THERMAL ADJUSTMENTS TO NONEXERTIONAL HEAT-STRESS IN MATURE AND SENESCENT FISCHER-344 RATS

The purpose of this study was to test the hypothesis that the rise in colonic temperature (T(c)) during nonexertional heat stress is exaggerated in senescent (SEN, 24 mo, n = 12) vs. mature (MAT, 12 mo, n = 15) conscious unrestrained Fischer 344 rats. On 2 separate days (48 h aart) each SEN and MAT animal was exposed to an ambient temperature (T(a)) of 42°C (relative humidity 20%) until a T(c) of 41°C was attained and then cooled at a T(a) of 26°C until T(c) returned to the initial control level. Control T(c) was similar in the two groups for both trials. The rate of T(c) change during heating was 63% greater (0.070 ± 0.005 vs. 0.043 ± 0.004°C/min, P < 0.05) and the time to 41°C reduced by 36% (54 ± 6 vs. 85 ± 10 min, P < 0.05) in MAT vs. SEN animals during the first exposure, although the cooling rate was slower in the MAT (0.048 ± 0.004°C/min) vs. SEN (0.062 ± 0.006°C/min) animals (P < 0.05). The heating rate was unchanged in MAT animals between trials 1 and 2. However, SEN animals had a 95% increase in heating rate in trial 2 compared with trial 1 (P < 0.05), and the corresponding time to 41°C was decreased by 44% (P < 0.05). As a result, rate of heating and time to 41°C were similar in the two groups during trial 2. The cooling rate was similar between trials within each group. Mortality rates after the two heating trials were 27% (MAT) and 50% (SEN). These findings indicate that the rate of heat gain is lower during an initial heat exposure with advancing age in conscious Fischer 344 rats. However, older animals demonstrate a marked increase in the rate of heat gain in response to a subsequent thermal challenge and a strong tendency for decreased survival compared with their mature counterparts.