An isotopic method for measurement of muscle protein fractional breakdown rate in vivo.

We have developed a novel method to measure the fractional breakdown rate (FBR) of muscle protein. This method involves infusing isotope tracer to reach an isotopic equilibrium and then observing its decay in the arterial blood and muscle intracellular pool. The calculation of FBR is based on the rate at which tracee released from breakdown dilutes the intracellular enrichment using a modified precursor-product equation. To test this method, L-[1,2-13C2]leucine and L-[ring-13C6]phenylalanine were infused into six dogs for measurement of FBR and fractional synthesis rate (FSR), respectively. Leucine and phenylalanine kinetics in the hindlimb were measured simultaneously using the arteriovenous (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 approach for measurement of muscle protein FBR. This method can be combined with the tracer incorporation method to measure both breakdown and synthesis in the same infusion study.