Effects of activity, genetic selection and their interaction on muscle metabolic capacities and organ masses in mice.

Chronic voluntary exercise elevates total daily energy expenditure and food consumption, potentially resulting in organ compensation supporting nutrient extraction/utilization. Additionally, species with naturally higher daily energy expenditure often have larger processing organs, which may represent genetic differences and/or phenotypic plasticity. We tested for possible adaptive changes in organ masses of four replicate lines of house mice selected (37 generations) for high running (HR) compared with four non-selected control (C) lines. Females were housed with or without wheel access for 13-14 weeks beginning at 53-60 days of age. In addition to organ compensation, chronic activity may also require an elevated aerobic capacity. Therefore, we also measured hematocrit and both citrate synthase activity and myoglobin concentration in heart and gastrocnemius. Both selection (HR versus C) and activity (wheels versus no wheels) significantly affected morphological and biochemical traits. For example, with body mass as a covariate, mice from HR lines had significantly higher hematocrit and larger ventricles, with more myoglobin. Wheel access lengthened the small intestine, increased relative ventricle and kidney size, and increased skeletal muscle citrate synthase activity and myoglobin concentration. As compared with C lines, HR mice had greater training effects for ventricle mass, hematocrit, large intestine length and gastrocnemius citrate synthase activity. For ventricle and gastrocnemius citrate synthase activity, the greater training was quantitatively explainable as a result of greater wheel running (i.e. 'more pain, more gain'). For hematocrit and large intestine length, differences were not related to amount of wheel running and instead indicate inherently greater adaptive plasticity in HR lines.