The Na+/K+ pump of cardiac Purkinje cells is preferentially fuelled by glycolytic ATP production

Loss of Cardiac PFKFB2 Drives Metabolic, Functional, and Electrophysiological Remodeling in the Heart

BACKGROUND

Phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2) is a critical glycolytic regulator responsible for upregulation of glycolysis in response to insulin and adrenergic signaling. PFKFB2, the cardiac isoform of PFK-2, is degraded in the heart in the absence of insulin signaling, contributing to diabetes-induced cardiac metabolic inflexibility. However, previous studies have not examined how the loss of PFKFB2 affects global cardiac metabolism and function.

METHODS AND RESULTS

To address this, we have generated a mouse model with a cardiomyocyte-specific knockout of PFKFB2 (cKO). Using 9-month-old cKO and control mice, we characterized the impacts of PFKFB2 on cardiac metabolism, function, and electrophysiology. cKO mice have a shortened life span of 9 months. Metabolically, cKO mice are characterized by increased glycolytic enzyme abundance and pyruvate dehydrogenase activity, as well as decreased mitochondrial abundance and beta oxidation, suggesting a shift toward glucose metabolism. This was supported by a decrease in the ratio of palmitoyl carnitine to pyruvate-dependent mitochondrial respiration in cKO relative to control animals. Metabolomic, proteomic, and Western blot data support the activation of ancillary glucose metabolism, including pentose phosphate and hexosamine biosynthesis pathways. Physiologically, cKO animals exhibited impaired systolic function and left ventricular dilation, represented by reduced fractional shortening and increased left ventricular internal diameter, respectively. This was accompanied by electrophysiological alterations including increased QT interval and other metrics of delayed ventricular conduction.

CONCLUSIONS

Loss of PFKFB2 results in metabolic remodeling marked by cardiac ancillary pathway activation. This could delineate an underpinning of pathologic changes to mechanical and electrical function in the heart.

Loss of cardiac PFKFB2 drives Metabolic, Functional, and Electrophysiological Remodeling in the Heart

Background

Phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2) is a critical glycolytic regulator responsible for upregulation of glycolysis in response to insulin and adrenergic signaling. PFKFB2, the cardiac isoform of PFK-2, is degraded in the heart in the absence of insulin signaling, contributing to diabetes-induced cardiac metabolic inflexibility. However, previous studies have not examined how the loss of PFKFB2 affects global cardiac metabolism and function.

Methods

To address this, we have generated a mouse model with a cardiomyocyte-specific knockout of PFKFB2 (cKO). Using 9-month-old cKO and control (CON) mice, we characterized impacts of PFKFB2 on cardiac metabolism, function, and electrophysiology.

Results

cKO mice have a shortened lifespan of 9 months. Metabolically, cKO mice are characterized by increased glycolytic enzyme abundance and pyruvate dehydrogenase (PDH) activity, as well as decreased mitochondrial abundance and beta oxidation, suggesting a shift toward glucose metabolism. This was supported by a decrease in the ratio of palmitoyl carnitine to pyruvate-dependent mitochondrial respiration in cKO relative to CON animals. Metabolomic, proteomic, and western blot data support the activation of ancillary glucose metabolism, including pentose phosphate and hexosamine biosynthesis pathways. Physiologically, cKO animals exhibited impaired systolic function and left ventricular (LV) dilation, represented by reduced fractional shortening and increased LV internal diameter, respectively. This was accompanied by electrophysiological alterations including increased QT interval and other metrics of delayed ventricular conduction.

Conclusions

Loss of PFKFB2 results in metabolic remodeling marked by cardiac ancillary pathway activation. This could delineate an underpinning of pathologic changes to mechanical and electrical function in the heart.

Clinical Perspective

What is New?: We have generated a novel cardiomyocyte-specific knockout model of PFKFB2, the cardiac isoform of the primary glycolytic regulator Phosphofructokinase-2 (cKO).The cKO model demonstrates that loss of cardiac PFKFB2 drives metabolic reprogramming and shunting of glucose metabolites to ancillary metabolic pathways.The loss of cardiac PFKFB2 promotes electrophysiological and functional remodeling in the cKO heart.What are the Clinical Implications?: PFKFB2 is degraded in the absence of insulin signaling, making its loss particularly relevant to diabetes and the pathophysiology of diabetic cardiomyopathy.Changes which we observe in the cKO model are consistent with those often observed in diabetes and heart failure of other etiologies.Defining PFKFB2 loss as a driver of cardiac pathogenesis identifies it as a target for future investigation and potential therapeutic intervention.

Specific ATPases drive compartmentalized glycogen utilization in rat skeletal muscle

Glycogen is a key energy substrate in excitable tissue, including in skeletal muscle fibers where it also contributes to local energy production. Transmission electron microscopy imaging has revealed the existence of a heterogenic subcellular distribution of three distinct glycogen pools in skeletal muscle, which are thought to reflect the requirements for local energy stores at the subcellular level. Here, we show that the three main energy-consuming ATPases in skeletal muscles (Ca2+, Na+,K+, and myosin ATPases) utilize different local pools of glycogen. These results clearly demonstrate compartmentalized glycogen metabolism and emphasize that spatially distinct pools of glycogen particles act as energy substrate for separated energy requiring processes, suggesting a new model for understanding glycogen metabolism in working muscles, muscle fatigue, and metabolic disorders. These observations suggest that the distinct glycogen pools can regulate the functional state of mammalian muscle cells and have important implications for the understanding of how the balance between ATP utilization and ATP production is regulated at the cellular level in general and in skeletal muscle fibers in particular.

Glucose-Sparing Action of Ketones Boosts Functions Exclusive to Glucose in the Brain

The ketogenic diet (KD) has been successfully used for a century for treating refractory epilepsy and is currently seen as one of the few viable approaches to the treatment of a plethora of metabolic and neurodegenerative diseases. Empirical evidence notwithstanding, there is still no universal understanding of KD mechanism(s). An important fact is that the brain is capable of using ketone bodies for fuel. Another critical point is that glucose's functions span beyond its role as an energy substrate, and in most of these functions, glucose is irreplaceable. By acting as a supplementary fuel, ketone bodies may free up glucose for its other crucial and exclusive function. We propose that this glucose-sparing effect of ketone bodies may underlie the effectiveness of KD in epilepsy and major neurodegenerative diseases, which are all characterized by brain glucose hypometabolism.

Myocyte [Na+]i Dysregulation in Heart Failure and Diabetic Cardiomyopathy

By controlling the function of various sarcolemmal and mitochondrial ion transporters, intracellular Na⁺ concentration ([Na⁺]_i) regulates Ca²⁺ cycling, electrical activity, the matching of energy supply and demand, and oxidative stress in cardiac myocytes. Thus, maintenance of myocyte Na⁺ homeostasis is vital for preserving the electrical and contractile activity of the heart. [Na⁺]_i is set by the balance between the passive Na⁺ entry through numerous pathways and the pumping of Na⁺ out of the cell by the Na⁺/K⁺-ATPase. This equilibrium is perturbed in heart failure, resulting in higher [Na⁺]_i. More recent studies have revealed that [Na⁺]_i is also increased in myocytes from diabetic hearts. Elevated [Na⁺]_i causes oxidative stress and augments the sarcoplasmic reticulum Ca²⁺ leak, thus amplifying the risk for arrhythmias and promoting heart dysfunction. This mini-review compares and contrasts the alterations in Na⁺ extrusion and/or Na⁺ uptake that underlie the [Na⁺]_i increase in heart failure and diabetes, with a particular emphasis on the emerging role of Na⁺ - glucose cotransporters in the diabetic heart.

The vicious circle of hypometabolism in neurodegenerative diseases: Ways and mechanisms of metabolic correction

Hypometabolism, characterized by decreased brain glucose consumption, is a common feature of many neurodegenerative diseases. Initial hypometabolic brain state, created by characteristic risk factors, may predispose the brain to acquired epilepsy and sporadic Alzheimer's and Parkinson's diseases, which are the focus of this review. Analysis of available data suggests that deficient glucose metabolism is likely a primary initiating factor for these diseases, and that resulting neuronal dysfunction further promotes the metabolic imbalance, establishing an effective positive feedback loop and a downward spiral of disease progression. Therefore, metabolic correction leading to the normalization of abnormalities in glucose metabolism may be an efficient tool to treat the neurological disorders by counteracting their primary pathological mechanisms. Published and preliminary experimental results on this approach for treating Alzheimer's disease and epilepsy models support the efficacy of metabolic correction, confirming the highly promising nature of the strategy. © 2017 Wiley Periodicals, Inc.

Long-Term β-Blocker Therapy Decreases Blood Lactate Concentration in Severely Septic Patients

OBJECTIVES

Measurement of blood lactate concentration in the early management of sepsis is an important step in severity assessment. High blood lactate levels in the early phase of sepsis have classically been thought to be related to tissue hypoxia, but other factors could intervene. We hypothesized that the activation of glycolysis through β-adrenergic stimulation by endogenous catecholamines plays an important role in lactate production and that long-term β-blocker therapy could affect the lactate concentration in patients with severe sepsis and septic shock.

DESIGN

Retrospective cohort study.

SETTING

Emergency department.

PATIENTS

Two hundred sixty patients with severe sepsis or septic shock were included. Twenty-five percent were previously treated with β-blockers.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

We recorded initial vital signs, the source of infection, mortality at 28 days, blood lactate concentration, and Predisposition Insult Response of Organ failure and Sequential Organ Failure Assessment scores using an electronic database. Blood lactate concentration was significantly lower in patients previously treated with β-blockers (3.9 ± 2.3 mmol/L vs 5.6 ± 3.6 mmol/L; p < 0.001). This difference was still significant after controlling for mortality (p < 0.005), for the level of the Predisposition Insult Response of Organ failure (p < 0.05) and Sequential Organ Failure Assessment (p < 0.05) scores, and for the source of infection (p < 0.05). Nearly four times more patients treated with β-blockers had normal blood lactate levels (p< 0.001). Only two factors were significantly and independently associated with normal blood lactate concentration during severe sepsis and septic shock: survival (p = 0.03) and β-blocker therapy (p = 0.01).

CONCLUSIONS

Long-term β-blocker therapy decreases blood lactate concentration of severely ill septic patients at presentation. We conclude that the use of blood lactate measurement as a triage tool in the initial assessment of septic patients with β-blocker therapy may underestimate the severity of the sepsis.

Separation of metabolic supply and demand: aerobic glycolysis as a normal physiological response to fluctuating energetic demands in the membrane

Background

Cancer cells, and a variety of normal cells, exhibit aerobic glycolysis, high rates of glucose fermentation in the presence of normal oxygen concentrations, also known as the Warburg effect. This metabolism is considered abnormal because it violates the standard model of cellular energy production that assumes glucose metabolism is predominantly governed by oxygen concentrations and, therefore, fermentative glycolysis is an emergency back-up for periods of hypoxia. Though several hypotheses have been proposed for the origin of aerobic glycolysis, its biological basis in cancer and normal cells is still not well understood.

Results

We examined changes in glucose metabolism following perturbations in membrane activity in different normal and tumor cell lines and found that inhibition or activation of pumps on the cell membrane led to reduction or increase in glycolysis, respectively, while oxidative phosphorylation remained unchanged. Computational simulations demonstrated that these findings are consistent with a new model of normal physiological cellular metabolism in which efficient mitochondrial oxidative phosphorylation supplies chronic energy demand primarily for macromolecule synthesis and glycolysis is necessary to supply rapid energy demands primarily to support membrane pumps. A specific model prediction was that the spatial distribution of ATP-producing enzymes in the glycolytic pathway must be primarily localized adjacent to the cell membrane, while mitochondria should be predominantly peri-nuclear. The predictions were confirmed experimentally.

Conclusions

Our results show that glycolytic metabolism serves a critical physiological function under normoxic conditions by responding to rapid energetic demand, mainly from membrane transport activities, even in the presence of oxygen. This supports a new model for glucose metabolism in which glycolysis and oxidative phosphorylation supply different types of energy demand. Cells use efficient but slow-responding aerobic metabolism to meet baseline, steady energy demand and glycolytic metabolism, which is inefficient but can rapidly increase adenosine triphosphate (ATP) production, to meet short-timescale energy demands, mainly from membrane transport activities. In this model, the origin of the Warburg effect in cancer cells and aerobic glycolysis in general represents a normal physiological function due to enhanced energy demand for membrane transporters activity required for cell division, growth, and migration.

Intracellular Na⁺ and cardiac metabolism

In heart failure, alterations of excitation-contraction underlie contractile dysfunction. One important defect is an elevation of the intracellular Na(+) concentration in cardiac myocytes ([Na(+)]i), which has an important impact on cytosolic and mitochondrial Ca(2+) homeostasis. While elevated [Na(+)]i is thought to compensate for decreased Ca(2+) load of the sarcoplasmic reticulum (SR), it yet negatively affects energy supply-and-demand matching and can even induce mitochondrial oxidative stress. Here, we review the mechanisms underlying these pathophysiological changes. The chain of events may constitute a vicious cycle of ion dysregulation, oxidative stress and energetic deficit, resembling characteristic cellular deficits that are considered key hallmarks of the failing heart. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

Compartmentation of GAPDH

The concept of the cytosol as a space that contains discrete zones of metabolites is discussed relative to the contribution of GAPDH. GAPDH is directed to very specific cell compartments. This chapter describes the utilization of GAPDH's enzymatic function for focal demands (i.e. ATP/ADP and NAD(+)/NADH), and offers a speculative role for GAPDH as perhaps moderating local concentrations of inorganic phosphate and hydrogen ions (i.e. co-substrate and co-product of the glycolytic reaction, respectively). Where known, the structural features of the binding between GAPDH and the compartment components are discussed. The nuances, which are associated with the intracellular distribution of GAPDH, appear to be specific to the cell-type, particularly with regards to the various plasma membrane proteins to which GAPDH binds. The chapter includes discussion on the curious observation of GAPDH being localized to the external surface of the plasma membrane in a human cell type. The default perspective has been that GAPDH localization is synonymous with compartmentation of glycolytic energy. The chapter discusses GAPDH translocation to the nucleus and to non-nuclear cellular structures, emphasizing its glycolytic function. Nevertheless, it is becoming clear that alternate functions of GAPDH play a role in compartmentation, particularly in the translocation to the nucleus.

Effects of β₂-agonists on force during and following anoxia in rat extensor digitorum longus muscle

Electrical stimulation of isolated muscles may lead to membrane depolarization, gain of Na(+), loss of K(+) and fatigue. These effects can be counteracted with β(2)-agonists possibly via activation of the Na(+)-K(+) pumps. Anoxia induces loss of force; however, it is not known whether β(2)-agonists affect force and ion homeostasis in anoxic muscles. In the present study isolated rat extensor digitorum longus (EDL) muscles exposed to anoxia showed a considerable loss of force, which was markedly reduced by the β(2)-agonists salbutamol (10(-6) M) and terbutaline (10(-6) M). Intermittent stimulation (15-30 min) clearly increased loss of force during anoxia and reduced force recovery during reoxygenation. The β(2)-agonists salbutamol (10(-7)-10(-5) M) and salmeterol (10(-6) M) improved force development during anoxia (25%) and force recovery during reoxygenation (55-262%). The effects of salbutamol on force recovery were prevented by blocking the Na(+)-K(+) pumps with ouabain or by blocking glycolysis with 2-deoxyglucose. Dibutyryl cAMP (1 mM) or theophylline (1 mM) also improved force recovery remarkably. In anoxic muscles, salbutamol decreased intracellular Na(+) and increased (86)Rb uptake and K(+) content, indicating stimulation of the Na(+)-K(+) pumps. In fatigued muscles salbutamol induced recovery of excitability. Thus β(2)-agonists reduce the anoxia-induced loss of force, leading to partial force recovery. These data strongly suggest that this effect is mediated by cAMP stimulation of the Na(+)-K(+) pumps and that it is not related to recovery of energy status (PCr, ATP, lactate).

Creatine kinase B deficient neurons exhibit an increased fraction of motile mitochondria

BACKGROUND

Neurons require an elaborate system of intracellular transport to distribute cargo throughout axonal and dendritic projections. Active anterograde and retrograde transport of mitochondria serves in local energy distribution, but at the same time also requires input of ATP. Here we studied whether brain-type creatine kinase (CK-B), a key enzyme for high-energy phosphoryl transfer between ATP and CrP in brain, has an intermediary role in the reciprocal coordination between mitochondrial motility and energy distribution. Therefore, we analysed the impact of brain-type creatine kinase (CK-B) deficiency on transport activity and velocity of mitochondria in primary murine neurons and made a comparison to the fate of amyloid precursor protein (APP) cargo in these cells, using live cell imaging.

RESULTS

Comparison of average and maximum transport velocities and global transport activity showed that CK-B deficiency had no effect on speed of movement of mitochondria or APP cargo, but that the fraction of motile mitochondria was significantly increased by 36% in neurons derived from CK-B knockout mice. The percentage of motile APP vesicles was not altered.

CONCLUSION

CK-B activity does not directly couple to motor protein activity but cells without the enzyme increase the number of motile mitochondria, possibly as an adaptational strategy aimed to enhance mitochondrial distribution versatility in order to compensate for loss of efficiency in the cellular network for ATP distribution.

Cyclic GMP signaling in cardiomyocytes modulates fatty acid trafficking and prevents triglyceride accumulation

While the balance between carbohydrates and fatty acids for energy production appears to be crucial for cardiac homeostasis, much remains to be learned about the molecular mechanisms underlying this relationship. Given the reported benefits of cGMP signaling on the myocardium, we investigated the impact of its chronic activation on cardiac energy metabolism using mice overexpressing a constitutively active cytoplasmic guanylate cyclase (GC(+/0)) in cardiomyocytes. Ex vivo working GC(+/0) heart perfusions with (13)C-labeled substrates revealed an altered pattern of exogenous substrate fuel selection compared to controls, namely a 38+/-9% lower contribution of exogenous fatty acids to acetyl-CoA formation, while that of carbohydrates remains unchanged despite a two-fold increase in glycolysis. The lower contribution of exogenous fatty acids to energy production is not associated with changes in energy demand or supply (contractile function, oxygen consumption, tissue acetyl-CoA or CoA levels, citric acid cycle flux rate) or in the regulation of beta-oxidation (acetyl-CoA carboxylase activity, tissue malonyl-CoA levels). However, GC(+/0) hearts show a two-fold increase in the incorporation of exogenous oleate into triglycerides. Furthermore, the following molecular data are consistent with a concomitant increase in triglyceride hydrolysis: (i) increased abundance of hormone sensitive lipase (HSL) protein (24+/-11%) and mRNA (22+/-4%) as well as (ii) several phosphorylation events related to HSL inhibitory (AMPK) and activation (ERK 1/2) sites, which should contribute to enhance its activity. These changes in exogenous fatty acid trafficking in GC(+/0) hearts appear to be functionally relevant, as demonstrated by their resistance to fasting-induced triglyceride accumulation. While the documented metabolic profile of GC(+/0) mouse hearts is partly reminiscent of hypertrophied hearts, the observed changes in lipid trafficking have not been previously documented, and may be part of the molecular mechanism underlying the benefits of cGMP signaling on the myocardium.

Na+-K+ pumps in the transverse tubular system of skeletal muscle fibers preferentially use ATP from glycolysis

The Na(+)-K(+) pumps in the transverse tubular (T) system of a muscle fiber play a vital role keeping K(+) concentration in the T-system sufficiently low during activity to prevent chronic depolarization and consequent loss of excitability. These Na(+)-K(+) pumps are located in the triad junction, the key transduction zone controlling excitation-contraction (EC) coupling, a region rich in glycolytic enzymes and likely having high localized ATP usage and limited substrate diffusion. This study examined whether Na(+)-K(+) pump function is dependent on ATP derived via the glycolytic pathway locally within the triad region. Single fibers from rat fast-twitch muscle were mechanically skinned, sealing off the T-system but retaining normal EC coupling. Intracellular composition was set by the bathing solution and action potentials (APs) triggered in the T-system, eliciting intracellular Ca(2+) release and twitch and tetanic force responses. Conditions were selected such that increased Na(+)-K(+) pump function could be detected from the consequent increase in T-system polarization and resultant faster rate of AP repriming. Na(+)-K(+) pump function was not adequately supported by maintaining cytoplasmic ATP concentration at its normal resting level ( approximately 8 mM), even with 10 or 40 mM creatine phosphate present. Addition of as little as 1 mM phospho(enol)pyruvate resulted in a marked increase in Na(+)-K(+) pump function, supported by endogenous pyruvate kinase bound within the triad. These results demonstrate that the triad junction is a highly restricted microenvironment, where glycolytic resynthesis of ATP is critical to meet the high demand of the Na(+)-K(+) pump and maintain muscle excitability.

The importance of limitations in aerobic metabolism, glycolysis, and membrane excitability for the development of high-frequency fatigue in isolated rat soleus muscle

We investigated the role of limitations in aerobic metabolism, glycolysis, and membrane excitability for development of high-frequency fatigue in isolated rat soleus muscle. Muscles mounted on force transducers were incubated in buffer bubbled with 5% CO2 and either 95% O2 (oxygenated) or 95% N2 (anoxic) and stimulated at 60 Hz continuously for 30–120 s or intermittently for 120 s. Cyanide (2 mM) and 2-deoxyglucose (10 mM) were used to inhibit aerobic metabolism and both glycolysis and aerobic metabolism, respectively. Excitability was reduced by carbacholine (10 μM), a nicotinic ACh receptor agonist, or ouabain (10 μM), an Na+-K+ pump inhibitor. Membrane excitability was measured by recording M waves. Intracellular Na+ and K+ contents and membrane potentials were measured by flame photometry and microelectrodes, respectively. During 120 s of continuous stimulation, oxygenated and anoxic muscles showed the same force loss. In oxygenated muscles, cyanide did not alter force loss for up to 90 s, whereas 2-deoxyglucose increased force loss (by 19–69%; P < 0.01) from 14 s of stimulation. In oxygenated muscles, 60 s of stimulation reduced force, M wave area, and amplitude by 70–90% ( P < 0.001). Carbacholine or ouabain increased intracellular Na+ content ( P < 0.001), induced a 7- to 8-mV membrane depolarization ( P < 0.001), and accelerated the rate of force loss (by 250–414%) during 30 s of stimulation ( P < 0.001). Similar effects were seen with intermittent stimulation. In conclusion, limitations in glycolysis and subsequently also in aerobic metabolism, as well as membrane excitability but not aerobic metabolism alone, appear to play an important role in the development of high-frequency fatigue in isolated rat soleus muscle.

Role of glycolytically generated ATP for CaMKII-mediated regulation of intracellular Ca2+ signaling in bovine vascular endothelial cells

The role of glycolytically generated ATP in Ca(2+)/calmodulin-dependent kinase II (CaMKII)-mediated regulation of intracellular Ca(2+) signaling was examined in cultured calf pulmonary artery endothelial (CPAE) cells. Exposure of cells (extracellular Ca(2+) concentration = 2 mM) to glycolytic inhibitors 2-deoxy-D-glucose (2-DG), pyruvate (pyr) + beta-hydroxybutyrate (beta-HB), or iodoacetic acid (IAA) caused an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)). CaMKII inhibitors (KN-93, W-7) triggered a similar increase of [Ca(2+)](i). The rise of [Ca(2+)](i) was characterized by a transient spike followed by a small sustained plateau of elevated [Ca(2+)](i). In the absence of extracellular Ca(2+) 2-DG caused an increase in [Ca(2+)](i), suggesting that inhibition of glycolysis directly triggered release of Ca(2+) from intracellular endoplasmic reticulum (ER) Ca(2+) stores. The inositol-1,4,5-trisphosphate receptor (IP(3)R) inhibitor 2-aminoethoxydiphenyl borate abolished the KN-93- and 2-DG-induced Ca(2+) response. Ca(2+) release was initiated in peripheral cytoplasmic processes from which activation propagated as a [Ca(2+)](i) wave toward the central region of the cell. Focal application of 2-DG resulted in spatially confined elevations of [Ca(2+)](i). Propagating [Ca(2+)](i) waves were preceded by [Ca(2+)](i) oscillations and small, highly localized elevations of [Ca(2+)](i) (Ca(2+) puffs). Inhibition of glycolysis with 2-DG reduced the KN-93-induced Ca(2+) response, and vice versa during inhibition of CaMKII 2-DG-induced Ca(2+) release was attenuated. Similar results were obtained with pyr + beta-HB and W-7. Furthermore, 2-DG and IAA caused a rapid increase of intracellular Mg(2+) concentration, indicating a concomitant drop of cellular ATP levels. In conclusion, CaMKII exerts a profound inhibition of ER Ca(2+) release in CPAE cells, which is mediated by glycolytically generated ATP, possibly through ATP-dependent phosphorylation of the IP(3)R.

Cytosolic energy reserves determine the effect of glycolytic sugar phosphates on sarcoplasmic reticulum Ca2+ release in cat ventricular myocytes

Localization of glycolytic enzymes in close proximity to Ca(2+) transport systems of the sarcoplasmic reticulum (SR) in cardiac cells suggests an important functional role for glycolysis in intracellular [Ca(2+)] regulation and, consequently, excitation-contraction coupling. Here, we investigated the mechanisms of regulation of SR Ca(2+) release by glycolytic sugar phosphate intermediates in cat ventricular myocytes. Experiments with permeabilized myocytes revealed that with normal cytosolic energy reserves (mm: ATP 5, ADP 0.01, phosphocreatine (CrP) 10) fructose-1,6-bisphosphate (FBP; 1 mm) and fructose-6-phosphate (F6P; 1 mm) caused a transient increase of Ca(2+) spark frequency by 62 and 42%, respectively. This effect of sugar phosphates was associated with a 13% decrease in SR Ca(2+) load. Pretreatment of the cells with an inhibitor of glycolysis, iodoacetate (IAA; 0.5 mm), did not prevent the effects of FBP and F6P on Ca(2+) sparks. Recording of single ryanodine receptor (RyR) channel activity indicated that FBP and F6P significantly increased RyR open probability. Reduction of cytosolic energy reserves decreased Ca(2+) spark activity. Increasing [ADP] to 0.4 mm or removal of CrP ([ATP] was kept constant) caused a slowly developing decrease of Ca(2+) spark frequency by 29 and 42%, respectively. Changing [ADP] and [CrP] simultaneously decreased Ca(2+) spark frequency by 66%. This inhibition of Ca(2+) sparks was associated with a 40% decrease in SR Ca(2+) load. The subsequent addition of FBP (1 mm) partially restored Ca(2+) spark frequency and SR Ca(2+) load. This recovery of Ca(2+) sparks was blocked completely by IAA. These data suggest that at physiological ATP, ADP and CrP levels accumulation of sugar phosphates from glycolysis can stimulate SR Ca(2+) release. This effect does not require the activity of downstream glycolytic enzymes, but rather is the result of direct activation of RyRs. However, under conditions associated with depletion of cellular energy reserves (e.g. myocardial ischaemia), ATP generated from glycolysis may play an important role in maintaining myocardial Ca(2+) homeostasis by improving SR Ca(2+) uptake.

Mitochondrial modulation of Ca2+ -induced Ca2+ -release in rat sensory neurons

Ca2+ -induced Ca2+ -release (CICR) from ryanodine-sensitive Ca2+ stores provides a mechanism to amplify and propagate a transient increase in intracellular calcium concentration ([Ca2+]i). A subset of rat dorsal root ganglion neurons in culture exhibited regenerative CICR when sensitized by caffeine. [Ca2+]i oscillated in the maintained presence of 5 mM caffeine and 25 mM K+. Here, CICR oscillations were used to study the complex interplay between Ca2+ regulatory mechanisms at the cellular level. Oscillations depended on Ca2+ uptake and release from the endoplasmic reticulum (ER) and Ca2+ influx across the plasma membrane because cyclopiazonic acid, ryanodine, and removal of extracellular Ca2+ terminated oscillations. Increasing caffeine concentration decreased the threshold for action potential-evoked CICR and increased oscillation frequency. Mitochondria regulated CICR by providing ATP and buffering [Ca2+]i. Treatment with the ATP synthase inhibitor, oligomycin B, decreased oscillation frequency. When ATP concentration was held constant by recording in the whole cell patch-clamp configuration, oligomycin no longer affected oscillation frequency. Aerobically derived ATP modulated CICR by regulating the rate of Ca2+ sequestration by the ER Ca2+ pump. Neither CICR threshold nor Ca2+ clearance by the plasma membrane Ca2+ pump were affected by inhibition of aerobic metabolism. Uncoupling electron transport with carbonyl cyanide p-trifluoromethoxy-phenyl-hydrazone or inhibiting mitochondrial Na+/Ca2+ exchange with CGP37157 revealed that mitochondrial buffering of [Ca2+]i slowed oscillation frequency, decreased spike amplitude, and increased spike width. These findings illustrate the interdependence of energy metabolism and Ca2+ signaling that results from the complex interaction between the mitochondrion and the ER in sensory neurons.

Myocardial hypoperfusion/reperfusion tolerance with exercise training in hypertension

The purpose of this study was to examine whether exercise training, superimposed on compensated-concentric hypertrophy, could increase myocardial hypoperfusion-reperfusion (H/R) tolerance. Female Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) (age: 4 mo; N = 40) were placed into a sedentary (SED) or exercise training (TRD) group (treadmill running; 25 m/min, 1 h/day, 5 days/wk for 16 wk). Four groups were studied: WKY-SED ( n = 10), WKY-TRD ( n = 10), SHR-SED ( n = 10), and SHR-TRD ( n = 10). Blood pressure and heart rate were determined, and in vitro isolated heart performance was measured with a retrogradely perfused, Langendorff isovolumic preparation. The H/R protocol consisted of a 75% reduction in coronary flow for 17 min followed by 30 min of reperfusion. Although the rate-pressure product was significantly elevated in SHR relative to WKY, training-induced bradycardia reduced the rate-pressure product in SHR-TRD ( P < 0.05) without an attenuation in systolic blood pressure. Heart-to-body weight ratio was greater in both groups of SHR vs. WKY-SED ( P < 0.001). Absolute and relative myocardial tolerance to H/R was greater in WKY-TRD and both groups of SHR relative to WKY-SED ( P < 0.05). Endurance training superimposed on hypertension-induced compensated hypertrophy conferred no further cardioprotection to H/R. Postreperfusion 72-kDa heat shock protein abundance was enhanced in WKY-TRD and both groups of SHR relative to WKY-SED ( P < 0.05) and was highly correlated with absolute left ventricular functional recovery during reperfusion ( R2= 0.86, P < 0.0001). These data suggest that both compensated hypertrophy associated with short-term hypertension and endurance training individually improved H/R and that increased postreperfusion 72-kDa heat shock protein abundance was, in part, associated with the cardioprotective phenotype observed in this study.

Sprint training improves postischemic, left ventricular diastolic performance

We examined the effects of sprint training on left ventricular diastolic stiffness during normoxia and after ischemia-reperfusion (I/R). Thirty-seven, male Sprague-Dawley rats, weighing 150-175 g at the initiation of the experiment, were randomly assigned to a sedentary, control group (n = 20) or to a high-intensity, sprint-trained group (n = 17). Animals were trained 5 days/wk on a motor-driven treadmill for 6 wk. High-intensity sprint training consisted of running five 1-min sprints at 75 m/min, 15% grade, interspersed with 1-min active recovery runs at a speed of 20 m/min, 15% grade. Langendorff-derived isolated heart performance was measured before and after 20 min of no-flow ischemia followed by 30 min of reperfusion. Isolated myocytes were harvested from a subset of postischemic hearts. Sprint training reduced Langendorff-derived LV chamber stiffness (P < 0.05) and induced a rightward shift in the LV pressure-volume relationship during both normoxic perfusion and after I/R. LV developed pressure after I/R was also better preserved in hearts obtained from sprint-trained animals (P < 0.05), a result that is in part related to a lower postischemic LV chamber stiffness in sprint-trained hearts. The putative impact of sprint training on postischemic LV chamber stiffness was masked by glycolytic inhibition with iodoacetate, suggesting that glycolysis was involved in the better postischemic recovery observed in sprint-trained hearts. There was a tendency for enhanced postischemic cardiomyocyte shortening in sprint-trained cardiomyocytes compared with control. The rate of myocyte relaxation, i.e., time for 50% relaxation of the Ca(2+) transient amplitude, was similar between groups. These data suggest that additional mechanisms unrelated to Ca(2+) were involved in sprint-induced protection from ischemia-reperfusion-induced LV diastolic dysfunction.

The glycolytic enzymes, glyceraldehyde-3-phosphate dehydrogenase, triose-phosphate isomerase, and pyruvate kinase are components of the K(ATP) channel macromolecular complex and regulate its function

The regulation of ATP-sensitive potassium (K(ATP)) channel activity is complex and a multitude of factors determine their open probability. Physiologically and pathophysiologically, the most important of these are intracellular nucleotides, with a long-recognized role for glycolytically derived ATP in regulating channel activity. To identify novel regulatory subunits of the K(ATP) channel complex, we performed a two-hybrid protein-protein interaction screen, using as bait the mouse Kir6.2 C terminus. Screening a rat heart cDNA library, we identified two potential interacting proteins to be the glycolytic enzymes, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and triose-phosphate isomerase. The veracity of interaction was verified by co-immunoprecipitation techniques in transfected mammalian cells. We additionally demonstrated that pyruvate kinase also interacts with Kir6.2 subunits. The physiological relevance of these interactions is illustrated by the demonstration that native Kir6.2 protein similarly interact with GAPDH and pyruvate kinase in rat heart membrane fractions and that Kir6.2 protein co-localize with these glycolytic enzymes in rat ventricular myocytes. The functional relevance of our findings is demonstrated by the ability of GAPDH or pyruvate kinase substrates to directly block the K(ATP) channel under patch clamp recording conditions. Taken together, our data provide direct evidence for the concept that key enzymes involved in glycolytic ATP production are part of a multisubunit K(ATP) channel protein complex. Our data are consistent with the concept that the activity of these enzymes (possibly by ATP formation in the immediate intracellular microenvironment of this macromolecular K(ATP) channel complex) causes channel closure.

Steady-state effects of temperature acclimation on the transcriptome of the rainbow trout heart

Cold-acclimated (CA) phenotype of trout heart was induced by 4-wk acclimation at 4°C and was characterized by 32.7% increase in relative heart mass and 49.8% increase in ventricular myocyte size compared with warm-acclimated (WA; 18°C) fish ( P < 0.001). Effect of temperature acclimation on transcriptome of the rainbow trout heart was examined using species-specific microarray chips containing 1,380 genes. After 4 wk of temperature acclimation, 8.8% (122) of the genes were differently expressed in CA and WA hearts, and most of them (82%) were upregulated in the cold ( P < 0.01). Transcripts of genes engaged in protein synthesis and intermediary metabolism were most strongly upregulated, whereas genes contributing to the connective tissue matrix were clearly repressed. Extensive upregulation of the genes coding for ribosomal proteins and translation elongation and initiation factors suggest that the protein synthesis machinery of the trout heart is enhanced in the cold and is an essential part of the compensatory mechanism causing and maintaining the hypertrophy of cardiac myocytes. The prominent depression of collagen genes may be indicative of a reduced contribution of extracellular matrix to the remodeling of the CA fish heart. Temperature-related changes in transcripts of metabolic enzymes suggest that at mRNA level, glycolytic energy production from carbohydrates is compensated in the heart of CA rainbow trout, while metabolic compensation is absent in mitochondria. In addition, the analysis revealed three candidate genes: muscle LIM protein, atrial natriuretic peptide B, and myosin light chain 2, which might be central for induction and maintenance of the hypertrophic phenotype of the CA trout heart. These findings indicate that extensive modification of gene expression is needed to maintain the temperature-specific phenotype of the fish heart.

Glyceraldehyde-3-phosphate dehydrogenase is a GABAA receptor kinase linking glycolysis to neuronal inhibition

Protein phosphorylation is crucial for regulating synaptic transmission. We describe a novel mechanism for the phosphorylation of the GABA(A) receptor, which mediates fast inhibition in the brain. A protein copurified and coimmunoprecipitated with the phosphorylated receptor alpha1 subunit; this receptor-associated protein was identified by purification and microsequencing as the key glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Molecular constructs demonstrated that GAPDH directly phosphorylates the long intracellular loop of GABA(A) receptor alpha1 subunit at identified serine and threonine residues. GAPDH and the alpha1 subunit were found to be colocalized at the neuronal plasma membrane. In keeping with the GAPDH/GABA(A) receptor molecular association, glycolytic ATP produced locally at plasma membranes was consumed for this alpha1 subunit phosphorylation, possibly within a single macrocomplex. The membrane-attached GAPDH is thus a dual-purpose enzyme, a glycolytic dehydrogenase, and a receptor-associated kinase. In acutely dissociated cortical neurons, the rundown of the GABA(A) responses was essentially attributable to a Mg(2+)-dependent phosphatase activity, which was sensitive to vanadate but insensitive to okadaic acid or fluoride. Rundown was significantly reduced by the addition of GAPDH or its reduced cofactor NADH and nearly abolished by the addition of its substrate glyceraldehyde-3-phosphate (G3P). The prevention of rundown by G3P was abolished by iodoacetamide, an inhibitor of the dehydrogenase activity of GAPDH, indicating that the GABA(A) responses are maintained by a glycolysis-dependent phosphorylation. Our results provide a molecular mechanism for the direct involvement of glycolysis in neurotransmission.

Modulation of sarcoplasmic reticulum Ca2+ release by glycolysis in cat atrial myocytes

In cardiac myocytes, glycolysis and excitation-contraction (E-C) coupling are functionally coupled. We studied the effects of inhibitors (2-deoxy-D-glucose (2-DG), iodoacetate (IAA)), intermediates (glucose-6-phosphate (G6P), fructose-6-phosphate (F6P), fructose-1,6-bisphosphate (FBP), phosphoenolpyruvate (PEP)) and products (pyruvate, L-lactate) of glycolysis on sarcoplasmic reticulum (SR) Ca(2+) release and uptake in intact and permeabilized cat atrial myocytes. In field-stimulated (0.5-0.7 Hz) intact myocytes, 2-DG (10 mm) and IAA (1 mm) caused elevation of diastolic [Ca(2+)](i) and [Ca(2+)](i) transient alternans (Ca(2+) alternans) followed by a decrease of the amplitude of the [Ca(2+)](i) transient. Focal application of 2-DG resulted in local Ca(2+) alternans that was confined to the region of exposure. 2-DG and IAA slowed the decay kinetics of the [Ca(2+)](i) transient and delayed its recovery (positive staircase) after complete SR depletion, suggesting impaired activity of the SR Ca(2+)-ATPase (SERCA). 2-DG and IAA reduced the rate of reuptake of Ca(2+) into the SR which was accompanied by a 15-20% decrease of SR Ca(2+) load. Major changes of mitochondrial redox state (measured as FAD autofluorescence) were not observed after inhibition of glycolysis. Pyruvate (10 mm) and L-lactate (10 mm) elicited similar changes of the [Ca(2+)](i) transient. Pyruvate, L-lactate and IAA - but not 2-DG - induced intracellular acidosis. Recording of single channel activity of ryanodine receptors (RyRs) incorporated into lipid bilayers revealed complex modulation by glycolytic intermediates and products (1 mm each): some were without effect (G6P, PEP, L-lactate) while others either increased (F6P, +40%; FBP, +265%) or decreased (pyruvate, -58%) the open probability of the RyR. Consistent with these findings, spontaneous SR Ca(2+) release (Ca(2+) sparks) in permeabilized myocytes was facilitated by FBP and inhibited by pyruvate. The results indicate that in atrial myocytes glycolysis regulates Ca(2+) release from the SR by multiple mechanisms including direct modulation of RyR activity by intermediates and products of glycolysis and modulation of SERCA activity through local changes of glycolytically derived ATP.

Na+-K+ Pump Regulation and Skeletal Muscle Contractility

Clausen, Torben. Na+-K+ Pump Regulation and Skeletal Muscle Contractility. Physiol Rev 83: 1269-1324, 2003; 10.1152/physrev.00011.2003.—In skeletal muscle, excitation may cause loss of K+, increased extracellular K+ ([K+]o), intracellular Na+ ([Na+]i), and depolarization. Since these events interfere with excitability, the processes of excitation can be self-limiting. During work, therefore, the impending loss of excitability has to be counterbalanced by prompt restoration of Na+-K+ gradients. Since this is the major function of the Na+-K+ pumps, it is crucial that their activity and capacity are adequate. This is achieved in two ways: 1) by acute activation of the Na+-K+ pumps and 2) by long-term regulation of Na+-K+ pump content or capacity. 1) Depending on frequency of stimulation, excitation may activate up to all of the Na+-K+ pumps available within 10 s, causing up to 22-fold increase in Na+ efflux. Activation of the Na+-K+ pumps by hormones is slower and less pronounced. When muscles are inhibited by high [K+]o or low [Na+]o, acute hormone- or excitation-induced activation of the Na+-K+ pumps can restore excitability and contractile force in 10-20 min. Conversely, inhibition of the Na+-K+ pumps by ouabain leads to progressive loss of contractility and endurance. 2) Na+-K+ pump content is upregulated by training, thyroid hormones, insulin, glucocorticoids, and K+ overload. Downregulation is seen during immobilization, K+ deficiency, hypoxia, heart failure, hypothyroidism, starvation, diabetes, alcoholism, myotonic dystrophy, and McArdle disease. Reduced Na+-K+ pump content leads to loss of contractility and endurance, possibly contributing to the fatigue associated with several of these conditions. Increasing excitation-induced Na+ influx by augmenting the open-time or the content of Na+ channels reduces contractile endurance. Excitability and contractility depend on the ratio between passive Na+-K+ leaks and Na+-K+ pump activity, the passive leaks often playing a dominant role. The Na+-K+ pump is a central target for regulation of Na+-K+ distribution and excitability, essential for second-to-second ongoing maintenance of excitability during work.

Pregnancy alters glucose-6-phosphate dehydrogenase trafficking, cell metabolism, and oxidant release of maternal neutrophils

Pregnancy is associated with changes in host susceptibility to infections and inflammatory disease. We hypothesize that metabolic enzyme trafficking affects maternal neutrophil activation. Specifically, immunofluorescence microscopy has shown that glucose-6-phosphate dehydrogenase (G-6-PDase), the rate-controlling step of the hexose monophosphate shunt (HMS), is located near the cell periphery in control neutrophils but is found near the microtubule-organizing centers in cells from pregnant women. Cytochemical studies confirmed that the distribution of the G-6-PDase antigen is coincident with functional G-6-PDase activity. Metabolic oscillations within activated pregnancy neutrophils are higher in amplitude, though lower in frequency, than activated control neutrophils, suggesting limited HMS activity. Analysis of radioisotope-labeled carbon flux from glucose to CO(2) indicates that the HMS is intact in leukocytes from pregnant women, but its level is not enhanced by cell stimulation. Using extracellular fluorescent markers, activated pregnancy neutrophils were found to release reactive oxygen metabolites (ROMs) at a lower rate than activated control neutrophils. However, basal levels of ROM production in polarized pregnancy neutrophils were greater than in control neutrophils. Microtubule-disrupting agents reversed the observed changes in G-6-PDase trafficking, metabolic oscillations, and ROM production by maternal neutrophils. G-6-PDase trafficking appears to be one mechanism regulating ROM production by maternal neutrophils.

Local calcium gradients during excitation-contraction coupling and alternans in atrial myocytes

Subcellular Ca(2+) signalling during normal excitation-contraction (E-C) coupling and during Ca(2+) alternans was studied in atrial myocytes using fast confocal microscopy and measurement of Ca(2+) currents (I(Ca)). Ca(2+) alternans, a beat-to-beat alternation in the amplitude of the [Ca(2+)](i) transient, causes electromechanical alternans, which has been implicated in the generation of cardiac fibrillation and sudden cardiac death. Cat atrial myocytes lack transverse tubules and contain sarcoplasmic reticulum (SR) of the junctional (j-SR) and non-junctional (nj-SR) types, both of which have ryanodine-receptor calcium release channels. During E-C coupling, Ca(2+) entering through voltage-gated membrane Ca(2+) channels (I(Ca)) triggers Ca(2+) release at discrete peripheral j-SR release sites. The discrete Ca(2+) spark-like increases of [Ca(2+)](i) then fuse into a peripheral 'ring' of elevated [Ca(2+)](i), followed by propagation (via calcium-induced Ca(2+) release, CICR) to the cell centre, resulting in contraction. Interrupting I(Ca) instantaneously terminates j-SR Ca(2+) release, whereas nj-SR Ca(2+) release continues. Increasing the stimulation frequency or inhibition of glycolysis elicits Ca(2+) alternans. The spatiotemporal [Ca(2+)](i) pattern during alternans shows marked subcellular heterogeneities including longitudinal and transverse gradients of [Ca(2+)](i) and neighbouring subcellular regions alternating out of phase. Moreover, focal inhibition of glycolysis causes spatially restricted Ca(2+) alternans, further emphasising the local character of this phenomenon. When two adjacent regions within a myocyte alternate out of phase, delayed propagating Ca(2+) waves develop at their border. In conclusion, the results demonstrate that (1) during normal E-C coupling the atrial [Ca(2+)](i) transient is the result of the spatiotemporal summation of Ca(2+) release from individual release sites of the peripheral j-SR and the central nj-SR, activated in a centripetal fashion by CICR via I(Ca) and Ca(2+) release from j-SR, respectively, (2) Ca(2+) alternans is caused by subcellular alterations of SR Ca(2+) release mediated, at least in part, by local inhibition of energy metabolism, and (3) the generation of arrhythmogenic Ca(2+) waves resulting from heterogeneities in subcellular Ca(2+) alternans may constitute a novel mechanism for the development of cardiac dysrhythmias.

Pregnancy alters glucose-6-phosphate dehydrogenase trafficking, cell metabolism, and oxidant release of maternal neutrophils

Pregnancy is associated with changes in host susceptibility to infections and inflammatory disease. We hypothesize that metabolic enzyme trafficking affects maternal neutrophil activation. Specifically, immunofluorescence microscopy has shown that glucose-6-phosphate dehydrogenase (G-6-PDase), the rate-controlling step of the hexose monophosphate shunt (HMS), is located near the cell periphery in control neutrophils but is found near the microtubule-organizing centers in cells from pregnant women. Cytochemical studies confirmed that the distribution of the G-6-PDase antigen is coincident with functional G-6-PDase activity. Metabolic oscillations within activated pregnancy neutrophils are higher in amplitude, though lower in frequency, than activated control neutrophils, suggesting limited HMS activity. Analysis of radioisotope-labeled carbon flux from glucose to CO(2) indicates that the HMS is intact in leukocytes from pregnant women, but its level is not enhanced by cell stimulation. Using extracellular fluorescent markers, activated pregnancy neutrophils were found to release reactive oxygen metabolites (ROMs) at a lower rate than activated control neutrophils. However, basal levels of ROM production in polarized pregnancy neutrophils were greater than in control neutrophils. Microtubule-disrupting agents reversed the observed changes in G-6-PDase trafficking, metabolic oscillations, and ROM production by maternal neutrophils. G-6-PDase trafficking appears to be one mechanism regulating ROM production by maternal neutrophils.

Subcellular Ca2+ alternans represents a novel mechanism for the generation of arrhythmogenic Ca2+ waves in cat atrial myocytes

Ca(2+) alternans is a potentially arrhythmogenic beat-to-beat alternation of the amplitude of the action potential-induced [Ca(2+)](i) transient in cardiac myocytes. Despite its pathophysiological significance the cellular mechanisms underlying Ca(2+) alternans are poorly understood. Recent evidence, however, points to the modulation of Ca(2+)-induced Ca(2+) release (CICR) from the sarcoplasmic reticulum (SR) by localized alterations in energy metabolism as an important determinant of Ca(2+) alternans. We therefore studied the subcellular properties of Ca(2+) alternans in field-stimulated cat atrial myocytes employing fast two-dimensional fluorescence confocal microscopy. Ca(2+) alternans was elicited by an increase in stimulation frequency or by metabolic interventions targeting glycolysis. Marked subcellular variations in the time of onset, the magnitude, and the phase of alternans were observed. Longitudinal and transverse gradients of Ca(2+) alternans were found as well as neighbouring subcellular regions alternating out-of-phase. Moreover, focal inhibition of glycolysis resulted in spatially restricted Ca(2+) alternans. When two adjacent regions within a myocyte alternated out-of-phase, steep [Ca(2+)](i) gradients developed at their border giving rise to delayed propagating Ca(2+) waves. The results demonstrate that Ca(2+) alternans is a subcellular phenomenon caused by modulation of SR Ca(2+) release, which is mediated, at least in part, by local inhibition of energy metabolism. The generation of arrhythmogenic Ca(2+) waves by subcellular variations in the phase of Ca(2+) alternans represents a novel mechanism for the development of atrial disrhythmias.

Hexokinase translocation during neutrophil activation, chemotaxis, and phagocytosis: disruption by cytochalasin D, dexamethasone, and indomethacin

Neutrophils expend large amounts of energy to perform demanding cell functions. To better understand energy production and flow during cell activation, immunofluorescence microscopy was employed to determine the location of the key metabolic enzyme hexokinase during various conditions. Hexokinase is translocated from the neutrophil's cytosol to its periphery in response to N-formyl-methionyl-leucyl-phenylalanine (fMLP) and other activating stimuli, but not during exposure to the formyl peptide receptor antagonist N-tert-BOC-phe-leu-phe-leu-phe (Boc-PLPLP). Translocation was observed from 10(-6) to 10(-9)M fMLP. However, fMLP did not affect the intracellular distribution of lactate dehydrogenase. Hexokinase accumulated at the lamellipodium of cells exposured to an fMLP gradient whereas it localized to the phagosome after latex bead uptake. Thus, hexokinase is differentially translocated within cells depending upon the prevailing physiological conditions. Further studies noted that cytochalasin D, dexamethasone, and indomethacin blocked hexokinase translocation. Parallel regulation of reactive oxygen metabolite (ROM) production was shown. We speculate that hexokinase translocation participates in neutrophil activation.

Hypoxia-induced increase in intracellular calcium concentration in endothelial cells: role of the Na(+)-glucose cotransporter

Hypoxia is a common denominator of many vascular disorders, especially those associated with ischemia. To study the effect of oxygen depletion on endothelium, we developed an in vitro model of hypoxia on human umbilical vein endothelial cells (HUVEC). Hypoxia strongly activates HUVEC, which then synthesize large amounts of prostaglandins and platelet-activating factor. The first step of this activation is a decrease in ATP content of the cells, followed by an increase in the cytosolic calcium concentration ([Ca(2+)](i)) which then activates the phospholipase A(2) (PLA(2)). The link between the decrease in ATP and the increase in [Ca(2+)](i) was not known and is investigated in this work. We first showed that the presence of extracellular Na(+) was necessary to observe the hypoxia-induced increase in [Ca(2+)](i) and the activation of PLA(2). This increase was not due to the release of Ca(2+) from intracellular stores, since thapsigargin did not inhibit this process. The Na(+)/Ca(2+) exchanger was involved since dichlorobenzamil inhibited the [Ca(2+)](i) and the PLA(2) activation. The glycolysis was activated, but the intracellular pH (pH(i)) in hypoxic cells did not differ from control cells. Finally, the hypoxia-induced increase in [Ca(2+)](i) and PLA(2) activation were inhibited by phlorizin, an inhibitor of the Na(+)-glucose cotransport. The proposed biochemical mechanism occurring under hypoxia is the following: glycolysis is first activated due to a requirement for ATP, leading to an influx of Na(+) through the activated Na(+)-glucose cotransport followed by the activation of the Na(+)/Ca(2+) exchanger, resulting in a net influx of Ca(2+).

Both glucose-insulin-potassium and glutamine in warm blood cardioplegia increase the rates of myocardial glucose and free fatty acid oxidation

OBJECTIVE

We wanted to assess the effect of glucose-insulin-potassium (GIK) and glutamine on the oxidative metabolism during and after prolonged warm continuous blood cardioplegia (WB).

DESIGN

WB was given to 21 pigs divided into three equally sized groups: WB (control), WB and GIK, or WB and glutamine. Oxidation rates of radiolabeled glucose (14C) and free fatty acid (FFA) (3H) were assessed before, during, and at 30 and 60 min after 3 h of cardiac arrest with WB.

RESULTS

During standstill the substrate oxidation dropped markedly (<60%), glucose oxidation was highest in the WB + GIK group (p < 0.05) and FFA oxidation highest in the WB + glutamine group (NS). During recovery the GIK group had an elevated glucose oxidation (47 and 40% vs WB at 30 and 60 min recovery, respectively -p < 0.05). Following 30 min recovery the addition of GIK suppressed FFA oxidation some 60%. Glutamine increased the oxidation of both glucose (30%) and FFA (150%) following 60 min recovery (p < 0.05). During the whole recovery phase the relative FFA oxidation was significantly lowered in the GIK group. There were no differences between the groups regarding arterial levels or uptake of substrates, except for a higher myocardial oxygen consumption (MVO2) during cardioplegia in the glutamine group. All the hearts performed similarly.

CONCLUSION

Addition of GIK or glutamine to the well-perfused and oxygenated heart during WB led to a postcardioplegic increase in oxidative metabolism and MVO2. GIK resulted in a significant metabolic shift from FFA to glucose.

Electrophysiology of the sodium-potassium-ATPase in cardiac cells

Like several other ion transporters, the Na(+)-K(+) pump of animal cells is electrogenic. The pump generates the pump current I(p). Under physiological conditions, I(p) is an outward current. It can be measured by electrophysiological methods. These methods permit the study of characteristics of the Na(+)-K(+) pump in its physiological environment, i.e., in the cell membrane. The cell membrane, across which a potential gradient exists, separates the cytosol and extracellular medium, which have distinctly different ionic compositions. The introduction of the patch-clamp techniques and the enzymatic isolation of cells have facilitated the investigation of I(p) in single cardiac myocytes. This review summarizes and discusses the results obtained from I(p) measurements in isolated cardiac cells. These results offer new exciting insights into the voltage and ionic dependence of the Na(+)-K(+) pump activity, its effect on membrane potential, and its modulation by hormones, transmitters, and drugs. They are fundamental for our current understanding of Na(+)-K(+) pumping in electrically excitable cells.

ATP from glycolysis is required for normal sodium homeostasis in resting fast-twitch rodent skeletal muscle

Myocellular sodium homeostasis is commonly disrupted during critical illness for unknown reasons. Recent data suggest that changes in intracellular sodium content and the amount of ATP provided by glycolysis are closely related. The role of glycolysis and oxidative phosphorylation in providing fuel to the Na(+)-K(+) pump was investigated in resting rat extensor digitorum longus muscles incubated at 30 degrees C for 1 h. Oxidative inhibition with carbonyl cyanide m-chlorophenylhydrazone, known as CCCP (0.2 microM), or by hypooxygenation did not alter myocellular sodium or potassium content ([Na(+)](i), [K(+)](i), respectively), whereas treatment with iodoacetic acid (0.3 mM), which effectively blocked glycolysis, dramatically increased [Na(+)](i) and the [Na(+)](i)/[K(+)](i) ratio. Experiments using ouabain and measurements of myocellular high-energy phosphates indicate that Na(+)-K(+)-ATPase activity is only impaired when glycolysis is inhibited. The data suggest that normal glycolysis is required to regulate intracellular sodium in fast-twitch skeletal muscles, because it is the predominant source of the fuel for the Na(+)-K(+)-ATPase.

The importance of glycolytically-derived ATP for the Na+/H+ exchange activity in guinea pig ventricular myocytes

The cardiac subtype of Na+/H+ exchanger (NHE-1) plays an important role in the regulation of intracellular pH (pHi) and also can be a major route for Na+ influx. Although intracellular ATP is required for the optimal function of NHE-1, the regulation of the exchanger by ATP is less well characterized. This study was designed to investigate which intracellular ATP generated by oxidative phosphorylation or by glycolysis is dominant for the activation of NHE-1 in intact cardiac myocytes. Isolated guinea pig ventricular myocytes were loaded with the pHi-sensitive fluorescent indicator, 2'-7'-bis(carboxyl)-5',6'-carboxy fluorescein (BCECF), and exposed to 20 mM 2-deoxyglucose (2-DG) or 2 mM sodium cyanide (CN) to inhibit glycolysis or oxidative phosphorylation, respectively. The activity of NHE-1 was estimated with pHi recovery following transient application of 15 mM NH4Cl (NH4Cl prepulse). After the NH4Cl prepulse, pHi decreased from 7.00 +/- 0.03 (mean +/- S.E.) to 6.60 +/- 0.06 and recovered to 6.94 +/- 0.13 at 10 min (n = 7). The pHi recovery was suppressed in the presence of 2-DG (6.67 +/- 0.05, p < 0.01, n = 7), but was not changed in the presence of CN (6.88 +/- 0.18, n = 6). Since there was no difference in the intrinsic H+ buffering power, the estimation of the net acid efflux demonstrated that the activity of NHE-1 was significantly depressed in 2-DG. The inhibitory effect of 2-DG was not due to more extensive depletion of global intracellular ATP or secondary to the change in either intracellular Na+ or Ca2+ concentration. We concluded that ATP generated by glycolysis rather than by oxidative phosphorylation is essential to activate NHE-1 in ventricular myocytes.

Altered metabolism causes cardiac dysfunction in perfused hearts from diabetic (db/db) mice

Contractile function and substrate metabolism were characterized in perfused hearts from genetically diabetic C57BL/KsJ-lepr(db)/lepr(db) (db/db) mice and their non-diabetic lean littermates. Contractility was assessed in working hearts by measuring left ventricular pressures and cardiac power. Rates of glycolysis, glucose oxidation, and fatty acid oxidation were measured using radiolabeled substrates ([5-(3)H]glucose, [U-(14)C]glucose, and [9,10-(3)H]palmitate) in the perfusate. Contractile dysfunction in db/db hearts was evident, with increased left ventricular end diastolic pressure and decreased left ventricular developed pressure, cardiac output, and cardiac power. The rate of glycolysis from exogenous glucose in diabetic hearts was 48% of control, whereas glucose oxidation was depressed to only 16% of control. In contrast, palmitate oxidation was increased twofold in db/db hearts. The hypothesis that altered metabolism plays a causative role in diabetes-induced contractile dysfunction was tested using perfused hearts from transgenic db/db mice that overexpress GLUT-4 glucose transporters. Both glucose metabolism and palmitate metabolism were normalized in hearts from db/db-human insulin-regulatable glucose transporter (hGLUT-4) hearts, as was contractile function. These findings strongly support a causative role of impaired metabolism in the cardiomyopathy observed in db/db diabetic hearts.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Functional coupling between glycolysis and excitation-contraction coupling underlies alternans in cat heart cells

Electromechanical alternans was characterized in single cat atrial and ventricular myocytes by simultaneous measurements of action potentials, membrane current, cell shortening and changes in intracellular Ca2+ concentration ([Ca2+]i). Using laser scanning confocal fluorescence microscopy, alternans of electrically evoked [Ca2+]i transients revealed marked differences between atrial and ventricular myocytes. In ventricular myocytes, electrically evoked [Ca2+]i transients during alternans were spatially homogeneous. In atrial cells Ca2+ release started at subsarcolemmal peripheral regions and subsequently spread toward the centre of the myocyte. In contrast to ventricular myocytes, in atrial cells propagation of Ca2+ release from the sarcoplasmic reticulum (SR) during the small-amplitude [Ca2+]i transient was incomplete, leading to failures of excitation-contraction (EC) coupling in central regions of the cell. The mechanism underlying alternans was explored by evaluating the trigger signal for SR Ca2+ release (voltage-gated L-type Ca2+ current, ICa,L) and SR Ca2+ load during alternans. Voltage-clamp experiments revealed that peak ICa,L was not affected during alternans when measured simultaneously with changes of cell shortening. The SR Ca2+ content, evaluated by application of caffeine pulses, was identical following the small-amplitude and the large-amplitude [Ca2+]i transient. These results suggest that the primary mechanism responsible for cardiac alternans does not reside in the trigger signal for Ca2+ release and SR Ca2+ load. beta-Adrenergic stimulation with isoproterenol (isoprenaline) reversed electromechanical alternans, suggesting that under conditions of positive cardiac inotropy and enhanced efficiency of EC coupling alternans is less likely to occur. The occurrence of electromechanical alternans could be elicited by impairment of glycolysis. Inhibition of glycolytic flux by application of pyruvate, iodoacetate or beta-hydroxybutyrate induced electromechanical and [Ca2+]i transient alternans in both atrial and ventricular myocytes. The data support the conclusion that in cardiac myocytes alternans is the result of periodic alterations in the gain of EC coupling, i. e. the efficacy of a given trigger signal to release Ca2+ from the SR. It is suggested that the efficiency of EC coupling is locally controlled in the microenvironment of the SR Ca2+ release sites by mechanisms utilizing ATP, produced by glycolytic enzymes closely associated with the release channel.

Activation of the cAMP-protein kinase A pathway facilitates Na+ translocation by the Na+-K+ pump in guinea-pig ventricular myocytes

1. The effects of the adenylyl cyclase activator forskolin on steady-state and transient currents generated by the Na+-K+ pump were studied in guinea-pig ventricular myocytes by means of whole-cell voltage clamp at 30 C. 2. In external solution containing 144 mM Na+ (Na+o) and 10 mM K+ (K+o), steady-state Na+-K+ pump current (Ip) activated by 5 mM pipette Na+ (Na+pip) at -20 mV was reversibly augmented by forskolin (4 microM) to 133 +/- 4 % of the control current (n = 15). The forskolin analogue 1, 9-dideoxyforskolin (10 microM), which does not activate adenylyl cyclases, did not increase Ip (n = 2). Application of the protein kinase A (PKA) inhibitor H-89 (10 microM) in the continued presence of forskolin reversed the forskolin-induced elevation of Ip (n = 3). 3. The forskolin effect on Ip persisted in the presence of 50 mM Na+pip which ensured that the internal Na+-binding sites of the Na+-K+ pump were nearly saturated. Under these conditions, the drug increased Ip to 142 +/- 3 % of the control Ip when the pipette free Ca2+ concentration ([Ca2+]pip) was 0.013 nM (n = 5) and to 138 +/- 4 % of the control Ip when free [Ca2+]pip was 15 nM (n = 9). 4. In Na+-free external solution, Ip activated by 50 mM Na+pip and 1.5 mM K+o was likewise increased by forskolin but to a lesser extent than in Na+-containing medium (116 +/- 3 % of control, n = 10). 5. In order to investigate exclusively partial reactions in the Na+ limb of the pump cycle, transient pump currents under conditions of electroneutral Na+-Na+ exchange were studied. Transient pump currents elicited by voltage jumps displayed an initial peak and then decayed monoexponentially. Moved charge (Q) and the rate constant of current decay varied with membrane potential (V). The Q-V relationship followed a Boltzmann distribution characterized by the midpoint voltage (V0.5) and the maximum amount of movable charge (DeltaQmax). Forskolin (2-10 microM) shifted V0.5 to more negative values while DeltaQmax was not affected (n = 11). The effects of forskolin on transient pump currents were mimicked by 8-bromo-cAMP (500 microM; n = 2) and abolished by a peptide inhibitor of PKA (PKI, 10 microM; n = 5). 6. We conclude that activation of the cAMP-PKA pathway in guinea-pig ventricular myocytes increases Na+-K+ pump current at least in part by modulating partial reactions in the Na+ limb of the pump cycle. Under physiological conditions, the observed stimulation of the cardiac Na+-K+ pump may serve to shorten the action potential duration and to counteract the increased passive sarcolemmal Na+ and K+ fluxes during sympathetic stimulation of the heart.

K+ transport in resting rat hind-limb skeletal muscle in response to paraxanthine, a caffeine metabolite

This study tested the hypothesis that paraxanthine, a caffeine metabolite, stimulates skeletal muscle potassium (K+) transport by an increase in Na+-K+ ATPase activity. The unidirectional transport of K+ into muscle (JinK) was studied using a perfused rat hind limb technique. Using 12 hind limbs, we examined the response to 20 min of paraxanthine perfusion (0.1 mM), followed by 20 min perfusion with 0.1 mM paraxanthine and 5 mM ouabain (n = 5) to irreversibly inhibit Na+-K+ ATPase activity. Paraxanthine stimulated JinK by 23 ± 5% within 20 min. Ouabain abolished the paraxanthine-induced stimulation of JinK, suggesting the increase in K+ uptake was due to activation of the Na+-K+ ATPase. To confirm the role of the Na+-K+ ATPase, 14 hind limbs were perfused for 20 min with 5 mM ouabain prior to 20 min perfusion with 0.1 mM paraxanthine and 5 mM ouabain (n = 6). Ouabain alone resulted in a 41 ± 7% decrease in JinK within 15 min. Inhibition of ouabain-sensitive JinK prevented the paraxanthine-induced increase in JinK. Hind limbs (n = 3) were also perfused with 0.1 mM paraxanthine for 60 min to examine the response to longer duration paraxanthine perfusion. The paraxanthine-induced increase in JinK continued for the entire 60 min. In another series, hind limbs were perfused with 0.01 (n = 9), 0.1 (n = 9), or 0.5 (n = 6) mM paraxanthine for 15 min. There was no concentration-dependent relationship between JinK and paraxanthine concentration, and 0.01, 0.1, and 0.5 mM paraxanthine increased JinK similarly (25 ± 5, 22 ± 4, and 27 ± 6%, respectively). The effect of paraxanthine on JinK could not be reversed by subsequent perfusion with paraxanthine-free perfusate. Caffeine (0.05-1.0 mM) had no effect on K+ transport. It is concluded that paraxanthine increases JinK in resting skeletal muscle by stimulating ouabain-sensitive Na+-K+ ATPase activity.Key words: caffeine, methylxanthine, ouabain, potassium transport, sodium pump, Na-K ATPase, VO2, glycolysis.

A single cell model of myocardial reperfusion injury: changes in intracellular Na+ and Ca2+ concentrations in guinea pig ventricular myocytes

To investigate the contribution of the changes in intracellular Na+ and Ca2+ concentrations ([Na+]i and [Ca2+]i) to myocardial reperfusion injury, we made an ischemia/reperfusion model in intact guinea pig myocytes. Myocardial ischemia was simulated by the perfusion of metabolic inhibitors (3.3 mM amobarbital and 5 microM carbonyl cyanide m-chlorophenylhydrazone) with pH 6.6 and reperfusion was achieved by the washout of them with pH 7.4. [Na+]i increased from 7.9 +/- 2.0 to 14.0 +/- 3.4 mM (means +/- S.E., p < 0.01 ) during 7.5 min of simulated ischemia (SI) and increased further to 18.8 +/- 3.0 mM at 7.5 min after reperfusion. [Ca2+]i, expressed as the ratio of fluo 3 fluorescence intensity, increased to 133 +/- 8% (p < 0.01) during SI and gradually returned to the control level after reperfusion. Intracellular pH decreased from 7.53 +/- 0.04 to 6.31 +/- 0.04 (p < 0.01) and recovered quickly after reperfusion. Reperfusion with the acidic solution or the continuous perfusion of hexamethylene amiloride (2 microM) prevented the reperfusion-induced increase in [Na+]i. When the duration of SI was prolonged to 15 min, the cell response after reperfusion varied, 16 of 37 cells kept quiescent, 21 cells showed spontaneous Ca2+ waves, and 4 cells out of these 21 cells became hypercontracted. In quiescent cells, both [Na+]i and [Ca2+]i decreased immediately after reperfusion. In cells with Ca2+ waves, [Na+]i transiently increased further at the early phase of reperfusion, while [Ca2+]i declined. In hypercontracted cells, [Na+]i increased as much as in 'Ca2+ wave' cells, but [Ca2+]i increased extensively and both ion concentrations continued to increase. Reperfusion with the Ca2+-free solution prevented both the [Ca2+]i increase and morphological change. In the presence of ryanodine (10 microM), the increase in [Ca2+]i after reperfusion was augmented and some cells became hypercontracted. We concluded that (1) Na+/H+ exchange is active both during SI and reperfusion, resulting in the additional [Na+]i elevation on reperfusion, (2) the [Na+]i level after reperfusion and the following Ca2+ influx via Na+/Ca2+ exchange are crucial for reperfusion cell injury, and (3) the Ca2+ buffering capacity of sarcoplasmic reticulum would also contribute to the Ca2+ regulation and cell injury after reperfusion.

5-hydroxydecanoate selectively reduces the initial increase in extracellular K+ in ischemic guinea-pig heart

The aim of the present study was to determine the effect of 5-hydroxydecanoate (5-HD) on extracellular K+ levels during global ischemia for 30 min employing K+-sensitive electrodes in isolated guinea-pig hearts. 5-HD (100 microM) reduced the K+ accumulation during the early period of ischemia, but did not inhibit the elevation of extracellular K+ in the latter half of the ischemic period which was selectively enhanced by ouabain (3 microM). Thus, 5-HD appears to exert a similar mode of action as glibenclamide on extracellular K+ accumulation in the ischemic guinea-pig hearts. The present study also strengthens the previous conclusion that an ATP-sensitive K+ channel contributes only to the initial increasing phase of extracellular K+ accumulation during ischemia in guinea-pig hearts.

Metabolic inhibition in the perfused rat heart: evidence for glycolytic requirement for normal sodium homeostasis

Subcellular compartmentalization of energy stores to support different myocardial processes has been exemplified by the glycolytic control of the ATP-sensitive K+ channel. Recent data suggest that the control of intracellular sodium (Nai) may also rely on glycolytically derived ATP; however, the degree of this dependence is unclear. To examine this question, isolated, perfused rat hearts were exposed to hypoxia, to selectively inhibit oxidative metabolism, or iodoacetate (IAA, 100 mumol/l), to selectively inhibit glycolysis. Nai and myocardial high-energy phosphate levels were monitored using triple-quantum-filtered (TQF) 23Na and 31P magnetic resonance spectroscopy, respectively. The effects of ion exchange mechanisms (Na+/Ca2+, Na+/H+) on Nai were examined by pharmacological manipulation of these channels. Nai, as monitored by shift reagent-aided TQF 23Na spectral amplitudes, increased by approximately 220% relative to baseline after 45 min of perfusion with IAA, with or without rapid pacing. During hypoxia, Nai increased by approximately 200% during rapid pacing but did not increase in unpaced hearts or when the Na+/H+ exchange blocker ethylisopropylamiloride (EIPA, 10 mumol/l) was used. Neither EIPA nor a low-Ca2+ perfusate (50 mumol/l) could prevent the rise in Nai during perfusion with IAA. Myocardial function and high-energy phosphate stores were preserved during inhibition of glycolysis with IAA and continued oxidative metabolism. These results suggest that glycolysis is required for normal Na+ homeostasis in the perfused rat heart, possibly because of preferential fueling of Na-K-adenosinetriphosphatase by glycolytically derived ATP.

Cation pumps in skeletal muscle: potential role in muscle fatigue

Two membrane bound pumps in skeletal muscle, the sarcolemma Na+-K+ adenosine triphosphatase (ATPase) and the sarcoplasmic reticulum Ca2+-ATPase, provide for the maintenance of transmembrane ionic gradients necessary for excitation and activation of the myofibrillar apparatus. The rate at which the pumps are capable of establishing ionic homeostasis depends on the maximal activity of the enzyme and the potential of the metabolic pathways for supplying adenosine triphosphate (ATP). The activity of the Ca2+-ATPase appears to be expressed in a fibre type specific manner with both the amount of the enzyme and the isoform type related to the speed of contraction. In contrast, only minimal differences exist between slow-twitch and fast-twitch fibres in Na+-K+ ATPase activity. Evidence is accumulating that both active transport of Na+ and K+ across the sarcolemma and Ca2+-uptake by the sarcoplasmic reticulum may be impaired in vivo in a task specific manner resulting in loss of contractile function. In contrast to the Ca2+-ATPase, the Na+-K+ ATPase can be rapidly upregulated soon after the onset of a sustained pattern of activity. Similar programmes of activity result in a downregulation of Ca2+-ATPase but at a much later time point. The manner in which the metabolic pathways reorganize following chronic activity to meet the changes in ATP demand by the cation pumps and the degree to which these adaptations are compartmentalized is uncertain.

Diltiazem inhibits the late increase in extracellular potassium by maintaining glycolytic ATP synthesis during myocardial ischemia

During myocardial ischemia, the extracellular potassium concentration increases in a triphasic pattern, an initial early increase, a constant phase, and a late increasing phase. The aim of this study was to determine whether diltiazem inhibits the late increasing phase by maintaining glycolytic adenosine triphosphate (ATP) synthesis in ischemic myocardium. The extracellular potassium concentration and pH were measured simultaneously with ion-sensitive electrodes during 30 min of global ischemia in isolated guinea-pig hearts. In the control hearts, the late increasing phase occurred 13 min after the onset of ischemia when the change in extracellular pH had reached a plateau. There was a sharp increase in the myocardial lactate level in the control hearts, which was maintained for approximately 8 min after the onset of ischemia. Iodoacetate (1 mM) led to a ATP depletion and rapid accumulation in extracellular potassium shortly after the onset of ischemia without a decrease in extracellular pH. The preischemic treatment with diltiazem (3 microM) reduced cardiac function both before ischemia and during the early period of ischemia. Diltiazem almost completely abolished the late increasing phase with a continuous decrease in extracellular pH throughout the ischemic period. The myocardial lactate level in the diltiazem-treated group increased sharply between 2 and 15 min after the onset of ischemia. The myocardial ATP level was preserved throughout the ischemic period. This study shows that diltiazem inhibits the late increasing phase in extracellular potassium by maintaining glycolytic ATP synthesis during ischemia.

Effects of hypothermia on energy metabolism in rat and Richardson's ground squirrel hearts

Glycolysis, glucose oxidation, palmitate oxidation, and cardiac function were measured in isolated working hearts from ground squirrels and rats subjected to a hypothermia-rewarming protocol. Hearts were perfused initially for 30 min at 37 degrees C, followed by 2 h of hypothermic perfusion at 15 degrees C, after which hearts were rewarmed to 37 degrees C and further perfused for 30 min. Functional recovery in ground squirrel hearts was greater than in rat hearts after rewarming. Hypothermia-rewarming had a similar general effect on the various metabolic pathways in both species. Despite these similarities, total energy substrate metabolic rates were greater in rat than ground squirrel hearts during hypothermia despite a lower level of work being performed by the rat hearts, indicating that rat hearts are less efficient than ground squirrel hearts during hypothermia. After rewarming, energy substrate metabolism recovered completely in both species, although cardiac work remained depressed in rat hearts. The difference in functional recovery between rat and ground squirrel hearts after rewarming cannot be explained by general differences in energy substrate metabolism during hypothermia or after rewarming.

Sarcolemmal phosphatidylethanolamine reorganization during simulated ischaemia and reperfusion: reversibility and ATP dependency

Exposure of cultured neonatal rat heart cells to simulated ischaemia results in a cessation of the spontaneous contractile activity and changes at both the level of sarcolemmal phospholipid topology and the ultrastructural level. Reperfusion at a timepoint before irreversible cell damage develops leads to a recovery of contractile activity. Furthermore, the shift in transbilayer distribution of sarcolemmal phosphatidylethanolamine in favour of the outer membrane leaflet, due to the ischaemic period, is reversed during subsequent reperfusion. Also the morphological changes (mitochondrial oedema, reorganization of the mitochondrial cristae and the formation of extrusions at the sarcolemma) are reversible. At the same time total intracellular ATP levels are restored to 80% of control. The role of cellular ATP content on sarcolemmal phospholipid topology was further studied by the use of the calcium antagonist verapamil (10 microM), which preserved cellular ATP content by inhibiting cell contractility before the onset of ischaemia. After 120 min of ischaemia, cell ATP content was still 63% of control in the presence of verapamil, versus 20% of control in untreated cells. Verapamil treatment also prevented the loss of the asymmetrical distribution of phosphatidylethanolamine and sarcolemmal disruption, the latter occurring during 120 min of ischaemia in untreated cells. It is proposed that maintenance of phospholipid asymmetry of the sarcolemma of the myocytes depends on the cellular ATP concentrations, indicating the involvement of an ATP dependent aminophospholipid translocase.

Change of Na+ pump current reversal potential in sheep cardiac Purkinje cells with varying free energy of ATP hydrolysis

1. The Na(+)-K+ pump current, Ip, of cardioballs from isolated sheep cardiac Purkinje cells was measured at 30-34 degrees C by means of whole-cell recording. 2. Under physiological conditions Ip is an outward current. Experimental conditions which cause a less negative free energy of intracellular ATP hydrolysis (delta GATP) and steeper sarcolemmal gradients for the pumped Na+ and Cs+ ions evoked an Ip in the inward direction over a wide range of membrane potentials. The reversal of the Ip direction was reversible. 3. The inwardly directed Ip increased with increasingly negative membrane potentials and amounted to -0.13 +/- 0.03 microA cm-2 (mean +/- S.E.M.; n = 6) at -95 mV. 4. The reversal potential (Erev) of Ip was studied as a function of delta GATP at constant sarcolemmal gradients of the pumped cations. 5. In order to vary delta GATP the cell interior was dialysed with patch pipette solutions containing 10 mM ATP and different concentrations of ADP and inorganic phosphate. The media were composed to produce delta GATP levels of about -58, -49 and -39 kJ mol-1. 6. A less negative delta GATP shifted Erev to more positive membrane potentials. From measurements of Ip as a function of membrane potential Erev was estimated to be -195, -115 and -60 mV at delta GATP levels of approximately -58, -49 and -39 kJ mol-1, respectively. The calculated Erev amounted to -224 mV at delta GATP approximately -58 kJ mol-1, -126 mV at delta GATP approximately 49 kJ mol-1 and -24 mV at delta GATP approximately -39 kJ mol-1. 7. Possible reasons for the discrepancy between estimated and calculated Erev values are discussed. 8. Shifting delta GATP to less negative values not only altered Erev but also diminished Ip at each membrane potential tested. The maximal Ip (Ip,max), which can be activated by external Cs+ (Cs+o), decreased under these conditions, whereas [Cs+]o causing half-maximal Ip activation remained unchanged. Similarly, the voltage dependence of Ip activation by Cs+o was unaffected. 9. It is concluded that Erev of Ip varies with delta GATP at constant sarcolemmal gradients of the pumped cations. This agrees with thermodynamic considerations.

Glycolysis vs. respiration as ATP source for the shape of quiescent cardiomyocytes

ATP concentration ([ATP]) and the fraction of rod-shaped myocytes (Rrod) were measured for quiescent ventricular cells isolated from the rat heart. The myocytes were gently shaken with metabolic inhibitors for 30 min at 37 degrees C, and then [ATP] was assayed enzymatically. [ATP] (mean +/- SE in nmol.(10(6) cells)-1) of the control group (80.2 +/- 6.3) was significantly reduced to 32.3 +/- 3.6 and 21.9 +/- 4.5 by inhibiting glycolysis with 2-deoxyglucose (DG, 10 mM) and iodoacetic acid (IAA, 2 mM), respectively. Inhibition of the oxidative phosphorylation by sodium cyanide (CN, 2 mM) or 2,4-dinitrophenol (DNP, 40 microM) did not induce significant [ATP] reduction (56.5 +/- 5.6 or 53.2 +/- 7.5). Rrod were 54.4 +/- 1.9, 47.3 +/- 5.1, 43.3 +/- 3.3, 28.5 +/- 3.6 and 11.6 +/- 1.8% for the control, CN, DNP, DG and IAA groups, respectively. Under glycolytic inhibition with DG, addition of pyruvate (5 mM) with a pyruvate dehydrogenase activator (dichloroacetate (DCA, 1 mM) or (-)isoproterenol (ISO, 10 microM)) restored [ATP] to 52.5 +/- 8.3 and 67.7 +/- 7.2, respectively, but Rrod still remained depressed (34.6 +/- 2.1 and 35.4 +/- 1.3%). We conclude that the normal shape of quiescent myocytes is maintained primarily by glycolytic ATP.