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Chronic reduction of gravitational load in the rear limbs of rats to simulate the influence of near-zero gravity in skeletal muscles has been shown previously to elicit atrophy in the soleus muscle. Use of this model by the present investigation indicates that soleus atrophy was characterized by a decline in the number of fibers in groups that contained the slow isoenzyme of myosin and which were classified as type I from intensity of staining to myofibrillar actomyosin adenosinetriphosphatase (ATPase) and to NADH tetrazolium reductase. Furthermore total fiber number was not changed, whereas fibers containing the intermediate isoenzyme and those classified as type IIa increased. There results could be explained by either a change in the composition within existing fibers or a simultaneous loss of slow fibers and de novo synthesis of intermediate and fast fibers. Evidence for transformation included an absence of embryonic or neonatal myosin in muscles from suspended rats and the constant fiber number that was unchanged by 4 wk of suspension. Furthermore although fiber areas of both groups of type I and IIa fibers declined during suspension, variability of the fiber areas within each group did not increase.

Muscle fiber number, cross-sectional area, and composition were studied in response to enlargement produced by synergistic ablation in the mouse soleus muscle. The effect of the location of a histological section on the number of fibers that appear in the section was also studied using the mouse soleus muscle. Enlargement was produced in the soleus muscle of 15 male and 15 female mice by ablation of the ipsilateral gastrocnemius muscle. Fiber counts, using the nitric acid digestion method, revealed no difference between control and enlarged muscles in male and female mice. Mean fiber area, determined by planimetry, was 49.1 and 34.5% greater following enlargement in male and female mice, respectively. Increase in muscle weight could be totally accounted for by the increase in fiber area following enlargement. A transformation of type II to type I fibers occurred following enlargement for both sexes. Counts of fibers from histological sections revealed that there was a progressive decrease in the fiber number as the section was moved from the belly to the distal end of the muscle. The results of these studies indicate that muscle enlargement in the mouse soleus muscle is due to hypertrophy of the existing muscle fibers.