Comparison of different techniques for estimating rates of protein synthesis in vivo in healthy and bacteraemic rats

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Abstract
Previous studies reported that use of a flooding dose of radiolabeled amino acid is a more precise technique than the constant infusion of tracer quantities for determining rates of protein synthesis in rapidly turning-over tissues in the rat. There was little direct investigation comparing different methods under comparable conditions. Initially, 12 healthy male Sprague-Dawley rats, weighing .apprx. 100 g, were randomized to receive either a bolus i.v. injection of 100 .mu.mol of L-leucine (containing 30 .mu.Ci of [1-14C]leucine)/100 g body wt., or a continuous 2 h tracer infusion of [14C]leucine. In the 2nd phase of the experiment, 12 additional rats were i.v. injected with 1 .times. 108 colony-forming units of Pseudomonas aeruginosa and 16 h later randomized to receive 1 of 2 infusions described above. Total protein synthesis as well as fractional synthesis rates were determined in liver, rectus muscle and whole body. Synthesis rates measured in liver, muscle and whole body were significantly higher in bacteraemic rats than in healthy rats. The flooding-dose methodology gave significantly higher estimates of protein synthesis in the liver, skeletal muscle and whole body than did the continuous-infusion method using direct measurement of the acid-soluble fraction from the respective tissue. Indirect estimates of whole-body protein synthesis based on plasma enrichments and stochastic modeling gave the lowest values.