other groups (P < 0.001). In a methylcholine injection test on days immediately before and after the acclimation, the order of sweat output per gland (SGO) on chest, back and thigh was Y > HO > NO, and on the forearm Y = HO > NO. No group differences were observed for activated sweat gland density at any site. The SGO at the respective sites increased in the post-acclimation test regardless of group (P < 0.01), but on the thigh the magnitude of the increase was lower in the NO (P < 0.02) and HO (P = 0.07) groups than in the Y group. These findings suggest that heat tolerance and the improvement with acclimation are little impaired not only in highly fit older but also normally fit older men, when the subjects exercised at the same relative exercise intensity. Furthermore, the changes induced by acclimation appear associated with an age-related decrease in VO2max. However methylcholine-activated SGO and the magnitude of improvement of SGO with acclimation are related not only to VO2max but also to aging, suggesting that sensitivity to cholinergic stimulation decreases with aging.

Ten older (aged 64-76 years) and ten younger (aged 20-24 years) healthy men were exposed to a standard heat stress [by placing the lower legs and feet in a water bath at 42 degrees C while sitting in a controlled environment (ambient temperature 35 degrees C and 45% relative humidity) for 60 min]. During passive heating, the rectal temperature of the older men was significantly greater (P < 0.05) and mean skin temperature was lower (P < 0.001), compared to the younger men. Skin blood flow by laser Doppler flowmetry (LDF) was significantly lower on the chest and thigh for the older men (P < 0.001), but forehead LDF did not differ between the groups. The percentages of total LDF in the older men to total LDF in the younger men for the last 30 min were 99%, 58% and 50% on the forehead, chest and thigh, respectively. The age-related differences in LDF responses mirrored cutaneous vascular conductances (CVC), since no group and time effects were observed in mean arterial blood pressure during the test. During the last 30 min the local sweat rates (msw) on the back and thigh were significantly lower for the older men (P < 0.02), but not on forehead, chest and forearm, although the older men had lower msw during the first 30 min exposure regardless of site (P < 0.03). The percentages of total msw in the older men to total msw in the younger men during the last 30 min were 105%, 99%, 63%, 106% and 88% on the forehead, chest, thigh, forearm, and back, respectively. During the latter half of the exposure, the older men had similar LDF, CVC and msw on the forehead, lower LDF and CVC and a similar msw on the chest, and lower LDF, CVC and msw on the thigh, compared to the younger men. These results suggest firstly that regional differences exist in the age-related decrement of cutaneous vasodilatation as well as sweat gland function, secondly that the age-related decrement in cutaneous vascular function may precede a decrement in sweat gland function, and thirdly that the successive decrements may develop sequentially from the lower limbs to the upper body, and head.

To examine sex- and menstrual cycle-related differences in thermoregulatory responses to heat exposure, ten young women and six young men were heated passively by immersing their legs in water heated to 42 degrees C for 60 min (in ambient conditions of 30 degrees C and 45% relative humidity). The women underwent heat exposure during the mid-follicular (F) and mid-luteal (L) phases of the menstrual cycle, which were confirmed by assaying plasma female reproductive hormones. The rectal and mean body (T(b)) temperatures of women in the L phase were significantly greater than those of women in the F phase and of men during a pre-heating equilibration period (28 degrees C) and during heat exposure. During heat exposure, the local sweat rates (m(sw)) on the forehead, chest, back, and forearm of women in either phase were significantly lower than those of men, but the thigh (m(sw)) was similar to that of men. The m(sw) did not change at any site during the different phases of the menstrual cycle. The cutaneous blood flow (%LDF) was significantly greater on the thigh for women in either phase compared with men, but no difference was found at any other site (forehead, chest, back, and forearm). The %LDF on the back was significantly greater for women in the L phase than in the F phase, but those at other sites were similar in both phases. We conclude that, compared with men, heat loss from women depends more on cutaneous vasodilation (especially on the thigh) than on sweating, irrespective of the phase of the menstrual cycle. This phenomenon was due to peripheral mechanisms, as reflected in the greater slope of the relationship between %LDF and T(b) lower slope of the relationship between m(sw)) and frequency of sweat expulsion, and lower sweat output per gland. The menstrual cycle modified the T(b) threshold for vasodilation and sweat onset in women. Therefore, the sex difference in the T(b) threshold was more marked for women during the L phase than during the F phase. Moreover, the menstrual cycle modified the slope of the relationship between %LDF on the back and T(b).

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.

We assessed sex differences in the sweat gland response to changes in exercise intensity with respect to subjects' physical training status. In total, 37 subjects participated (10 trained and 10 untrained females, and 8 trained and 9 untrained males). Each subject cycled continuously at 35, 50 and 65% of their maximal O 2 uptake (V O 2 max ) for 60 min at an ambient temperature of 30• C and a relative humidity of 45%. The mean local sweating rate (SR) on the forehead, chest, back, forearm and thigh was significantly greater in the trained subjects than in the untrained subjects of both sexes. The degree of the increase in SR with physical training was greater in males than in females at higher levels of exercise intensity. This increase in SR depended primarily on an increase in the sweat output per gland (SGO) in both sexes. However, control of the SR increase with increasing exercise intensity was altered by training in females, i.e. the increase in SR from exercise at 50 to 65%V O 2 max depended only on an increase in SGO in trained females and males and untrained males, but it depended on increases in activated sweat glands and the SGO in untrained females. It was concluded that training improved the sweating response, and a sex difference was observed in the degree of improvement in the sweating response due to physical training. This sex difference became more pronounced with increasing exercise intensity. A sex difference was observed in the control of sweating rate to an increase in exercise intensity, i.e. the maximal activated sweat gland responses of untrained females required a higher body temperature or work intensity than the other groups.

. Effects of menstrual cycle and physical training on heat loss responses during dynamic exercise at moderate intensity in a temperate environment. Am J Physiol Regul Integr Comp Physiol 288: R1347-R1353, 2005. First published January 27, 2005 doi:10.1152/ajpregu.00547.2004.-We evaluated the effects of the menstrual cycle and physical training on heat loss (sweating and cutaneous vasodilation) responses during moderate exercise in a temperate environment. Ten untrained (group U) and seven endurance-trained (group T) women (maximal O2 uptake of 36.7 Ϯ 1.1 vs. 49.4 Ϯ 1.7 ml ⅐ kg Ϫ1 ⅐ min Ϫ1 , respectively; P Ͻ 0.05) performed a cycling exercise at 50% maximal O 2 uptake for 30 min during both the midfollicular and midluteal menstrual phase in a temperate environment (ambient temperature of 25°C, relative humidity of 45%). In group U, plasma levels of estrone, estradiol, and progesterone at rest and esophageal temperature (Tes) during exercise were significantly higher during the midluteal than during the midfollicular phase (P Ͻ 0.05). Sweating rate and cutaneous blood flow (measured via laserDoppler flowmetry) on the chest, back, forearm, and thigh were lower during the midluteal than during the midfollicular phase during exercise. Tes threshold for heat loss responses was significantly higher and sensitivity of the heat loss responses was significantly lower in the midluteal than in the midfollicular phase, regardless of body site. These effects of the menstrual cycle in group U were not observed in group T. The sweating rate and cutaneous blood flow were significantly higher in group T than in group U, regardless of menstrual phase or body site. Tes threshold for heat loss responses was significantly lower and sensitivity of heat loss responses was significantly greater in group T than in group U in the midluteal phase; however, sensitivity of the sweating response was significantly greater in the midfollicular phase. These results suggest that heat loss responses in group U were inhibited in the midluteal phase compared with in the midfollicular phase. Menstrual cycle had no remarkable effects in group T. Physical training improved heat loss responses, which was more marked in the midluteal than in the midfollicular phase. thermoregulation; estradiol; progesterone; long-term endurance training; heat acclimatization HEAT LOSS RESPONSES, SUCH as sweating and cutaneous vasodilation, during exercise in women differ from those in men because female hormones modify the responses after puberty (3). All studies of the effects of the menstrual cycle on heat loss responses during exercise have demonstrated that the core body temperature thresholds for sweating and cutaneous vasodilation are higher during the luteal phase of the menstrual cycle than during the follicular phase (14,15,17,18,23,31,33,35). In addition, the rise in the core body temperature threshold for sweating and cutaneous vasodilation during the luteal phase is mainly due to elevated progesterone concentrations (8, 31).Conversely, there are conflictin...

Cross-section comparisons of the effect of age on physiological responses to heat stress have yielded conflicting results, in part because of the inability to separate chronological age from factors which change in concert with the biological aging process. The present study was designed to examine the relative influence of age on cardiovascular and thermoregulatory responses to low intensity cycle exercise (60 W for 1 h) in a warm humid environment (35 degrees C, 80% relative humidity). Specifically, the relative importance of age compared to other individual characteristics [maximal oxygen uptake (VO2max), physical activity level, anthropometry, and adiposity] was determined by multiple regression analysis in a heterogeneous sample of 56 subjects in which age (20-73 years) and VO2max (1.86-4.44 l.min-1) were not interrelated. Dependent variables (with ranges) included final values of thermoregulatory responses [rectal temperature (Tre, 37.8-39.2 degrees C), calculated heat storage (S, 3.4-8.1 J.g-1), sweat loss (238-847 g.m-2)] and cardiovascular responses [heart rate (HR, 94-176 beats.min-1), forearm blood flow (FBF, 5.3-31.3 ml.100 ml-1.min-1), mean arterial blood pressure (MAP, 68-122 mmHg), and forearm vascular conductance (FVC = FBF.MAP-1, 0.06-0.44 ml.100 ml-1.min-1.mmHg-1). Age had no significant influence on Tre, S, or sweat loss, all of which were closely related to VO2max. On the other hand, HR, MAP, FBF, and FVC were related to both age and VO2max. Anthropometric variables and adiposity had secondary, but statistically significant, effects on MAP, FBF, FVC, and sweat loss.(ABSTRACT TRUNCATED AT 250 WORDS)

This paper addresses the ways in which heat loss effector functions change with maturation and aging, using data obtained in our laboratory. Prepubertal children have an underdeveloped sweat function compared with young adults; this is compensated by a greater surface area-to-mass ratio and relatively greater heat loss from cutaneous vasodilation on the head and trunk when the air temperature is lower than the skin temperature. As the heat dissipation depends greatly on the evaporation of sweat, the core temperature of prepubertal children is greater than that of young adults owing to the underdevelopment of sweating. In the elderly the heat loss effector function decreases with aging. The decrease may first involve cutaneous vasodilation, then sweat output per gland, and finally active sweat gland density; and it may proceed from the lower limbs to the back of the upper body, the front of the upper body, then the upper limbs and finally to the head.