Impact of separating amino acids between plasma, extracellular and intracellular compartments on estimating protein synthesis in rodents

Three models representing different separations of amino acid sources were used to simulate experimental specific radioactivity data and to predict protein fractional synthesis rate (FSR). Data were from a pulse dose of ¹⁴C-U Leu given to a non-growing 20g mouse and a flooding dose of ³H Phe given to a non-growing 200g rat. Protein synthesis rates estimated using the combined extracellular and intracellular (Ec + Ic) source pool and extracellular and plasma (Ec + Pls) source pool mouse models were 78 and 120%d^-1 in liver, 14 and 16%d^-1 in brain and 15 and 14%d^-1 in muscle. Predicted protein synthesis rates using the Ec + Ic, Ec + Ic + Tr (combined extracellular, intracellular and aminoacyl tRNA source pool) and Ec + Pls rat models were 57, 3.4 and 57%d^-1 in gastrocnemius, 58, 71 and 62%d^-1 in gut, 8.3, 8.4 and 7.9%d^-1 in heart, 32, 23 and 25%d^-1 in kidney, 160, 90 and 80%d^-1 in liver, 57, 5.5 and 57%d^-1 in soleus and 56, 3.4 and 57%d^-1 in tibialis. The Ec + Ic + Tr model underestimated protein synthesis rates in mouse tissues (5.0, 27 and 2.5%d^-1 for brain, liver and muscle) and rat muscles (3.4, 5.5 and 3.4%d^-1 for gastrocnemius, soleus and tibialis). The Ec + Pls model predicted the mouse pulse dose data best and the Ec + Ic model predicted the rat flooding dose data best. Model predictions of FSR imply that identification and separation of the source specific radioactivity is critical to accurately estimate FSR.