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

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.