Protein dynamics in skeletal muscle after trauma: local and systemic effects

Injury is attended by accelerated skeletal muscle proteolysis. Accurate definition of this hypercatabolic response and its mediation is requisite for specific therapy. We measured protein dynamics in the incubated and intact epitrochlearis and soleus muscles excised from both forelimbs and both hindlimbs of rats 4 days after injury by either a single hind limb scald (90 degrees C water for 3 seconds; metabolic rate (MR) + 15%, urinary urea nitrogen (UUN) + 10%) or a 5% excision (dorsal skin removed to fascia; MR + 40%, UUN + 90%). Protein synthesis (3H phenylalanine incorporation) increased only in the injured soleus from the scalded hind limb (+100%). Actin and myosin breakdown (3-methylhistidine release) increased in all muscles tested and was consistently larger in epitrochlearis than in soleus muscles. Breakdown of the mixed protein pool (tyrosine release) increased but less so than 3-methylhistidine and did not reach significance in the uninjured soleus muscle of scalded rats. With respect to fiber type, white fiber epitrochlearis muscle demonstrated a more pronounced elevation of both measures of breakdown but at a lower metabolic rate than did red fiber soleus muscle. Increasing MR was associated with a linear increase in soleus proteolysis but no further change in epitrochlearis breakdown. We conclude that protein breakdown is increased in skeletal muscle distant from injury; however, even when metabolic stress is severe, synthesis is unchanged. Muscles of different fiber composition are not equally labile. Furthermore, myofibrillar protein is more labile than the mixed protein pool.