COMPARISON OF THE MOLECULAR, ANTIGENIC AND ATPASE DETERMINANTS OF FAST MYOSIN HEAVY-CHAINS IN RAT AND HUMAN - A SINGLE-FIBER STUDY

Combined methodologies of histochemistry, immunohistochemistry, sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE), rever se transcriptase polymerase chain reaction (RT-PCR) and histochemical method specific for myofibrillar ATPase (mATPase) of the type IIX myos in heavy chain (MyHC) isoform were used to study human and rat single fibres to examine the homology between type II MyHC isoform-based fibr es of both species. We demonstrate that human type II fibres exhibit a ntigenic mATPase and 3'-untranslated region (3'-UTR) sequence determin ants homologous to the IIA and IIX but not the IIB MyHC isoforms of th e rat. Both immunolabelling with anti-MyHC monoclonal antibodies and t he mATPase method used with frozen sections confirmed that all human t ype II fibres express type IIA and/or type IIX MyHC, Quantitative immu nohistochemistry failed to recognize human fibres with antigenic chara cteristics corresponding to hybrid IIXB MyHC-based fibres. Ca2+-stimul ated maximum myosin ATPase activity, determined by quantitative histoc hemistry, revealed that human IIX fibres (with an optical density or O D = 0.707) display enzyme activity which is comparable to that of the rat type IIX (OD = 0.687) but lower than that of the rat type IIB fibr es (OD = 0.836). The results do not support the notion that MyHC IIB i s ex pressed in human limb muscles, even in hybrid fibres. We conclude that human type II fibres have been misclassified in numerous previou s publications and that this has important implications in attempts to compare the physiological characteristics of fibre types, particularl y when animal models are used.