Respiratory control in heart muscle during fatty acid oxidation. Energy state or substrate-level regulation by Ca2+?

Increased Energy Demand during Adrenergic Receptor Stimulation Contributes to Ca(2+) Wave Generation

While β-adrenergic receptor (β-AR) stimulation ensures adequate cardiac output during stress, it can also trigger life-threatening cardiac arrhythmias. We have previously shown that proarrhythmic Ca(2+) waves during β-AR stimulation temporally coincide with augmentation of reactive oxygen species (ROS) production. In this study, we tested the hypothesis that increased energy demand during β-AR stimulation plays an important role in mitochondrial ROS production and Ca(2+)-wave generation in rabbit ventricular myocytes. We found that β-AR stimulation with isoproterenol (0.1 μM) decreased the mitochondrial redox potential and the ratio of reduced to oxidated glutathione. As a result, β-AR stimulation increased mitochondrial ROS production. These metabolic changes induced by isoproterenol were associated with increased sarcoplasmic reticulum (SR) Ca(2+) leak and frequent diastolic Ca(2+) waves. Inhibition of cell contraction with the myosin ATPase inhibitor blebbistatin attenuated oxidative stress as well as spontaneous SR Ca(2+) release events during β-AR stimulation. Furthermore, we found that oxidative stress induced by β-AR stimulation caused the formation of disulfide bonds between two ryanodine receptor (RyR) subunits, referred to as intersubunit cross-linking. Preventing RyR cross-linking with N-ethylmaleimide decreased the propensity of Ca(2+) waves induced by β-AR stimulation. These data suggest that increased energy demand during sustained β-AR stimulation weakens mitochondrial antioxidant defense, causing ROS release into the cytosol. By inducing RyR intersubunit cross-linking, ROS can increase SR Ca(2+) leak to the critical level that can trigger proarrhythmic Ca(2+) waves.

Role of mitochondrial Ca2+ in the regulation of cellular energetics

Calcium is an important signaling molecule involved in the regulation of many cellular functions. The large free energy in the Ca(2+) ion membrane gradients makes Ca(2+) signaling inherently sensitive to the available cellular free energy, primarily in the form of ATP. In addition, Ca(2+) regulates many cellular ATP-consuming reactions such as muscle contraction, exocytosis, biosynthesis, and neuronal signaling. Thus, Ca(2+) becomes a logical candidate as a signaling molecule for modulating ATP hydrolysis and synthesis during changes in numerous forms of cellular work. Mitochondria are the primary source of aerobic energy production in mammalian cells and also maintain a large Ca(2+) gradient across their inner membrane, providing a signaling potential for this molecule. The demonstrated link between cytosolic and mitochondrial Ca(2+) concentrations, identification of transport mechanisms, and the proximity of mitochondria to Ca(2+) release sites further supports the notion that Ca(2+) can be an important signaling molecule in the energy metabolism interplay of the cytosol with the mitochondria. Here we review sites within the mitochondria where Ca(2+) plays a role in the regulation of ATP generation and potentially contributes to the orchestration of cellular metabolic homeostasis. Early work on isolated enzymes pointed to several matrix dehydrogenases that are stimulated by Ca(2+), which were confirmed in the intact mitochondrion as well as cellular and in vivo systems. However, studies in these intact systems suggested a more expansive influence of Ca(2+) on mitochondrial energy conversion. Numerous noninvasive approaches monitoring NADH, mitochondrial membrane potential, oxygen consumption, and workloads suggest significant effects of Ca(2+) on other elements of NADH generation as well as downstream elements of oxidative phosphorylation, including the F(1)F(O)-ATPase and the cytochrome chain. These other potential elements of Ca(2+) modification of mitochondrial energy conversion will be the focus of this review. Though most specific molecular mechanisms have yet to be elucidated, it is clear that Ca(2+) provides a balanced activation of mitochondrial energy metabolism that exceeds the alteration of dehydrogenases alone.

Domestication of the cardiac mitochondrion for energy conversion

The control of mitochondria energy conversion by cytosolic processes is reviewed. The nature of the cytosolic and mitochondrial potential energy homeostasis over wide ranges of energy utilization is reviewed and the consequences of this homeostasis in the control network are discussed. An analysis of the major candidate cytosolic signaling molecules ADP, Pi and Ca(2+) are reviewed based on the magnitude and source of the cytosolic concentration changes as well as the potential targets of action within the mitochondrial energy conversion system. Based on this analysis, Ca(2+) is the best candidate as a cytosolic signaling molecule for this process based on its ability to act as both a feedforward and feedback indicator of ATP hydrolysis and numerous targets within the matrix to provide a balanced activation of ATP production. These targets include numerous dehydrogenases and the F1-F0-ATPase. Pi is also a good candidate since it is an early signal of a mismatch between cytosolic ATP production and ATP synthesis in the presence of creatine kinase and has multiple targets within oxidative phosphorylation including NADH generation, electron flux in the cytochrome chain and a substrate for the F1-F0-ATPase. The mechanism of the coordinated activation of oxidative phosphorylation by these signaling molecules is discussed in light of the recent discoveries of extensive protein phosphorylation sites and other post-translational modifications. From this review it is clear that the control network associated with the maintenance of the cytosolic potential energy homeostasis is extremely complex with multiple pathways orchestrated to balance the sinks and sources in this system. New tools are needed to image and monitor metabolites within sub-cellular compartments to resolve many of these issues as well as the functional characterization of the numerous matrix post-translational events being discovered along with the enzymatic processes generating and removing these protein modifications.

Maintenance of the metabolic homeostasis of the heart: developing a systems analysis approach

The heart is almost unique in the body with a constant requirement to conduct work well beyond the normal maintenance of cellular integrity. With this constant workload, it is not surprising that cardiac energy conversion is highly specialized to maintain a constant supply of energy. This maintenance of cellular metabolites during alterations in workload has been termed metabolic homeostasis. Here we discuss our efforts to understand the cellular and mitochondrial control network that orchestrates the metabolic homeostasis of the heart. This begins with a better definition of the metabolic pathways, acute posttranslational control sites, and proper kinetic evaluation of the reaction steps in the intact mitochondrial environment. First, a quantitative model of mitochondrial energy conversion is presented and demonstrates several serious gaps in our knowledge of this process. Toward filling these gaps, screens of the entire mitochondrial proteome have been conducted to establish the metabolic pathways that need to be considered. In addition, the dynamic phosphoproteome of intact mitochondria, using 2D gel electrophoresis coupled to (32)P labeling, has revealed a remarkably extensive protein phosphorylation network throughout the mitochondrial metabolic network that has essentially been overlooked. Initial studies on evaluating the functional significance of these protein phosphorylations and the kinase-phosphatase system involved will be reviewed. One of the major deficits in the consensus quantitative model of oxidative phosphorylation to explain intact mitochondria activities is in complex I, where even the initiation of Nicotinamide Adenine Dinucleotide (reduced) (NADH) oxidation is problematical using in vitro kinetic data. Studies will be described where the NADH binding and oxidation kinetics at complex I in the intact mitochondria were determined using fluorescence lifetime and enzyme dependent-fluorescence recovery after photo-oxidation (ED-FRAP) techniques. These later studies suggest that matrix NADH binding characteristics are much different (>10(3) binding constant errors) than isolated proteins. In addition, complex I is far from equilibrium and may play an important role in regulating the rate of reducing equivalent delivery to the cytochromes.

Improved mitochondrial bioenergetics by anesthetic preconditioning during and after 2 hours of 27 degrees C ischemia in isolated hearts

We examined if sevoflurane given before cold ischemia of intact hearts (anesthetic preconditioning, APC) affords additional protection by further improving mitochondrial energy balance and if this is abolished by a mitochondrial KATP blocker. NADH and FAD fluorescence was measured within the left ventricular wall of 5 groups of isolated guinea pig hearts: (1) hypothermia alone; (2) hypothermia+ischemia; (3) APC (4.1% sevoflurane)+cold ischemia; (4) 5-HD+cold ischemia, and (5) APC+5-HD+cold ischemia. Hearts were exposed to sevoflurane for 15 minutes followed by 15 minutes of washout at 37 degrees C before cooling, 2 hours of 27 degrees C ischemia, and 2 hours of 37 degrees C reperfusion. The KATP channel inhibitor 5-HD was perfused before and after sevoflurane. Ischemia caused a rapid increase in NADH and a decrease in FAD that waned over 2 hours. Warm reperfusion led to a decrease in NADH and an increase in FAD. APC attenuated the changes in NADH and FAD and further improved postischemic function and reduced infarct size. 5-HD blocked the cardioprotective effects of APC but not APC-induced alterations of NADH and FAD. Thus, APC improves redox balance and has additive cardioprotective effects with mild hypothermic ischemia. 5-HD blocks APC-induced cardioprotective effects but not improvements in mitochondrial bioenergetics. This suggests that mediation of protection by KATP channel opening during cold ischemia and reperfusion is downstream from the APC-induced improvement in redox state or that these changes in redox state are not attenuated by KATP channel antagonism.

Influence of calcium-induced workload transitions and fatty acid supply on myocardial substrate selection

Because of differences in energy yield and oxygen demand, the selection of oxidative fuels is important in the hypoxic or ischemic heart muscle. The aim of the present study was to clarify the contradictions observed in the effects of workload and fatty acid supply on myocardial fuel preference in isolated perfused rat hearts. Nuclear magnetic resonance spectroscopy combined with the administration of substrates labeled with the stable isotope carbon 13 and isotopomer analysis of glutamate labeling offers an opportunity to simultaneously measure metabolic fluxes in pathways feeding into the tricarboxylic acid (TCA) cycle. The work output was modulated by changes in extracellular calcium. In the presence of 5 mmol/L glucose, 0.5 mmol/L octanoate in the perfusate dominated the oxidative metabolism, and workload had little effect on the ratio of glucose to fatty acid utilization. This was the case even when the octanoate concentration was lowered to 50 micromol/L. The relative rate of replenishment of the TCA cycle intermediates was higher at a low workload. The redox state of flavoproteins in the intact heart was monitored fluorometrically to obtain an estimate of the mitochondrial reduced/oxidized nicotinamide-adenine dinucleotide ratio (NADH/NAD ratio) for assessment of the dominant level of regulation of cell respiration, and the myoglobin spectrum was simultaneously monitored to evaluate the oxygenation status of the myocardium. Commencement of octanoate infusion (50 micromol/L or 0.5 mmol/L) caused a large but transient reduction of mitochondrial NAD and, conversely, its cessation elicited NADH oxidation and rebound reduction. During glucose oxidation, an increase in workload led to oxidation of the mitochondrial NADH, but this effect was much smaller in the presence of 50 micromol/L octanoate and absent in the presence of 0.5 mmol/L. This indicates that strong control of oxygen consumption during glucose oxidation is exerted in the mitochondrial respiratory chain, whereas equal control during fatty acid oxidation is exerted within the metabolic pathway upstream from the respiratory chain. It is concluded that when a medium-chain fatty acid is available, myocardial workload and energy consumption have little influence on fuel preference and glucose oxidation remains suppressed.

Warm ischemic preconditioning improves mitochondrial redox balance during and after mild hypothermic ischemia in guinea pig isolated hearts

Ischemic preconditioning (IPC) induces distinctive changes in mitochondrial bioenergetics during warm (37 degrees C) ischemia and improves function and tissue viability on reperfusion. We examined whether IPC before 2 h of hypothermic (27 degrees C) ischemia affords additive cardioprotection and improves mitochondrial redox balance assessed by mitochondrial NADH and flavin adenine dinucleotide (FAD) autofluorescence in intact hearts. A mediating role of ATP-sensitive K(+) (K(ATP)) channel opening was investigated. NADH and FAD fluorescence was measured in the left ventricular wall of guinea pig isolated hearts assigned to five groups of eight animals each: hypothermia alone, hypothermia with ischemia, IPC with cold ischemia, 5-hydroxydecanoic acid (5-HD) alone, and 5-HD with IPC and cold ischemia. IPC consisted of two 5-min periods of warm global ischemia spaced 5 min apart and 15 min of reperfusion before 2 h of ischemia at 27 degrees C and 2 h of warm reperfusion. The K(ATP) channel inhibitor 5-HD was perfused from 5 min before until 5 min after IPC. IPC before 2 h of ischemia at 27 degrees C led to better recovery of function and less tissue damage on reperfusion than did 27 degrees C ischemia alone. These improvements were preceded by attenuated increases in NADH and decreases in FAD during cold ischemia and the reverse changes during warm reperfusion. 5-HD blocked each of these changes induced by IPC. This study indicates that IPC induces additive cardioprotection with mild hypothermic ischemia by improving mitochondrial bioenergetics during and after ischemia. Because effects of IPC on subsequent changes in NADH and FAD were inhibited by 5-HD, this suggests that mitochondrial K(ATP) channel opening plays a substantial role in improving mitochondrial bioenergetics throughout mild hypothermic ischemia and reperfusion.

Pyruvate modulates cardiac sarcoplasmic reticulum Ca2+ release in rats via mitochondria-dependent and -independent mechanisms

The glycolytic product pyruvate has beneficial effects on cardiac contractile function. The postulated cellular mechanisms underlying the positive inotropic effect of pyruvate, however, are contradictory or have remained elusive. Therefore, we studied the effects of pyruvate on cardiac Ca2+ regulation, intracellular pH (pHi) and flavoprotein oxidation using fluorescence confocal microscopy in intact and permeabilized rat ventricular myocytes and single channel recordings from rat cardiac ryanodine receptors (RyRs) incorporated into planar lipid bilayers. In intact cells extracellular pyruvate (10 mM) elevated diastolic [Ca2+]i, which was due, at least in part, to a concomitant acidification of the cytosol. Furthermore, pyruvate increased the amplitude and slowed the kinetics of the electrically evoked [Ca2+]i transient, and augmented sarcoplasmic reticulum (SR) Ca2+ content. Recording of flavoprotein (FAD) fluorescence indicated that pyruvate caused a reduction of mitochondrial redox potential, which is proportional to an increase of the rate of ATP synthesis. Inhibitors of mitochondrial monocarboxylate transport (alpha-cyano-4-hydroxycinnamate, 0.5 mM), adenine nucleotide translocation (atractyloside, 0.3 mM) and the electron transport chain (cyanide, 4 mM) abolished or attenuated the pyruvate-mediated increase of the amplitude of the [Ca2+]i transient, but did not change the effect of pyruvate on diastolic [Ca2+]i. Results from experiments with permeabilized myocytes indicated a direct correlation between ATP/ADP ratio and SR Ca2+ content. Furthermore, pyruvate (4 mM) reduced the frequency of spontaneous Ca2+ sparks by approximately 50%. Single RyR channel recordings revealed a approximately 60% reduction of the open probability of the channel by pyruvate (1 mM), but no change in conductance. This effect of pyruvate on RyR channel activity was neither Ca2+ nor ATP dependent. Taken together, these findings suggest that, in cardiac tissue, pyruvate has a dual effect on SR Ca2+ release consisting of a direct inhibition of RyR channel activity and elevation of SR Ca2+ content. The latter effect was most probably mediated by an enhanced SR Ca2+ uptake due to an augmentation of mitochondria-dependent ATP synthesis.

Role of calcium in metabolic signaling between cardiac sarcoplasmic reticulum and mitochondria in vitro

The role of Ca(2+) as a cytosolic signaling molecule between porcine cardiac sarcoplasmic reticulum (SR) ATPase and mitochondrial ATP production was evaluated in vitro. The Ca(2+) sensitivity of these processes was determined individually and in a reconstituted system with SR and mitochondria in a 0.5:1 protein-to-cytochrome aa(3) ratio. The half-maximal concentration (K(1/2)) of SR ATPase was 335 nM Ca(2+). The ATP synthesis dependence was similar with a K(1/2) of 243 nM for dehydrogenases and 114 nM for overall ATP production. In the reconstituted system, Ca(2+) increased thapsigargin-sensitive ATP production (maximum approximately 5-fold) with minimal changes in mitochondrial reduced nicotinamide adenine dinucleotide (NADH). NADH concentration remained stable despite graded increases in NADH turnover induced over a wide range of Ca(2+) concentrations (0 to approximately 500 nM). These data are consistent with a balanced activation of SR ATPase and mitochondrial ATP synthesis by Ca(2+) that contributes to a homeostasis of energy metabolism metabolites. It is suggested that this balanced activation by cytosolic Ca(2+) is partially responsible for the minimal alteration in energy metabolism intermediates that occurs with changes in cardiac workload in vivo.

Simulation of cardiac work transitions, in vitro: effects of simultaneous Ca2+ and ATPase additions on isolated porcine heart mitochondria

During increases in cardiac work there are net increases in cytosolic [Ca(2+)] and ATP hydrolysis by myofiliments and ion transport ATPases. However, it is still unclear what role Ca(2+)or the ATP hydrolysis products, ADP and Pi, have on the regulation of mitochondrial ATP production. In this study, work jumps were simulated by simultaneous additions of Ca(2+) and ATPase to porcine heart mitochondria. The net effects on the mitochondrial ATP production were monitored by simultaneously monitoring respiration (mVo2), [NADH], [ADP] and membrane potential (deltapsi) at 37 degrees C. Addition of exogenous ATPase (300 mlU.ml(-1))]ATP (3.4 mM) was used to generate a 'resting' background production of ADP. This resting metabolic rate was 200% higher than the quiescent rate while [NADH] and deltapsi were reduced. Subsequent ATPase additions (1.3IU.ml(-)) were made with varying amounts of Ca(2+)(0 to 535 nM) to simulate step increases in cardiac work. Ca(2+) additions increased mVo2 and depolarized deltapsi, and were consistent with an activation of Fo/F1)ATPase. In contrast, Ca(2+) reduced the [NADH] response to the ATPase addition, consistent with Ca(2+)-sensitive dehydrogenase activity (CaDH). The calculated free ADP response to ATPase decreased \2-fold in the presence of Ca(2+). The addition of 172nM free Ca(2+)] ATPase increased mVo2 by 300% (P<0.05, n=8) while deltapsi decreased by 14.9+/-0.1 mV without changes in [NADH] (P > or =0.05, n=8), consistent with working heart preparations. The addition of Ca(2+) and ATPase combined increased the mitochondrial ATP production rate with changes in deltapsi, NADH and [ADP], consistent with an activation of CaDH and F o /F(1)ATPase activity. These balancing effects of ATPase activity and [Ca(2+)] may explain several aspects of metabolic regulation in the heart during work transitions in vivo.

Mitochondrial NAD(P)H, ADP, oxidative phosphorylation, and contraction in isolated heart cells

To examine the relationship between mitochondrial NADH (NADH(m)) and cardiac work output, NADH(m) and the amplitude and frequency of the contractile response of electrically paced rat heart cells were measured at 25 degrees C. With 5.4 mM glucose plus 2 mM beta-hydroxybutyrate, NADH(m) was reversibly decreased by 23%, and the amplitude of contraction was reversibly decreased by 27% during 4-Hz pacing. With glucose plus 2 mM pyruvate or with 10 mM 2-deoxy-D-glucose, NADH(m) was maintained during rapid pacing, and the contractile amplitude remained high. Phosphocreatine levels decreased with 2-deoxy-D-glucose administration but not with rapid pacing. Respiration increased to meet the increased ATP demand at 30 degrees C. The data suggest that 1) when NADH(m) is decreased during rapid pacing with defined substrates, the amplitude of contraction is decreased; 2) the amplitude of contraction during electrical pacing does not change with rate of pacing when both the ATP and NADH(m) levels are continuously replenished; and 3) the replenishment of NADH(m) during pacing with physiological substrates may be rate-limited by substrate supply to mitochondrial dehydrogenases. During activation of mitochondrial dehydrogenases, or a significant increase in free ADP induced by 2-deoxy-D-glucose, this rate limitation is bypassed or overcome.

Functional imaging of mitochondria in saponin-permeabilized mice muscle fibers

Confocal laser-scanning and digital fluorescence imaging microscopy were used to quantify the mitochondrial autofluorescence changes of NAD(P)H and flavoproteins in unfixed saponin-permeabilized myofibers from mice quadriceps muscle tissue. Addition of mitochondrial substrates, ADP, or cyanide led to redox state changes of the mitochondrial NAD system. These changes were detected by ratio imaging of the autofluorescence intensities of fluorescent flavoproteins and NAD(P)H, showing inverse fluorescence behavior. The flavoprotein signal was colocalized with the potentiometric mitochondria-specific dye dimethylaminostyryl pyridyl methyl iodide (DASPMI), or with MitoTrackerTM Green FM, a constitutive marker for mitochondria. Within individual myofibers we detected topological mitochondrial subsets with distinct flavoprotein autofluorescence levels, equally responding to induced rate changes of the oxidative phosphorylation. The flavoprotein autofluorescence levels of these subsets differed by a factor of four. This heterogeneity was substantiated by flow-cytometric analysis of flavoprotein and DASPMI fluorescence changes of individual mitochondria isolated from mice skeletal muscle. Our data provide direct evidence that mitochondria in single myofibers are distinct subsets at the level of an intrinsic fluorescent marker of the mitochondrial NAD-redox system. Under the present experimental conditions these subsets show similar functional responses.

Mechanisms of force inhibition by halothane and isoflurane in intact rat cardiac muscle

1. We investigated the mechanisms underlying the negative inotropic effect of the volatile anaesthetics halothane and isoflurane using twenty-two intact, right ventricular trabeculae of rat. [Ca2+]1 was measured qualitatively using either fluo-3 or fura-2, loaded into the cytosol via the acetoxymethyl (AM) ester form. Diastolic sarcomere length was adjusted to 2.1-2.2 micrograms and experiments were performed at 21-23 degrees C. 2. Halothane (0.25-3%) and isoflurane (0.48-4%) produced dose-dependent decreases in the amplitudes of the intracellular Ca2+ transients and twitch force. When the fluorescent Ca2+ indicator signals were corrected for changes in autofluorescence, neither volatile anaesthetic significantly changed diastolic [Ca2+]. 3. The ability of halothane and isoflurane to induce Ca2+ release from the sarcoplasmic reticulum of quiescent trabeculae was examined. When the superfusate was Ca2+ ad Na+ free (thereby preventing Na(+)-Ca2+ exchange and Ca2+ influx), 2% halothane, but not 4% isoflurane, evoked a transient increase in [Ca2+]i. 4. Halothane and isoflurane produced reversible, dose-dependent changes in cellular autofluorescence, the pattern of which was consistent with an increase in concentration of the reduced forms of nicotinamide adenine nucleotides and flavoproteins. This observation supports the putative inhibitory action of volatile anaesthetics at the site of Complex I of the mitochondrial electron transport chain. 5. Addition of the fatty acid hexanoate, a substrate that can be metabolized in the face of Complex I inhibition, did not appreciably attenuate the anaesthetic-induced negative inotropy; however, it greatly diminished autofluorescence changes. 6. To determine whether direct actions of the volatile anaesthetics on the contractile system contributed to the negative inotropy, external [Ca2+] was varied to modulate the amplitude of the Ca2+ transient. In the presence of 2% halothane or 4% isoflurane, restoration of the peak Ca2+ transient to control levels did not restore peak force. Moreover, halothane (1%) and isoflurane (16%) each reduced maximal Ca2(+)-activated force (attained using ryanodine tetani and a high external [Ca2+]) by around 15%. 7. We conclude that the negative inotropic actions of halothane and isoflurane on intact cardiac muscle reflect both reduced availability of Ca2+ and decreased responsiveness of the contractile system to Ca2+. The inhibitory action of the volatile anaesthetics on mitochondrial function does not contribute significantly to the negative inotropy but may lead to changes in cellular autofluorescence and misinterpretation of fluorescent Ca2+ indicator signals.