Masters, Haley

The amino acid enrichment in blood (E b ) is often used in clinically relevant stable isotope studies to calculate muscle protein fractional synthesis rates (FSR), but it underestimates the FSR compared to methods using intracellular enrichment in muscle (E i ). We compared the FSR calculated with intravenous infusion of the novel tracer L‐[2,3,3,4,5,5,5,6,6,6‐ 2 H 10 ]leucine ([ 2 H 10 ]leu) to infusion of the traditional tracer L‐[ 13 C 6 ]phenylalanine ([ 13 C 6 ]phe) using E b and E i . Because [ 2 H 10 ]leu is intracellularly metabolized to [ 2 H 9 ]leu prior to reentry into the blood, we hypothesize that using [ 2 H 10 ]leu will minimize the discrepancy between FSR values calculated using E b or E i compared to [ 13 C 6 ]phe‐calculated FSR values using the corresponding E b or E i . To test our hypothesis, six healthy subjects received simultaneous infusions of the two stable‐isotope tracers following standard protocols. The [ 2 H 9 ]leu‐calculated FSR (%h ‐1 ) was 0.08±0.01 based on E b and 0.18±0.01 based on E i . The corresponding [ 13 C 6 ]phe‐calculated FSR values were 0.06±0.01 and 0.25±0.02, respectively. Moreover, the difference in the [ 2 H 9 ]leu‐calculated FSR values using E i vs E b was significantly less ( P <0.01; 0.10±0.01) compared to the difference in the corresponding [ 13 C 6 ]phe‐calculated FSR values (0.19±0.02). We conclude that, compared to [ 13 C 6 ]phe, the [ 2 H 10 ]leu tracer offers a more accurate approach to calculating FSR when E b is sought as an alternative to the E i. Grant Funding Source : Supported by NIH/NIDDK Grant# R01 DK094062; NCATS Grant# UL1 TR000135

Multiple conditions are linked to perturbed skeletal muscle protein (SMP) metabolism, including excess adiposity and insulin resistance. Here we delineate the precise roles of body fat and insulin sensitivity on SMP metabolism, and test the hypothesis that hyperaminoacidemia can eliminate the influence of these factors. Six healthy individuals underwent a DEXA scan to determine percent body fat (%BF; 16.3‐29.4%) and an oral glucose tolerance test (OGTT) to evaluate insulin sensitivity via Matsuda index (MI; 1.21‐12.0). Subjects received a primed, continuous infusion of L‐[2,3,3,4,5,5,5,6,6,6‐ 2 H 10 ]leucine to determine SMP fractional synthesis rate (FSR) in biopsies from the vastus lateralis . Myostatin (MSTN) mRNA, a marker of muscle wasting, was quantified in similar biopsies, and MSTN activation was assessed in plasma as the percent of active over latent protein. To evaluate the effect of hyperaminoacidemia, samples were collected before and during infusions of high amino acids (AA). In our subjects, %BF was not correlated with MI ( P =0.11). The %BF was only associated with MSTN mRNA ( P =0.02; r=0.81). However, MI was a key determinant in FSR ( P =0.02; r=0.83) and MSTN activation ( P <0.01; r=‐0.94). AA infusion eliminated all discrepancies in FSR and MSTN quantitations. Thus, SMP metabolism is more strongly linked to insulin sensitivity than body fat, but the influence is attenuated with hyperaminoacidemia. Grant Funding Source : Supported by NIH/NIDDK Grant# R01 DK094062; NCATS Grant# UL1 TR000135