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 C ₆ ] phenylalanine ( ¹³ C ₆ -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 C ₆ -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 K _i 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 K _i (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 K _i 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.