EFFECTS OF RACTOPAMINE IN PIG MUSCLES - HISTOLOGY, CALPAINS AND BETA-ADRENERGIC RECEPTORS

Ractopamine is a beta-adrenergic agonist which stimulates growth in pi gs, but this effect appears to decline during prolonged treatment. Thi s study was conducted to examine possible mechanisms for the growth-en hancing activity of ractopamine, and its attenuation. Finishing gilts (47.0 kg body weight) were allocated to one of five experimental group s: CON, control diet for 6 weeks; RAC, ractopamine (20 mg/kg feed) for 6 weeks; C/R, CON for 3 weeks and RAC for 3 weeks; R/C, RAC for 3 wee ks and CON for 3 weeks; and ALT, CON for weeks 1, 3 and 5, and RAC in weeks 2, 4 and 6. All pigs were fed 2.25 kg/day of a commercial grower ration (13 MJ DE/kg, 16% crude protein, 0.6 g available lysine/MJ DE) . Samples of the m. longissimus dorsi were taken at slaughter and anal ysed for fibre histology, calpain activities and beta-adrenergic recep tor populations. Ractopamine had no effect on muscle fibre proportions , but Type II fibres were smaller in pigs treated for the first 3 week s only (R/C; 2477 mum2, P < 0.05), and tended to be larger in pigs tre ated for the second 3 weeks only (C/R; 3670 mum2, p < 0.10), as compar ed to controls (3086 mum2). Giant fibres were also larger in C/R pigs as compared to controls (9027 v. 6917 mum2, p < 0.05). Type I fibres w ere unaffected by any of the treatments. Calpain II and calpastatin ac tivities were unaffected by any treatment, but calpain I activities we re reduced in C/R pigs as compared to controls (0.331 v. 0.504 U/g, P < 0.05), indicating that reduced capacity for protein degradation migh t partially account for the hypertrophic effect of ractopamine. Pigs t reated with ractopamine for 3 weeks before slaughter had reduced numbe rs of beta-adrenoceptors in muscle than controls (2.34 v. 3.17 fmol/mg protein, P < 0.05), which may be responsible for the observed attenua tion in growth enhancement over time.