Chronic activation of peroxisome proliferator-activated receptor-alpha with fenofibrate prevents alterations in cardiac metabolic phenotype without changing the onset of decompensation in pacing-induced heart failure.

Severe heart failure (HF) is characterized by profound alterations in cardiac metabolic phenotype, with down-regulation of the free fatty acid (FFA) oxidative pathway and marked increase in glucose oxidation. We tested whether fenofibrate, a pharmacological agonist of peroxisome proliferator-activated receptor-alpha, the nuclear receptor that activates the expression of enzymes involved in FFA oxidation, can prevent metabolic alterations and modify the progression of HF. We administered 6.5 mg/kg/day p.o. fenofibrate to eight chronically instrumented dogs over the entire period of high-frequency left ventricular pacing (HF + Feno). Eight additional HF dogs were not treated, and eight normal dogs were used as a control. [3H]Oleate and [14C]Glucose were infused intravenously to measure the rate of substrate oxidation. At 21 days of pacing, left ventricular end-diastolic pressure was significantly lower in HF + Feno (14.1 +/- 1.6 mm Hg) compared with HF (18.7 +/- 1.3 mm Hg), but it increased up to 25 +/- 2 mm Hg, indicating end-stage failure, in both groups after 29 +/- 2 days of pacing. FFA oxidation was reduced by 40%, and glucose oxidation was increased by 150% in HF compared with control, changes that were prevented by fenofibrate. Consistently, the activity of myocardial medium chain acyl-CoA dehydrogenase, a marker enzyme of the FFA beta-oxidation pathway, was reduced in HF versus control (1.46 +/- 0.25 versus 2.42 +/- 0.24 micromol/min/gram wet weight (gww); p < 0.05) but not in HF + Feno (1.85 +/- 0.18 micromol/min/gww; N.S. versus control). Thus, preventing changes in myocardial substrate metabolism in the failing heart causes a modest improvement of cardiac function during the progression of the disease, with no effects on the onset of decompensation.

Glucose-6-phosphate dehydrogenase-derived NADPH fuels superoxide production in the failing heart.

In the failing heart, NADPH oxidase and uncoupled NO synthase utilize cytosolic NADPH to form superoxide. NADPH is supplied principally by the pentose phosphate pathway, whose rate-limiting enzyme is glucose 6-phosphate dehydrogenase (G6PD). Therefore, we hypothesized that cardiac G6PD activation drives part of the excessive superoxide production implicated in the pathogenesis of heart failure. Pacing-induced heart failure was performed in eight chronically instrumented dogs. Seven normal dogs served as control. End-stage failure occurred after 28 +/- 1 days of pacing, when left ventricular end-diastolic pressure reached 25 mm Hg. In left ventricular tissue homogenates, spontaneous superoxide generation measured by lucigenin (5 microM) chemiluminescence was markedly increased in heart failure (1338 +/- 419 vs. 419 +/- 102 AU/mg protein, P < 0.05), as were NADPH levels (15.4 +/- 1.5 vs. 7.5 +/- 1.5 micromol/gww, P < 0.05). Superoxide production was further stimulated by the addition of NADPH. The NADPH oxidase inhibitor gp91(ds-tat) (50 microM) and the NO synthase inhibitor L-NAME (1 mM) both significantly lowered superoxide generation in failing heart homogenates by 80% and 76%, respectively. G6PD was upregulated and its activity higher in heart failure compared to control (0.61 +/- 0.10 vs. 0.24 +/- 0.03 nmol/min/mg protein, P < 0.05), while superoxide production decreased to normal levels in the presence of the G6PD inhibitor 6-aminonicotinamide. We conclude that the activation of myocardial G6PD is a novel mechanism that enhances NADPH availability and fuels superoxide-generating enzymes in heart failure.

Altered cardiac metabolic phenotype after prolonged inhibition of NO synthesis in chronically instrumented dogs.

Acute inhibition of nitric oxide (NO) synthase causes a reversible alteration in myocardial substrate metabolism. We tested the hypothesis that prolonged NO synthase inhibition alters cardiac metabolic phenotype. Seven chronically instrumented dogs were treated with N(omega)-nitro-L-arginine methyl ester (L-NAME, 35 mg.kg(-1).day(-1) po) for 10 days to inhibit NO synthesis, and seven were used as controls. Cardiac free fatty acid, glucose, and lactate oxidation were measured by infusion of [(3)H]oleate, [(14)C]glucose, and [(13)C]lactate, respectively. After 10 days of L-NAME administration, despite no differences in left ventricular afterload, cardiac O(2) consumption was significantly increased by 30%, consistent with a marked enhancement in baseline oxidation of glucose (6.9 +/- 2.0 vs. 1.7 +/- 0.5 micromol.min(-1).100 g(-1), P < 0.05 vs. control) and lactate (21.6 +/- 5.6 vs. 11.8 +/- 2.6 micromol.min(-1).100 g(-1), P < 0.05 vs. control). When left ventricular afterload was increased by ANG II infusion to stimulate myocardial metabolism, glucose oxidation was augmented further in the L-NAME than in the control group, whereas free fatty acid oxidation decreased. Exogenous NO (diethylamine nonoate, 0.01 micromol.kg(-1).min(-1) iv) could not reverse this metabolic alteration. Consistent with the accelerated rate of carbohydrate oxidation, total myocardial pyruvate dehydrogenase activity and protein expression were higher (38 and 34%, respectively) in the L-NAME than in the control group. Also, protein expression of the constitutively active glucose transporter GLUT-1 was significantly elevated (46%) vs. control. We conclude that prolonged NO deficiency causes a profound alteration in cardiac metabolic phenotype, characterized by selective potentiation of carbohydrate oxidation, that cannot be reversed by a short-term infusion of exogenous NO. This phenomenon may constitute an adaptive mechanism to counterbalance cardiac mechanical inefficiency.