COMPOSITION AND SIZE OF TYPE-I, IIA, IID X, AND IIB FIBERS AND CITRATE SYNTHASE ACTIVITY OF RAT MUSCLE/

A population of muscle fibers containing a myosin heavy-chain isoform IId (or 2x) has recently been identified in rat muscle. The purpose of this study was to histochemically determine the relative population a nd size of muscle fibers composed of type IID/X fibers as well as type I, IIA, and IIB fibers to estimate the absolute mass of the different types of fibers in rat muscle. In addition, muscle citrate synthase a ctivity was measured to determine the relationship between fiber compo sition and muscle oxidative capacity. Seventy-six muscles or muscle pa rts from the face, neck, shoulder, arm, trunk, hip, thigh, and leg of three adult (4.5-5 mo of age) male Sprague-Dawley rats were removed, w eighed, and frozen for histochemical and biochemical analyses. The dat a demonstrated that type IIB fibers make up 71% of the total muscle ma ss, type IID/X fibers 18%, type IIA fibers 5%, and type I fibers 6%. T he mean cross-sectional area across all muscles was 5,078 +/- 175 mu m (2) for type IIB fibers, 3,078 +/- 105 mu m(2) for type IID/X fibers, 2,045 +/- 80 mu m(2) for type IIA fibers, and 1,898 +/- 90 mu m(2) for type I fibers. Citrate synthase activity, an indicator of muscle mito chondrial content, was most closely related to the population of type IIA fibers and was in the rank order of type IIA > I > IID/X > IIB. NA DH-tetrazolium reductase staining intensity also confirmed this order. These data demonstrate that type IID/X fibers make up a significant p ortion of the adult rat muscle mass and are intermediate to type IIA a nd IIB fibers in regard to fiber size and oxidative potential.