Use of type-specific antimyosins to demonstrate the transformation of individual fibers in chronically stimulated rabbit fast muscles.

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Abstract
Continuous stimulation of a rabbit fast muscle at 10 Hz changes its physiological and biochemical parameters to those of a slow muscle. These transformations include the replacement of myosin of 1 type by myosin of another type. Two hypotheses could explain the cellular basis of these changes. If fibers were permanently programmed to be fast or slow, but not both, a change from 1 muscle type to another would involve atrophy of 1 fiber type accompanied by de novo appearance of the other type. Alternatively, preexisting muscle fibers could be changing from the expression of one set of genes to the expression of another. Fluorescein-labeled antibodies against fast (AF) and slow (AS) muscle myosins of rabbits were prepared by procedures originally applied to chicken muscle. In the unstimulated fast peroneus longus muscle, most fibers stained only with AF, a small percentage stained only with AS, and no fibers stained with both antibodies. In stimulated muscles, most fibers were stained with both AF and AS. With increasing time of stimulation, there was a progressive decrease in staining intensity with AF and a progressive increase in staining intensity with AS within the same fibers. Individual preexisting muscle fibers can probably actually switch from the synthesis of fast myosin to the synthesis of slow myosin.