HISTOCHEMICAL CHANGES IN RAT SKELETAL MUSCLE AFTER EXERCISE

Previous studies led us to suspect that the proportion of fibers within a given muscle which show relatively high activity of the enzymes associated with mitochondria should be related to the nature of the work load placed on that muscle. Following a 52-day swimming program for white rats (one and two 30-min exercise bouts each day) the proportion of fibers having intermediate and high malate dehydrogenase, succinate dehydrogenase, and reduced nicotinamide adenine dinucleotide diaphorase was not altered in the soleus. Neither was the proportion of fibers having low and high mitochondrial α-glycerophosphate dehydrogenase (mito α-GPD) and myosin adenosine triphosphatase altered in the soleus. Myosin adenosine triphosphatase showed no changes in the plantaris. However, the plantaris of moderately and heavily exercised rats did show a greater proportion of fibers having high malate and succinate dehydrogenase, and nicotinamide adenine dinucleotide diaphorase than did the plantaris of sedentary rats. This increase in the proportion of fibers having these high enzyme activities appears to be one mechanism through which a muscle can adapt to a chronic repetitive overload. If one wishes to think in terms of fiber types as determined by these enzymes, this study demonstrates that muscle fiber types can be changed in a nonpathologic condition. © 1969.