CHARACTERIZATION OF HUMAN SKELETAL-MUSCLE FIBERS ACCORDING TO THE MYOSIN HEAVY-CHAINS THEY EXPRESS

Using a method of single muscle fibre analysis, we investigated the pr esence of RNA transcripts for various isoforms of the myosin heavy cha in (MyoHC) gene in histochemically, immunohistochemically and electrop horetically characterized individual muscle fibres (n = 65) from adult human vastus lateralis muscle. A cDNA clone isolated in this study wa s shown to contain the 3' end of a previously uncharacterized human My oHC gene which is expressed specifically in human fast IIA muscle fibr es and we conclude that this clone contains part of the human fast IIA MyoHC gene. In all the fibres histochemically, immunohistochemically and electrophoretically characterized as containing the previously cla ssified IIB MyoHC (n = 23), it was shown that the human equivalent to the rat type IIX MyoHC gene is expressed. This observation was taken t o suggest that the previously classified IIB muscle fibres in human mu scle express a MyoHC isoform equivalent to the rat IIX, not the IIB, a nd would therefore be more accurately classified as IIX fibres.