ATP consumption by uncoupled mitochondria activates sarcolemmal K(ATP) channels in cardiac myocytes

Drug-induced atrial fibrillation. A narrative review of a forgotten adverse effect

Atrial fibrillation (AF) is the most common cardiac arrhythmia and is associated with an increased morbidity and mortality. There is clinical evidence that an increasing number of cardiovascular and non-cardiovascular drugs, mainly anticancer drugs, can induce AF either in patients with or without pre-existing cardiac disorders, but drug-induced AF (DIAF) has not received the attention that it might deserve. In many cases DIAF is asymptomatic and paroxysmal and patients recover sinus rhythm spontaneously, but sometimes, DIAF persists, and it is necessary to perform a cardioversion. Furthermore, DIAF is not mentioned in clinical guidelines on the treatment of AF. The risk of DIAF increases in elderly and in patients treated with polypharmacy and with risk factors and comorbidities that commonly coexist with AF. This is the case of cancer patients. Under these circumstances ascribing causality of DIAF to a given drug often represents a clinical challenge. We review the incidence, the pathophysiological mechanisms, risk factors, clinical relevance, and treatment of DIAF. Because of the limited information presently available, further research is needed to obtain a deeper insight into DIAF. Meanwhile, it is important that clinicians are aware of the problem that DIAF represents, recognize which drugs may cause DIAF, and consider the possibility that a drug may be responsible for a new-onset AF episode.

Mitochondrial Dysfunction in Cardiac Arrhythmias

Electrophysiological and structural disruptions in cardiac arrhythmias are closely related to mitochondrial dysfunction. Mitochondria are an organelle generating ATP, thereby satisfying the energy demand of the incessant electrical activity in the heart. In arrhythmias, the homeostatic supply-demand relationship is impaired, which is often accompanied by progressive mitochondrial dysfunction leading to reduced ATP production and elevated reactive oxidative species generation. Furthermore, ion homeostasis, membrane excitability, and cardiac structure can be disrupted through pathological changes in gap junctions and inflammatory signaling, which results in impaired cardiac electrical homeostasis. Herein, we review the electrical and molecular mechanisms of cardiac arrhythmias, with a particular focus on mitochondrial dysfunction in ionic regulation and gap junction action. We provide an update on inherited and acquired mitochondrial dysfunction to explore the pathophysiology of different types of arrhythmias. In addition, we highlight the role of mitochondria in bradyarrhythmia, including sinus node dysfunction and atrioventricular node dysfunction. Finally, we discuss how confounding factors, such as aging, gut microbiome, cardiac reperfusion injury, and electrical stimulation, modulate mitochondrial function and cause tachyarrhythmia.

Loss of Mitochondrial Ca2+ Uniporter Limits Inotropic Reserve and Provides Trigger and Substrate for Arrhythmias in Barth Syndrome Cardiomyopathy

BACKGROUND

Barth syndrome (BTHS) is caused by mutations of the gene encoding tafazzin, which catalyzes maturation of mitochondrial cardiolipin and often manifests with systolic dysfunction during early infancy. Beyond the first months of life, BTHS cardiomyopathy typically transitions to a phenotype of diastolic dysfunction with preserved ejection fraction, blunted contractile reserve during exercise, and arrhythmic vulnerability. Previous studies traced BTHS cardiomyopathy to mitochondrial formation of reactive oxygen species (ROS). Because mitochondrial function and ROS formation are regulated by excitation-contraction coupling, integrated analysis of mechano-energetic coupling is required to delineate the pathomechanisms of BTHS cardiomyopathy.

METHODS

We analyzed cardiac function and structure in a mouse model with global knockdown of tafazzin (Taz-KD) compared with wild-type littermates. Respiratory chain assembly and function, ROS emission, and Ca²⁺ uptake were determined in isolated mitochondria. Excitation-contraction coupling was integrated with mitochondrial redox state, ROS, and Ca²⁺ uptake in isolated, unloaded or preloaded cardiac myocytes, and cardiac hemodynamics analyzed in vivo.

RESULTS

Taz-KD mice develop heart failure with preserved ejection fraction (>50%) and age-dependent progression of diastolic dysfunction in the absence of fibrosis. Increased myofilament Ca²⁺ affinity and slowed cross-bridge cycling caused diastolic dysfunction, in part, compensated by accelerated diastolic Ca²⁺ decay through preactivated sarcoplasmic reticulum Ca²⁺-ATPase. Taz deficiency provoked heart-specific loss of mitochondrial Ca²⁺ uniporter protein that prevented Ca²⁺-induced activation of the Krebs cycle during β-adrenergic stimulation, oxidizing pyridine nucleotides and triggering arrhythmias in cardiac myocytes. In vivo, Taz-KD mice displayed prolonged QRS duration as a substrate for arrhythmias, and a lack of inotropic response to β-adrenergic stimulation. Cellular arrhythmias and QRS prolongation, but not the defective inotropic reserve, were restored by inhibiting Ca²⁺ export through the mitochondrial Na⁺/Ca²⁺ exchanger. All alterations occurred in the absence of excess mitochondrial ROS in vitro or in vivo.

CONCLUSIONS

Downregulation of mitochondrial Ca²⁺ uniporter, increased myofilament Ca²⁺ affinity, and preactivated sarcoplasmic reticulum Ca²⁺-ATPase provoke mechano-energetic uncoupling that explains diastolic dysfunction and the lack of inotropic reserve in BTHS cardiomyopathy. Furthermore, defective mitochondrial Ca²⁺ uptake provides a trigger and a substrate for ventricular arrhythmias. These insights can guide the ongoing search for a cure of this orphaned disease.

Hydrogen peroxide diffusion and scavenging shapes mitochondrial network instability and failure by sensitizing ROS-induced ROS release

The mitochondrial network of cardiac cells is finely tuned for ATP delivery to sites of energy demand; however, emergent phenomena, such as mitochondrial transmembrane potential oscillations or propagating waves of depolarization have been observed under metabolic stress. While regenerative signaling by reactive oxygen species (ROS)-induced ROS release (RIRR) has been suggested as a potential trigger, it is unknown how it could lead to widespread responses. Here, we present a novel computational model of RIRR transmission that explains the mechanisms of this phenomenon. The results reveal that superoxide mediates neighbor-neighbor activation of energy-dissipating ion channels, while hydrogen peroxide distributes oxidative stress to sensitize the network to mitochondrial criticality. The findings demonstrate the feasibility of RIRR as a synchronizing factor across the dimensions of the adult heart cell and illustrate how a cascade of failures at the organellar level can scale to impact cell and organ level functions of the heart.

Myocardial death and dysfunction after ischemia-reperfusion injury require CaMKIIδ oxidation

Reactive oxygen species (ROS) contribute to myocardial death during ischemia-reperfusion (I/R) injury, but detailed knowledge of molecular pathways connecting ROS to cardiac injury is lacking. Activation of the Ca²⁺/calmodulin-dependent protein kinase II (CaMKIIδ) is implicated in myocardial death, and CaMKII can be activated by ROS (ox-CaMKII) through oxidation of regulatory domain methionines (Met281/282). We examined I/R injury in mice where CaMKIIδ was made resistant to ROS activation by knock-in replacement of regulatory domain methionines with valines (MMVV). We found reduced myocardial death, and improved left ventricular function 24 hours after I/R injury in MMVV in vivo and in vitro compared to WT controls. Loss of ATP sensitive K⁺ channel (KATP) current contributes to I/R injury, and CaMKII promotes sequestration of KATP from myocardial cell membranes. KATP current density was significantly reduced by H₂O₂ in WT ventricular myocytes, but not in MMVV, showing ox-CaMKII decreases KATP availability. Taken together, these findings support a view that ox-CaMKII and KATP are components of a signaling axis promoting I/R injury by ROS.

Mitochondrial dysfunction on sinoatrial node and pulmonary vein electrophysiological activities

Atrial fibrillation (AF) is associated with mitochondrial dysfunction. Sinoatrial node (SAN) dysfunction increases arrhythmogenesis of pulmonary veins (PVs), which is the most important trigger of AF; however, it is not clear whether mitochondrial dysfunction differentially regulates electrical activity of SANs and PVs. In the present study, conventional microelectrodes were used to record the action potentials (APs) in isolated rabbit PVs, SANs, left atrium (LA) and right atrium (RA) before and after application of trifluorocarbonylcyanide phenylhydrazone (FCCP; a mitochondrial uncoupling agent) at 10, 100 and 300 nM. FCCP application at 100 and 300 nM decreased spontaneous rates in PVs and in SANs at 10, 100 and 300 nM. FCCP shortened the 20, 50 and 90% AP durations in the LA, and shortened only the 20% AP duration in the RA. FCCP caused a greater rate reduction in SANs than in PVs; however, in the presence of coenzyme-Q₁₀ (10 µM), FCCP reduced the beating rate in PVs and SANs to a similar extent. In SAN-PV preparations with intact electrical connections, FCCP (100 nM) application shifted the SAN-PV electrical conduction into PV-SAN conduction in 5 (62.5%) of 8 preparations. In conclusion, mitochondrial dysfunction modulates PV and SAN electrical activities, which may contribute to atrial arrhythmogenesis.

Perspectives and challenges of antioxidant therapy for atrial fibrillation

Atrial fibrillation (AF) is the most common sustained arrhythmia associated with significant morbidity and mortality. The mechanisms underlying the pathogenesis of AF are poorly understood, although electrophysiological remodeling has been described as an important initiating step. There is growing evidence that oxidative stress is involved in the pathogenesis of AF. Many known triggers of oxidative stress, such as age, diabetes, smoking, and inflammation, are linked with an increased risk of arrhythmia. Numerous preclinical studies and clinical trials reported the importance of antioxidant therapy in the prevention of AF, using vitamins C and E, polyunsaturated fatty acids, statins, or nitric oxide donors. The aim of our work is to give a current overview and analysis of opportunities, challenges, and benefits of antioxidant therapy in AF.

β3-Adrenoceptor Impairs Mitochondrial Biogenesis and Energy Metabolism During Rapid Atrial Pacing-Induced Atrial Fibrillation

BACKGROUND

The β3-adrenoceptor (β3-AR) is implicated in cardiac remodeling. Since metabolic dysfunction due to loss of mitochondria plays an important role in heart diseases, we examined the effects of β3-AR on mitochondrial biogenesis and energy metabolism in atrial fibrillation (AF).

METHODS

Atrial fibrillation was created by rapid atrial pacing in adult rabbits. Rabbits were randomly divided into 4 groups: control, pacing (P7), β3-AR antagonist (L748337), and β3-AR agonist (BRL37344) groups. Atrial effective refractory period (AERP) and AF induction rate were measured. Atrial concentrations of adenine nucleotides and phosphocreatine were quantified through high-performance liquid chromatography. Mitochondrial DNA content was determined. Real-time polymerase chain reaction and Western blot were used to examine the expression levels of signaling intermediates related to mitochondrial biogenesis.

RESULTS

After pacing for 7 days, β3-AR was significantly upregulated, AERP was reduced, and the AF induction rate was increased. The total adenine nucleotides pool was significantly reduced due to the decrease in adenosine triphosphate (ATP). The P7 group showed decreased activity of F0F1-ATPase. Mitochondrial DNA content was decreased and mitochondrial respiratory chain subunits were downregulated after pacing. Furthermore, expression of transcription factors involved in mitochondrial biogenesis, including peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), nuclear respiratory factor 1 (NRF-1), and mitochondrial transcription factor A (Tfam), was lower in the P7 group in response to β3-AR activation. Further stimulation of β3-AR with BRL37344 exacerbated these effects, together with a significant decrease in the levels of phosphocreatine. In contrast, inhibition of β3-AR with L748337 partially restored mitochondrial biogenesis and energy metabolism of atria in the paced rabbits.

CONCLUSION

The activation of β3-AR contributes to atrial metabolic remodeling via transcriptional downregulation of PGC-1α/NRF-1/Tfam pathway that are involved in mitochondrial biogenesis, which ultimately perturbs mitochondrial function in rapid pacing-induced AF. The β3-AR is therefore a potential novel therapeutic target for the treatment or prevention of AF.

Mechanisms of sudden cardiac death: oxidants and metabolism

Ventricular arrhythmia is the leading cause of sudden cardiac death (SCD). Deranged cardiac metabolism and abnormal redox state during cardiac diseases foment arrhythmogenic substrates through direct or indirect modulation of cardiac ion channel/transporter function. This review presents current evidence on the mechanisms linking metabolic derangement and excessive oxidative stress to ion channel/transporter dysfunction that predisposes to ventricular arrhythmias and SCD. Because conventional antiarrhythmic agents aiming at ion channels have proven challenging to use, targeting arrhythmogenic metabolic changes and redox imbalance may provide novel therapeutics to treat or prevent life-threatening arrhythmias and SCD.

Mitochondrial depolarization and asystole in the globally ischemic rabbit heart: coordinated response to interventions affecting energy balance

Mitochondrial membrane potential (ΔΨm) depolarization has been implicated in the loss of excitability (asystole) during global ischemia, which is relevant for the success of defibrillation and resuscitation after cardiac arrest. However, the relationship between ΔΨm depolarization and asystole during no-flow ischemia remains unknown. We applied spatial Fourier analysis to confocally recorded fluorescence emitted by ΔΨm-sensitive dye tetramethylrhodamine methyl ester. The time of ischemic ΔΨm depolarization (tmito_depol) was defined as the time of 50% decrease in the magnitude of spectral peaks reflecting ΔΨm. The time of asystole (tasys) was determined as the time when spontaneous and induced ventricular activity ceased to exist. Interventions included tachypacing (150 ms), myosin II ATPase inhibitor blebbistatin (heart immobilizer), and the combination of blebbistatin and the inhibitor of glycolysis iodoacetate. In the absence of blebbistatin, confocal images were obtained during brief perfusion with hyperkalemic solution and after the contraction failed between 7 and 15 min of ischemia. In control, tmito_depol and tasys were 24.4 ± 6.0 and 26.0 ± 5.0 min, respectively. Tachypacing did not significantly affect either parameter. Blebbistatin dramatically delayed tmito_depol and tasys (51.4 ± 8.6 and 45.7 ± 5.3 min, respectively; both P < 0.0001 vs. control). Iodoacetate combined with blebbistatin accelerated both events (tmito_depol, 12.7 ± 1.8 min; and tasys, 6.5 ± 1.1 min; both P < 0.03 vs. control). In all groups pooled together, tasys was strongly correlated with tmito_depol (R(2) = 0.845; P < 0.0001). These data may indicate a causal relationship between ΔΨm depolarization and asystole or a similar dependence of the two events on energy depletion during ischemia. Our results urge caution against the use of blebbistatin in studies addressing pathophysiology of myocardial ischemia.

Mitochondrial DNA 4977bp deletion mutation in peripheral blood reflects atrial remodeling in patients with non-valvular atrial fibrillation

PURPOSE

Recently, mitochondrial DNA 4977bp deletion (mtDNA4977-mut), a somatic mutation related to oxidative stress, has been shown to be associated with atrial fibrillation (AF). We hypothesized that patient age, as well as electroanatomical characteristics of fibrillating left atrial (LA), vary depending on the presence of mtDNA4977-mut in peripheral blood among patients with non-valvular AF.

MATERIALS AND METHODS

Analyzing clinical and electroanatomical characteristics, we investigated the presence of the mtDNA4977-mut in peripheral blood of 212 patients (51.1±13.2 years old, 83.5% male) undergoing catheter ablation for non-valvular AF, as well as 212 age-matched control subjects.

RESULTS

The overall frequency of peripheral blood mtDNA4977-mut in patients with AF and controls was not significantly different (24.5% vs. 19.3%, p=0.197). When the AF patient group was stratified according to age, mtDNA4977-mut was more common (47.4% vs. 20.0%, p=0.019) in AF patients older than 65 years than their age-matched controls. Among AF patients, those with mtDNA4977-mut were older (58.1±11.9 years old vs. 48.8±11.9 years old, p<0.001). AF patients positive for the mtDNA mutation had greater LA dimension (p=0.014), higher mitral inflow peak velocity (E)/diastolic mitral annular velocity (Em) ratio (p<0.001), as well as lower endocardial voltage (p=0.035), and slower conduction velocity (p=0.048) in the posterior LA than those without the mutation. In multivariate analysis, E/Em ratio was found to be significantly associated with the presence of mtDNA4977-mut in peripheral blood.

CONCLUSION

mtDNA4977-mut, an age-related somatic mutation detected in the peripheral blood, is associated with advanced age and electro-anatomical remodeling of the atrium in non-valvular AF.

Mitochondrial instability during regional ischemia-reperfusion underlies arrhythmias in monolayers of cardiomyocytes

Regional depolarization of the mitochondrial network can alter cellular electrical excitability and increase the propensity for reentry, in part, through the opening of sarcolemmal KATP channels. Mitochondrial inner membrane potential (ΔΨm) instability or oscillation can be induced in myocytes by exposure to reactive oxygen species (ROS), laser excitation, or glutathione depletion, and is thought to be a major factor in arrhythmogenesis during ischemia-reperfusion. Nevertheless, the correlation between ΔΨm recovery kinetics and reperfusion-induced arrhythmias has been difficult to demonstrate experimentally. Here, we investigate the relationship between subcellular changes in ΔΨm, cellular glutathione redox potential, electrical excitability, and wave propagation during coverslip-induced ischemia-reperfusion (IR) in neonatal rat ventricular myocyte (NRVM) monolayers. Ischemia led to decreased action potential amplitude and duration followed by electrical inexcitability after ~15min of ischemia. ΔΨm depolarization occurred in two phases during ischemia: in phase 1 (<30min ischemia), mitochondrial clusters within individual NRVMs depolarized, while phase 2 ΔΨm depolarization (30-60min) was characterized by global functional collapse of the mitochondrial network across the whole ischemic region of the monolayer, typically involving a propagating metabolic wave. Oxidation of the glutathione (GSSG:GSH) redox potential occurred during ischemia, followed by recovery upon reperfusion (i.e., lifting the coverslip). ΔΨm recovered in the mitochondria of individual myocytes quite rapidly upon reperfusion (<5min), but was highly unstable, characterized by subcellular oscillations or flickering of clusters of mitochondria in NRVMs across the reperfused region. Electrical excitability also recovered in a heterogeneous manner, providing an arrhythmogenic substrate which led to formation of sustained reentry. Treatment with 4'-chlorodiazepam, a peripheral benzodiazepine receptor ligand, prevented ΔΨm oscillation, improved GSH recovery rate, and prevented reentry during reperfusion, indicating that stabilization of mitochondrial network dynamics is important for preventing post-ischemic arrhythmias. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease".

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Mitochondrial reactive oxygen species production and elimination

Reactive oxygen species (ROS) play an important role in cardiovascular diseases, and one important source for ROS are mitochondria. Emission of ROS from mitochondria is the net result of ROS production at the electron transport chain (ETC) and their elimination by antioxidative enzymes. Both of these processes are highly dependent on the mitochondrial redox state, which is dynamically altered under different physiological and pathological conditions. The concept of "redox-optimized ROS balance" integrates these aspects and implies that oxidative stress occurs when the optimal equilibrium of an intermediate redox state is disturbed towards either strong oxidation or reduction. Furthermore, mitochondria integrate ROS signals from other cellular sources, presumably through a process termed "ROS-induced ROS release" that involves mitochondrial ion channels. Here, we attempt to integrate these recent advances in our understanding of the control of mitochondrial ROS emission and develop a concept of how in heart failure, defects in ion handling can lead to mitochondrial oxidative stress. This article is part of a Special Issue entitled "Redox Signalling in the Cardiovascular System".

Effects of regional mitochondrial depolarization on electrical propagation: implications for arrhythmogenesis

BACKGROUND

Sudden cardiac death often involves arrhythmias triggered by metabolic stress. Loss of mitochondrial function is thought to contribute to the arrhythmogenic substrate, but how mitochondria contribute to uncoordinated electrical activity is poorly understood. It has been proposed that the formation of metabolic current sinks, caused by the nonuniform collapse of mitochondrial inner membrane potential (ΔΨm), contributes to re-entrant arrhythmias because ΔΨm depolarization is tightly coupled to the activation of sarcolemmal ATP-sensitive K(+) channels, hastening action potential repolarization and shortening the refractory period.

METHODS AND RESULTS

Here, we use computational and experimental methods to investigate how ΔΨm instability can induce re-entrant arrhythmias. We develop the first tissue-level model of cardiac electrical propagation incorporating cellular electrophysiology, excitation-contraction coupling, mitochondrial energetics, and reactive oxygen species balance. Simulations show that re-entry and fibrillation can be initiated by regional ΔΨm loss because of the disparity of refractory periods inside and outside the metabolic sink. Computational results are compared with the effects of a metabolic sink generated experimentally by local perfusion of a mitochondrial uncoupler in a monolayer of cardiac myocytes.

CONCLUSIONS

The results demonstrate that regional mitochondrial depolarization triggered by oxidative stress activates sarcolemmal ATP-sensitive K(+) currents to form a metabolic sink. Consequent shortening of the action potential inside, but not outside, the sink increases the propensity for re-entry. ΔΨm recovery during pacing can lead to novel mechanisms of ectopic activation. The findings highlight the importance of mitochondria as potential therapeutic targets for sudden death associated with cardiovascular disease.

Preconditioning by isoflurane elicits mitochondrial protective mechanisms independent of sarcolemmal KATP channel in mouse cardiomyocytes

Cardiac mitochondria and the sarcolemmal (sarc)KATP channels contribute to cardioprotective signaling of anesthetic-induced preconditioning. Changes in mitochondrial bioenergetics influence the sarcolemmal ATP-sensitive K (sarcKATP) channel function, but whether this channel has impacts on mitochondria is uncertain. We used the mouse model with deleted pore-forming Kir6.2 subunit of sarcKATP channel (Kir6.2 KO) to investigate whether the functional sarcKATP channels are necessary for isoflurane activation of mitochondrial protective mechanisms. Ventricular cardiomyocytes were isolated from C57Bl6 wild-type (WT) and Kir6.2 KO mouse hearts. Flavoprotein autofluorescence, mitochondrial reactive oxygen species production, and mitochondrial membrane potential were monitored by laser-scanning confocal microscopy in intact cardiomyocytes. Cell survival was assessed using H2O2-induced stress. Isoflurane (0.5 mM) increased flavoprotein fluorescence to 180% ± 14% and 190% ± 15% and reactive oxygen species production to 118% ± 2% and 124% ± 6% of baseline in WT and Kir6.2 KO myocytes, respectively. Tetramethylrhodamine ethyl ester fluorescence decreased to 84% ± 6% in WT and to 86% ± 4% in Kir6.2 KO myocytes. This effect was abolished by 5HD. Pretreatment with isoflurane decreased the stress-induced cell death from 31% ± 1% to 21% ± 1% in WT and from 44% ± 2% to 35% ± 2% in Kir6.2 KO myocytes. In conclusion, Kir6.2 deletion increases the sensitivity of intact cardiomyocytes to oxidative stress, but does not alter the isoflurane-elicited protective mitochondrial mechanisms, suggesting independent roles for cardiac mitochondria and sarcKATP channels in anesthetic-induced preconditioning by isoflurane.

Regulation of cardiac ATP-sensitive potassium channel surface expression by calcium/calmodulin-dependent protein kinase II

Cardiac ATP-sensitive potassium (K(ATP)) channels are key sensors and effectors of the metabolic status of cardiomyocytes. Alteration in their expression impacts their effectiveness in maintaining cellular energy homeostasis and resistance to injury. We sought to determine how activation of calcium/calmodulin-dependent protein kinase II (CaMKII), a central regulator of calcium signaling, translates into reduced membrane expression and current capacity of cardiac K(ATP) channels. We used real-time monitoring of K(ATP) channel current density, immunohistochemistry, and biotinylation studies in isolated hearts and cardiomyocytes from wild-type and transgenic mice as well as HEK cells expressing wild-type and mutant K(ATP) channel subunits to track the dynamics of K(ATP) channel surface expression. Results showed that activation of CaMKII triggered dynamin-dependent internalization of K(ATP) channels. This process required phosphorylation of threonine at 180 and 224 and an intact (330)YSKF(333) endocytosis motif of the K(ATP) channel Kir6.2 pore-forming subunit. A molecular model of the μ2 subunit of the endocytosis adaptor protein, AP2, complexed with Kir6.2 predicted that μ2 docks by interaction with (330)YSKF(333) and Thr-180 on one and Thr-224 on the adjacent Kir6.2 subunit. Phosphorylation of Thr-180 and Thr-224 would favor interactions with the corresponding arginine- and lysine-rich loops on μ2. We concluded that calcium-dependent activation of CaMKII results in phosphorylation of Kir6.2, which promotes endocytosis of cardiac K(ATP) channel subunits. This mechanism couples the surface expression of cardiac K(ATP) channels with calcium signaling and reveals new targets to improve cardiac energy efficiency and stress resistance.

Acetyl-L-carnitine supplementation reverses the age-related decline in carnitine palmitoyltransferase 1 (CPT1) activity in interfibrillar mitochondria without changing the L-carnitine content in the rat heart

The aging heart displays a loss of bioenergetic reserve capacity partially mediated through lower fatty acid utilization. We investigated whether the age-related impairment of cardiac fatty acid catabolism occurs, at least partially, through diminished levels of L-carnitine, which would adversely affect carnitine palmitoyltransferase 1 (CPT1), the rate-limiting enzyme for fatty acyl-CoA uptake into mitochondria for β-oxidation. Old (24-28 mos) Fischer 344 rats were fed±acetyl-L-carnitine (ALCAR; 1.5% [w/v]) for up to four weeks prior to sacrifice and isolation of cardiac interfibrillar (IFM) and subsarcolemmal (SSM) mitochondria. IFM displayed a 28% (p<0.05) age-related loss of CPT1 activity, which correlated with a decline (41%, p<0.05) in palmitoyl-CoA-driven state 3 respiration. Interestingly, SSM had preserved enzyme function and efficiently utilized palmitate. Analysis of IFM CPT1 kinetics showed both diminished V(max) and K(m) (60% and 49% respectively, p<0.05) when palmitoyl-CoA was the substrate. However, no age-related changes in enzyme kinetics were evident with respect to L-carnitine. ALCAR supplementation restored CPT1 activity in heart IFM, but not apparently through remediation of L-carnitine levels. Rather, ALCAR influenced enzyme activity over time, potentially by modulating conditions in the aging heart that ultimately affect palmitoyl-CoA binding and CPT1 kinetics.

Reduction in number of sarcolemmal KATP channels slows cardiac action potential duration shortening under hypoxia

The cardiovascular system operates under demands ranging from conditions of rest to extreme stress. One mechanism of cardiac stress tolerance is action potential duration shortening driven by ATP-sensitive potassium (K(ATP)) channels. K(ATP) channel expression has a significant physiologic impact on action potential duration shortening and myocardial energy consumption in response to physiologic heart rate acceleration. However, the effect of reduced channel expression on action potential duration shortening in response to severe metabolic stress is yet to be established. Here, transgenic mice with myocardium-specific expression of a dominant negative K(ATP) channel subunit were compared with littermate controls. Evaluation of K(ATP) channel whole cell current and channel number/patch was assessed by patch clamp in isolated ventricular cardiomyocytes. Monophasic action potentials were monitored in retrogradely perfused, isolated hearts during the transition to hypoxic perfusate. An 80-85% reduction in cardiac K(ATP) channel current density results in a similar magnitude, but significantly slower rate, of shortening of the ventricular action potential duration in response to severe hypoxia, despite no significant difference in coronary flow. Therefore, the number of functional cardiac sarcolemmal K(ATP) channels is a critical determinant of the rate of adaptation of myocardial membrane excitability, with implications for optimization of cardiac energy consumption and consequent cardioprotection under conditions of severe metabolic stress.

Dystrophin is required for the normal function of the cardio-protective K(ATP) channel in cardiomyocytes

Duchenne and Becker muscular dystrophy patients often develop a cardiomyopathy for which the pathogenesis is still unknown. We have employed the murine animal model of Duchenne muscular dystrophy (mdx), which develops a cardiomyopathy that includes some characteristics of the human disease, to study the molecular basis of this pathology. Here we show that the mdx mouse heart has defects consistent with alteration in compounds that regulate energy homeostasis including a marked decrease in creatine-phosphate (PC). In addition, the mdx heart is more susceptible to anoxia than controls. Since the cardio-protective ATP sensitive potassium channel (K_ATP) complex and PC have been shown to interact we investigated whether deficits in PC levels correlate with other molecular events including K_ATP ion channel complex presence, its functionality and interaction with dystrophin. We found that this channel complex is present in the dystrophic cardiac cell membrane but its ability to sense a drop in the intracellular ATP concentration and consequently open is compromised by the absence of dystrophin. We further demonstrate that the creatine kinase muscle isoform (CKm) is displaced from the plasma membrane of the mdx cardiac cells. Considering that CKm is a determinant of K_ATP channel complex function we hypothesize that dystrophin acts as a scaffolding protein organizing the K_ATP channel complex and the enzymes necessary for its correct functioning. Therefore, the lack of proper functioning of the cardio-protective K_ATP system in the mdx cardiomyocytes may be part of the mechanism contributing to development of cardiac disease in dystrophic patients.

Metabolic stress, reactive oxygen species, and arrhythmia

Cardiac arrhythmias can cause sudden cardiac death (SCD) and add to the current heart failure (HF) health crisis. Nevertheless, the pathological processes underlying arrhythmias are unclear. Arrhythmic conditions are associated with systemic and cardiac oxidative stress caused by reactive oxygen species (ROS). In excitable cardiac cells, ROS regulate both cellular metabolism and ion homeostasis. Increasing evidence suggests that elevated cellular ROS can cause alterations of the cardiac sodium channel (Na(v)1.5), abnormal Ca(2+) handling, changes of mitochondrial function, and gap junction remodeling, leading to arrhythmogenesis. This review summarizes our knowledge of the mechanisms by which ROS may cause arrhythmias and discusses potential therapeutic strategies to prevent arrhythmias by targeting ROS and its consequences. This article is part of a Special Issue entitled "Local Signaling in Myocytes".

Mitochondrial uncoupling agents trigger ventricular fibrillation in isolated rat hearts

Sudden cardiac death resulting from ventricular fibrillation (VF) remains a major cause of mortality. The purpose of this study was to investigate the roles of loss of oxidative phosphorylation and activation of the mitochondrial ATP-sensitive K+ channel and permeability transition pore in VF development during myocardial ischemia by using mitochondrial uncoupling agents (carbonyl cyanide m-chlorophenylhydrazone and 2,4-dinitrophenol) and channel blockers (5-hydroxydecanoate and cyclosporine A) at concentrations that have been demonstrated to block the intended targets selectively. Isolated rat hearts (n = 8 per group) were perfused with 0.3 μM carbonyl cyanide m-chlorophenylhydrazone, 100 μM 2,4-dinitrophenol, 0.2 μM cyclosporine A, 100 μM 5-hydroxydecanoate, or vehicle solution and regional ischemia induced after 10 minutes. Carbonyl cyanide m-chlorophenylhydrazone and 2,4 dinitrophenol caused profound QT shortening and triggered VF in 100% of hearts before ischemia. During ischemia, neither cyclosporine A (88%) nor 5-hydroxydecanoate (100%) reduced VF incidence compared with control (100% VF). In separate hearts, carbonyl cyanide m-chlorophenylhydrazone decreased tissue ATP content, and glibenclamide or glimepiride delayed the QT shortening and onset of VF triggered by carbonyl cyanide m-chlorophenylhydrazone. In conclusion, mitochondrial uncoupling agents trigger VF, likely as a result of ATP depletion with subsequent activation of sarcolemmal ATP-sensitive K+ currents. The mechanism of VF in ischemia does not involve activation of the mitochondrial ATP-sensitive K+ channel or permeability transition pore.

Mitochondria are sources of metabolic sink and arrhythmias

Mitochondria have long been recognized for their central role in energy transduction and apoptosis. More recently, extensive work in multiple laboratories around the world has significantly extended the role of cardiac mitochondria from relatively static arbitrators of cell death and survival pathways to highly dynamic organelles that form interactive functional networks across cardiomyocytes. These coupled networks were shown to strongly affect cardiomyocyte responses to oxidative stress by modulating cell signaling pathways that strongly impact physiological properties. Of particular importance is the role of mitochondria in modulating key electrophysiological and calcium cycling properties in cardiomyocytes, either directly through activation of a myriad of mitochondrial ion channels or indirectly by affecting cell signaling cascades, ATP levels, and the over-all redox state of the cardiomyocyte. This important recognition has ushered a renewed interest in understanding, at a more fundamental level, the exact role that cardiac metabolism, in general and mitochondria, in particular, play in both health and disease. In this article, we provide an overview of recent advances in our growing understanding of the fundamental role that cardiac mitochondria play in the genesis of lethal arrhythmias.

Mitochondrial consumption of cytosolic ATP: not so fast

In various pathologic circumstances depolarized mitochondria are thought to precipitate cell death by avidly consuming cytosolic ATP. However, for as long as the inner mitochondrial membrane remains intact the reversal potentials of the adenine nucleotide translocase (ANT) and that of F(0)-F(1) ATP synthase are strategically positioned so that they oppose import of cytosolic ATP into the matrix of respiration-impaired mitochondria. This arrangement also seems to protect against a hysteretic consumption of cytosolic ATP accumulating in the mitochondrial matrix, in view of the depolarization caused by inhibition of F(0)-F(1) ATP synthase by the endogenous protein IF1, yielding fast ANT reversal rates.

Exercise-induced expression of cardiac ATP-sensitive potassium channels promotes action potential shortening and energy conservation

Physical activity is one of the most important determinants of cardiac function. The ability of the heart to increase delivery of oxygen and metabolic fuels relies on an array of adaptive responses necessary to match bodily demand while avoiding exhaustion of cardiac resources. The ATP-sensitive potassium (K(ATP)) channel has the unique ability to adjust cardiac membrane excitability in accordance with ATP and ADP levels, and up-regulation of its expression that occurs in response to exercise could represent a critical element of this adaption. However, the mechanism by which K(ATP) channel expression changes result in a beneficial effect on cardiac excitability and function remains to be established. Here, we demonstrate that an exercise-induced rise in K(ATP) channel expression enhanced the rate and magnitude of action potential shortening in response to heart rate acceleration. This adaptation in membrane excitability promoted significant reduction in cardiac energy consumption under escalating workloads. Genetic disruption of normal K(ATP) channel pore function abolished the exercise-related changes in action potential duration adjustment and caused increased cardiac energy consumption. Thus, an expression-driven enhancement in the K(ATP) channel-dependent membrane response to alterations in cardiac workload represents a previously unrecognized mechanism for adaptation to physical activity and a potential target for cardioprotection.

Cardiac mitochondria and arrhythmias

Despite a high prevalence of sudden cardiac death throughout the world, the mechanisms that lead to ventricular arrhythmias are not fully understood. Over the last 20 years, a growing body of evidence indicates that cardiac mitochondria are involved in the genesis of arrhythmia. In this review, we have attempted to describe the role that mitochondria play in altering the heart's electrical function by introducing heterogeneity into the cardiac action potential. Specifically, we have focused on how the energetic status of the mitochondrial network can alter sarcolemmal potassium fluxes through ATP-sensitive potassium channels, creating a 'metabolic sink' for depolarizing wave-fronts and introducing conditions that favour catastrophic arrhythmia. Mechanisms by which mitochondria depolarize under conditions of oxidative stress are characterized, and the contributions of several mitochondrial ion channels to mitochondrial depolarization are presented. The inner membrane anion channel in particular opens upstream of other inner membrane channels during metabolic stress, and may be an effective target to prevent the metabolic oscillations that create action potential lability. Finally, we discuss therapeutic strategies that prevent arrhythmias by preserving mitochondrial membrane potential in the face of oxidative stress, supporting the notion that treatments aimed at cardiac mitochondria have significant potential in attenuating electrical dysfunction in the heart.

Forward operation of adenine nucleotide translocase during F0F1-ATPase reversal: critical role of matrix substrate-level phosphorylation

In pathological conditions, F(0)F(1)-ATPase hydrolyzes ATP in an attempt to maintain mitochondrial membrane potential. Using thermodynamic assumptions and computer modeling, we established that mitochondrial membrane potential can be more negative than the reversal potential of the adenine nucleotide translocase (ANT) but more positive than that of the F(0)F(1)-ATPase. Experiments on isolated mitochondria demonstrated that, when the electron transport chain is compromised, the F(0)F(1)-ATPase reverses, and the membrane potential is maintained as long as matrix substrate-level phosphorylation is functional, without a concomitant reversal of the ANT. Consistently, no cytosolic ATP consumption was observed using plasmalemmal K(ATP) channels as cytosolic ATP biosensors in cultured neurons, in which their in situ mitochondria were compromised by respiratory chain inhibitors. This finding was further corroborated by quantitative measurements of mitochondrial membrane potential, oxygen consumption, and extracellular acidification rates, indicating nonreversal of ANT of compromised in situ neuronal and astrocytic mitochondria; and by bioluminescence ATP measurements in COS-7 cells transfected with cytosolic- or nuclear-targeted luciferases and treated with mitochondrial respiratory chain inhibitors in the presence of glycolytic plus mitochondrial vs. only mitochondrial substrates. Our findings imply the possibility of a rescue mechanism that is protecting against cytosolic/nuclear ATP depletion under pathological conditions involving impaired respiration. This mechanism comes into play when mitochondria respire on substrates that support matrix substrate-level phosphorylation.

Inhibition of oxygen consumption in skeletal muscle-derived mitochondria by pinacidil, diazoxide, and glibenclamide, but not by 5-hydroxydecanoate

Cell intermediary metabolism and energy production succeeds by means of mitochondria, whose activity is in relation to transmembrane potential and/or free radical production. Adenosine triphosphate (ATP)-dependent potassium channels (K(ATP)) in several cell types have shown to couple cell metabolism to membrane potential and ATP production. In this study, we explore whether oxygen consumption in isolated skeletal-muscle mitochondria differs in the presence of distinct respiration substrates and whether these changes are affected by K(ATP)-channel inhibitors such as glibenclamide, 5-Hydroxydecanoate (5-HD), and K(ATP) channel activators (pinacidil and diazoxide). Results demonstrate a concentration-dependent diminution of respiration rate by glibenclamide (0.5-20 microM), pinacidil (1-50 microM), and diazoxide (50-200 microM), but no significant differences were found when the selective mitochondrial K(ATP)-channel inhibitor (5-HD, 10-500 microM) was used. These results suggest that these K(ATP)-channel agonists and antagonists exert an effect on mitochondrial respiration and that they could be acting on mito-K(ATP) or other respiratory-chain components.

Modeling cardiac action potential shortening driven by oxidative stress-induced mitochondrial oscillations in guinea pig cardiomyocytes

Ischemia-induced shortening of the cardiac action potential and its heterogeneous recovery upon reperfusion are thought to set the stage for reentrant arrhythmias and sudden cardiac death. We have recently reported that the collapse of mitochondrial membrane potential (DeltaPsi(m)) through a mechanism triggered by reactive oxygen species (ROS), coupled to the opening of sarcolemmal ATP-sensitive potassium (K(ATP)) channels, contributes to electrical dysfunction during ischemia-reperfusion. Here we present a computational model of excitation-contraction coupling linked to mitochondrial bioenergetics that incorporates mitochondrial ROS-induced ROS release with coupling between the mitochondrial energy state and electrical excitability mediated by the sarcolemmal K(ATP) current (I(K,ATP)). Whole-cell model simulations demonstrate that increasing the fraction of oxygen diverted from the respiratory chain to ROS production triggers limit-cycle oscillations of DeltaPsi(m), redox potential, and mitochondrial respiration through the activation of a ROS-sensitive inner membrane anion channel. The periods of transient mitochondrial uncoupling decrease the cytosolic ATP/ADP ratio and activate I(K,ATP), consequently shortening the cellular action potential duration and ultimately suppressing electrical excitability. The model simulates emergent behavior observed in cardiomyocytes subjected to metabolic stress and provides a new tool for examining how alterations in mitochondrial oxidative phosphorylation will impact the electrophysiological, contractile, and Ca(2+) handling properties of the cardiac cell. Moreover, the model is an important step toward building multiscale models that will permit investigation of the role of spatiotemporal heterogeneity of mitochondrial metabolism in the mechanisms of arrhythmogenesis and contractile dysfunction in cardiac muscle.

Philosophical basis and some historical aspects of systems biology: from Hegel to Noble - applications for bioenergetic research

We live in times of paradigmatic changes for the biological sciences. Reductionism, that for the last six decades has been the philosophical basis of biochemistry and molecular biology, is being displaced by Systems Biology, which favors the study of integrated systems. Historically, Systems Biology - defined as the higher level analysis of complex biological systems - was pioneered by Claude Bernard in physiology, Norbert Wiener with the development of cybernetics, and Erwin Schrödinger in his thermodynamic approach to the living. Systems Biology applies methods inspired by cybernetics, network analysis, and non-equilibrium dynamics of open systems. These developments follow very precisely the dialectical principles of development from thesis to antithesis to synthesis discovered by Hegel. Systems Biology opens new perspectives for studies of the integrated processes of energy metabolism in different cells. These integrated systems acquire new, system-level properties due to interaction of cellular components, such as metabolic compartmentation, channeling and functional coupling mechanisms, which are central for regulation of the energy fluxes. State of the art of these studies in the new area of Molecular System Bioenergetics is analyzed.

Oxidative stress and inflammation in atrial fibrillation: role in pathogenesis and potential as a therapeutic target

Atrial fibrillation (AF) is one of the most prevalent and vexing cardiovascular conditions. Available treatments for AF based on ion channel blockade are only poorly effective. The fundamental mechanisms that underlie AF are still not clearly understood, and likely vary depending on the etiology of AF. In older individuals with senile AF, likely mechanisms include abnormal calcium cycling, oxidant stress, and deleterious inflammatory responses. Clinical and experimental evidence is provided to support the role of oxidant and inflammatory mechanisms in AF. On the basis of these studies, the prospects of manipulating oxidant and inflammatory pathways as targets for therapeutic intervention are discussed.

Glycolytic oscillations in isolated rabbit ventricular myocytes

Previous studies have shown that glycolysis can oscillate periodically, driven by feedback loops in regulation of key glycolytic enzymes by free ADP and other metabolites. Here we show both theoretically and experimentally in cardiac myocytes that when the capacity of oxidative phosphorylation and the creatine kinase system to buffer the cellular ATP/ADP ratio is suppressed, glycolysis can cause large scale periodic oscillations in cellular ATP levels (0.02-0.067 Hz), monitored from glibenclamide-sensitive changes in action potential duration or intracellular free Mg2+. Action potential duration oscillations originate primarily from glycolysis, since they 1) occur in the presence of cyanide or rotenone, 2) are suppressed by iodoacetate, 3) are accompanied by at most very small mitochondrial membrane potential oscillations, and 4) exhibit an anti-phase relationship to NADH fluorescence. By uncoupling energy supply-demand balance, glycolytic oscillations may promote injury and electrophysiological heterogeneity during acute metabolic stresses, such as acute myocardial ischemia in which both oxidative phosphorylation and creatine kinase activity are inhibited.

E-NTPDases and ecto-5'-nucleotidase expression profile in rat heart left ventricle and the extracellular nucleotide hydrolysis by their nerve terminal endings

In this study, we have identified the E-NTPDase family members and ecto-5'-nucleotidase/CD73 in rat heart left ventricle. Moreover, we characterize the biochemical properties and enzyme activities from synaptosomes of the nerve terminal endings of heart left ventricle. We observe divalent cation-dependent enzymes that presented optimum pH of 8.0 for ATP and ADP hydrolysis, and 9.5 for AMP hydrolysis. The apparent K(M) values are 40 microM, 90 microM and 39 microM and apparent V(max) values are 537, 219 and 111 nmol Pi released/min/mg of protein for ATP, ADP and AMP hydrolysis, respectively. Ouabain, orthovanadate, NEM, lanthanum and levamisole do not affect ATP and ADP hydrolysis in rat cardiac synaptosomes. Oligomycin (2 microg/mL) and sodium azide (0.1 mM), both mitochondrial ATPase inhibitors, inhibit only the ATP hydrolysis. High concentrations of sodium azide and gadolinium chloride show an inhibition on both, ATP and ADP hydrolysis. Suramin inhibit more strongly ATP hydrolysis than ADP hydrolysis whereas Evans blue almost abolish both hydrolysis. AMP hydrolysis is not affected by levamisole and tetramisole, whereas 0.1 mM ammonium molybdate practically abolish the ecto-5'-nucleotidase activity. RT-PCR analysis from left ventricle tissue demonstrate different levels of expression of Entpd1 (Cd39), Entpd2 (Cd39L1), Entpd3 (Cd39L3), Entpd5 (Cd39L4) Entpd6, (Cd39L2) and 5'-NT/CD73. By quantitative real-time PCR we identify the Entpd2 as the enzyme with the highest expression in rat left ventricle. Our results contribute to the understanding about the control of the extracellular nucleotide levels in and cardiac system.

ATP-sensitive potassium channels: metabolic sensing and cardioprotection

The cardiovascular system operates under a wide scale of demands, ranging from conditions of rest to extreme stress. How the heart muscle matches rates of ATP production with utilization is an area of active investigation. ATP-sensitive potassium (K(ATP)) channels serve a critical role in the orchestration of myocardial energetic well-being. K(ATP) channel heteromultimers consist of inwardly-rectifying K(+) channel 6.2 and ATP-binding cassette sulfonylurea receptor 2A that translates local ATP/ADP levels, set by ATPases and phosphotransfer reactions, to the channel pore function. In cells in which the mobility of metabolites between intracellular microdomains is limited, coupling of phosphotransfer pathways with K(ATP) channels permits a high-fidelity transduction of nucleotide fluxes into changes in membrane excitability, matching energy demands with metabolic resources. This K(ATP) channel-dependent optimization of cardiac action potential duration preserves cellular energy balance at varying workloads. Mutations of K(ATP) channels result in disruption of the nucleotide signaling network and generate a stress-vulnerable phenotype with excessive susceptibility to injury, development of cardiomyopathy, and arrhythmia. Solving the mechanisms underlying the integration of K(ATP) channels into the cellular energy network will advance the understanding of endogenous cardioprotection and the development of strategies for the management of cardiovascular injury and disease progression.

A wave of reactive oxygen species (ROS)-induced ROS release in a sea of excitable mitochondria

Once considered simply as the main source of ATP, mitochondria are now implicated in the control of many additional aspects of cell physiology, such as calcium signaling, and pathology, as in injury incurred on ischemia and subsequent reperfusion (I/R). Mitochondrial respiration is ordinarily accompanied by low-level ROS production, but they can respond to elevated ROS concentrations by increasing their own ROS production, a phenomenon termed ROS-induced ROS release (RIRR). Two modes of RIRR have been described. In the first mode of RIRR, enhanced ROS leads to mitochondrial depolarization via activation of the MPTP, yielding a short-lived burst of ROS originating from the mitochondrial electron transport chain (ETC). The second mode of RIRR is MPTP independent but is regulated by the mitochondrial benzodiazepine receptor (mBzR). Increased ROS in the mitochondrion triggers opening of the inner mitochondrial membrane anion channel (IMAC), resulting in a brief increase in ETC-derived ROS. Both modes of RIRR have been shown to transmit localized mitochondrial perturbations throughout the cardiac cell in the form of oscillations or waves but are kinetically distinct and may involve different ROS that serve as second messengers. In this review, we discuss the mechanisms of these different modes of RIRR.

Cardiac system bioenergetics: metabolic basis of the Frank-Starling law

The fundamental principle of cardiac behaviour is described by the Frank-Starling law relating force of contraction during systole with end-diastolic volume. While both work and respiration rates increase linearly with imposed load, the basis of mechano-energetic coupling in heart muscle has remained a long-standing enigma. Here, we highlight advances made in understanding of complex cellular and molecular mechanisms that orchestrate coupling of mitochondrial oxidative phosphorylation with ATP utilization for muscle contraction. Cardiac system bioenergetics critically depends on an interrelated metabolic infrastructure regulating mitochondrial respiration and energy fluxes throughout cellular compartments. The data reviewed indicate the significance of two interrelated systems regulating mitochondrial respiration and energy fluxes in cells: (1) the creatine kinase, adenylate kinase and glycolytic pathways that communicate flux changes generated by cellular ATPases within structurally organized enzymatic modules and networks; and (2) a secondary system based on mitochondrial participation in cellular calcium cycle, which adjusts substrate oxidation and energy-transducing processes to meet increasing cellular energy demands. By conveying energetic signals to metabolic sensors, coupled phosphotransfer reactions provide a high-fidelity regulation of the excitation-contraction cycle. Such integration of energetics with calcium signalling systems provides the basis for 'metabolic pacing', synchronizing the cellular electrical and mechanical activities with energy supply processes.

Effects of hypoxia, anoxia, and metabolic inhibitors on KATP channels in rat femoral artery myocytes

Vascular ATP-sensitive potassium (KATP) channels have an important role in hypoxic vasodilation. Because KATP channel activity depends on intracellular nucleotide concentration, one hypothesis is that hypoxia activates channels by reducing cellular ATP production. However, this has not been rigorously tested. In this study we measured KATP current in response to hypoxia and modulators of cellular metabolism in single smooth muscle cells from the rat femoral artery by using the whole cell patch-clamp technique. KATP current was not activated by exposure of cells to hypoxic solutions (Po2 approximately 35 mmHg). In contrast, voltage-dependent calcium current and the depolarization-induced rise in intracellular calcium concentration ([Ca2+]i) was inhibited by hypoxia. Blocking mitochondrial ATP production by using the ATP synthase inhibitor oligomycin B (3 microM) did not activate current. Blocking glycolytic ATP production by using 2-deoxy-D-glucose (5 mM) also did not activate current. The protonophore carbonyl cyanide m-chlorophenylhydrazone (1 microM) depolarized the mitochondrial membrane potential and activated KATP current. This activation was reversed by oligomycin B, suggesting it occurred as a consequence of mitochondrial ATP consumption by ATP synthase working in reverse mode. Finally, anoxia induced by dithionite (0.5 mM) also depolarized the mitochondrial membrane potential and activated KATP current. Our data show that: 1) anoxia but not hypoxia activates KATP current in femoral artery myocytes; and 2) inhibition of cellular energy production is insufficient to activate KATP current and that energy consumption is required for current activation. These results suggest that vascular KATP channels are not activated during hypoxia via changes in cell metabolism. Furthermore, part of the relaxant effect of hypoxia on rat femoral artery may be mediated by changes in [Ca2+]i through modulation of calcium channel activity.

Glibenclamide depletes ATP in renal proximal tubular cells by interfering with mitochondrial metabolism

Sulfonylurea drugs, like glibenclamide, stimulate insulin secretion by blocking ATP-sensitive potassium channels on pancreatic beta cells. Renal tubular epithelial cells possess a different class of K(ATP) channels with much lower affinities for sulfonylurea drugs, necessitating the use of micromolar glibenclamide concentrations to study these channels. Here, we describe the toxic effects of these concentrations on mitochondrial energy metabolism in freshly isolated renal proximal tubular cells. Glibenclamide, at concentrations of 50 and 100 microM, reduced intracellular ATP levels by 28+/-4 and 53+/-5%, respectively (P<0.01). This decline in ATP could be attributed to a dose-dependent inhibition of mitochondrial respiration. Glibenclamide (10-500 microM) inhibited ADP-stimulated mitochondrial oxygen consumption. In addition to this toxic effect, specific association of radiolabeled and fluorescent glibenclamide to renal mitochondria was found. Association of [(3)H]glibenclamide with renal mitochondria revealed a low-affinity site with a K(D) of 16+/-6 microM and a B(max) of 167+/-29 pmol mg(-1). We conclude that micromolar concentrations of glibenclamide interfere with mitochondrial bioenergetics and, therefore, should be used with care for inhibition of epithelial K(ATP) channels.

Cardioprotection afforded by chronic exercise is mediated by the sarcolemmal, and not the mitochondrial, isoform of the KATP channel in the rat

This study was conducted to examine the role of myocardial ATP-sensitive potassium (K(ATP)) channels in exercise-induced protection from ischaemia-reperfusion (I-R) injury. Female rats were either sedentary (Sed) or exercised for 12 weeks (Tr). Hearts were excised and underwent a 1-2 h regional I-R protocol. Prior to ischaemia, hearts were subjected to pharmacological blockade of the sarcolemmal K(ATP) channel with HMR 1098 (SedHMR and TrHMR), mitochondrial blockade with 5-hydroxydecanoic acid (5HD; Sed5HD and Tr5HD), or perfused with buffer containing no drug (Sed and Tr). Infarct size was significantly smaller in hearts from Tr animals (35.4 +/- 2.3 versus 44.7 +/- 3.0% of the zone at risk for Tr and Sed, respectively). Mitochondrial K(ATP) blockade did not abolish the training-induced infarct size reduction (30.0 +/- 3.4 versus 38.0 +/- 2.6 in Tr5HD and Sed5HD, respectively); however, sarcolemmal K(ATP) blockade completely eradicated the training-induced cardioprotection. Infarct size was 71.2 +/- 3.3 and 64.0 +/- 2.4% of the zone at risk for TrHMR and Sed HMR. The role of sarcolemmal K(ATP) channels in Tr-induced protection was also supported by significant increases in both subunits of the sarcolemmal K(ATP) channel following training. LV developed pressure was better preserved in hearts from Tr animals, and was not influenced by addition of HMR 1098. 5HD decreased pressure development regardless of training status, from 15 min of ischaemia through the duration of the protocol. This mechanical dysfunction was likely to be due to a 5HD-induced increase in myocardial Ca2+ content following I-R. The major findings of the present study are: (1) unlike all other known forms of delayed cardioprotection, infarct sparing following chronic exercise was not abolished by 5HD; (2) pharmacological blockade of the sarcolemmal K(ATP) channel nullified the cardioprotective benefits of exercise training; and (3) increased expression of sarcolemmal K(ATP) channels was observed following chronic training.

A biophysical model of the mitochondrial respiratory system and oxidative phosphorylation

A computational model for the mitochondrial respiratory chain that appropriately balances mass, charge, and free energy transduction is introduced and analyzed based on a previously published set of data measured on isolated cardiac mitochondria. The basic components included in the model are the reactions at complexes I, III, and IV of the electron transport system, ATP synthesis at F₁F₀ ATPase, substrate transporters including adenine nucleotide translocase and the phosphate–hydrogen co-transporter, and cation fluxes across the inner membrane including fluxes through the K⁺/H⁺ antiporter and passive H⁺ and K⁺ permeation. Estimation of 16 adjustable parameter values is based on fitting model simulations to nine independent data curves. The identified model is further validated by comparison to additional datasets measured from mitochondria isolated from rat heart and liver and observed at low oxygen concentration. To obtain reasonable fits to the available data, it is necessary to incorporate inorganic-phosphate-dependent activation of the dehydrogenase activity and the electron transport system. Specifically, it is shown that a model incorporating phosphate-dependent activation of complex III is able to reasonably reproduce the observed data. The resulting validated and verified model provides a foundation for building larger and more complex systems models and investigating complex physiological and pathophysiological interactions in cardiac energetics.

Cells are able to perform tasks that consume energy (such as producing mechanical force in muscle contraction) by using chemical energy delivered in the form of a chemical compound called adenosine triphosphate, or ATP. Two Nobel Prizes were awarded (in 1978 to Peter D. Mitchell and in 1997 to Paul D. Boyer and John E. Walker) for the determination of how ATP is synthesized from the components adenosine diphosphate (ADP) and inorganic phosphate in a subcellular body called the mitochondrion. The operating theory, called the chemiosmotic theory, describes how a driving force called the proton motive force, which arises from the sum of contributions from the electrical potential and the hydrogen ion concentration difference across the mitochondrial inner membrane, is developed by reactions catalyzed by certain enzymes and consumed in generating ATP. Yet, to date, no computer model has successfully described the development and consumption of both the chemical and electrical components of the proton motive force in a thermodynamically balanced simulation. Beard introduces such a model, which is extensively validated based on previously published sets of data obtained on isolated mitochondria. The model is used to test hypotheses about how intracellular respiration is regulated; this model could serve as a foundation for investigating the control of mitochondrial function and for developing larger integrated simulations of cellular metabolism.

Cardioprotective effects of estradiol include the activation of large-conductance Ca(2+)-activated K(+) channels in cardiac mitochondria

The molecular components of the large-conductance Ca(2+)-activated K(+) channels that are functionally expressed in mitochondria (mitoK(Ca)) in cardiac myocytes have not been identified. Our experimental results show that the transcript corresponding to the large-conductance Ca(2+)-activated K(+) channel beta1-subunit (BK-beta1) is substantially expressed in mammalian heart. A yeast two-hybrid assay showed the BK-beta1 protein can interact with a mitochondrial protein, cytochrome c oxidase subunit I (Cco1). Results from immunocytochemical experiments also demonstrated that BK-beta1 interacted with Cco1 and colocalized in rat cardiac mitochondria. Furthermore, 17beta-estradiol, which enhances the activity of the BK channel alpha-subunit only in the presence of the beta1-subunit, significantly increased flavoprotein oxidation in rat ventricle myocytes and decreased the rate of cell death under simulated ischemia. Single-channel recordings from mitochondrial inner membrane indicated that the activity of mitoK(Ca), which had a conductance of approximately 270 pS, was enhanced by 17beta-estradiol and blocked by paxilline. In combination, the present study revealed a new mechanism for the cardioprotective effects of 17beta-estradiol, which include the activation of mitoK(Ca) via the interaction with BK-beta1. BK-beta1 may be an important molecular component that functionally couples with both Cco1 and mitoK(Ca) pore-forming alpha-subunit.

KATP channel: relation with cell metabolism and role in the cardiovascular system

ATP-sensitive potassium channel (K(ATP)) is one kind of inwardly rectifying channel composed of two kinds of subunits: the pore forming subunits and the regulatory subunits. K(ATP) channels exist in the sarcolemmal, mitochondrial and nuclear membranes of various tissues. Cell metabolism regulates K(ATP) gene expression and metabolism products regulate the channel by direct interactions, while K(ATP) controls membrane potentials and regulate cell activities including energy metabolism, apoptosis and gene expression. K(ATP) channels from different cell organelles are linked by some signal molecules and they can respond to common stimulation in a coordinate way. In the cardiovascular system K(ATP) has important functions. The most prominent is that opening of this channel can protect cardiac myocytes against ischemic injuries. The sarcolemmal K(ATP) may provide a basic protection against ischemia by energy sparing, while both the sarcolemmal K(ATP) and mitochondrial K(ATP) channels are necessary for the ischemia preconditioning. K(ATP) channels also have important functions including homeostasis maintenance and vascular tone regulation under physiological conditions. Further elucidation of the role of K(ATP) in the cardiovascular system will help us to regulate cell metabolism or prevent damage caused by abnormal channel functions.

ATP-sensitive K+ channel channel/enzyme multimer: metabolic gating in the heart

Cardiac ATP-sensitive K(+) (K(ATP)) channels, gated by cellular metabolism, are formed by association of the inwardly rectifying potassium channel Kir6.2, the potassium conducting subunit, and SUR2A, the ATP-binding cassette protein that serves as the regulatory subunit. Kir6.2 is the principal site of ATP-induced channel inhibition, while SUR2A regulates K(+) flux through adenine nucleotide binding and catalysis. The ATPase-driven conformations within the regulatory SUR2A subunit of the K(ATP) channel complex have determinate linkage with the states of the channel's pore. The probability and life-time of ATPase-induced SUR2A intermediates, rather than competitive nucleotide binding alone, defines nucleotide-dependent K(ATP) channel gating. Cooperative interaction, instead of independent contribution of individual nucleotide binding domains within the SUR2A subunit, serves a decisive role in defining K(ATP) channel behavior. Integration of K(ATP) channels with the cellular energetic network renders these channel/enzyme heteromultimers high-fidelity metabolic sensors. This vital function is facilitated through phosphotransfer enzyme-mediated transmission of controllable energetic signals. By virtue of coupling with cellular energetic networks and the ability to decode metabolic signals, K(ATP) channels set membrane excitability to match demand for homeostatic maintenance. This new paradigm in the operation of an ion channel multimer is essential in providing the basis for K(ATP) channel function in the cardiac cell, and for understanding genetic defects associated with life-threatening diseases that result from the inability of the channel complex to optimally fulfill its physiological role.

Mechanisms by which KATP channel openers produce acute and delayed cardioprotection

Mitochondria are being increasingly studied for their critical role in cell survival. Multiple diverse signaling pathways have been shown to converge on the K+-sensitive ATP channels as the effectors of cytoprotection against necrosis and apoptosis. The role of potassium channel openers in regulation and transformation of cell membrane excitability, action potential and electrolyte transfer has been extensively studied. Cardiac mitoK(ATP) channels are the key effectors in cardioprotection during ischemic preconditioning, as yet with an undefined mechanism. They have been hypothesized to couple myocardial metabolism with membrane electrical activity and provide an excellent target for drug therapy. A number of K(ATP) channel openers have been characterized for their beneficial effects on the myocardium against ischemic injury. This review updates recent progress in understanding the physiological role of K(ATP) channels in cardiac protection induced by preconditioning and highlights relevant questions and controversies in the light of published data.

Structural and functional adaptations of striated muscles to CK deficiency

In adult mammalian muscle cells, energy consuming processes are mainly localized to the sarcolemma, sarcoplasmic reticulum (SR) and myofibrillar compartments, while energy production occurs within mitochondria or glycolytic complexes. Due to the restricted diffusion of adenine nucleotides near the active sites of ATPases involved in contractile activity and calcium homeostasis, there are multiple local systems that can locally rephosphorylate ADP and provide ATP. The creatine kinase (CK) system, with specific isoenzymes localized within each compartment, efficiently controls local adenylate pools and links energy production and utilization. However, mice lacking one or both of the MM-CK and mi-CK isoforms (CK-/-) are viable and develop almost normal cardiac and skeletal muscle function under the conditions of moderate workload, suggesting adaptations or other mechanisms that may ensure efficient energy transfer. While fixed CK is essentially important, other systems could also be involved as well, such as bound glycolytic enzymes or adenylate kinase. We have shown that, additionally, a direct functional interplay exists between mitochondria and sarcoplasmic reticulum, or between mitochondria and myofilaments in muscle cells, that catalyzes direct energy and signal transfer between organelles. In cardiac cells of CK-/- mice, marked cytoarchitectural modifications were observed, and direct adenine nucleotide channeling between mitochondria and organelles was very effective to rescue SR and myofilament functions. In fast skeletal muscles, increased oxidative capacity also indicates compensatory mechanisms. In mutant mice, mitochondrial capacity increases and a direct energy channeling occurs between mitochondria on one hand and ATP consuming sites on the other. However, these systems appear to be insufficient to fully compensate for the lack of CK at high workload. It can be concluded that local rephosphorylation of ADP is a crucial regulatory point in highly differentiated and organized muscle cells to ensure contractile diversity and efficiency and that the CK system is important to control energy fluxes and energy homeostasis.

Nucleotide-gated KATP channels integrated with creatine and adenylate kinases: amplification, tuning and sensing of energetic signals in the compartmentalized cellular environment

Transmission of energetic signals to membrane sensors, such as the ATP-sensitive K+ (KATP) channel, is vital for cellular adaptation to stress. Yet, cell compartmentation implies diffusional hindrances that hamper direct reception of cytosolic energetic signals. With high intracellular ATP levels, KATP channels may sense not bulk cytosolic, but rather local submembrane nucleotide concentrations set by membrane ATPases and phosphotransfer enzymes. Here, we analyzed the role of adenylate kinase and creatine kinase phosphotransfer reactions in energetic signal transmission over the strong diffusional barrier in the submembrane compartment, and translation of such signals into a nucleotide response detectable by KATP channels. Facilitated diffusion provided by creatine kinase and adenylate kinase phosphotransfer dissipated nucleotide gradients imposed by membrane ATPases, and shunted diffusional restrictions. Energetic signals, simulated as deviation of bulk ATP from its basal level, were amplified into an augmented nucleotide response in the submembrane space due to failure under stress of creatine kinase to facilitate nucleotide diffusion. Tuning of creatine kinase-dependent amplification of the nucleotide response was provided by adenylate kinase capable of adjusting the ATP/ADP ratio in the submembrane compartment securing adequate KATP channel response in accord with cellular metabolic demand. Thus, complementation between creatine kinase and adenylate kinase systems, here predicted by modeling and further supported experimentally, provides a mechanistic basis for metabolic sensor function governed by alterations in intracellular phosphotransfer fluxes.

Acute desensitization of GIRK current in rat atrial myocytes is related to K+ current flow

We have investigated the acute desensitization of acetylcholine-activated GIRK current (I(K(ACh))) in cultured adult rat atrial myocytes. Acute desensitization of I(K(ACh)) is observed as a partial relaxation of current with a half-time of < 5 s when muscarinic M2 receptors are stimulated by a high concentration (> 2 micromol l(-1)) of ACh. Under this condition experimental manoeuvres that cause a decrease in the amplitude of I(K(ACh)), such as partial block of M2 receptors by atropine, intracellular loading with GDP-beta-S, or exposure to Ba2+, caused a reduction in desensitization. Acute desensitization was also identified as a decrease in current amplitude and a blunting of the response to saturating [ACh] (20 micromol l(-1)) when the current had been partially activated by a low concentration of ACh or by stimulation of adenosine A1 receptors. A reduction in current analogous to acute desensitization was observed when ATP-dependent K+ current (I(K(ATP))) was activated either by mitochondrial uncoupling using 2,4-dinitrophenole (DNP) or by the channel opener rilmakalim. Adenovirus-driven overexpression of Kir2.1, a subunit of constitutively active inwardly rectifying K+ channels, resulted in a large Ba2+-sensitive background K+ current and a dramatic reduction of ACh-activated current. Adenovirus-driven overexpression of GIRK4 (Kir3.4) subunits resulted in an increased agonist-independent GIRK current paralleled by a reduction in I(K(ACh)) and removal of the desensitizing component. These data indicate that acute desensitization depends on K+ current flow, independent of the K+ channel species, suggesting that it reflects a reduction in electrochemical driving force rather than a bona fide signalling mechanism. This is supported by the observation that desensitization is paralleled by a significant negative shift in reversal potential of I(K(ACh)). Since the ACh-induced hyperpolarization shows comparable desensitization properties as I(K(ACh)), this novel current-dependent desensitization is a physiologically relevant process, shaping the time course of parasympathetic bradycardia.

Role of reactive oxygen species in ischemic preconditioning of subcellular organelles in the heart

Ischemic preconditioning (IPC) is an endogenous adaptive mechanism and is manifested by early and delayed phases of cardioprotection. Brief episodes of ischemia-reperfusion during IPC cause some subtle functional and structural alterations in sarcolemma, mitochondria, sarcoplasmic reticulum, myofibrils, glycocalyx, as well as nucleus, which render these subcellular organelles resistant to subsequent sustained ischemia-reperfusion insult. These changes occur in functional groups of various receptors, cation transporters, cation channels, and contractile and other proteins, and may explain the initial effects of IPC. On the other hand, induction of various transcriptional factors occurs to alter gene expression and structural changes in subcellular organelles and may be responsible for the delayed effects of IPC. Reactive oxygen species (ROS), which are formed during the IPC period, may cause these changes directly and indirectly and act as a trigger of IPC-induced cardioprotection. As ROS may be one of the several triggers proposed for IPC, this discussion is focused on the current knowledge of both ROS-dependent and ROS-independent mechanisms of IPC. Furthermore, some events, which are related to functional preservation of subcellular organelles, are described for a better understanding of the IPC phenomenon.

Anaesthetics and cardiac preconditioning. Part I. Signalling and cytoprotective mechanisms

Cardiac preconditioning represents the most potent and consistently reproducible method of rescuing heart tissue from undergoing irreversible ischaemic damage. Major milestones regarding the elucidation of this phenomenon have been passed in the last two decades. The signalling and amplification cascades from the preconditioning stimulus, be it ischaemic or pharmacological, to the putative end-effectors, including the mechanisms involved in cellular protection, are discussed in this review. Volatile anaesthetics and opioids effectively elicit pharmacological preconditioning. Anaesthetic-induced preconditioning and ischaemic preconditioning share many fundamental steps, including activation of G-protein-coupled receptors, multiple protein kinases and ATP-sensitive potassium channels (K(ATP) channels). Volatile anaesthetics prime the activation of the sarcolemmal and mitochondrial K(ATP) channels, the putative end-effectors of preconditioning, by stimulation of adenosine receptors and subsequent activation of protein kinase C (PKC) and by increased formation of nitric oxide and free oxygen radicals. In the case of desflurane, stimulation of alpha- and beta-adrenergic receptors may also be of importance. Similarly, opioids activate delta- and kappa-opioid receptors, and this also leads to PKC activation. Activated PKC acts as an amplifier of the preconditioning stimulus and stabilizes, by phosphorylation, the open state of the mitochondrial K(ATP) channel (the main end-effector in anaesthetic preconditioning) and the sarcolemmal K(ATP) channel. The opening of K(ATP) channels ultimately elicits cytoprotection by decreasing cytosolic and mitochondrial Ca(2+) overload.