TRANSIENT CHANGES IN GROWTH AND IN CALPAIN AND CALPASTATIN EXPRESSIONIN OVINE SKELETAL-MUSCLE AFTER SHORT-TERM DIETARY INCLUSION OF CIMATEROL

Prolonged dietary inclusion of beta-adrenergic agonists can induce ske letal muscle hypertrophy in meat animals, by a mechanism probably rela ted to the calcium-dependent proteolytic enzymes, or calpains, and in particular to their specific inhibitor calpastatin. Calpain and calpas tatin activities are also believed to be important factors during post -mortem tenderisation of meat. Beta-Agonist treatment is generally ass ociated with increased calpastatin activity, which may lead to meat to ughness. The aim of the present study was to examine the effect of a s hort 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 tre atment with cimaterol. After 24 further days on a control diet, both L D wet weight and calpastatin activity were not significantly different (P > 0.05) from untreated controls of the same age, although calpasta tin mRNA stayed surprisingly high. In contrast to several earlier stud ies, 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 a ctivity rather than the activity of either calpain isoform is closely linked to beta-agonist-induced muscle hypertrophy. Changes in calpasta tin mRNA are not directly proportional to inhibitory activity, suggest ing that variable mRNA species may be transcribed, spliced or stabilis ed, but not necessarily translated as part of the beta-agonist respons e.