AN ISOTOPIC METHOD FOR MEASUREMENT OF MUSCLE PROTEIN FRACTIONAL BREAKDOWN RATE IN-VIVO

We have developed a novel method to measure the fractional breakdown r ate (FBR) of muscle protein. This method involves infusing isotope tra cer to reach an isotopic equilibrium and then observing its decay in t he arterial blood and muscle intracellular pool. The calculation of FB R is based on the rate at which tracee released from breakdown dilutes the intracellular enrichment using a modified precursor-product equat ion. To test this method, L-[1,2-C-13(2)]leucine and L-[ring-C-13(6)]p henylalanine were infused into six dogs for measurement of FBR and fra ctional synthesis rate (FSR), respectively. Leucine and phenylalanine kinetics in the hindlimb were measured simultaneously using the arteri ovenous (A-V) balance method. The measured FBR (0.17 +/- 0.02%/h) and FSR (0.10 +/- 0.01%/h) were in agreement with the results from the A-V balance method. In conclusion, our new method provides a feasible app roach for measurement of muscle protein FBR. This method can be combin ed with the tracer incorporation method to measure both breakdown and synthesis in the same infusion study.