Effects of dietary clenbuterol on metabolism of the hindquarters in steers.

The objective of this study was to measure acute (d 1) and chronic (d 9) effects of dietary clenbuterol on heart rate, blood flow, oxygen uptake, and net uptake/release of metabolites in the hindquarters of growing steers. The design was a single reversal with two 9-d periods of control or 8 mg clenbuterol/d with 5 d between periods. Within 2 h of initial consumption of 2 mg clenbuterol (d 1), heart rate and blood flow doubled and arterial plasma concentrations of glucose, L-lactate and nonesterified fatty acid (NEFA) increased, whereas alpha-NH2 N and NH3 concentrations decreased, demonstrating an acute response. Uptake of oxygen increased and net uptake of alpha-NH2 N decreased. Net release of both L-lactate and NEFA increased. On d 9, there were no acute responses to clenbuterol consumption; however, heart rate, blood flow, and NEFA concentration remained chronically elevated, and plasma concentrations of acetate and propionate decreased compared with control feeding. Net uptake of alpha-NH2 N, oxygen and release of L-lactate by the hindquarters chronically increased on d 9 of clenbuterol feeding. Changes in both blood flow and arteriovenous (AV) concentration difference contributed to changes in uptake/release. The chronic metabolic changes and oxygen uptake were consistent with increased N retention in the hindquarters through increased protein synthesis, decreased use of acetate and increased reliance on NEFA for cellular energy. In conclusion, the data show that the perturbation of homeostatic regulation by dietary clenbuterol on d 1 evolved to establishment of homeorhetic regulation by d 9 that is consistent with increased skeletal protein accretion in growing steers.