[5-3H]glucose overestimates glycolytic flux in isolated working rat heart: role of the pentose phosphate pathway.
We set out to study the pentose phosphate pathway (PPP) in isolated rat hearts perfused with [5-3H]glucose and [1-14C]glucose or [6-14C]glucose (crossover study with 1- then 6- or 6- then 1-14C-labeled glucose). To model a physiological state, hearts were perfused under working conditions with Krebs-Henseleit buffer containing 5 mM glucose, 40 microU/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 [1-14C]glucose to [6-14C]glucose) of metabolites released by the heart, an index of oxidative PPP, was not different from 1 (1.06 +/- 0.19 for 14CO2, and 1.00 +/- 0.01 for [14C]lactate + [14C]pyruvate, mean +/- SE, n = 8). Hearts exhibited contractile, metabolic, and 14C-isotopic steady state for glucose oxidation (14CO2 production). Net glycolytic flux (net release of lactate + pyruvate) and efflux of [14C]lactate + [14C]pyruvate were the same and also exhibited steady state. In contrast, flux based on 3H2O production from [5-3H]glucose increased progressively, reaching 260% of the other measures of glycolysis after 30 min. The 3H/14C 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 <1 (0.36 +/- 0.01 and 0.43 +/- 0.05 respectively, n = 8, P < 0.05 vs. 1). We conclude that both transaldolase and the L-type PPP permit hexose detritiation in the absence of net glycolytic flux by allowing interconversion of glycolytic hexose and triose phosphates. Thus apparent glycolytic flux obtained by 3H2O production from [5-3H]glucose overestimates the true glycolytic flux in rat heart.
Goodwin
G W
GW
Division of Cardiology, University of Texas-Houston Medical School, Department of Pediatrics, Baylor College of Medicine, Houston, TX 77030, USA.
Cohen
D M
DM
Taegtmeyer
H
H
D000818:Animals; D002245:Carbon Dioxide / Q000378:metabolism; D002250:Carbon Radioisotopes; D005189:False Positive Reactions; D005947:Glucose / Q000378:metabolism*; D006003:Glycogen / Q000378:metabolism; D006019:Glycolysis; D019344:Lactic Acid / Q000378:metabolism; D008297:Male; D009200:Myocardial Contraction; D009206:Myocardium / Q000378:metabolism*; D010427:Pentose Phosphate Pathway; D019289:Pyruvic Acid / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013403:Sugar Phosphates / Q000378:metabolism; D014153:Transaldolase / Q000378:metabolism; D014316:Tritium
D002250:Carbon Radioisotopes; D013403:Sugar Phosphates; D014316:Tritium; D002245:Carbon Dioxide; D019344:Lactic Acid; D019289:Pyruvic Acid; D006003:Glycogen; D014153:Transaldolase; D005947:Glucose