METABOLIC COMPARTMENTATION OF LACTATE IN THE GLUCOSE-PERFUSED RAT-HEART

Several studies using exogenous pyruvate as substrate have suggested t hat there are separate intracellular pyruvate pools in cardiac cells. Such heterogeneity in intracellular pyruvate has important implication s for our understanding of both oxidative and nonoxidative glucose met abolism. Because pyruvate is not a major substrate for the heart in vi vo, we wished to determine if there was pyruvate compartmentation with glucose as substrate. Hearts were isolated from male Sprague-Dawley r ats and retrogradely perfused (Langendorff) with a modified Krebs-Hens eleit buffer containing [1-C-13]glucose for 5-115 min. At the end of e ach experiment, hearts were freeze-clamped and extracted for determina tion of the fractional enrichment of alanine, lactate, and acetyl CoA using H-1- and C-13-nuclear magnetic resonance spectroscopy. The fract ional enrichment of alanine at the C-3 position was significantly high er than that for lactate at 55 min [43.6 +/- 2.8 vs. 27.2 +/- 4.6% (SD ), n = 5, P < 0.003]. The differences in steady-state enrichment betwe en lactate and alanine were not due to differences in the rate of labe ling of these metabolites. The mean steady-state lactate enrichment wa s higher in the perfusate samples compared with the tissue samples fro m the same experiments (46.6 +/- 2.2 vs. 30.5 +/- 2.5%, n = 3, P < 0.0 05). Because fractional enrichment of alanine, acetyl CoA, and perfusa te lactate are similar, we suggest that there is a separate nonexchang ing pool of lactate rather than cytosolic compartmentation of pyruvate that has been previously proposed.