Abstract
Comparative microphotometric determinations of succinate dehydrogenase, triosephosphate dehydrogenase and lactate dehydrogenase activities in single white, red and intermediate fibers in cross-sections of rat rectus femoris muscle and soleus muscle were performed. White fibers have the highest triosephosphate and lactate dehydrogenase activities and the lowest succinate dehydrogenase activity. Red fibers are highest in succinate dehydrogenase activity and relatively low in triosephosphate and lactate dehydrogenase activities. The intermediate fibers have medium succinate dehydrogenase activity and relatively low triosephosphate dehydrogenase activity. In the soleus muscle, the triosephosphate dehydrogenase activity of the red fibers exceeds that of the intermediate fibers. A parallel increase in succinate dehydrogenase is observed in the red fibers of musculus soleus and musculus rectus femoris after thyroid hormone administration. There is some evidence, however, that the red fibers of the two muscles investigated vary with regard to their absolute succinate dehydrogenase activities and also the triosephosphate dehydrogenase to succinate dehydrogenase activity ratio.