Aging impairs myocardial fatty acid and ketone oxidation and modifies cardiac functional and metabolic responses to insulin in mice

Aging presumably initiates shifts in substrate oxidation mediated in part by changes in insulin sensitivity. Similar shifts occur with cardiac hypertrophy and may contribute to contractile dysfunction. We tested the hypothesis that aging modifies substrate utilization and alters insulin sensitivity in mouse heart when provided multiple substrates. In vivo cardiac function was measured with microtipped pressure transducers in the left ventricle from control (4–6 mo) and aged (22–24 mo) mice. Cardiac function was also measured in isolated working hearts along with substrate and anaplerotic fractional contributions to the citric acid cycle (CAC) by using perfusate containing 13C-labeled free fatty acids (FFA), acetoacetate, lactate, and unlabeled glucose. Stroke volume and cardiac output were diminished in aged mice in vivo, but pressure development was preserved. Systolic and diastolic functions were maintained in aged isolated hearts. Insulin prompted an increase in systolic function in aged hearts, resulting in an increase in cardiac efficiency. FFA and ketone flux were present but were markedly impaired in aged hearts. These changes in myocardial substrate utilization corresponded to alterations in circulating lipids, thyroid hormone, and reductions in protein expression for peroxisome proliferator-activated receptor (PPAR)α and pyruvate dehydrogenase kinase (PDK)4. Insulin further suppressed FFA oxidation in the aged. Insulin stimulation of anaplerosis in control hearts was absent in the aged. The aged heart shows metabolic plasticity by accessing multiple substrates to maintain function. However, fatty acid oxidation capacity is limited. Impaired insulin-stimulated anaplerosis may contribute to elevated cardiac efficiency, but may also limit response to acute stress through depletion of CAC intermediates

Superior cardiac function via anaplerotic pyruvate in the immature swine heart after cardiopulmonary bypass and reperfusion

Pyruvate produces inotropic responses in the adult reperfused heart. Pyruvate oxidation and anaplerotic entry into the tricarboxylic acid (TCA) cycle via carboxylation are linked to the stimulation of contractile function. The goals of this study were to determine if these metabolic pathways operate and are maintained in the developing myocardium after reperfusion. Immature male swine (age: 10–18 days) were subjected to cardiopulmonary bypass (CPB). Intracoronary infusion of [2-13C]pyruvate (to achieve an estimated final concentration of 8 mM) was given for 35 min, starting either during weaning (group I) and after its discontinuation (group II) or without (control) CPB. Hemodynamic data were collected. 13C NMR spectroscopy was used to determine the fraction of pyruvate entering the TCA cycle via pyruvate carboxylation (PC) to total TCA cycle entry (PC plus decarboxlyation via pyruvate dehydrogenase). Liquid chromatography-mass spectrometry was used to determine total glutamate enrichment. Pyruvate infusion starting during the weaning of mechanical circulatory support improved maximum dP/dt (P < 0.05) but waiting to start the infusion until after the discontinuation of CPB did not. Glutamate fractional enrichment was confirmed by liquid chromatography-mass spectroscopy as adequate (>5%) to provide signal to noise in the NMR experiment in all groups. The ratio of pyruvate carboxylase to total pyruvate entry into the TCA cycle did not differ between groups (group I: 20 ± 4%, group II: 23 ± 7%, and control: 27 ± 7%). These data show that robust PC operates in the neonatal pig heart and is maintained during reperfusion under conditions that emulate CPB and reperfusion in human infants

Cardioselective dominant-negative thyroid hormone receptor (Δ337T) modulates myocardial metabolism and contractile efficiency

Dominant-negative thyroid hormone receptors (TRs) show elevated expression relative to ligand-binding TRs during cardiac hypertrophy. We tested the hypothesis that overexpression of a dominant-negative TR alters cardiac metabolism and contractile efficiency (CE). We used mice expressing the cardioselective dominant-negative TRβ1 mutation Δ337T. Isolated working Δ337T hearts and nontransgenic control (Con) hearts were perfused with 13C-labeled free fatty acids (FFA), acetoacetate (ACAC), lactate, and glucose at physiological concentrations for 30 min. 13C NMR spectroscopy and isotopomer analyses were used to determine substrate flux and fractional contributions (Fc) of acetyl-CoA to the citric acid cycle (CAC). Δ337T hearts exhibited rate depression but higher developed pressure and CE, defined as work per oxygen consumption (MV̇o2). Unlabeled substrate Fc from endogenous sources was higher in Δ337T, but ACAC Fc was lower. Fluxes through CAC, lactate, ACAC, and FFA were reduced in Δ337T. CE and Fc differences were reversed by pacing Δ337T to Con rates, accompanied by an increase in FFA Fc. Δ337T hearts lacked the ability to increase MV̇o2. Decreases in protein expression for glucose transporter-4 and hexokinase-2 and increases in pyruvate dehydrogenase kinase-2 and -4 suggest that these hearts are unable to increase carbohydrate oxidation in response to stress. These data show that Δ337T alters the metabolic phenotype in murine heart by reducing substrate flux for multiple pathways. Some of these changes are heart rate dependent, indicating that the substrate shift may represent an accommodation to altered contractile protein kinetics, which can be disrupted by pacing stress