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Limited transfer of cytosolic NADH into mitochondria at high cardiac workload

¹Program in Integrative Cardiac Metabolism, Department of Physiology and Biophysics, University of Illinois, College of Medicine, Chicago, Illinois 60612; ²Tufts University School of Veterinary Medicine, North Grafton, Massachusetts 01536; ³Cardiovascular Institute, Department of Cellular and Molecular Medicine, University of Medicine and Dentistry of New Jersey-New Jersey Medical School, Newark, New Jersey 07103; and ⁴Department of Molecular and Cellular Physiology, Pennsylvania State University Medical School, Hershey, Pennsylvania 17033

Submitted 30 November 2003 ; accepted in final form 28 January 2004

Glycolysis supplements energy synthesis at high cardiac workloads, producing not only ATP but also cytosolic NADH and pyruvate for oxidative ATP synthesis. Despite adequate PO₂, speculation exists that not all cytosolic NADH is oxidized by the mitochondria, leading to lactate production. In this study, we elucidate the mechanism for limited cytosolic NADH oxidation and increased lactate production at high workload despite adequate myocardial blood flow and oxygenation. Reducing equivalents from glycolysis enter mitochondria via exchange of mitochondrial -ketoglutarate (-KG) for cytosolic malate. This exchange was monitored at baseline and at high workloads by comparing ¹³C enrichment between the products of -KG oxidation (succinate) and -KG efflux from mitochondria (glutamate). Under general anesthesia, a left thoracotomy was performed on 14 dogs and [2-¹³C]acetate was infused into the left anterior descending artery for 40 min. The rate-pressure product was 9,035 ± 1,972 and 21,659 ± 5,266 mmHg·beats·min^–1 (n = 7) at baseline (n = 7) and with dobutamine, respectively. ¹³C enrichment of succinate was 57 ± 10% at baseline and 45 ± 13% at elevated workload (not significant), confirming oxidation of [2-¹³C]acetate. However, cytosolic glutamate enrichment, a marker of cytosolic NADH transfer to mitochondria, was dramatically reduced at high cardiac workload (11 ± 1%) vs. baseline (50 ± 14%, P < 0.05). This reduced exchange of ¹³C from -KG to cytosolic glutamate at high work indicates reduced shuttling of cytosolic reducing equivalents into the mitochondria. Myocardial tissue lactate increased 78%, countering this reduced oxidation of cytosolic NADH. The findings elucidate a contributing mechanism to glycolysis outpacing glucose oxidation in the absence of myocardial ischemia.

heart; ¹³C nuclear magnetic resonance; malate-aspartate shuttle; metabolism

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