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Effects of Diabetes, Fatty Acids, and Ketone Bodies on Tricarboxylic Acid Cycle Metabolism in the Perfused Rat Heart

From the ¹ From the Department of Physiology, Vanderbilt University School of Medicine, Nashville 5, Tennessee

An attempt has been made to determine the mechanisms involved in regulating citrate levels in the rat heart. Citric acid cycle intermediates have been measured in perfused and nonperfused hearts of normal and alloxan-diabetic rats. Levels of citrate, isocitrate, -ketoglutarate, and malate were found, in general, to be elevated by diabetes and by perfusion with octanoate, acetate, and ß-hydroxybutyrate. Octanoate also caused a large increase in the concentration of acetyl coenzyme A. This latter effect was greatly diminished by anoxia, and anoxia likewise prevented any large effect of octanoate on citrate and isocitrate levels. In the absence of octanoate, anoxia resulted in a slight fall in citrate, no change in isocitrate, a 90% decrease in -ketoglutarate, and a doubling of malate.

The increased concentration of citrate in hearts perfused with octanoate was not dependent upon the presence of glucose or bicarbonate in the perfusion medium. In hearts perfused with ¹⁴C-bicarbonate the specific activity of citrate was many times that of malate, suggesting that the immediate product of CO₂ fixation under these conditions was not oxalacetate or malate.

Under all conditions in which fatty acids or ß-hydroxybutyrate induced a rise in citrate there was a fall in aspartate and, in most instances, a rise in glutamate. In hearts perfused with ¹⁴C-aspartate, radioactivity was incorporated into citrate, and this incorporation was greatly increased when octanoate was added to the perfusion medium.

It is concluded that elevated levels of citric acid cycle intermediates in hearts of diabetic rats and in normal hearts perfused with fatty acids and ketone bodies are due largely to increased availability of acetyl-CoA. It is proposed that the oxalacetate required for extra citrate synthesis is derived to a major extent from aspartate by transamination. Carbon dioxide fixation, perhaps by -ketoglutarate or changes in the aconitase equilibrium, or both, may be factors in determining the citrate level. Altered activity of dehydrogenase enzymes associated with the cycle may also contribute to changes in the concentration of intermediates.

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