Effect of age, weight, and metabolic rate on endurance, hyperthermia, and heatstroke mortality in a small animal model.

Since exercise performance in man is a function of the age, body weight, and metabolic efficiency interrelationship, this investigation was designed to investigate in rats the effects of these variables on endurance, thermoregulation, and heatstroke mortality at a relatively mild environmental temperature (26 degrees C). The results indicate that light rats (250 g) are able to run (11 m X min-1, 6 degrees incline) longer (221 min), have the lowest rate of heat gain (0.019 C degrees X min-1) than the other groups and thereby experience low heatstroke mortality (20%) despite a large proportion of fluid loss (6.1%). Alternatively, heavier and older rats (350 g-12 week, 500 g-16 week, 500 g-30 week) demonstrated significantly reduced run times (108, 67, and 54 min), more rapid accumulation of metabolic heat (0.040, 0.057 and 0.062 degrees C X min-1) and much higher mortalities (50, 69, and 50%) despite lower percentages of fluid loss (5.2, 3.7, or 4.2%). Although the mechanisms responsible for these varying thermoregulatory responses to exercise-induced exhaustion are not fully understood, the present data indicate that the rate of fluid loss, body weight, and age are important variables in the etiology of thermoregulatory failure culminating in heatstroke and death.