Effects of genetic selection and voluntary activity on the medial gastrocnemius muscle in house mice

In a previous study (J. G, Swallow, T. Garland, Jr,, P.A. Garter, W-Z. Zhan , and G. C. Sieck, J. Appl. Physiol. 84: 69-76, 1998), we found that in hou se mice both genetic selection (10 generations of artificial selection for high voluntary activity on running wheels) and access to running wheels (7- 8 weeks) elicited a modest increase In maximal oxygen consumption. Based on these results, we hypothesized that genetic selection would affect the cha nges in endurance and oxidative capacity of the medial gastrocnemius (MCT) muscle induced by wheel access (training response). Wheel access increased the isotonic endurance of the MG in both genetically selected and random-br ed (control) mice. However, this exercise-induced improvement in isotonic e ndurance of the MG was similar between genetically selected and control mic e. Wheel access also increased the succinate dehydrogenase activity of MG m uscle fibers in both selected and control Lines. However, this exercise-ind uced increase in succinate dehydrogenase activity was comparable between ge netically selected and control animals. Taken together, these results indic ate that the modest increase in maximal oxygen consumption associated with genetic selection is not reflected by the training-induced changes in oxida tive capacity and endurance of MG muscle fibers.