Both muscle inactivity and direct thermal injury increase net proteolysis and amino acid release by muscle. To assess their relative contribution to the altered protein metabolism by thermally injured muscle, rats were scalded on one hindlimb and half of these rats were fitted with casts immobilizing the lower half of the body. Three days later, soleus muscles from the burned and unburned limbs of all burned rats and from controls (uninjured, without casts) were studied in vitro. Protein synthesis was estimated by [14C]tyrosine incorporation into muscle proteins and net proteolysis was measured by tyrosine release. Protein synthesis by uncasted unburned limb muscles did not differ from that by controls but that by burned limb muscles was enhanced 134% (P < 0.001). Protein synthesis by casted unburned and burned limb muscles was augmented 56 and 309% (P < 0.003), respectively, above control. Tyrosine release by uncasted unburned muscles of burned rats did not differ from that of controls while that of burned limb muscles was elevated 163% (P < 0.007). Tyrosine release by casted unburned and burned limb muscles was increased 70 and 203% (P < 0.002), respectively, above control. It is concluded that both inactivity of and thermal injury to muscle stimulate its protein turnover. The acceleration of protein turnover by muscle underlying the burn wound far exceeds that produced by mere inactivity, suggesting that disuse is only a minor contributor to enhanced protein turnover in thermally injured muscle.
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