EFFECTS OF CLENBUTEROL ON RABBIT GROWTH, NITROGEN-BALANCE AND SKELETAL-MUSCLE FIBERS

27 male N.Z.W. rabbits (1953 +/- 32 g initial live weight) were housed in individual metabolic cages in a controlled environment maintained at a temperature of 18-degrees-C, illuminated between 8 AM and 8 PM. T he animals were divided into 3 homogenous groups fed ad libitum (from 6 PM to 9 AM) diets containing different levels of clenbuterol (CB) (' 'C'': control diet, ''T-0.5'': control diet plus 0.5 mg/kg CB, ''T-1'' : control diet plus 1 mg/kg CB). Two N-balance trials were performed: days 4 through 8 and days 27 through 31 from the beginning of administ ration of experimental diet. CB treatment significantly improved final live weight (about + 8.9%), with no dose-effect. Treatment did not af fect feed intake, while a better feed efficiency was observed for grou ps receiving CB. During the N-balance study no treatment-effects were observed on N intake nor on fecal N. The beta-adrenergic agonist incre ased N-retained (P < 0.01) and reduced N-urinary excretion (P < 0.01). The effects of CB treatment on reducing urea, OH-proline and alpha-am ino-N excretion were initially greater. Our data indicate that the bet a-agonist reduces amino acid oxidation, collagen protein degradation a nd spares amino acids, thus contributing to enhanced efficiency of N d eposition. Creatinine-N excretion was significantly increased only dur ing second period of N-balance (P < 0.001). Treatment with CB signific antly improved the dressing percentage. The semimembranosus muscle was significantly heavier in the treated group (T-1) than in the controls , due to muscle hypertrophy. The muscle hypertrophy caused by CB inclu ded a general effect on the cross-sectional area of all fibre types an d a transformation of fibre types, especially type IIA to IIB.