DISTRIBUTION OF FIBER TYPES DETERMINED BY IN-SITU HYBRIDIZATION OF MYOSIN HEAVY-CHAIN MESSENGER-RNA AND ENZYME-HISTOCHEMISTRY IN RAT SKELETAL-MUSCLES

We analyzed fiber types in rat skeletal muscles using a novel combinat ion of in situ hybridization of myosin heavy chain (MyHC) mRNA, and en zyme histochemistry for succinate dehydrogenase (SD), which displayed metabolic properties. The fiber types were classified into the four ma jor subtypes of I(beta/slow>, IIA, IIX and IIB, and their intermediate types coexpressed two MyHC mRNAs: I and IIA, IIA and IIX, or IIX and IIB. The distribution of fiber types differed markedly in each skeleta l muscle. The superficial region of limb muscles was composed mainly o f fast-twitch fibers with oxidative-glycolytic and glycolytic activiti es, such as type IM and type IIB. In contrast, the deep region was com posed almost exclusively of type I and type IIA fibers both with oxida tive activity. In this region, type IIA/IIX hybrid fibers were noted m ore frequently than type I/IIA and IIX/IIB hybrid fibers. In axial mus cles, slow-twitch fibers and fast-twitch fibers composed predominantly of type IIB were distributed dispersively. The diaphragm and masseter showed a high proportion of type IIX and type IIB, respectively, to a dapt to tissue-specific functional requirements and more frequently co ntained type IIX/IIB hybrid fibers than other observed muscles.