PROTEIN-SYNTHESIS AND DEGRADATION CHANGE RAPIDLY IN RESPONSE TO FOOD-INTAKE IN MUSCLE OF FOOD-DEPRIVED MICE

The short-term changes in muscle protein synthesis and degradation aft er food intake are unclear. We investigated muscle protein metabolism after food intake in mice that were starved for 18 h and refed for 1 h . Protein synthesis activity was estimated by the polysome profiles, a nd protein degradation was estimated by plasma N-tau-methylhistidine ( MeHis) concentration, reflecting translational activity and myofibrill ar protein degradation, respectively. MeHis is an index of myofibrilla r protein degradation because it is not reused for protein synthesis a nd it is not metabolized. Stimulation of protein synthesis (polysome p rofile) and the reduction of protein degradation (plasma N-tau-methylh istidine concentration) were observed immediately after feeding began. Protein synthesis returned to the prefeeding level by 6 h after refee ding, whereas protein degradation remained at a low level. The decreas ed plasma MeHis concentration after refeeding was not due to a decreas e in MeHis release from muscle cells and an increase in the free MeHis pool size, because the changes in free MeHis concentration in muscle were similar to that of plasma. Plasma insulin concentration immediate ly rose with feeding and it returned to the prefeeding level by 3 h af ter refeeding, These results suggest that responses of postprandial pr otein metabolism are very rapid and that protein synthesis is regulate d by insulin, whereas degradation is regulated by insulin and other di etary factors. Thus the ability of skeletal muscle to use nutrients mo re effectively by stimulating protein synthesis and reducing protein d egradation may cause the accelerated rate of protein accretion in skel etal muscle during the short postprandial period.