Rat adductor fiber groups, predominantly composed of either red or white muscle fibers, have been utilized to demonstrate metabolic differences in these two types of voluntary skeletal muscle. Glycogen concentrations of muscle obtained immediately after sacrificing the rats were higher in white than in red muscle, but at the end of 2-hr incubation this relationship was reversed. No difference could be demonstrated in glucose uptake in the two types of muscle, while lactate production was greater in white than in red muscle. Acetoacetic acid uptake by red muscle was higher than that by white muscle as measured by chemical analysis and uptake of acetoacetic acid-C14. A lower percentage of acetoacetic acid-C14 uptake appeared in the lactate fraction in white as compared to red muscle fiber groups. In the case of glycogen, this relationship was reversed and a larger percentage of C14 uptake appeared in the glycogen fraction of white as compared to red muscle. Oxygen uptake and the counts from acetoacetic acid-C14 appearing in C14O2 were higher in red than in white muscle. The creatine phosphate level, after 2-hr incubation, was higher in white muscle. These data demonstrate striking differences in the relative distribution of various metabolic pathways in red and white muscle.

There was a direct correlation between the qualitative histochemical classification by staining intensity for succinic dehydrogenase and the quantitative measurements of succinic dehydrogenase activity for the quadratus femoris (red), soleus (red), sartorius (predominantly red) and the superficial portion of the brachioradialis (predominantly white) muscles of the rhesus monkey. The relative succinic dehydrogenase activities were quadratus femoris > soleus > sartorius > brachioradialis, the quadratus femoris having 7 times more enzyme activity than the brachioradialis. The sartorius of male rhesus monkeys had a higher enzyme activity than that of the female. Muscle samples were stained with sirius red and graded for amounts of connective tissue as follows: soleus < sartorius < brachioradialis. These histochemical results were verified by chemical analyses. The soleus, sartorius and brachioradialis from 10 other species of primates had the same relative succinic dehydrogenase activities and histochemical staining patterns as the rhesus.

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