Mitochondrial pyruvate transport in working guinea-pig heart. Work-related vs. carrier-mediated control of pyruvate oxidation

Myocardial pyruvate oxidation is work- or calcium-load-related, but control of pyruvate dehydrogenase (PDH) by the specific mitochondrial pyruvate transporter has also been proposed. To test the transport hypothesis distribution of pyruvate across the cell membrane as well as rates of mitochondrial pyruvate net transport plus oxidation were examined in isolated perfused but stable and physiologically working guinea-pig hearts. 150 μM-1.2 mM α-cyanohydroxycinnamate proved to specifically block mitochondrial pyruvate uptake in these hearts. When perfusate glucose as cytosolic pyruvate precursor was supplied in combination with octanoate (0.2 or 0.5 mM) as diffusible alternative fatty acid substrate, α-cyanohydroxycinnamate produced up to 20- and 3-fold increases in pyruvate and lactate efflux, respectively. Cinnamates did not alter myocardial hemodynamics nor sarcolemmal pyruvate and lactate export. In contrast the tested concentrations of cinnamate produced reversible, dose-dependent decreases in ¹⁴CO₂ production from [1-¹⁴C]pyruvate or [U-¹⁴C]glucose by inhibiting mitochondrial pyruvate uptake. Linear least-squares estimates of available cinnamate-sensitive total pyruvate transport potential yielded rates close to 110 μmol/min per g dry mass at S_0.5 ≈ 120 μM, which compared reasonably well with literature values from isolated cardiac mitochondria. This transport potential was severalfold larger than total extractable myocardial PDH activity of ≈ 32 μmol/min per g dry mass at 37°C. Even when cytosolic pyruvate levels were in the lower physiologic range of about 90 μM, pyruvate oxidation readily kept pace with mitochondrial respiration over a wide range of workload and inotropism. Furthermore, dichloroacetate, a selective activator of PDH, stimulated pyruvate oxidation without affecting myocardial O₂ consumption, regardless of the metabolic or inotropic state of the hearts. Consequently, little or no regulatory function with regard to pyruvate oxidation could be assigned to the native mitochondrial pyruvate carrier of the working heart. Therefore, mitochondrial pyruvate-H⁺ symport was the normal, highly efficient (rather than controlling) mechanism for pyruvate entry into the mitochondria where PDH regulation controlled pyruvate oxidation.