Jaas. Pereira et al., COMPARISON OF THE MOLECULAR, ANTIGENIC AND ATPASE DETERMINANTS OF FAST MYOSIN HEAVY-CHAINS IN RAT AND HUMAN - A SINGLE-FIBER STUDY, Pflugers Archiv, 435(1), 1997, pp. 151-163
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.