Comparison of bolus injection and constant infusion methods for measuring muscle protein fractional synthesis rate in humans

Background The use of stable isotope tracer techniques to measure muscle protein fractional synthesis rate (FSR) has been well established and widely used. The most common method that has been utilized so far is a primed constant infusion (CI) method, which requires 3-4 h of tracer infusion. However, recently our group has developed a bolus injection (BI) method, which requires an injection of bolus of tracer and can be completed within 1 h. In this study, we compared calf (gastrocnemius) muscle protein FSR measured using these two different methods - CI and BI. Method FSRs were measured in eight people (5 men and 3 women; age: 62.3 ± 6.9 years (mean ± SD); body weight: 75.4 ± 21.5 kg) at basal, postabsorptive state using L-[ring-²H₅]-phenylalanine. In the CI protocol, a primed continuous infusion was given for 4 h, and muscle biopsies were taken at 120 and 240 min; in the BI, a bolus injection of the tracer was given at 0 min and biopsies were taken at 5 and 60 min. Tracer enrichments in blood and muscle tissue were determined by gas chromatography-mass spectrometry. Data are expressed as mean ± SE; t-test, linear regression and Levene Median equal variance test analyses were performed. Results CI FSR was 0.066 ± 0.006%/h, whereas BI FSR was 0.058 ± 0.008%/h, p = NS. The linear regression analysis showed a significant relationship between BI and CI, p = 0.038. The intra-class correlation coefficient was 0.83. The standard deviation of the differences in the measurements was 0.015%/h. The Levene Median equal variance test demonstrated no difference in variance between the CI and BI measurements (p = 0.722). Conclusion No difference could be detected in calf muscle protein FSR measured by CI and BI methods; the BI method can be used for the measurement of muscle protein FSR in humans.