Regulation of protein degradation in chick muscle by several hormones and metabolites.

Experiments were conducted to examine the effect of various regulatory agents on the rate of protein degradation in chick extensor digiti communis (EDC), and ulnaris lateralis (UL) muscles in vitro. Muscles were incubated in an oxygenated Krebs-Ringer bicarbonate buffer. Rates of protein degradation were calculated from the rate at which tyrosine was released from protein. Protein synthesis was blocked with cycloheximide to prevent reutilization of tyrosine. Insulin (bovine) decreased protein degradation in the EDC and UL muscles by 11.3 and 10.5%, respectively, when glucose was present in the incubation medium and by 11.0 and 10.3% when glucose was omitted. This suggested the action of insulin on protein degradation was not secondary to effects on glucose transport. Glucose alone decreased protein degradation by 17.3 and 14.8% in EDC and UL, respectively. Glucagon (bovine), cortisol, corticosterone, and dexamethasone did not have significant effects on the rate of protein degradation in either muscle. Branched chain amino acids (BCAA) decreased the rate of protein degradation by 21.6% in the presence of insulin but did not significantly change the rate when insulin was omitted. These results demonstrate that chicken muscle, like that of rats, responds to insulin, glucose, and BCAA by decreasing protein degradative rates. The rates of protein degradation in three muscles, EDC, UL, and extensor digitorum longus (EDL), were compared utilizing a medium containing insulin, glucose, and BCAA to minimize protein degradation. All three muscles released tyrosine at a linear rate for 2.25 hr of incubation. The rate of protein degradation in the slowest growing EDC muscle was significantly greater than that in the UL, whereas the fastest growing EDL had the slowest rate of protein degradation.