EVALUATION OF THE ARTERIOVENOUS DIFFERENCE TECHNIQUE TO SIMULTANEOUSLY ESTIMATE PROTEIN-SYNTHESIS AND DEGRADATION IN THE HINDLIMB OF FED AND CHRONICALLY UNDERFED STEERS

Our objectives were to validate a technique to measure protein turnove r based on net fluxes of tyrosine radioactivity and mass across the hi ndlimb, and to describe the impact of nutrition on hindlimb protein ki netics and nutrient utilization in growing Holstein steers. External i liac vessels were catheterized to allow for measurements of hindlimb a rteriovenous (AV) differences and blood flow. Animals were used in a s ingle reversal design with 16-d periods. Treatments were either 60% (0 .6M) or 220% (2.2M) of the energy required for maintenance. On d 12 an d 14 of each period, a primed-continuous infusion of L-[side chain-2,3 -H-3]tyrosine was initiated. followed by a 4-h sampling period to asse ss hindlimb nutrient utilization and protein kinetics. Well-fed steers exhibited rates of hindlimb utilization for non-nitrogenous energy su bstrates and amino acids that were consistent with values previously r eported for well-fed ruminants. Underfeeding decreased hindlimb blood flow (62%), oxygen consumption (51%) and utilization of respiratory fu els (64-73%). Protein synthesis was also reduced (49%), leading to net nitrogen loss from the hindlimb. The estimated energy cost of protein synthesis accounted for 13% of hindlimb oxygen consumption across int ake levels. Hindlimb protein balance was consistent with whole-body ba lance, adding credence to derived rates of degradation. Overall, data support the validity of the AV difference technique to estimate compon ents of hindlimb protein turnover.