Protein depletion and replenishment in mice: different roles of muscle and liver

Fully grown male CD-1 mice, fed a protein-free diet for 3 days, received 1 g of starch with or without 300 mg casein by intragastric intubation. We surveyed the acute effects of these nutrients on protein synthesis in all tissues (by extrapolating to infinity the incorporation of radioactive leucine after its injection in massive doses) and protein degradation in skeletal muscle and liver (by the accumulation of bestatin-induced peptide intermediates). Muscle proteolysis was the major source of N during depletion. Compared with postabsorptive animals, starch suppressed muscle protein loss (synthesis +21%, degradation -24%, P < 0.01) and stimulated hepatic proteolysis (degradation +28%, P < 0.01). Addition of casein to the starch was anabolic in liver (synthesis +41%, degradation -33%, P < 0.01), gastrointestinal tract, pancreas, and skin (synthesis +38, +69 and +38%, respectively, P < 0.01) but had no effect on muscle. Protein turnover proved uniquely sensitive to the dietary supply of carbohydrates in muscle and to the endogenous or exogenous supply of amino acids in liver.