Expression of eight distinct MHC isoforms in bovine striated muscles:evidence for MHC-2B presence only in extraocular muscles

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Abstract
SUMMARY This study aimed to analyse the expression of myosin heavy chain (MHC)isoforms in bovine muscles, with particular attention to the MHC-2Bgene. Diaphragm, longissimus dorsi, masseter, several laryngeal muscles and two extraocular muscles (rectus lateralis and retractor bulbi) were sampled in adult male Bos taurus (age 18-24 months, mass 400-500 kg) and analysed by RT-PCR, gel electrophoresis and immunohistochemistry. Transcripts and proteins corresponding to eight MHC isoforms were identified: MHC-αand MHC-β/slow (or MHC-1), two developmental isoforms (MHC-embryonic and MHC-neonatal), three adult fast isoforms (MHC-2A, MHC-2X and MHC-2B) and the extraocular isoform MHC-Eo. All eight MHC isoforms were found to be co-expressed in extrinsic eye muscles, retractor bulbi and rectus lateralis,four (β/slow, 2A, 2X, neonatal) in laryngeal muscles, three (β/slow,2A and 2X) in trunk and limb muscles and two (β/slow and α) in masseter. The expression of MHC-2B and MHC-Eo was restricted to extraocular muscles. Developmental MHC isoforms (neonatal and embryonic) were only found in specialized muscles in the larynx and in the eye. MHC-α was only found in extraocular and masseter muscle. Single fibres dissected from masseter, diaphragm and longissimus were classified into five groups(expressing, respectively, β/slow, α, slow and 2A, 2A and 2X) on the basis of MHC isoform electrophoretical separation, and their contractile properties [maximum shortening velocity (v0) and isometric tension (P0)] were determined. v0increased progressively from slow to fast 2A and fast 2X, whereas hybrid 1-2A fibres and fibres containing MHC-α were intermediate between slow and fast 2A.