Protein degradation in muscle: response to feeding and fasting in growing rats.

Degradation rates of muscle proteins were determined in young rats allowed access to standard rat chow 12 h/day. degradation was assessed by determination of 3-methylhistidine (3MH) excretion rates. 3MH is a nonreutilized amino acid produced almost exclusively within the actin and myosin of skeletal and cardiac muscle. Because plasma levels of 3MH are low and renal clearance is high, excretion reflects myofibrillar degradative rates. Excretion of 3MH was determined for 4-h periods beginning 12 and 20 h after initiation of feeding and after 24-and 48-h fasts. Excretion of 3MH per 4-h period increased with time after the last feeding. Because creatinine excretion is a function of muscle mass dividing 3MH excretion by creatinine excretion represents myofibrillar degradation per unit muscle mass, the fractional degradative rate. Degradation rates rose from 4.6 to 14.5%/day between 12 and 16 and 60 and 64 h after the beginning of the last meal. These results support the presence of a diurnal pattern of protein degradation as well as increased muscle degradation during starvation.