Burn injury stimulates multiple proteolytic pathways in skeletal muscle, including the ubiquitin-energy-dependent pathway.

BACKGROUND

Burn injury is associated with increased muscle protein breakdown. However, the role of different intracellular proteolytic pathways in burn-induced muscle proteolysis is not known.

METHODS

A 30 percent total body surface area burn injury was inflicted on rats. Total and myofibrillar proteolysis was determined in incubated extensor digitorum longus muscles as release of tyrosine and 3-methylhistidine, respectively. Lysosomal proteolysis was assessed by using the lysosomotropic agents leupeptin and methylamine. Calcium-dependent proteolysis was determined by incubating muscles in the absence or presence of calcium or by blocking the calcium-dependent proteases calpain I and II. Energy-dependent proteolysis was determined in muscles depleted of adenosine triphosphate (ATP) by 2-deoxyglucose and 2,4-dinitrophenol. Muscle ubiquitin messenger RNA (mRNA) was determined by Northern blot analysis to assess ATP-ubiquitin-dependent proteolysis.

RESULTS

Calcium-dependent total protein breakdown was stimulated in muscles from burned rats. However, the sensitivity to calcium in vitro was not increased after burn. The lysosomal and energy-dependent components of total protein breakdown were doubled in muscles from burned rats and the energy-dependent myofibrillar protein breakdown was increased almost seven-fold. Ubiquitin mRNA was increased in muscles from burned rats.

CONCLUSIONS

Burn injury stimulates multiple proteolytic pathways in skeletal muscle. The ubiquitin-energy-dependent pathway may be particularly important for the breakdown of myofibrillar proteins.