Skeletal muscle interstitial fluid metabolomics at rest and associated with an exercise bout: Application in rats and humans

Authors: ZHANG, JIE - University Of Utah; BHATTACHARYYA, SUDEEPA - Arkansas Children'S Nutrition Research Center (ACNC); HICKNER, ROBERT - Florida State University; LIGHT, ALAN - University Of Utah; LAMBERT, CHRISTOPHER - University Of Utah; GALE, BRUCE - University Of Utah; FIEHN, OLIVER - University Of California, Davis; Ferruzzi, Mario

Submitted to: American Journal of Physiology - Endocrinology and Metabolism
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2018
Publication Date: 11/6/2018
Citation: Zhang, J., Bhattacharyya, S., Hickner, R.C., Light, A.R., Lambert, C.J., Gale, B.K., Fiehn, O., Adams, S.H. 2018. Skeletal muscle interstitial fluid metabolomics at rest and associated with an exercise bout: Application in rats and humans. American Journal of Physiology - Endocrinology and Metabolism. https://doi.org/10.1152/ajpendo.00156.2018.
DOI: https://doi.org/10.1152/ajpendo.00156.2018

Interpretive Summary: The body's skeletal muscles are critical in regulating overall metabolic health, since they are primary sites for energy consumption (calorie burning) and uptake of fuels such as blood sugar and fats. Exercise has important health benefits that are thought to stem from signaling factors from muscle, as well as changes in the muscle metabolism itself. Understanding these factors on a molecular scale is challenging. Blood- or tissue biopsy-based approaches are often applied to characterize specific metabolites that are modulated by muscle metabolism with exercise. Yet, blood reflects a large pool with inputs derived from multiple tissues, making it difficult to determine muscle-specificity. Biopsies cannot discriminate secreted vs. intracellular metabolites, and the invasive nature of this method introduces challenges for frequent collections in sensitive populations (e.g., children, pregnant women). Thus, minimally-invasive collections of interstitial fluid (IF) that surrounds muscle cells, for metabolite analysis under resting and challenged conditions would be of great value. A catheter was designed to collect gastrocnemius muscle ("calf muscle") IF from anesthetized adult male rats, at rest or immediately following moderate-intensity 20 min. exercise. Using a non-targeted, metabolomics analysis (measurement of many metabolites at one time), 299 metabolites were detected, including non-annotated metabolites, select sugars, fatty acids, amino acids, purine metabolites and derivatives. Just 43% of all detected metabolites were in common between IF and blood plasma, and only 4% of exercise-modified metabolites were shared in both pools, highlighting that the blood pool does not reflect the comprehensive metabolic outcomes in muscle IF. A preliminary study of human muscle IF was conducted using samples collected from a microdialysis catheter placed in the vastus lateralis (quadriceps femoris) of 5 healthy adults at rest and during exercise (60% of VO2max, 30 min). Approximately 70% of commonly-detected metabolites discriminating rest and exercise in rats had shared directionality in humans. Interstitium metabolomics reveals how exercise alters muscle metabolism, and may aid in the identification of molecules that signal exertion, fatigue, and muscle health. Since IF catheters are minimally invasive, the use of this approach should prove helpful in studying how exercise and metabolic health status influence muscle function in children, pregnant mothers, and other sensitive populations.

Technical Abstract: Blood or biopsies are often applied to characterize metabolites that are modulated by exercising muscle. However, blood has inputs derived from multiple tissues, biopsies cannot discriminate secreted vs. intracellular metabolites and their invasive nature is challenging for frequent collections in sensitive populations (e.g., children, pregnant women). Thus, minimally-invasive approaches to interstitial fluid (IF) metabolomics would be valuable. A catheter was designed to collect gastrocnemius IF from acutely anesthetized adult male rats, at rest or immediately following 20 min. exercise (~60% VO2max). Using non-targeted, gas chromatography/time-of-flight mass spectrometry analysis, 299 metabolites were detected, including non-annotated metabolites, sugars, fatty acids, amino acids, purine metabolites and derivatives. Just 43% of all detected metabolites were in common between IF and blood plasma, and only 20% of exercise-modified metabolites were shared in both pools, highlighting that the blood does not fully reflect the metabolic outcomes in muscle. Notable exercise patterns included increased IF amino acids (except Leu and Isoleu), increased a-ketoglutarate and citrate (which may reflect tricarboxylic acid cataplerosis or shifts in non-mitochondrial pathways), and higher concentration of the signaling lipid oleamide. A preliminary study of human muscle IF was conducted using a 20 kDa microdialysis catheter placed in the vastus lateralis of 5 healthy adults at rest and during exercise (60% VO2max, 30 min). Approximately 70% of commonly-detected metabolites discriminating rest and exercise in rats were also changed in exercising humans. Interstitium metabolomics may aid in the identification of molecules that signal muscle work (e.g., exertion, fatigue) and muscle health.