CHEMICAL VERSUS ISOTOPIC EQUILIBRIUM AND THE METABOLIC-FATE OF GLYCOLYTIC END-PRODUCTS IN THE HEART

Recent studies of isotope exchange across lactate dehydrogenase (LDH) and alanine aminotransferase (AAT) in hearts call into question whethe r both reactions are in equilibrium. To compare the oxidative and non- oxidative fates of glycolytic end products, isolated rabbit hearts wer e perfused with 5 mM [2-C-13] glucose and 2.5 mM [3-C-13] pyruvate: wi th (n = 6) and without (n = 7) stimulation of pyruvate oxidation using dichloroacetate (DCA), and during normal perfusion or hypoxia (n = 7/ n = 6, + / - DCA). C-13 NMR spectroscopy of intact hearts confirmed a steady-state enrichment level in both alanine and lactate. H-1- and C- 13-NMR spectroscopy of tissue extracts identified the fractions of lac tate, alanine and glutamate pools formed from each exogenous substrate . Glycolysis from glucose accounted for 22 +/- 7% of lactate formed an d 10 +/- 2% of alanine formed in control hearts, and 16 +/- 2% lactate and 15 +/- 2% alanine in hypoxic hearts (mean +/- S.E.M.). In contras t, exogenous pyruvate formed 36 +/- 5% of the lactate pool, and 86 +/- 3% of the alanine pool in controls and 47 +/- 3% of lactate and of 67 +/- 3% alanine during hypoxia. [2-C-13] glucose did not contribute to oxidative energy production via the TCA cycle as determined from low C-13 enrichment of glutamate C5 from glucose (<2%), while [3-C-13] pyr uvate accounted for 84 +/- 7% of labeled glutamate C4. Thus, exogenous pyruvate out-competed the metabolism of glucose, indicating low glyco lytic activity. At 40 min, 96 +/- 2% of the total alanine was labeled from either glucose or pyruvate, confirming equilibrium at AAT. Howeve r, only 55 +/- 10% of total lactate was labeled, suggesting that the L DH reaction is not in rapid equilibrium within the myocardium. (C) 199 6 Academic Press Limited