THE MOLECULAR REGULATION OF PROTEIN BREAKDOWN FOLLOWING BURN INJURY IS DIFFERENT IN FAST-TWITCH AND SLOW-TWITCH SKELETAL-MUSCLE

We compared the effect of burn injury on the energy-ubiquitin-dependen t proteolytic pathway in the fast-twitch extensor digitorum longus (ED L) and the slow-twitch soleus muscle in rats. Rats were subjected to a 30% total body surface area full-thickness burn or sham procedure. At various time points after injury, total and myofibrillar protein brea kdown rates were determined in incubated EDL and soleus muscles. The e nergy-dependent component of protein breakdown was determined by incub ating muscles in energy-depleting medium. Messenger RNA levels for ubi quitin and RC3, a 20S proteasome subunit, were measured by Northern bl ot analysis. Burn injury resulted in an approximately 50% increase in total protein breakdown and a 3-4 fold increase in myofibrillar protei n breakdown in EDL muscles, and this response reflected increased ener gy-dependent protein breakdown. In contrast, protein breakdown rates w ere not significantly influenced by the burn injury in soleus muscles. Ubiquitin mRNA levels were increased almost 10-fold in EDL and approx imately 4.5-fold in soleus muscles following burn injury. Burn injury resulted in a 2-fold increase in RC3 mRNA in EDL with no significant c hanges noted in soleus muscles. The results suggest that the more pron ounced effect of burn injury on protein breakdown in fast-twitch than in slow-twitch muscle may reflect different regulation of proteolysis at the molecular level.