RT Journal Article SR Electronic T1 Fast fibres in a large animal: fibre types, contractile properties and myosin expression in pig skeletal muscles JF Journal of Experimental Biology JO J. Exp. Biol. FD The Company of Biologists Ltd SP 1875 OP 1886 DO 10.1242/jeb.00950 VO 207 IS 11 A1 Toniolo, Luana A1 Patruno, Marco A1 Maccatrozzo, Lisa A1 Pellegrino, Maria A. A1 Canepari, Monica A1 Rossi, Rosetta A1 D'Antona, Giuseppe A1 Bottinelli, Roberto A1 Reggiani, Carlo A1 Mascarello, Francesco YR 2004 UL http://jeb.biologists.org/content/207/11/1875.abstract AB Little is known about the influence of Myosin Heavy Chain (MHC) isoforms on the contractile properties of single muscle fibres in large animals. We have studied MHC isoform composition and contractile properties of single muscle fibres from the pig. Masseter, diaphragm, longissimus, semitendinosus, rectractor bulbi and rectus lateralis were sampled in female pigs (aged 6 months, mass 160 kg). RT-PCR, histochemistry, immunohistochemistry and gel electrophoresis were combined to identify and separate four MHC isoforms: MHC-slow and three fast MHC (2A, 2X, 2B). Maximum shortening velocity (Vo) and isometric tension (Po) were measured in single muscle fibres with known MHC isoform composition. Six groups of fibres (pure: slow, 2A, 2X and 2B, and hybrid: 2A-2X and 2X-2B) with large differences in Vo and Po were identified. Slow fibres had mean Vo=0.17±0.01 length s-1 and Po=25.1±3.3 mN mm-2. For fast fibres 2A, 2X and 2B, mean Vo values were 1.86±0.18, 2.55±0.19 and 4.06±0.33 length s-1 and mean Po values 74.93±8.36, 66.85±7.58 and 32.96±7.47 mN mm-2, respectively. An in vitro motility assay confirmed that Vo strictly reflected the functional properties of the myosin isoforms. We conclude that pig muscles express high proportions of fast MHC isoforms, including MHC-2B, and that Vo values are higher than expected on the basis of the scaling relationship between contractile parameters and body size.