Figure 1. Succinate Is Released Selectively by Exercising Muscle in Mice and Humans.
(A) Comparative metabolomics approach to identify exercise-responsive metabolites released locally by muscle. Extracellular fluids are a combination of interstitial fluids and local circulation.
(B–D) Summary results from the comparative approach, illustrating all annotated metabolites (gray), metabolites fulfilling each individual criterion (black), and metabolites fulfilling all criteria (red).
(B) Metabolites accumulated in muscle extracellular fluid after exercise were defined as fold change > 2 and −log10p>3 versus sedentary extracellular fluid (n = 8).
(C) Metabolites enriched post-exercise in extracellular fluid compared with whole muscle (n = 8).
(D) Metabolites selectively enriched in local muscle extracellular fluid post-exercise were defined as fold change > 2 and −log10p > 3 versus post-exercise bulk plasma (n = 8).
(E) Local release of succinate post-exercise occurs in tibialis anterior (TA) and gastrocnemius (GA) muscle. A selective increase in muscle extracellular fluid is observed in both muscle groups but not in whole muscle (n = 8).
(F) Selective accumulation of succinate in muscle extracellular fluid post-exercise. The relative change in abundance in muscle and interstitial fluid when sedentary and post-exercise is determined separately. Then, to query relative accumulation in extracellular fluid versus muscle, the ratio of relative differences is plotted (n = 8).
(G) Local accumulation of succinate in muscle extracellular fluid post-exercise. The relative change in abundance in plasma and interstitial fluid when sedentary and post-exercise is determined separately. Then, to query relative accumulation in extracellular fluid versus bulk plasma, the ratio of relative differences is plotted (n = 8).
(H) Experimental design to quantify succinate release by human exercising muscle.
(I) Femoral artery and vein succinate concentration during human exercise (n = 10).
(J) Femoral artery-vein difference in succinate concentration during human exercise (n = 10).
(K) Comparison of post-exercise enrichment of mitochondrial TCA cycle metabolites in extracellular fluid compared with whole muscle (n = 8). The relative change in abundance in muscle and extracellular fluid when sedentary and post-exercise is determined separately. Then, to query relative accumulation in interstitial fluid versus muscle, the ratio of relative differences is plotted.
Data are represented as mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.005 (two-tailed Student’s t test for pairwise comparisons, one-way ANOVA for multiple comparisons involving one independent variable).