Fiber type composition of four hindlimb muscles of adult Fisher 344 rats

The limb and trunk muscles of adult rats express four myosin heavy chain (M HC) isoforms, one slow (MHCI) and three fast (MHCIIa, MHCIId, and MHCIIb). The distribution of these isoforms correlates with fiber types delineated u sing myofibrillar actomyosin adenosine triphosphatase (mATPase) histochemis try. For example, type I fibers express MHCI and fiber types IIA, IID, and IIB express MHCIIa, MHCIId, and MHC IIb, respectively. Fibers containing on ly one MHC isoform have been termed "pure" fibers. Recent evidence suggests that a population of "hybrid" fibers exist in rat skeletal muscle which co ntain two MHC isoforms. The purpose of the present investigation was to doc ument the entire range of histochemically defined "pure" and "hybrid" fiber types in untreated muscles of the young adult Fisher 344 rat hindlimb. The selected hindlimb muscles (soleus, tibialis anterior, extensor digitorum l ongus, and gastrocnemius muscles) were removed from 12 male rats and analyz ed for muscle fiber type distribution, cross-sectional area, and MHC conten t. Care was taken to delineate eight fiber types (I, IC, IIC, IIA, IIAD, II D, IIDB, and IIB) using refined histochemical techniques. Hybrid fibers wer e found to make up a considerable portion of the muscles examined (a range of 8.8-17.8% of the total). The deep red portion of the gastrocnemius muscl e contained the largest number of hybrid fibers, most of which were the fas t types IIAD (8.5+/-2.8%) and IIDB (5.2+/-2.3%). In conclusion, hybrid fibe rs make up a considerable portion of normal rat limb musculature and are an important population that should not be ignored.