Effects of voluntary and forced exercise on thermoregulation and survival in aged C57BL/6J mice

To evaluate the effect of exercise on thermoregulation in senescent animals, three groups of male C57BL/6J mice aged 28-30 months were tested for cold tolerance, defined as the rate of decline in colonic temperature during 3 h exposure to 10 degrees C ambient temperature. Following this test, the mice were exposed to one of the following exercise conditions: forced exercise on a treadmill for 60 min daily at a rate of 5 m/min; continuous access to voluntary exercise in wheel-activity cages, which resulted in a mean rate of 1.1 m/min; or no expressed exercise with 60 min daily placement on the non-activated treadmill. After 3 weeks, assessment of cold tolerance was repeated. A combined mortality rate of 36% was observed in the exercise groups for this period, while there were no deaths in the non-exercised group. The high mortality rate among exercised animals indicated that these regimens were hazardous for aged mice. Moreover, between tests the non-exercised group exhibited a 0.6 degree C increase in body temperature and 38% improvement in cold tolerance which could be interpreted as a normal adaptation for repeated cold exposure. In contrast, no significant change in either of these variables was observed among survivors in the exercise groups. Thus, introduction of these exercise regimens in senescent mice decreased survival and did not improve the age-related impairment in thermoregulation.