Muscle fiber types and their metabolic and size properties, key determinants of muscle function, can be altered by variety of factors. The aim of this study was to evaluate and compare the fiber type composition and size of fibers in the tibialis anterior (TA) muscle of young adult male and female rats, using histochemical and morphometric methods. Combined myofibrillar ATPase after preincubation at pH 4.3, 4.5, 9.4 and 10.4, and metabolic enzyme histochemistry were performed on serial cross-sections of TA from 10 rats (12- week-old).
Three main fiber types (I, IIA, and IIB) and IIX-like fibers were identified by myofibrillar ATPase histochemistry. These fiber populations showed differences in their oxidative potential, too. The succinate dehydrogenase activity decreased in the rank order IIA>I> IIX-like>IIB. There was no significant difference between males and females in the fiber type composition. The TA muscle of both groups contained 48.5% type IIA and IIX-like fibers (the proportion of which was approximately equal), 47.2 % type IIB and only 4.3% type I fibers.
In conclusion, this study demonstrates that there is no sex-related difference in fiber type composition of the rat TA muscle and confirms an inverse correlation between fiber size and oxidative potential, and that IIX-like fibers make a significant population of fast twitch fibers.
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