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

The purpose of this investigation was to examine the effects of aging and a erobic fitness on exercise- and methylcholine-induced sweating responses du ring heat acclimation. Five younger [Y group - age: 23+/-1 (SEM) years; max imal oxygen consumption ((V) over dot(2max)): 47+/-3 ml.kg(-1).min(-1)], fo ur highly fit older (HO group 63+/-3 years; 48+/-3 ml.kg(-1).min(-1)) and f ive 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 ac climated by daily cycle exercise (approximate to 35% (V) over dot(2max) 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 temper ature (T-re) and percentage maximal heart rate (%HRmax), improved thermal c omfort and decreased sweat sodium concentration similarly in all groups. Al though total body sweating rates ((M over dot)(sw)) 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 (M) over dot (sw) did not change in all groups with the acclimation sessions. Neither we re local sweating rates ((m) over dot(sw)) on chest, back, forearm and thig h changed in all groups by the acclimation. The HO group presented greater forearm (m) overdot(sw) (30-90 min) values and the Y group had greater back and thigh (m) over dot(sw) (early in exercise) values, compared to the oth er 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 su eat gland density at any site. The SGO at the respective sites increased in the post-acclimation test rega rdless of group (P<0.01), but on the thigh the magnitude of the increase wa s lower in the NO (P<0.02) and HO (P=0.07) groups than in the Y group. Thes e findings suggest that heat tolerance and the improvement with acclimation are little impaired not only in highly fit older but also normally fit old er 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 (V) over dot(2max). However methylcholine-activate d SGO and the magnitude of improvement of SGO with acclimation are related not only to (V) over dot(2max),, but also to aging, suggesting that sensiti vity to cholinergic stimulation decreases with aging.