Abstract 237: The First Direct Measure of Chylomicron Metabolism in Heart Reveals a Reciprocal Balance Between Mitochondrial Oxidation of Fats From Chylomicron vs Albumin Sources

Abstract

Nearly all studies of fat metabolism in healthy and diseased heart are based on albumin sources of fat, even though lipoproteins (chylomicrons, VLDL) are thought to be a major source of fat. If there are distinct differences in uptake, oxidation, and storage of fatty acids (FA) from lipoproteins compared to albumin sources, then our understanding of disease is far from complete. This study provides the very first investigation of the fate of 13C labeled FA from chylomicrons (CM) in heart, with quantified results of the direct competition between CM-FA vs albumin-FA to support mitochondrial ATP synthesis. Chylomicrons were biosynthetically labeled with 13C-oleate by delivering to the duodenum of rat either (a) emulsified in olive oil, or (b) incorporated into micelles. The labeled chylomicrons were collected as lymph via the thoracic duct for six hours. The olive oil approach, used traditionally to prepare CM to contain radiolabeled fats, resulted in a trace fraction of 13C-labeled FA (<2%). The micelle approach enabled 50% enrichment of CM-FA store. Metabolic studies: Isolated rat hearts were perfused with KH buffer containing 13C-chylomicrons, FA free albumin, and glucose; or 13C-chylomicrons, unlabeled oleate complexed to albumin (equimolar FA from CM and albumin; 0.4 mM), and glucose. Hearts were freeze-clamped, and FA oxidation was assessed via high resolution 13C NMR of heart samples. CM-FA provided 50 ± 5 % of the acetyl CoA to support citric acid cycle synthesis of ATP when FA free albumin was used. In the presence of albumin-FA, only 12 ± 2% of acetyl CoA entering the cycle was from the CM-FA. These direct observations of the competition between CM-FA vs albumin-FA counter prior interpretations based on indirect measures. Those studies suggested no change in CM-FA oxidation when albumin-FA was added. Our novel approach enables direct measurements of CM-FA and albumin-FA oxidation in the mitochondria of intact hearts. The uptake and distribution of fat from either exogenous source were found to contribute to a common intracellular source of activated FA, with mitochondrial oxidation being proportional to the availability of the FA source. Importantly, this new evidence for a common pool significantly simplifies our metabolic paradigm of cardiac disease.