A rodent model of protein turnover used to design an experiment for measuring the rates of channeling, recycling and protein synthesis.

We described previously a mechanistic model of whole-body protein turnover in rodents. Channeling was defined as the flow of amino acids from the extracellular compartment to aminoacyl tRNA and protein synthesis. Recycling was defined as the flow of amino acids from protein degradation to aminoacyl tRNA (protein synthesis) without mixing with the intracellular pool of amino acids. In this paper, the model is applied to tissues and whole body and is used to develop an experimental protocol for estimating protein fractional synthesis rate, recycling and channeling. Channeling, recycling and protein synthesis must be estimated simultaneously because changes in specific radioactivities over time are highly dependent on the rate of protein synthesis. Injection-specific radioactivities, body weights and experimental variation were used with the model to generate data at different rates of recycling and channeling. The data generated were then used to determine the best time points and experimental method to estimate percentages of recycling, channeling and protein synthesis rate by the iterative Method of Maximum Likelihood. Specific radioactivity at each time point was based on simulated data from three rodents at each of six time points. Predicted protein synthesis rates were within 5%/d of observed rates for all methods. Predicted rates of recycling and channeling were generally within 15% of observed rates except recycling in muscle at high channeling and high recycling. Standard deviations of the predictions of percentages of channeling and recycling were between 0.148 and 44.5% for the pulse dose method, 0.0655 and 197% for the continuous infusion method and 0.351 and 962% for the flooding dose method. The experimental design that yields the best estimates of channeling, recycling and protein synthesis is the pulse dose. Changes in amino acid specific radioactivities in the extracellular, aminoacyl tRNA and protein pools were greatest and should be measured at 2, 6, 10, 40, 70 and 100 min in the pulse method.