Compartmented pyruvate in perfused working heart

Pyruvate compartmentation and lactate dehydrogenase (LDH) were studied in isolated perfused working guinea pig hearts. The mean intracellular pyruvate (Pyr) contents increased with perfusate Pyr (0-2 mM) but varied only slightly with glucose (0-10 mM) and additional insulin (0.04-5 U/l), respectively. With 5-10 mM glucose plus 5 U/l insulin, but not with Pyr or lactate (Lac) as substrates, a near equilibrium between the LDH and the glycerol-3-phosphate dehydrogenase seemed to exist. Evidence for an inhibitory effect of Pyr on the activity of the LDH system of the perfused hearts was not obtained. With [U-14C]glucose as sole substrate, the specific activity of coronary venous Lac was near half that of precursor glucose. 14CO2 production was thus in quantitative agreement with rates of pyruvate oxidation that were determined as glucose uptake minus (Pyr + Lac) release. In contrast, with 0.2 mM [1-14C]Pyr plus 5 mM glucose, the ratio of 14CO2 production to specific activity of Lac overestimated Pyr oxidation judged from myocardial substrate balances and O2 uptake, respectively; here, at least three pools of [14C]HCO-3 and [14C]lac, respectively, were kinetically demonstrable during washout of trace amounts of 14C-labeled Pyr. Evidently, the specific activity of Lac was equivalent to that of mitochondrial oxidized Pyr provided [14C]glucose was the sole or major precursor of cellular pyruvate. However, exogenously applied [1-14C]Pyr of high specific activity seemed to induce intracellular formation of both a highly and lowly labeled Pyr; the latter Pyr compartment did not seem in ready equilibrium with the cell physiologically prevailing highly labeled Pyr pool.