11 C-L-methyl methionine dynamic PET/CT of skeletal muscle: response to protein supplementation compared to L-[ring 13 C 6 ] phenylalanine infusion with serial muscle biopsy.

OBJECTIVE: The objective of this study was to determine if clinical dynamic PET/CT imaging with 11 C-L-methyl-methionine (11 C-MET) in healthy older women can provide an estimate of tissue-level post-absorptive and post-prandial skeletal muscle protein synthesis that is consistent with the more traditional method of calculating fractional synthesis rate (FSR) of muscle protein synthesis from skeletal muscle biopsies obtained during an infusion of L-[ring 13 C6 ] phenylalanine (13 C6 -Phe).

METHODS: Healthy older women (73 ± 5 years) completed both dynamic PET/CT imaging with 11 C-MET and a stable isotope infusion of 13 C6 -Phe with biopsies to measure the skeletal muscle protein synthetic response to 25 g of a whey protein supplement. Graphical estimation of the Patlak coefficient Ki from analysis of the dynamic PET/CT images was employed as a measure of incorporation of 11 C-MET in the mid-thigh muscle bundle.

RESULTS: Post-prandial values [mean ± standard error of the mean (SEM)] were higher than post-absorptive values for both Ki (0.0095 ± 0.001 vs. 0.00785 ± 0.001 min-1 , p < 0.05) and FSR (0.083 ± 0.008 vs. 0.049 ± 0.006%/h, p < 0.001) in response to the whey protein supplement. The percent increase in Ki and FSR in response to the whey protein supplement was significantly correlated (r = 0.79, p = 0.015).

CONCLUSIONS: Dynamic PET/CT imaging with 11 C-MET provides an estimate of the post-prandial anabolic response that is consistent with a traditional, invasive stable isotope, and muscle biopsy approach. These results support the potential future use of 11 C-MET imaging as a non-invasive method for assessing conditions affecting skeletal muscle protein synthesis.