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 turnover based on net fluxes of tyrosine radioactivity and mass across the hindlimb, and to describe the impact of nutrition on hindlimb protein kinetics and nutrient utilization in growing Holstein steers. External iliac vessels were catheterized to allow for measurements of hindlimb arteriovenous (AV) differences and blood flow. Animals were used in a single 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 and 14 of each period, a primed-continuous infusion of L-[side chain-2,3-3H]tyrosine was initiated, followed by a 4-h sampling period to assess hindlimb nutrient utilization and protein kinetics. Well-fed steers exhibited rates of hindlimb utilization for non-nitrogenous energy substrates and amino acids that were consistent with values previously reported for well-fed ruminants. Underfeeding decreased hindlimb blood flow (62%), oxygen consumption (51%) and utilization of respiratory fuels (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 intake levels. Hindlimb protein balance was consistent with whole-body balance, adding credence to derived rates of degradation. Overall, data support the validity of the AV difference technique to estimate components of hindlimb protein turnover.