Protein turnover, body composition, muscle characteristics, and blood hormones in response to different direction of growth selection in mice

Whole body protein turnover and tissue traits were studied in growing male mice long-term selected (78 generations) for carcass protein mass (DU-6P, p rotein line), for body weight (DU-6, growth line) or for an index combining body weight and treadmill performance (DU-6+LB, growth+fitness line) and i n the unselected randomly bred control (DU-Ks). For the estimation of prote in turnover data six mice of each line were housed individually in metaboli c cages and were fed ad libitum a commercial stock diet (crude protein 268 g, gross energy 19 MJ/kg DM). At the end of the experiment body weight of a ll selected mice was about twice that of the controls. Fractional rates of protein synthesis (scaled to the corresponding body protein pools) were sig nificantly higher in selected mice than in controls, but were not significa ntly different between the protein and growth line in contrast to the prote in deposition rates. Chemical body composition was changed in dependence on selection traits. In the growth line body fat content was highest at the e xpense of protein content; in the protein line the proportion of lean body mass (muscle protein) was highest. This is supported by results of a previo us experiment (generation 68) on 60-days old mice showing an enlargement of cross sectional area of the EDL muscle. The number of capillaries per fibr e as indicator for nutrient and oxygen supply of the muscle was adequately increased with the enlargement of muscle cross section in all selection lin es, but it was even doubled in the growth+fitness line. No significant alte rations in blood thyroid hormone levels (T-3, T-4) or in free glutamine con tent of muscles extracts were observed in response to selection. Plasma IGF -I levels were higher in selected lines than in controls.