The relationship between slow and fast myosin heavy chain content, calpastatin and meat tenderness in different ovine skeletal muscles

The present study investigated the relationship between fibre type distribution and slow (MHC-s) and fast (MHC-f) myosin heavy chain content on calpastatin and meat tenderness in longissimus dorsi (LD), tensor fasciae latae (TFL), semitendinosus (ST), trapezius (TZ) and supraspinatus (SS) muscles from six Mule × Charolais rams. Samples taken at slaughter were frozen either in liquid N2 for analysis of MHC-s and MHC-f by immunoblotting, or in cooled isopentane for histochemical fibre typing. Calpastatin activity and an immunoreactive 135 kDa calpastatin band were analysed in samples taken 24 h postmortem. Shear force was determined on muscle chops taken at 24 h postmortem and conditioned until day 14. The intensity of MHC-s and MHC-f immunopositive bands correlated with %Type I and %Type II fibres identified histochemically (r2 = 0.612 and 0.366, respectively, p < 0.001). Muscle specific differences were observed in MHC-s and MHC-f immunoreactivity, fibre type distribution, calpastatin activity, calpastatin 135 kDa immunoreactivity and shear force. MHC-s correlated positively with calpastatin activity (r2 = 0.725, p < 0.001) and 135 kDa calpastatin (r2 = 0.228, p < 0.01) across all muscle types. The data show that detection of MHC-s can be used to identify fibre type differences between ovine muscles and that this correlates with differences in calpastatin content and inhibitory activity, but not tenderness.