The rate of protein degradation in isolated skeletal muscle does not correlate with reduction-oxidation status

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Abstract
It has been suggested that the cytoplasmic reduction-oxidation state correlates with, and may regulate, rates of protein breakdown in skeletal muscle. To test whether an increased lactate/pyruvate ratio is in fact generally associated with low proteolytic rates, this ratio was measured in rat extensor digitorum longus muscles incubated under conditions that rates of protein breakdown. Treatment with the calcium ionophore A23187 caused similar large increases in the lactate/pyruvate ratio at 2 microM, where proteolysis did not change, and at 20 microM, where proteolysis was greatly accelerated. Omission of Ca2+ from the medium slowed proteolysis, but decreased the lactate/pyruvate ratio. In muscles incubated at 40 degrees C, rates of proteolysis were faster, but the lactate/pyruvate ratios were higher than 37 degrees C. Thus alterations in the redox status do not necessarily correlate with, and can occur independently of, changes in proteolysis. Furthermore, insulin and inhibitors of lysosomal proteinases decreased proteolysis but, in contrast with previous reports, failed to alter the lactate/pyruvate ratio. In addition, protein breakdown decreased in muscles maintained under tension, although redox state did not change. Thus protein degradation can fall without a concomitant change in the reduction-oxidation state.