Metabolism of Cardiac Muscle

Both pyruvate and lactate are extracted by the intact myocardium at physiological levels (1, 2) and are rapidly burned to CO2 in the intact cardiac muscle slice. They are interconverted by the diphosphopyridine nucleotide-dependent lactic acid dehydrogenase system and share a common final pathway for terminal oxidation (3, 4). Since these substrates are so closely related in carbohydrate metabolism, a metabolic competition under certain conditions might be expected. Very little has been done to explore the possibility of such a competition, although BraunMenendez et al. (5) concluded that each substrate was used independently as a function of concentration in the heart-lung preparation. The purpose of this study was to determine t.he rates of utilization, oxidation, and interconversion of pyruvate and L(+)-lact,ate when incubated together at various concentrations with cardiac muscle slices from ducklings. By tagging one substrate at a time with Cl4 it was possible to determine the rates of exchange between substrates as well as the over-all rates of oxidation in an open system.