[5-3H]glucose overestimates glycolytic flux in isolated working rat heart: role of the pentose phosphate pathway

1 Division of Cardiology, University of Texas-Houston Medical School; and 2 Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, Texas 77030 We set out to study the pentose phosphate pathway (PPP) in isolated rat hearts perfused with [5- 3 H]glucose and [1- 14 C]glucose or [6- 14 C]glucose (crossover study with 1- then 6- or 6- then 1- 14 C-labeled glucose). To model a physiological state, hearts were perfused under working conditions with Krebs-Henseleit buffer containing 5 mM glucose, 40 µU/ml insulin, 0.5 mM lactate, 0.05 mM pyruvate, and 0.4 mM oleate/3% albumin. The steady-state C1/C6 ratio (i.e., the ratio from [1- 14 C]glucose to [6- 14 C]glucose) of metabolites released by the heart, an index of oxidative PPP, was not different from 1 (1.06 ± 0.19 for 14 CO 2 , and 1.00 ± 0.01 for [ 14 C]lactate + [ 14 C]pyruvate, mean ± SE, n = 8). Hearts exhibited contractile, metabolic, and 14 C-isotopic steady state for glucose oxidation ( 14 CO 2 production). Net glycolytic flux (net release of lactate + pyruvate) and efflux of [ 14 C]lactate + [ 14 C]pyruvate were the same and also exhibited steady state. In contrast, flux based on 3 H 2 O production from [5- 3 H]glucose increased progressively, reaching 260% of the other measures of glycolysis after 30 min. The 3 H/ 14 C ratio of glycogen (relative to extracellular glucose) and sugar phosphates (representing the glycogen precursor pool of hexose phosphates) was not different from each other and was