Hind-limb protein metabolism and calpain system activity influence post-mortem change in meat quality in lamb

This study is concerned with the rate of protein turnover in the hind limb muscle bed of intact lambs, the activity of calpain proteolytic system in the M. biceps femoris, and subsequent rates of myofibre breakdown and tenderisation in the M. longissimus dorsi. Feed restriction increased protein degradation in hind-limb muscle of lambs (p<0.1), with a concominant decrease in the extractable activity of calpastatin (p<0.01), the endogenous inhibitor of calpain. IGF-1 analog treatment decreased both protein degradation and assayed μ-calpain activity (p<0.05) with no effect on the activity of calpastatin. β-Agonist treatment increased hind-limb protein synthesis (p<0.01), calpastatin activity (p<0.1) and decreased (p<0.01) μ-calpain activity, but did not effect protein degradation. Significant correlations were observed between Myofibril Fragmentation Index (MFI) values during post-mortem storage and initial post-slaughter calpastatin activity at days 3 (r=-0.34, p<0.1), 5 (r=-0.58, p<0.01) and 9 (r=-0.58, p<0.1), and μ-calpain activity at days 5 (r=0.35, p<0.1) and 9 (r=0.41, p<0.05). However, stronger correlations were observed between the ratio of μ-calpain to calpastatin, an estimate of potential μ-calpain proteolytic activity, and the rate of myofibril fragmentation (r=0.75, p<0.001) and tenderisation (r=-0.64, p<0.01) during aging. These results are consistent with the calpain system being the major proteolytic system involved in myofibril fragmentation and hence aging-related tenderisation of meat.