Influence of physical exercise on old rats: changes in patterns of spontaneous activity and connective tissues.

It has been shown that life-long, regular physical exercise has benefits for humans as well as laboratory animals. Population studies have shown that the longevity of humans is increased due to the decrease in all-cause mortality. Further, the functional capacities of organ systems, especially the cardiovascular, are maintained better. Body fat content stays lower. Similar conclusions have been drawn from animal studies, most of them using voluntary exercise in running wheels. We have previously shown that spontaneous activity, measured in an open field setting, is better preserved, suggesting a slowing of sensorimotor impairment with age, and possibly improved maintenance of dopaminergic and cholinergic systems. Further, the systemic effect on connective tissues was a slowing of the age-dependent increase of stability by changes in the cross-linking patterns. The purpose of the present investigation was to analyze whether late-onset training programs had any such effects, i.e., whether the aging effects seen in sedentary animals could be reversed to some extent. We trained male Sprague-Dawley rats from the age of 18 and 20 months until the age of 22 months, i.e., for 4 and 2 months, in a treadmill for 800 m/day. Spontaneous activity in an open field was assessed at the ages of 18, 20, and 22 months. For systemic changes in connective tissues, tail tendons were analyzed with respect to thermal stability and biomechanical strength parameters. The rats trained for 4 months lost weight significantly, which suggests that most of the fat accumulated during a sedentary life can be removed by physical exercise. Two months of training, either from the age of 18 or 20 months, had a positive effect on spontaneous activity, while the last 2 months of a 4-month training period had a negative influence. We conclude that the exercise program was too strenuous to be maintained for 4 months, which should be interpreted as a failure to increase the functional capacity sufficiently. Analysis of the thermal stability and biomechanical properties showed that both training programs moved these properties in a "younger" direction. We conclude that training starting late in life also influences the aging rat in a positive way, although there are limitations to the intensity of training that is beneficial.