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Glutamine cycling in isolated working rat heart

¹Department of Chemistry, University of Houston, Houston 77204; ²Division of Cardiology, Department of Internal Medicine, University of Texas Medical Center, Houston, Texas 77204; and ³Institute of Life Sciences, Ajinomoto Co., Inc., Kawasaki, 210-8681 Japan

Submitted 11 December 2002 ; accepted in final form 23 August 2003

ABSTRACT

To what extent does glutamine turnover keep pace with oxidative metabolism in the rat heart? To address this question, the following groups of substrates were presented to the isolated, working rat heart: 1) glucose (5 mM), insulin (40 µU/ml), and [2-¹³C]acetate (5 mM; high workload, n = 5); 2) pyruvate (2.5 mM) and [2-¹³C]acetate (5 mM; normal workload, n = 5); or 3) propionate (1 mM) and [2-¹³C]acetate (2.5 mM; normal workload, n = 3). In a subset of these experiments, the exchange of glutamate and glutamine was quantified by separation with ion exchange chromatography and analysis by GC-MS. There was an apparent equilibration of mass isotopomers of glutamate and glutamine after 50 min of perfusion, although the extent of equilibration was not determined. The fractional enrichment in glutamine was 31% of the enrichment of glutamate with the three different perfusates. From high-resolution nuclear magnetic resonance spectra, we found a ratio of glutamine to glutamate content of 94.1, 53.4, and 96.9%, respectively, for each experimental group. In experiments for which L-[1-¹³C]glutamine (5 mM) was included in the perfusate of group 2, [1-¹³C]glutamine was detected in the heart, but transfer of ¹³C from glutamine to glutamate was not detected (n = 4). We conclude that, in the perfused working heart, production of glutamine by amidation of glutamate takes place and can be detected, whereas the reverse process, generation of glutamate from glutamine, remains undetected.

perfused heart; turnover; nuclear magnetic resonance; anaplerosis

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