On the interdependence of ketone body oxidation, glycogen content, glycolysis and energy metabolism in the heart

In heart, glucose and glycolysis are important for anaplerosis and potentially therefore for d-beta-hydroxybutyrate (beta HB) oxidation. As a glucose store, glycogen may also furnish anaplerosis. We determined the effects of glycogen content on beta HB oxidation and glycolytic rates, and their downstream effects on energetics, in the isolated rat heart. High glycogen (HG) and low glycogen (LG) containing hearts were perfused with 11 mM [5-H-3]glucose and/or 4 mM [C-14]beta HB to measure glycolytic rates or beta HB oxidation, respectively, then freeze-clamped for glycogen and metabolomic analyses. Free cytosolic [NAD(+)]/[NADH] and mitochondrial [Q(+)]/[QH(2)] ratios were estimated using the lactate dehydrogenase and succinate dehydrogenase reaction, respectively. Phosphocreatine (PCr) and inorganic phosphate (P-i) concentrations were measured using P-31-nuclear magnetic resonance spectroscopy. Rates of beta HB oxidation in LG hearts were half that in HG hearts, with beta HB oxidation directly proportional to glycogen content. beta HB oxidation decreased glycolysis in all hearts. Glycogenolysis in glycogen-replete hearts perfused with beta HB alone was twice that of hearts perfused with beta HB and glucose, which had significantly higher levels of the glycolytic intermediates fructose 1,6-bisphosphate and 3-phosphoglycerate, and higher free cytosolic [NAD(+)]/[NADH]. beta HB oxidation increased the Krebs cycle intermediates citrate, 2-oxoglutarate and succinate, the total NADP/H pool, reduced mitochondrial [Q(+)]/[QH(2)], and increased the calculated free energy of ATP hydrolysis ( delta G(ATP)). Although beta HB oxidation inhibited glycolysis, glycolytic intermediates were not depleted, and cytosolic free NAD remained oxidised. beta HB oxidation alone increased Krebs cycle intermediates, reduced mitochondrial Q and increased delta G(ATP). We conclude that glycogen facilitates cardiac beta HB oxidation by anaplerosis.