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First published online April 23, 2004
Journal of Experimental Biology 207, 1875-1886 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00950

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Fast fibres in a large animal: fibre types, contractile properties and myosin expression in pig skeletal muscles

Luana Toniolo¹, Marco Patruno², Lisa Maccatrozzo², Maria A. Pellegrino³, Monica Canepari³, Rosetta Rossi³, Giuseppe D'Antona³, Roberto Bottinelli³, Carlo Reggiani^1,* and Francesco Mascarello²

¹ Department of Anatomy and Physiology, University of Padova, 35131 Padova, Italy
² Department of Experimental Veterinary Sciences, University of Padova, 35131 Padova, Italy
³ Department of Experimental Medicine, University of Pavia, Italy

^* Author for correspondence: (e-mail: carlo.reggiani{at}unipd.it)

Accepted 17 February 2004

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 (V_o) and isometric tension (P_o) 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 V_o and P_o were identified. Slow fibres had mean V_o=0.17±0.01 length s^-1 and P_o=25.1±3.3 mN mm^-2. For fast fibres 2A, 2X and 2B, mean V_o values were 1.86±0.18, 2.55±0.19 and 4.06±0.33 length s^-1 and mean P_o 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 V_o 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 V_o values are higher than expected on the basis of the scaling relationship between contractile parameters and body size.

Key words: myosin heavy chain, isoform, shortening velocity, pig

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