Effect of insulin and leucine on protein turnover in rat soleus muscle after burn injury.

To investigate the effects of thermal injury on muscle protein turnover, net protein breakdown and incorporation of leucine into protein was measured in vitro in rat soleus at 3 days following a 3 sec burn to one hindlimb. The weight gain and food consumption of the burn injured animals was similar to that unburned animals. However, the burn caused an 11% decrease in soleus muscle weight and protein content. The levels of ATP, phosphocreatine, and the phosphocreatine/creatine ratio were all normal. Net protein breakdown from the burn-injured muscle was elevated 35% while the incorporation of leucine into protein was unchanged. Thus the increase in protein breakdown in the muscle from the burned region appears to be responsible for the loss in muscle protein. Leucine oxidation was also stimulated by burn. Since protein turnover and leucine oxidation in the contralateral muscle of the burned animal was identical to those from unburned animals, the effects appear to result from direct thermal injury to the muscle from the burned hindlimb rather than from systemic alterations in the metabolic or endocrine environment. In addition, a physiological concentration of insulin (100 microU/ml) was found to stimulate incorporation of leucine into protein and inhibit net proteolysis to the same extent in soleus from burned and unburned limbs. While lower insulin concentrations need to be tested, there appears to be no evidence for insulin resistance with respect to protein turnover under these conditions.