NUTRIENT UTILIZATION AND PROTEIN-TURNOVER IN THE HINDLIMB OF CATTLE TREATED WITH BOVINE SOMATOTROPIN

Our objectives were to assess the effects of chronic administration of recombinant bovine somatotropin (bST) on nutrient utilization and pro tein turnover in the hindlimb of growing Holstein steers. External ili ac vessels were catheterized to allow for hindlimb measurements of art eriovenous differences and blood flow. Animals were used in a single-r eversal design with 16-d treatment periods of daily intramuscular inje ction of either excipient or 120 mu g/kg body wt of bST. On d 11 and 1 3 of each period, a primed-continuous infusion of L-[sidechain-2,3-H-3 ]tyrosine was initiated, followed by a 4-h sampling period to assess h indlimb nutrient utilization and protein kinetics. Somatotropin did no t alter blood flow or the consumption of acetate and oxygen across the hindlimb. In contrast, glucose uptake was reduced by 22% despite incr eases in arterial concentrations of glucose and insulin of 10 and 114% , respectively. Treatment with bST increased hindlimb protein accretio n (estimated from net uptake of tyrosine) and whole-body N balance, ea ch by similar to 40%. A modest increase (10%) in the absolute rate of protein synthesis seemed to account for the improved N retention in th e hindlimb with no change in the rate of protein degradation. Thus, bS T reduced the responses of the hindlimb to insulin, and a small altera tion in protein synthesis was sufficient to explain substantial improv ement in protein deposition.