ROLE OF PROTEIN-INTAKE ON PROTEIN-SYNTHESIS AND FIBER DISTRIBUTION INTHE UNWEIGHTED SOLEUS MUSCLE

Protein turnover in skeletal muscle is very sensitive to protein intak e. To examine whether protein intake is able to affect protein synthes is in the atrophied soleus muscle, the effects of a high-protein (30%, HP) and a medium-protein (15%, MP) diet were studied in rats after 21 days of hindlimb unweighting. Three weeks of unweighting induced a sh arp decrease in food intake (30%). The fractional rate of protein synt hesis (k(s)) was determined in vivo in the slow-twitch soleus muscle b y use of a flooding-dose method. With respect to pair-fed animals, a s ignificant reduction in k(s) occurred (33%) in MP non-weight-bearing r ats, whereas it was of lesser magnitude and not significant in HP rats . In the atrophied soleus muscle of non-weight-bearing MP rats, a larg e decrease (42%) in type I fiber distribution was accompanied by an in crease in intermediate and type IIa fibers. By contrast, a higher perc entage of type I fiber was maintained with the HP diet. However, the H P diet had no beneficial effect in preventing the decrease in either t ype I fiber cross-sectional area (65%) or the average decrease in abso lute myofibrillar and mitochondrial volumes (69 and 52%, respectively) . These results demonstrate that an HP intake did not prevent soleus m uscle atrophy but may sustain protein synthesis and partly preserve fi ber type distribution without affecting the ultrastructural compositio n of fibers. Because the circulating level of free 3,5,3'-triiodothyro nine was reduced by 14% with the HP diet, this effect on fiber type di stribution, and possibly protein synthesis, may involve thyroid hormon es.