Phenotypic and evolutionary plasticity of body composition in rats selectively bred for high endurance capacity.

We investigated the effects of genetic selection and prolonged wheel access (8 wk) on food consumption and body composition in lines of rats selected for high and low intrinsic (untrained) endurance running capacity (HCR and LCR, respectively) to test the generality of phenotypic correlations between physical activity levels, aerobic capacity, and body composition. HCR rats ran more minutes per day on activity wheels than LCR rats, supporting the hypothesis that voluntary activity and physiological capacity are genetically correlated (self-induced adaptive plasticity). Both treatments (selection and wheel access) significantly affected food consumption. HCR rats consumed and digested more food than LCR rats. Access to running wheels did not result in changes in overall body mass, but lean body mass increased and percent body fat decreased in both lines. Selection for high endurance capacity resulted in hypertrophy of the heart and kidneys and decreased long intestine length. We found significant phenotypic flexibility in a number of organ masses after wheel running. Specifically, access to running wheels resulted in hypertrophy of the heart, liver, kidney, stomach, and small and large intestines in LCR and HCR rats. The selected line×wheel access interaction was significantly greater in HCR rats in relative mass for the heart and lung. Compared with LCR rats, HCR rats fortify wheel running with increased food consumption along with greater hypertrophy of key organs for O2 transport.