Accumulation of Intramyocellular Acetylcarnitine is Associated with Glycolytic ATP Production In Vivo During Incremental Contractions in Human Skeletal Muscle
Authors: Hayden Christopher M. , Nagarajan Rajakumar , Gilmore Samantha , Smith Zoe H. , Kent Jane A. 
Published: May 2022

The interplay in vivo between muscle bioenergetics (ATP production) and intermediary metabolism in working skeletal muscle is not fully understood. During contractions, acetylcarnitine (AC) production may be an important modulator of energetic pathway and fuel selection for the support of ATP production. As contraction intensity and energetic demand increase, free carnitine in the mitochondria buffers excess acetyl‐CoA by converting it into AC and free CoA. This process enables greater flux of pyruvate into the mitochondria by lessening acetyl‐CoA’s product inhibition of pyruvate dehydrogenase. Further, as AC accumulates in the mitochondria, it will move into the cytosol, where by its conversion to malonyl‐CoA it can have the additional effect of inhibiting fatty acid translocation into the mitochondria. Here, a new method combining noninvasive magnetic resonance spectroscopic (MRS) measures of AC (by 1H MRS) with phosphorus MRS measures of ATP production was used to investigate the link between [AC] and energy production in vivo. Our primary aim was to test the novel hypothesis that AC accumulation during muscular work would be directly associated with nonoxidative (i.e., wherein the fate of pyruvate is lactate) glycolytic ATP production. Using a 3‐Tesla MR system with a dual‐tuned (1H/31P) surface coil, intracellular phosphorus metabolites and pH were measured in the vastus lateralis muscle of 8 males (mean 28.4 yr, range 25‐45) during 8 min of incremental isotonic contractions (0.5 Hz, 2‐min stages at 6, 9, 12, and 15% maximal torque). These metabolites and pH were then used to calculate ATP production (mM·s‐1) via oxidative phosphorylation, nonoxidative glycolysis, and the creatine kinase reaction (CK) throughout the contraction protocol. [AC] was measured in resting muscle pre‐ and post‐contractions. Spearman rank correlation coefficients (rs) were calculated to evaluate associations between post‐contraction [AC] and ATP production during the incremental protocol. On average, 13.1% (range 8.6 ‐ 17.3) of ATP production during the contraction protocol was from CK, 11.3% (0.4 ‐ 29.3) through glycolysis, and 75.6% (55.3 ‐ 90) by oxidative metabolism. [AC] increased > 5‐fold, from 4.4 mM pre‐contractions (1.6‐11.6, n = 7) to 22.5 mM post (13.6 ‐ 33.8, n = 8; p = 0.004). Post‐contraction [AC] was positively correlated with absolute (mM·s‐1) and relative (% total) glycolytic ATP production (rs = 0.95, p = 0.001), supporting our primary hypothesis. Notably, post‐contraction [AC] was negatively correlated with relative (rs = ‐0.81 p = 0.02) but not absolute (rs = ‐0.14, p = 0.75) oxidative ATP production. Thus, cytosolic AC accumulated more when there was greater reliance on glycolysis and a relatively lower overall contribution from oxidative metabolism – a situation consistent with a buildup of acetyl‐CoA in the mitochondria. These results demonstrate, for the first time, the connection between energetic pathway selection and a key metabolic intermediate in working muscle in vivo. Intramyocellular AC may be a valuable tool for probing muscle energetics, substrate utilization, and metabolic flexibility in the future.