1. Lamb growth trials were designed to modify growth and protein content of muscle by diet and also by β-agonist treatment, and to correlate any changes to the activities of calpain proteinases (EC 22.214.171.124) and their inhibitor calpastatin.
2. Wether lambs in a control group were fed on a barley-based diet designed to give a growth rate of 350 g/d; a second group was fed on the same diet but the intake was restricted to give an expected gain of 44 g/d; a third group was fed on the same diet as the first group but the diet included 2 mg clenbuterol/g. At the end of a 6-week trial, longissimus dorsi wet weights were 635 (n6), 377 (n4) and 788 g (n6) (standard error of difference 53.0) in the three groups respectively.
3. Minced L. dorsi was extracted in low-salt buffers and analysed by a fast protein liquid-chromatographic system for calpain I (low calcium-requiring), calpain II (high Ca2+-requiring) and calpastatin activities. No significant changes in the three activities were associated with reduced muscle weight in the restricted-intake group. The inclusion of clenbuterol in the diet, however, led to highly significant increases (P < 0.001) in calpain II and calpastatin to approximately double the control values.
4. The results did not support a direct relation between these activities and muscle growth, except when protein accretion was stimulated by a β-agonist, suggesting a role for this enzyme system in the mechanism by which these agents exert their effect.