Exercise- and methylcholine-induced sweating responses in older and younger men: effect of heat acclimation and aerobic fitness

Abstract

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.

Thermoregulatory Efficiency is Increased after Heat Acclimation in Tropical Natives

“…Cotter et al (1997) did not observe sweat redistribution after a 6-day HA period and neither did Inoue et al (1999) after 8 days of HA. The duration of the protocols used in those studies might have been too short to induce local sudomotor adaptation.…”

Temperature Regulation, Heat Balance and Climatic Stress

“…With advancing age our ability to thermoregulate tends to decrease (Havenith et al, 1995b;Inoue et al, 1999;Kenney and Havenith, 1993). This is a multi-factorial process involving many of our physiological systems with an emphasis on the cardiovascular system.…”

Perspectives on Environmental Adaptability and Physiological Polymorphism in Thermoregulation

“…Subcutaneous fat is effective in the inhibition of heat radiation in a cold environment, and muscles affect basal metabolism and are closely related to metabolic heat production in cold. Of course, it is also known that repeated exposure to a hot environment will improve vasodilation and sweating responses (Inoue et al, 1999;Matsumoto, 2002), and repeated exposure to a cold environment will improve vasoconstriction and heat production responses (Baker, 1988). A number of hypotheses have been proposed considering changes due to age in thermoregulatory response to cold.…”