Effects of the beta-adrenergic agonist salbutamol and its withdrawal on protein metabolism of lambs.

Rasa Aragonesa lambs were used to evaluate the repartitioning effects of a beta-adrenergic agonist and its withdrawal on nutrient metabolism. One group of animals (T-I) was fed salbutamol at a dose of 2 mg x kg-1 diet x day-1 for 38 days, while in another group (T-II) the beta-adrenergic agonist was discontinued in the diet 5 days before slaughter on the 43rd day. The semitendinosus muscle protein content increased (p < 0.05), while fat and collagen content decreased (p < 0.05) in the T-I group. These differences were not apparent in the group from which salbutamol was withdrawn. Muscle protein degradation was diminished (p < 0.05) in both treated groups. The serum free fatty acid level was markedly enhanced (p < 0.05) in the T-I group. Total essential amino acid serum levels were reduced (p < 0.05) after the withdrawal period. Ketogenic and urea cycle amino acids were reduced (p < 0.05) in the salbutamol-fed T-I group and glycogenic amino acids were diminished (p < 0.05) in the T-II experimental group. The data show that salbutamol is able to increase muscle content at the expense of fat stores in productive animals. However, these repartitioning effects are circumvented by a 5-day period of withdrawal, in which concomitant metabolic changes in lipid and protein turnover and plasma amino acid profiles occur.