Myocardial mitochondrial dysfunction in mice lacking adiponectin receptor 1.

Hypoadiponectinemia is an independent predictor of cardiovascular disease, impairs mitochondrial function in skeletal muscle, and has been linked to the pathogenesis of Type 2 diabetes. In models of Type 2 diabetes, myocardial mitochondrial function is impaired, which is improved by increasing serum adiponectin levels. We aimed to define the roles of adiponectin receptor 1 (AdipoR1) and 2 (AdipoR2) in adiponectin-evoked regulation of mitochondrial function in the heart. In isolated working hearts in mice lacking AdipoR1, myocardial oxygen consumption was increased without a concomitant increase in cardiac work, resulting in reduced cardiac efficiency. Activities of mitochondrial oxidative phosphorylation (OXPHOS) complexes were reduced, accompanied by reduced OXPHOS protein levels, phosphorylation of AMP-activated protein kinase, sirtuin 1 activity, and peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) signaling. Decreased ATP/O ratios suggested myocardial mitochondrial uncoupling in AdipoR1-deficient mice, which was normalized by lowering increased mitochondrial 4-hydroxynonenal levels following treatment with the mitochondria-targeted antioxidant Mn (III) tetrakis (4-benzoic acid) porphyrin. Lack of AdipoR2 did not impair mitochondrial function and coupling in the heart. Thus, lack of AdipoR1 impairs myocardial mitochondrial function and coupling, suggesting that impaired AdipoR1 signaling may contribute to mitochondrial dysfunction and mitochondrial uncoupling in Type 2 diabetic hearts.

Myocardial mitochondrial and contractile function are preserved in mice lacking adiponectin.

Adiponectin deficiency leads to increased myocardial infarct size following ischemia reperfusion and to exaggerated cardiac hypertrophy following pressure overload, entities that are causally linked to mitochondrial dysfunction. In skeletal muscle, lack of adiponectin results in impaired mitochondrial function. Thus, it was our objective to investigate whether adiponectin deficiency impairs mitochondrial energetics in the heart. At 8 weeks of age, heart weight-to-body weight ratios were not different between adiponectin knockout (ADQ-/-) mice and wildtypes (WT). In isolated working hearts, cardiac output, aortic developed pressure and cardiac power were preserved in ADQ-/- mice. Rates of fatty acid oxidation, glucose oxidation and glycolysis were unchanged between groups. While myocardial oxygen consumption was slightly reduced (-24%) in ADQ-/- mice in isolated working hearts, rates of maximal ADP-stimulated mitochondrial oxygen consumption and ATP synthesis in saponin-permeabilized cardiac fibers were preserved in ADQ-/- mice with glutamate, pyruvate or palmitoyl-carnitine as a substrate. In addition, enzymatic activity of respiratory complexes I and II was unchanged between groups. Phosphorylation of AMP-activated protein kinase and SIRT1 activity were not decreased, expression and acetylation of PGC-1α were unchanged, and mitochondrial content of OXPHOS subunits was not decreased in ADQ-/- mice. Finally, increasing energy demands due to prolonged subcutaneous infusion of isoproterenol did not differentially affect cardiac contractility or mitochondrial function in ADQ-/- mice compared to WT. Thus, mitochondrial and contractile function are preserved in hearts of mice lacking adiponectin, suggesting that adiponectin may be expendable in the regulation of mitochondrial energetics and contractile function in the heart under non-pathological conditions.

Mitochondrial reactive oxygen species production and respiratory complex activity in rats with pressure overload-induced heart failure.

We investigated the impact of cardiac reactive oxygen species (ROS) during the development of pressure overload-induced heart failure. We used our previously described rat model where transverse aortic constriction (TAC) induces compensated hypertrophy after 2 weeks, heart failure with preserved ejection fraction at 6 and 10 weeks, and heart failure with systolic dysfunction after 20 weeks. We measured mitochondrial ROS production rates, ROS damage and assessed the therapeutic potential of in vivo antioxidant therapies. In compensated hypertrophy (2 weeks of TAC) ROS production rates were normal at both mitochondrial ROS production sites (complexes I and III). Complex I ROS production rates increased with the appearance of diastolic dysfunction (6 weeks of TAC) and remained high thereafter. Surprisingly, maximal ROS production at complex III peaked at 6 weeks of pressure overload. Mitochondrial respiratory capacity (state 3 respiration) was elevated 2 and 6 weeks after TAC, decreased after this point and was significantly impaired at 20 weeks, when contractile function was also impaired and ROS damage was found with increased hydroxynonenal. Treatment with the ROS scavenger α-phenyl-N-tert-butyl nitrone or the uncoupling agent dinitrophenol significantly reduced ROS production rates at 6 weeks. Despite the decline in ROS production capacity, no differences in contractile function between treated and untreated animals were observed. Increased ROS production occurs early in the development of heart failure with a peak at the onset of diastolic dysfunction. However, ROS production may not be related to the onset of contractile dysfunction.

Echocardiography alone allows the determination of heart failure stages in rats with pressure overload.

BACKGROUND: There is currently no standard for the assessment of contractile function in animals. We aimed to determine whether transthoracic echocardiography in rats with chronic pressure overload allows determining the stage of hypertrophy and heart failure (HF). METHODS: Pressure overload was created by placement of a metal clip around the thoracic aorta at a weight of 40 to 50 g. After 1, 2, 6, 10, and 20 weeks, we performed echocardiography according to the American Heart Association guidelines (n = 26, four to six rats for each time point). We also obtained heart, lung, and body weights and regularly evaluated clinical signs of HF. RESULTS: : Pressure overload caused significant hypertrophy within 1 week. Contractile function was normal until 6 weeks when diastolic dysfunction appeared. After 10 weeks of pressure overload, systolic function decreased. At 20 weeks, hearts were dilated and cardiac index was decreased. These findings correlated with increased lung-to-body weight ratio after 6 weeks and clinical signs of HF after 20 weeks. CONCLUSION: Echocardiography alone allows the reproducible determination of HF stages after aortic constriction in rats.

Alterations in mitochondrial function in cardiac hypertrophy and heart failure.

Normal cardiac function requires high and continuous supply with ATP. As mitochondria are the major source of ATP production, it is apparent that mitochondrial function and cardiac function need to be closely related to each other. When subjected to overload, the heart hypertrophies. Initially, the development of hypertrophy is a compensatory mechanism, and contractile function is maintained. However, when the heart is excessively and/or persistently stressed, cardiac function may deteriorate, leading to the onset of heart failure. There is considerable evidence that alterations in mitochondrial function are involved in the decompensation of cardiac hypertrophy. Here, we review metabolic changes occurring at the mitochondrial level during the development of cardiac hypertrophy and the transition to heart failure. We will focus on changes in mitochondrial substrate metabolism, the electron transport chain and the role of oxidative stress. We will demonstrate that, with respect to mitochondrial adaptations, a clear distinction between hypertrophy and heart failure cannot be made because most of the findings present in overt heart failure can already be found in the various stages of hypertrophy.

Pressure overload differentially affects respiratory capacity in interfibrillar and subsarcolemmal mitochondria.

Years ago a debate arose as to whether two functionally different mitochondrial subpopulations, subsarcolemmal mitochondria (SSM) and interfibrillar mitochondria (IFM), exist in heart muscle. Nowadays potential differences are often ignored. Presumably, SSM are providing ATP for basic cell function, whereas IFM provide energy for the contractile apparatus. We speculated that two distinguishable subpopulations exist that are differentially affected by pressure overload. Male Sprague-Dawley rats were subjected to transverse aortic constriction for 20 wk or sham operation. Contractile function was assessed by echocardiography. Heart tissue was analyzed by electron microscopy. Mitochondria were isolated by differential centrifugation, and respiratory capacity was analyzed using a Clark electrode. Pressure overload induced left ventricular hypertrophy with increased posterior wall diameter and impaired contractile function. Mitochondrial state 3 respiration in control was 50% higher in IFM than in SSM. Pressure overload significantly impaired respiratory rates in both IFM and SSM, but in SSM to a lower extent. As a result, there were no differences between SSM and IFM after 20 wk of pressure overload. Pressure overload reduced total citrate synthase activity, suggesting reduced total mitochondrial content. Electron microscopy revealed normal morphology of mitochondria but reduced total mitochondrial volume density. In conclusion, IFM show greater respiratory capacity in the healthy rat heart and a greater depression of respiratory capacity by pressure overload than SSM. The differences in respiratory capacity of cardiac IFM and SSM in healthy hearts are eliminated with pressure overload-induced heart failure. The strong effect of pressure overload on IFM together with the simultaneous appearance of mitochondrial and contractile dysfunction may support the notion of IFM primarily producing ATP for contractile function.

Targeted metabolic imaging to improve the management of heart disease.

Tracer techniques are powerful methods for assessing rates of biological processes in vivo. A case in point is intermediary metabolism of energy providing substrates, a central feature of every living cell. In the heart, the tight coupling between metabolism and contractile function offers an opportunity for the simultaneous assessment of cardiac performance at different levels in vivo: coronary flow, myocardial perfusion, oxygen delivery, metabolism, and contraction. Noninvasive imaging techniques used to identify the metabolic footprints of either normal or perturbed cardiac function are discussed.

Exercise can induce temporary mitochondrial and contractile dysfunction linked to impaired respiratory chain complex activity.

Exercise is considered to elicit a physiological response of the heart. Previous studies investigated the influence of repetitive exercise only at the end of the training period. We assessed the impact of 2 exercise protocols, differing in their treadmill inclination, on cardiac and mitochondrial function at different times during the training period. Within 10 weeks, animals trained with 16% incline developed hypertrophy (left ventricular posterior wall thickness: 1.6 ± 0.1 vs 2.4 ± 0.1 mm; P < .05) with normal function (ejection fraction: 75.2% ± 2.5% vs 75.6% ± 2.1%). However, at 6 weeks, there was temporary impairment of contractile function (ejection fraction: 74.5% ± 1.67% vs 65.8% ± 2.3%; P < .05) associated with decreased mitochondrial respiratory capacity (state 3 respiration: 326 ± 71 vs 161 ± 22 natoms/[min mg protein]; P < .05) and a gene expression shift from the adult (α) to the fetal (ß) myosin heavy chain isoform. Although peroxisome proliferator-activated receptor gamma coactivator-1α expression was normal, nuclear respiratory factors (NRFs)-1 and -2 were significantly reduced (NRF-1: 1.00 ± 0.16 vs 0.55 ± 0.09; NRF-2: 1.00 ± 0.11 vs 0.63 ± 0.07; P < .05) after 6 weeks. These findings were associated with a reduction of electron transport chain complexes I and IV activity (complex I: 1016 ± 67 vs 758 ± 71 nmol/[min mg protein]; complex IV: 18768 ± 1394 vs 14692 ± 960 nmol/[min mg protein]; P < .05). Messenger RNA expression of selected nuclear encoded subunits of the electron transport chain was unchanged at all investigated time points. In contrast, animals trained with 10% incline showed less hypertrophy and normal function in echocardiography, normal maximal respiratory capacity, and unchanged complex activities at all 3 time points. Repetitive exercise may cause contractile and mitochondrial dysfunction characterized by impaired respiratory chain complex activities. This activity reduction is temporary and intensity related.