HISTOCHEMICAL AND QUANTITATIVE CHANGES IN GLYCOGEN AND PHOSPHORYLASE DURING DISUSE ATROPHY OF THE PIGEON PECTORALIS

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Abstract
Histochemical and quantitative biochemical changes in glycogen content and phosphorylase activity of pigeon breast muscle were studied during development of disuse atrophy from one day to sixty days. Of the two types of muscle fibers present in the pigeon breast muscle the large white glycogen-loaded fibers, which are well adapted for an anaerobic metabolism, were found to lose their glycogen store and phosphorylase activity during the early days of atrophy. On the other hand, the smaller, red, fat-loaded fibers, which are well adapted for an aerobic metabolism, were found to acquire more glycogen and phosphorylase activity during the same period. The staining reaction of these fibers for phosphorylase thus, as with biochemically different branching enzymes, presented a mosaic. However, a few small fibers showed no increase in glycogen content or phosphorylase activity. The histochemical observations also revealed that the majority of the large white fibers were reduced in size, filled with fat, and separated by connective tissue during the later stages of atrophy. Fibers of this sort disappeared or were completely engulfed by connective tissue. Very few small fibers changed in this way. In certain regions an entire fasciculus containing both types of fiber was found to be replaced by fat and connective tissue, leaving a few narrow red fibers intact. There was no phosphorylase activity in the connective tissue. The quantitative studies showed a decrease in glycogen and phosphorylase levels in the muscle early in disuse. After 60 days of atrophy the phosphorylase level recovered somewhat but the glycogen level remained low.