Xj. Zhang et al., AN ISOTOPIC METHOD FOR MEASUREMENT OF MUSCLE PROTEIN FRACTIONAL BREAKDOWN RATE IN-VIVO, American journal of physiology: endocrinology and metabolism, 33(5), 1996, pp. 759-767
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.