EFFECTS OF AGE AND ACCLIMATION ON RESPONSES TO PASSIVE HEAT EXPOSURE

To examine the effect of chronological age on thermoregulation during passive heat exposure, six older (O, 61 +/- 1 yr) and six young (Y, 26 +/- 2 yr) men sat at rest during a 30-min baseline period (dry-bulb t emperature = 28-degrees-C), a 60-min thermal transient (28-46-degrees- C by 2-degrees-C steps every 5 min), and 30 min at 46-degrees-C dry-bu lb temperature. Subjects were matched for maximal O2 consumption, anth ropometry, and body composition. Testing was repeated after a 9-day ac tive heat acclimation protocol. There were no age differences in recta l (T(re)), mean skin (T(sk)BAR), or mean body temperature (T(b)BAR = 0 .8T(re) + 0.2T(sk)BAR) before or after acclimation, but heart rate was lower (P < 0.01) in the O group in both acclimation states. Heat accl imation resulted in a significantly lower baseline T(re) and T(b)BAR i n both groups, which remained lower throughout the passive heat stress (P < 0.05). To examine the effects of age and acclimation on thermore gulatory effector function, forearm blood flow (by venous occlusion pl ethysmography) and chest sweating rate (SR(ch), by dew-point hygrometr y) were plotted against T(b)BAR. The slope of the forearm blood flow-T (b)BAR relationship was significantly (P < 0.05) lower in the O group before and after acclimation. A lower maximal SR(ch) (P < 0.05) was ac hieved by the O group, but neither the slope of SR(ch)-T(b)BAR relatio nship nor the T(b)BAR threshold for sweating was affected by age. Pred ictably, acclimation resulted in a lower T(b)BAR threshold for the ons et of sweating and skin vasodilation. These results indicate that, dur ing passive heat stress, older men respond with a lower skin blood flo w at a given T(b)BAR and a lower maximal SR(ch) than matched young sub jects. These relatively decreased effector responses are still evident after rigorous heat acclimation, yet they do not result in poorer hea t tolerance or greater heat storage during passive heat stress of this magnitude.