Nonuniform myosin expression along single fibers of chronically stimulated and contralateral rabbit tibialis anterior muscles.

Using a combined histochemical and biochemical technique, single fiber analyses were performed on chronically stimulated and contralateral tibialis anterior (TA) muscles of the rabbit. The major fiber population (60%) in 30 days stimulated TA was transforming fibers (type IIC). Some of these fibers displayed a nonuniform distribution of histochemically assessed myofibrillar actomyosin ATPase (mATPase) activity. This heterogeneity of mATPase activity along the fibers was verified in longitudinal sections and by microphotometric evaluation of mATPase staining intensities in serial cross-sections. Biochemical analyses of single fiber segments revealed that these C fibers not only coexpressed fast- and slow-myosin subunits but did so nonuniformly along their length. The distribution of fast- and slow-myosin subunits in these fibers was not random but focal. Variations in myosin expression were also observed in some of the C fibers in the contralateral TA. As opposed to the transforming fibers in the stimulated TA, heterogeneities of mATPase activity and myosin subunits in these contralateral C fibers were less focal and more gradual. These findings suggest that muscle fibers in chronically stimulated TA and the contralateral muscle do not transform synchronously or uniformly along their length.