Mitochondrial metabolism of pyruvate is required for its enhancement of cardiac function and energetics

Pyruvate augmentation of contractile function and cytosolic free energy of ATP hydrolysis in myocardium could result from pyruvate catabolism in the mitochondria or from increased ratio of the cytosolic NAD⁺/NADH redox couple via the lactate dehydrogenase equilibrium. Objective: To test the hypothesis that cytosolic oxidation by pyruvate is sufficient to increase cardiac function and energetics. Methods: Isolated working guinea-pig hearts received 0.2 mM octanoate±2.5 mM pyruvate as fuels. α-Cyano-3-hydroxycinnamate (COHC, 0.6 mM) was administered to selectively inhibit mitochondrial pyruvate uptake without inhibiting pyruvate's cytosolic redox effects or octanoate oxidation. The effects of pyruvate and COHC on sarcoplasmic reticular Ca²⁺ handling were examined in ⁴⁵Ca-loaded hearts. Results: Pyruvate increased left ventricular stroke work and power 40%, mechanical efficiency 29%, and cytosolic ATP phosphorylation potential nearly fourfold. ¹⁴CO₂ formation from [1-¹⁴C]pyruvate was inhibited 65% by COHC, and octanoate oxidation, i.e. ¹⁴CO₂ formation from [1-¹⁴C]octanoate, concomitantly increased threefold. COHC prevented pyruvate enhancement of left ventricular function, mechanical efficiency and cytosolic phosphorylation potential, but did not alter respective levels in pyruvate-free control hearts and augmented cytosolic oxidation by pyruvate. Pyruvate increased sarcoplasmic reticular Ca²⁺ turnover, i.e. Ca²⁺ uptake and release, as indicated by 62% decrease in caffeine-induced ⁴⁵Ca release following 40 min ⁴⁵Ca washout (P<0.01). In presence of COHC, pyruvate did not lower caffeine-induced ⁴⁵Ca release; thus, COHC abrogated pyruvate enhancement of Ca²⁺ turnover (P<0.001). Conclusion: Pyruvate oxidation of cytosolic redox state is not sufficient to increase cardiac function, cytosolic energetics and sarcoplasmic reticular Ca²⁺ turnover when mitochondrial pyruvate transport is disabled; thus, mitochondrial metabolism of pyruvate is essential for its metabolic inotropism.