TABLE 1.

Concentrations of substrates and effectors during rest, mild aerobic exercise, and intense aerobic exercise in rat skeletal muscle cytosol and compositions of the three media mimicking these conditions ex vivo

Data are for all metabolites assumed to be relevant, from the literature for rat skeletal muscle as indicated (values for carnitine and acetylcarnitine include some human data; values for free Ca²⁺ include some mouse data). Where appropriate, calculations used Equation 3 of Ref. 60 and assumed that muscle wet weight is 77% water (60–64) and 81% intracellular (60–62, 64–66) or that 18% of muscle wet weight is protein (66, 67). Published values were corrected for plasma concentrations where appropriate and for the distribution of intracellular metabolites between cytosol and mitochondria, using the following matrix/cytosol ratios from heart and liver (or the assumed values for metabolites in parentheses): glycerol 3-phosphate, glutamine (dihydroxyacetone phosphate, taurine): 0 (68, 69); citrate, isocitrate: 10 (69–71); malate (succinate): 4 (69, 70, 72, 73); 2-oxoglutarate: 7 (69–73); pyruvate (acetoacetate, 3-hydroxybutyrate, arginine, lysine): 2 (70, 73); aspartate (neutral amino acids, acylcarnitines): 1 (69, 70, 72, 74); glutamate: 3.5 (69, 70, 72, 74); carnitine: 0.74 (75); ATP: 0.25 (69); phosphate: 5 (76). Mitochondrial volume was assumed to be 10% of intracellular (77–82). Values were rounded to convenient whole numbers. Total Mg²⁺ and Ca²⁺ concentrations to give the targeted free values were calculated using the software MaxChelator. Targeted free Ca²⁺ concentrations were 0.05 μm (rest), 1 μm (mild aerobic exercise), and 1 μm (intense aerobic exercise) (83–86). Targeted free Mg²⁺ concentrations were 600 μm (rest), 600 μm (mild aerobic exercise), and 1000 μm (intense aerobic exercise) (40, 87–89). Targeted Na⁺ concentrations were 16 mm for all media (65, 89). Media had K⁺ and Cl⁻ adjusted to give an osmolarity of 290 mosm (90–92). Total K⁺ concentrations were 80 mm (rest), 77 mm (mild aerobic exercise), and 74 mm (intense aerobic exercise). The “basic media” had the compositions shown; the “complex substrate mixes” contained all of the substrates in the colored boxes. *, no data found, so values are assumed; **, maximum value to avoid mitochondrial uncoupling in vitro.