Transient changes in growth and in calpain and calpastatin expression in ovine skeletal muscle after short-term dietary inclusion of cimaterol

Prolonged dietary inclusion of beta-adrenergic agonists can induce skeletal muscle hypertrophy in meat animals, by a mechanism probably related to the calcium-dependent proteolytic enzymes, or calpains, and in particular to their specific inhibitor calpastatin. Calpain and calpastatin activities are also believed to be important factors during post-mortem tenderisation of meat. beta-Agonist treatment is generally associated with increased calpastatin activity, which may lead to meat toughness. The aim of the present study was to examine the effect of a short period of cimaterol (feeding for 8 days, followed by reversion to a normal diet for a further 24 days) on muscle growth and on calpain isoform and calpastatin activities and specific mRNA abundance in the longissimus dorsi (LD) muscle. Significant changes were detected in LD wet weight and in calpastatin activity and mRNA after only 8 days treatment with cimaterol. After 24 further days on a control diet, both LD wet weight and calpastatin activity were not significantly different (P > 0.05) from untreated controls of the same age, although calpastatin mRNA stayed surprisingly high. In contrast to several earlier studies, changes in calpain I (or mu-calpain) and calpain II (or m-calpain) activity and calpain I mRNA were not significantly different (P > 0.05) from controls in any groups. These data suggest that calpastatin activity rather than the activity of either calpain isoform is closely linked to beta-agonist-induced muscle hypertrophy. Changes in calpastatin mRNA are not directly proportional to inhibitory activity, suggesting that variable mRNA species may be transcribed, spliced or stabilised, but not necessarily translated as part of the beta-agonist response.