Figure 2. pH-Gated Secretion of Succinate.

(A) Monocarboxylic pKa characteristics of TCA cycle metabolites. Distinctly, succinate exists in the monocarboxylic form at physiological acidic pH.

(B) Relationship between the TCA cycle metabolite monocarboxylic pKa and post-exercise enrichment in interstitial fluid (n = 8).

(C) A model for pH-gated release of the monocarboxylic form of succinate during exercise.

(D) C2C12 myotube hypoxia drives selective succinate secretion (n = 6).

(E) Promotion of glycolytic flux through acute inhibition of mitochondrial oxidative phosphorylation drives intracellular acidification in C2C12 myotubes. (n = 4).

(F) Promotion of glycolytic flux through acute inhibition of mitochondrial oxidative phosphorylation with antimycin A drives selective succinate secretion in C2C12 myotubes (n = 6).

(G) Promotion of glycolytic flux through acute inhibition of mitochondrial oxidative phosphorylation with atpenin A5 drives selective succinate secretion in C2C12 myotubes (n = 6).

(H) Monensin equilibrates intracellular pH with extracellular pH through H⁺/Na⁺ antiport.

(I) Monensin prevents intracellular acidification by acute inhibition of mitochondrial oxidative phosphorylation in C2C12 myotubes (n = 4).

(J) Monensin-driven cytosolic alkalinization prevents succinate secretion initiated by acute inhibition of mitochondrial oxidative phosphorylation in C2C12 myotubes (n = 6).

(K) Monensin-driven cytosolic alkalinization prevents succinate secretion initiated by hypoxia in C2C12 myotubes. pH values denote clamped extracellular pH (n = 6).

(L) Promotion of glycolytic flux through acute inhibition of mitochondrial oxidative phosphorylation with atpenin A5 drives succinate secretion from intact EDL muscle, which is prevented by monensin (n = 5).

Data are represented as mean ± SEM. *p or #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).