RESPONSE OF MUSCLE PROTEIN-SYNTHESIS TO FASTING IN SUCKLING AND WEANED RATS

Protein synthetic efficiency (K(RNA)) is low in immature skeletal muscle of suckling rats and increases toward the end of the suckling period. To determine whether immature skeletal muscle is able to further reduce K(RNA) in response to fasting, suckling (5, 10, and 16 days of age) and weaned (28 days of age) rats were fed, fasted for 10 h, or fasted for 18 h and injected with a flooding dose of L-[4-H-3]phenylalanine for measurement of muscle protein synthesis in vivo. In fed rats, fractional rates of protein synthesis (K(s)) and protein synthetic capacity decreased during the suckling period. K(RNA) increased toward the end of the suckling period. In 5-day-old rats, fasting for 10 h produced a 50% decline in K(s) of extensor digitorum longus and plantaris muscles, but K(s) did not change further after 18 h of fasting. In older suckled and weaned rats, 10 h of fasting decreased K(s) of extensor digitorum longus and plantaris muscles 30%; after 18 h of fasting, values had declined to 50% of those in fed animals. The reductions in K(s) in soleus muscles with 10 and 18 h of fasting were similar to those in other muscles at 5 and 10 days but were less than those in other muscles at 16 and 28 days. Changes in K(RNA) were similar to those for K(s) in all muscles from all age groups fasted for 10 and 18 h. Protein synthetic capacity decreased approximately 12% after 18 h of fasting, but this effect did not differ between age groups or muscle types. These results suggest that fasting decreases protein synthesis in the skeletal muscle of young suckled rats more rapidly than in that of older suckled or weaned rats. Protein synthesis becomes resistant to fasting in the soleus muscle toward the end of the suckling period in association with the maturation of locomotor function.