Mechanism of hypoxic K loss in rabbit ventricle

The mechanisms of potassium loss in acute myocardial ischemia: New insights from computational simulations

Acute myocardial ischemia induces hyperkalemia (accumulation of extracellular potassium), a major perpetrator of lethal reentrant ventricular arrhythmias. Despite considerable experimental efforts to explain this pathology in the last decades, the intimate mechanisms behind hyperkalemia remain partially unknown. In order to investigate these mechanisms, we developed a novel computational model of acute myocardial ischemia which couples a) an electrophysiologically detailed human cardiomyocyte model that incorporates modifications to account for ischemia-induced changes in transmembrane currents, with b) a model of cardiac tissue and extracellular K ⁺ transport. The resulting model is able to reproduce and explain the triphasic time course of extracellular K ⁺ concentration within the ischemic zone, with values of [ K + ] o close to 14 mmol/L in the central ischemic zone after 30 min. In addition, the formation of a [ K + ] o border zone of approximately 1.2 cm 15 min after the onset of ischemia is predicted by the model. Our results indicate that the primary rising phase of [ K + ] o is mainly due to the imbalance between K ⁺ efflux, that increases slightly, and K ⁺ influx, that follows a reduction of the NaK pump activity by more than 50%. The onset of the plateau phase is caused by the appearance of electrical alternans (a novel mechanism identified by the model), which cause an abrupt reduction in the K ⁺ efflux. After the plateau, the secondary rising phase of [ K + ] o is caused by a subsequent imbalance between the K ⁺ influx, which continues to decrease slowly, and the K ⁺ efflux, which remains almost constant. Further, the study shows that the modulation of these mechanisms by the electrotonic coupling is the main responsible for the formation of the ischemic border zone in tissue, with K ⁺ transport playing only a minor role. Finally, the results of the model indicate that the injury current established between the healthy and the altered tissue is not sufficient to depolarize non-ischemic cells within the healthy tissue.

Coordinate adaptations of skeletal muscle and kidney to maintain extracellular [K+] during K+-deficient diet

Extracellular fluid (ECF) potassium concentration ([K+]) is maintained by adaptations of kidney and skeletal muscle, responses heretofore studied separately. We aimed to determine how these organ systems work in concert to preserve ECF [K+] in male C57BL/6J mice fed a K+-deficient diet (0K) versus 1% K+ diet (1K) for 10 days (n = 5-6/group). During 0K feeding, plasma [K+] fell from 4.5 to 2 mM; hindlimb muscle (gastrocnemius and soleus) lost 28 mM K+ (from 115 ± 2 to 87 ± 2 mM) and gained 27 mM Na+ (from 27 ± 0.4 to 54 ± 2 mM). Doubling of muscle tissue [Na+] was not associated with inflammation, cytokine production or hypertension as reported by others. Muscle transporter adaptations in 0K- versus 1K-fed mice, assessed by immunoblot, included decreased sodium pump α2-β2 subunits, decreased K+-Cl- cotransporter isoform 3, and increased phosphorylated (p) Na+,K+,2Cl- cotransporter isoform 1 (NKCC1p), Ste20/SPS-1-related proline-alanine rich kinase (SPAKp), and oxidative stress-responsive kinase 1 (OSR1p) consistent with intracellular fluid (ICF) K+ loss and Na+ gain. Renal transporters' adaptations, effecting a 98% reduction in K+ excretion, included two- to threefold increased phosphorylated Na+-Cl- cotransporter (NCCp), SPAKp, and OSR1p abundance, limiting Na+ delivery to epithelial Na+ channels where Na+ reabsorption drives K+ secretion; and renal K sensor Kir 4.1 abundance fell 25%. Mass balance estimations indicate that over 10 days of 0K feeding, mice lose ~48 μmol K+ into the urine and muscle shifts ~47 μmol K+ from ICF to ECF, illustrating the importance of the concerted responses during K+ deficiency.

Cardiac sympathectomy and spinal cord stimulation attenuate reflex-mediated norepinephrine release during ischemia preventing ventricular fibrillation

The purpose of this study was to define the mechanism by which cardiac neuraxial decentralization or spinal cord stimulation (SCS) reduces ischemia-induced ventricular fibrillation (VF). Direct measurements of norepinephrine (NE) levels in the left ventricular interstitial fluid (ISF) by microdialysis, in response to transient (15-minute) coronary artery occlusion (CAO), were performed in anesthetized canines. Responses were studied in animals with intact neuraxes and were compared with those in which the intrathoracic component of the cardiac neuraxes (stellate ganglia) or the intrinsic cardiac neuronal (ICN) system was surgically delinked from the central nervous system and those with intact neuraxes with preemptive SCS (T1-T3). With intact neuraxes, animals with exaggerated NE release due to CAO were at increased risk for VF. During CAO, there was a 152% increase in NE when the neuraxes were intact compared with 114% following stellate decentralization and 16% following ICN decentralization. During SCS, CAO NE levels increased by 59%. Risk for CAO-induced VF was 38% in controls, 8% following decentralization, and 11% following SCS. These data indicate that ischemia-related afferent neuronal transmission differentially engages central and intrathoracic sympathetic reflexes and amplifies sympathoexcitation. Differences in regional ventricular NE release are associated with increased risk for VF. Surgical decentralization or SCS reduced NE release and VF.

Non-Mammalian Vertebrates: Distinct Models to Assess the Role of Ion Gradients in Energy Expenditure

Animals store metabolic energy as electrochemical gradients. At least 50% of mammalian energy is expended to maintain electrochemical gradients across the inner mitochondrial membrane (H⁺), the sarcoplasmic reticulum (Ca⁺⁺), and the plasma membrane (Na⁺/K⁺). The potential energy of these gradients can be used to perform work (e.g., transport molecules, stimulate contraction, and release hormones) or can be released as heat. Because ectothermic species adapt their body temperature to the environment, they are not constrained by energetic demands that are required to maintain a constant body temperature. In fact, ectothermic species expend seven to eight times less energy than similarly sized homeotherms. Accordingly, ectotherms adopt low metabolic rates to survive cold, hypoxia, and extreme bouts of fasting that would result in energy wasting, lactic acidosis and apoptosis, or starvation in homeotherms, respectively. Ectotherms have also evolved unique applications of ion gradients to allow for localized endothermy. Endothermic avian species, which lack brown adipose tissue, have been integral in assessing the role of H⁺ and Ca⁺⁺ cycling in skeletal muscle thermogenesis. Accordingly, the diversity of non-mammalian vertebrate species allows them to serve as unique models to better understand the role of ion gradients in heat production, metabolic flux, and adaptation to stressors, including obesity, starvation, cold, and hypoxia.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Ventricular repolarization markers for predicting malignant arrhythmias in clinical practice

Malignant cardiac arrhythmias which result in sudden cardiac death may be present in individuals apparently healthy or be associated with other medical conditions. The way to predict their appearance represents a challenge for the medical community due to the tragic outcomes in most cases. In the last two decades some ventricular repolarization (VR) markers have been found to be useful to predict malignant cardiac arrhythmias in several clinical conditions. The corrected QT, QT dispersion, Tpeak-Tend, Tpeak-Tend dispersion and Tp-e/QT have been studied and implemented in clinical practice for this purpose. These markers are obtained from 12 lead surface electrocardiogram. In this review we discuss how these markers have demonstrated to be effective to predict malignant arrhythmias in medical conditions such as long and short QT syndromes, Brugada syndrome, early repolarization syndrome, acute myocardial ischemia, heart failure, hypertension, diabetes mellitus, obesity and highly trained athletes. Also the main pathophysiological mechanisms that explain the arrhythmogenic predisposition in these diseases and the basis for the VR markers are discussed. However, the same results have not been found in all conditions. Further studies are needed to reach a global consensus in order to incorporate these VR parameters in risk stratification of these patients.

Multiscale computational analysis of the bioelectric consequences of myocardial ischaemia and infarction

Ischaemic heart disease is considered as the single most frequent cause of death, provoking more than 7 000 000 deaths every year worldwide. A high percentage of patients experience sudden cardiac death, caused in most cases by tachyarrhythmic mechanisms associated to myocardial ischaemia and infarction. These diseases are difficult to study using solely experimental means due to their complex dynamics and unstable nature. In the past decades, integrative computational simulation techniques have become a powerful tool to complement experimental and clinical research when trying to elucidate the intimate mechanisms of ischaemic electrophysiological processes and to aid the clinician in the improvement and optimization of therapeutic procedures. The purpose of this paper is to briefly review some of the multiscale computational models of myocardial ischaemia and infarction developed in the past 20 years, ranging from the cellular level to whole-heart simulations.

Effect of reoxygenation on the electrical stability of the rat heart in vivo: a chronobiological study

Reoxygenation following hypoxic episodes can increase the risk for the development of ventricular arrhythmias, which, in addition to circadian aspects of reoxygenation arrhythmias has not been studied extensively. The aim of the present study was to evaluate circadian changes in the electrical stability of the rat heart during reoxygenation following a hypoventilatory episode. The electrical stability of the heart, defined in the present study as the ventricular arrhythmia threshold (VAT), was measured at 3 h intervals at clock times 09:00, 12:00, 15:00, 18:00, 21:00, 24:00, 03:00, 06:00 and 09:00 during 20 min hypoventilation (20 breaths/min, tidal volume = 0.5 ml/100 g body weight [n=17]) and subsequent 20 min reoxygenation (50 breaths/min, tidal volume = 1 ml/100 g body weight [n=4]) intervals. The experiments were performed using pentobarbital-anesthetized (40 mg/kg intraperitoneally) female Wistar rats that first underwent a four-week adaptation to a 12 h light:12 h dark regimen. Detailed analysis showed that circadian VATs changed to biphasic rhythms at 10 min of hypoventilation. The VAT circadian rhythms were observed immediately following the commencement of reoxygenation, with the highest values measured between 12:00 and 15:00, and the lowest values between 24:00 and 03:00. These results suggest that myocardial vulnerability is dependent on the light:dark cycle and characteristics of pulmonary ventilation.

Nutrient restriction preserves calcium cycling and mitochondrial function in cardiac myocytes during ischemia and reperfusion

Nutrient restriction (NR) prolongs longevity via enhanced mitochondrial function. We tested the hypothesis that NR enhances resistance to ischemia/reperfusion (IR) arrhythmias via preserved calcium (Ca) cycling and mitochondrial function. We examined the protective effects of NR on regional IR in cultured neonatal rat ventricular myocyte monolayers. Optical mapping of intracellular Ca and mitochondrial membrane potential Δψ(m) was performed using Rhod 2-AM and TMRE, respectively. Regional ischemia was mimicked by covering a portion of monolayer with a glass coverslip until loss of Ca propagation, and reperfusion was mimicked by removing the coverslip. NR was mimicked by culture in serum- and glucose-free medium for 24 h. Relative to controls, NR monolayers: (1) sustained Ca oscillations during longer periods of ischemia (19.2 ± 1.8 min vs 10.4 ± 1.4 min, p<0.001); (2) had attenuated increases in Ca transient duration (CaD) and time decay constant (Tau) during ischemia; (3) had preserved conduction velocity (CV) during early reperfusion, leading to protection against reperfusion arrhythmias; (4) had minimal "rebound" decreased CaD and Tau during reperfusion; and (5) had no depolarization of Δψ(m) during IR. NR attenuates IR arrhythmias via (1) stable calcium cycling and (2) prevention of Δψ(m) depolarization during IR. Enhanced mitochondrial resistance to IR arrhythmias may play a role in NR-induced longevity prolongation.

Marked QT prolongation and ventricular tachycardia of a transient nature in young children with cardiomyopathy

Significant prolongation of the QT interval in pediatric patients with cardiomyopathy is rare. We report two cases of dilated myopathy with transient and dramatic QT prolongation. Both had associated ventricular arrhythmias, with one having torsade de pointes, and the other nonsustained ventricular tachycardia. Normalization of their QTc occurred as their ventricular function improved.

Increased myofilament Ca2+-sensitivity and arrhythmia susceptibility

Increased myofilament Ca(2+) sensitivity is a common attribute of many inherited and acquired cardiomyopathies that are associated with cardiac arrhythmias. Accumulating evidence supports the concept that increased myofilament Ca(2+) sensitivity is an independent risk factor for arrhythmias. This review describes and discusses potential underlying molecular and cellular mechanisms how myofilament Ca(2+) sensitivity affects cardiac excitation and leads to the generation of arrhythmias. Emphasized are downstream effects of increased myofilament Ca(2+) sensitivity: altered Ca(2+) buffering/handling, impaired energy metabolism and increased mechanical stretch, and how they may contribute to arrhythmogenesis.

Cariporide given during resuscitation promotes return of electrically stable and mechanically competent cardiac activity

Episodes of ventricular fibrillation (VF) and myocardial dysfunction commonly occur after cardiac resuscitation compromising the return of stable circulation. We investigated in a pig model of VF whether limiting Na(+)-induced cytosolic Ca(2+) overload using the sarcolemmal sodium-hydrogen exchanger isoform-1 (NHE-1) inhibitor cariporide promotes resuscitation with stable circulation.

METHODS

VF was electrically induced in 20 male pigs and left untreated for 6 min after which CPR was initiated and continued for 8 min before attempting defibrillation. Pigs were randomized to receive 3-mg/kg cariporide (n=10) or 0.9%-NaCl (n=10) before chest compression.

RESULTS

Seven of 10 pigs in each group were successfully resuscitated and survived 2h. Cariporide ameliorated post-resuscitation ventricular ectopic activity such that fewer singlets (5+/-5 vs. 26+/-21; p<0.05) and fewer bigemini (1+/-3 vs. 33+/-25; p<0.05) were observed during the initial 5 min post-resuscitation. Additionally, cariporide-treated pigs did not require additional post-resuscitation shocks for ventricular tachycardia or recurrent VF (0.0+/-0.0 vs. 5.3+/-7.8 shocks; p=0.073). During the initial 60 min cariporide-treated pigs had higher, cardiac index (6.1+/-0.7 vs. 4.4+/-1.1L/min/m(2); p<0.01), left ventricular stroke work index (45+/-9 vs. 36+/-10 gmm/beat/m(2); p<0.05), and numerically higher mean aortic pressure (104+/-11 vs. 91+/-12 mmHg; p=0.054).

CONCLUSION

Cariporide administered at the start of chest compression may help restore electrically and mechanically stable circulation after resuscitation from cardiac arrest.

Modeling of cardiac ischemia in human myocytes and tissue including spatiotemporal electrophysiological variations

Cardiac tissue exhibits spatially heterogeneous electrophysiological properties. In cardiac diseases, these properties also change in time. This study introduces a framework to investigate their role in cardiac ischemia using mathematical modeling and computational simulations at cellular and tissue level. Ischemia was incorporated by reproducing effects of hyperkalemia, acidosis, and hypoxia with a human electrophysiological model. In tissue, spatial heterogeneous ischemia was described by central ischemic (CIZ) and border zone. Anisotropic conduction was simulated with a bidomain approach in an anatomical ventricle model including realistic fiber orientation and transmural, apico-basal, and interventricular electrophysiological heterogeneities. A model of electrical conductivity in a human torso served for ECG calculations. Ischemia increased resting but reduced peak voltage, action potential duration, and upstroke velocity. These effects were strongest in subepicardial cells. In tissue, conduction velocity decreased towards CIZ but effective refractory period increased. At 10 min of ischemia 19% of subepi- and 100% of subendocardial CIZ cells activated with a delay of 34.6+/-7.8 ms and 55.9+/-18.8 ms, respectively, compared to normal. Significant ST elevation and premature T wave end appeared only with the subepicardial CIZ. The model reproduced effects of ischemia at cellular and tissue level. The results suggest that the presented in silico approach can complement experimental studies, e.g., in understanding the role of ischemia or the onset of arrhythmia.

Epoxyeicosatrienoic acid prevents postischemic electrocardiogram abnormalities in an isolated heart model

Cytochrome P450 epoxygenases metabolize arachidonic acid (AA) to epoxyeicosatrienoic acids (EETs) which are in turn converted to dihydroxyeicosatrienoic acids (DHETs) by soluble epoxide hydrolase (sEH). The main objective of this study was to investigate the protective effects of EETs following ischemic injury using an ex vivo electrocardiogram (EKG) model. Hearts from C57Bl/6, transgenic mice with cardiomyocyte-specific overexpression of CYP2J2 (Tr) and wildtype (WT) littermates were excised and perfused with constant pressure in a Langendorff apparatus. Electrodes were placed superficially at the right atrium and left ventricle to assess EKG waveforms. In ischemic reperfusion experiments hearts were subjected to 20 min of global no-flow ischemia followed by 20 min of reperfusion (R20). The EKG from C57Bl/6 hearts perfused with 1 microM 14,15-EET showed less QT prolongation (QTc) and ST elevation (STE) (QTc=41+/-3, STE=2.3+/-0.3; R20: QTc=42+/-2 ms, STE=1.2+/-0.2mv) than control hearts (QTc=36+/-2, STE=2.3+/-0.2; R20: QTc=53+/-3 ms; STE=3.6+/-0.4mv). Similar results of reduced QT prolongation and ST elevation were observed in EKG recording from CYP2J2 Tr mice (QTc=35+/-1, STE=1.9+/-0.1; R20: QTc=38+/-4 ms, STE=1.3+/-0.2mv) compared to WT hearts. The putative epoxygenase inhibitor MS-PPOH (50 microM) and EET antagonist 14,15-EEZE (10 microM) both abolished the cardioprotective response, implicating EETs in this process. In addition, separate exposure to the K(ATP) channel blockers glibenclamide (1 microM) and HMR1098 (10 microM), or the PKA protein inhibitor H89 (50 nM) during reperfusion abolished the improved repolarization in both the models. Consistent with a role of PKA, CYP2J2 Tr mice had an enhanced activation of the PKAalpha regulatory II subunit in plasma membrane following IR injury. The present data demonstrate that EETs can enhance the recovery of ventricular repolarization following ischemia, potentially by facilitating activation of K(+) channels and PKA-dependent signaling.

Phospholipid Lysophosphatidylcholine as a Metabolic Trigger and HERG as an Ionic Pathway for Extracellular K+ Accumulation and “Short QT Syndrome” in Acute Myocardial Ischemia

The most profound abnormalities during acute myocardial ischemia are extracellular K(+) accumulation ([K(+)](o)- upward arrow) and shortening of action potential duration or QT interval (APD- downward arrow or QT- downward arrow), which are pivotal in the genesis of ischemic arrhythmias and sudden cardiac death. The ionic mechanisms however remained obscured. We performed studies in a rabbit model of acute global myocardial ischemia in order to explore ionic and metabolic mechanisms for ischemic [K(+)](o)- upward arrow and QT- downward arrow. Exogenous 1-palmitoyl-lysophosphatidylcholine (LPC-16) mimicked the low-perfusion ischemia to produce significant [K(+)](o)- upward arrow and QT- downward arrow. The [K(+)](o)- upward arrow and QT- downward arrow induced by either LPC-16 or ischemia were prevented by dofetilide, a blocker of rapid delayed rectifier K(+) current (I(Kr)), but not by blockers for other K(+) channels. Consistently, dofetilide efficiently abolished the ventricular tachy-arrhythmias induced by ischemia or LPC-16. LPC-16 remarkably shortened APD and enhanced the function of I(Kr) and HERG (the pore-forming subunit of I(Kr)). The effects of LPC-16 manifested with shorter APD (faster repolarization rate) and at more negative potential (membrane repolarization). Dofetilide abolished the I(Kr)/HERG enhancing and APD shortening effects of LPC-16. Our results suggest that LPC-16 accumulation/HERG enhancement may be a link between metabolic trigger and ionic pathway for ischemic [K(+)](o)- upward arrow and QTc- downward arrow. This represents the first documentation of I(Kr)/HERG as the ionic mechanism in ischemic [K(+)](o)- upward arrow and QTc- downward arrow. Inhibition of LPC-16 production and accumulation and/or of I(Kr)/HERG may be a promising therapeutic strategy to attenuate the incidence of lethal arrhythmias associated with ischemic heart disease.

The balance between inactivation and activation of the Na+-K+ pump underlies the triphasic accumulation of extracellular K+ during myocardial ischemia

Ischemia-induced hyperkalemia (accumulation of extracellular K(+)) predisposes the heart to the development of lethal reentrant ventricular arrhythmias. This phenomenon exhibits a triphasic time course and is thought to be mediated by a combination of three mechanisms: 1) increased cellular K(+) efflux, 2) decreased cellular K(+) influx, and 3) shrinkage of the extracellular space. These ischemia-induced electrophysiological changes are driven by an impaired cellular metabolism. However, the relative contributions of these mechanisms, as well as the origin of the triphasic profile, have proven to be difficult to determine experimentally. In this study, the changes in metabolite concentrations that arise during 15 min of zero-flow global ischemia were incorporated into a dynamic model of cellular electrophysiology, which was extended to include a metabolically sensitive description of the Na(+)-K(+) pump and ATP-sensitive K(+) channel, in addition to cell volume regulation. The coupling of altered K(+) fluxes and cell volume regulation enables an integrative simulation of ischemic hyperkalemia. These simulations were able to quantitatively reproduce experimental measurements of the accumulation of extracellular K(+) during 15 min of simulated ischemia, both with respect to the degree of K(+) loss as well as the triphasic time course. Analysis of the model indicates that the inhibition of the Na(+)-K(+) pump is the dominant factor underlying this hyperkalemic behavior, accounting for approximately 85% of the observed extracellular K(+) accumulation. It was found that the balance between activation and inhibition of the Na(+)-K(+) pump, affected by the changing metabolite and ion concentrations (in particular, [ADP]), give rise to the triphasic profile associated with ischemic hyperkalemia.

[Coronary artery disease--relevance of total coronary revascularization on the incidence of malignant arrhythmias]

Myocardial ischemia induces redistribution of different ions (H(+), K(+), Na(+), Ca(++)) across the cardiomyocyte membrane, as well as the loss of intracellular ATP content. This results in changes in the electrical properties including shortening of the action potential, appearance of delayed afterpotentials, and a modified refractoriness of the cardiomyocyte. These changes may induce or support malignant cardiac arrhythmias. Supersensitivity of sympathetic denervated myocardium may further support the electrical instability of ischemic myocardium.Virtues of studies indicate that patients with coronary artery disease who develop complex arrhythmias during or after exercise bear a substantially increased risk for sudden cardiac death. Other studies report about arrhythmic stabilization and reduced mortality if patients with reversible myocardial ischemia receive complete revascularization. However, none of these studies is without methodological flaws. Due to the lack of methodologically sound studies in sufficiently large patient cohorts, the question whether complete coronary revascularisation improves the prognosis of patients with coronary artery disease and which strategy (medical, interventional, or surgical) warrants the best outcomes remains open.

Beneficial effects of carbon monoxide-releasing molecules on post-ischemic myocardial recovery

There is increasing evidence corroborating a protective role of carbon monoxide releasing molecules (CORMs) in injured tissues. Carbon monoxide (CO) carriers have been recently developed as a pharmacological tool to simulate the effect of heme oxygenase-1-derived CO. The effects of CORM-3, a water-soluble CO releaser, on the incidence of reperfusion-induced ventricular fibrillation (VF) and tachycardia (VT) were studied in isolated rat hearts. Hearts were treated with different doses of CORM-3 before the induction of 30 min global ischemia followed by 120 min reperfusion. We found that at concentrations of 25 microM and 50 microM of CORM-3 promoted a significant reduction in the incidence of VF and VT. Thus, the incidence of VF was reduced by 67% (p<0.05) and 92% (p<0.05) with 25 microM and 50 microM of CORM-3, respectively. The protective effect of CORM-3 on the incidence of VT followed the same pattern. The antiarrhythmic protection was associated with a marked attenuation in infarct size, significant decreases in cellular Na(+) and Ca(2+) gains and K(+) loss. Consequently, the recovery of post-ischemic function was significantly improved. In conclusion, CORM-3 exerts beneficial effects against ischemia/reperfusion-induced injury through its abilities to release CO which mediates a cardioprotective action by regulating tissue Na(+), K(+), and Ca(2+) levels.

Prolongation of the QTc interval is seen uniformly during early transmural ischemia

OBJECTIVES

In order to more clearly understand the electrocardiographic manifestations of early transmural ischemia, we studied electrocardiograms (ECGs) in patients undergoing balloon angioplasty.

BACKGROUND

Decisions regarding reperfusion strategies in patients with acute myocardial infarction rely largely on the presence of ST-segment elevation (STE) in the ECG, consequently with significant limitations. Studies of the "ischemic cascade" show that ST-segment changes occur well after the onset of wall motion abnormalities.

METHODS

We prospectively analyzed ECGs obtained at 20-s intervals in 74 patients undergoing elective balloon angioplasty. The ECGs were analyzed using 3 methodologies. In 74 patients, the ST-segment, the T-wave, and the QT-interval were analyzed using the MUSE (General Electric HC, Menomonee Falls, Wisconsin) automated system (MUSE). Fifty patients were also analyzed using the Interval Editor automated system (IE; General Electric HC). In 20 patients, measurements were made manually.

RESULTS

Transmural ischemia prolonged the QTc interval (using the Bazett's formula) in 100% of patients. In all 74 patients analyzed with MUSE, QTc interval prolonged from 423 +/- 25 ms to 455 +/- 34 ms (p < 0.001). In the 50 patients analyzed with IE, QTc interval prolonged in 50 of 50 (100%) patients (from 424 +/- 27 ms to 458 +/- 33 ms [p < 0.001]). Mean time to maximal QTc interval prolongation, changes in T-wave polarity, > or =1 mm STE, and ST-segment depression (STD) were 22, 24, 29, and 35 s, respectively. Although QTc interval prolonged in 100% of patients, T-wave changes, STE, and STD (> or =1 mm) occurred in 7%, 15%, and 7%, respectively.

CONCLUSIONS

The QTc interval prolongs in 100% of patients with early transmural ischemia. When compared with clinically accepted indexes of transmural ischemia (i.e., STD and STE [> or =1 mm]) it is the earliest ECG abnormality.

Thematic review series: Systems Biology Approaches to Metabolic and Cardiovascular Disorders. Network perspectives of cardiovascular metabolism

In this review, we examine cardiovascular metabolism from three different, but highly complementary, perspectives. First, from the abstract perspective of a metabolite network, composed of nodes and links. We present fundamental concepts in network theory, including emergence, to illustrate how nature has designed metabolism with a hierarchal modular scale-free topology to provide a robust system of energy delivery. Second, from the physical perspective of a modular spatially compartmentalized network. We review evidence that cardiovascular metabolism is functionally compartmentalized, such that oxidative phosphorylation, glycolysis, and glycogenolysis preferentially channel ATP to ATPases in different cellular compartments, using creatine kinase and adenylate kinase to maximize efficient energy delivery. Third, from the dynamics perspective, as a network of dynamically interactive metabolic modules capable of self-oscillation. Whereas normally, cardiac metabolism exists in a regime in which excitation-metabolism coupling closely matches energy supply and demand, we describe how under stressful conditions, the network can be pushed into a qualitatively new dynamic regime, manifested as cell-wide oscillations in ATP levels, in which the coordination between energy supply and demand is lost. We speculate how this state of "metabolic fibrillation" leads to cell death if not corrected and discuss the implications for cardioprotection.

Modeling cardiac ischemia

Myocardial ischemia is one of the main causes of sudden cardiac death, with 80% of victims suffering from coronary heart disease. In acute myocardial ischemia, the obstruction of coronary flow leads to the interruption of oxygen flow, glucose, and washout in the affected tissue. Cellular metabolism is impaired and severe electrophysiological changes in ionic currents and concentrations ensue, which favor the development of lethal cardiac arrhythmias such as ventricular fibrillation. Due to the burden imposed by ischemia in our societies, a large body of research has attempted to unravel the mechanisms of initiation, sustenance, and termination of cardiac arrhythmias in acute ischemia, but the rapidity and complexity of ischemia-induced changes as well as the limitations in current experimental techniques have hampered evaluation of ischemia-induced alterations in cardiac electrical activity and understanding of the underlying mechanisms. Over the last decade, computer simulations have demonstrated the ability to provide insight, with high spatiotemporal resolution, into ischemic abnormalities in cardiac electrophysiological behavior from the ionic channel to the whole organ. This article aims to review and summarize the results of these studies and to emphasize the role of computer simulations in improving the understanding of ischemia-related arrhythmias and how to efficiently terminate them.

Dynamic responses of single cardiomyocytes to graded ischemia studied by oxygen clamp in on-chip picochambers

Single mouse cardiomyocytes were exposed to defined ischemia. We designed chambers on glass chips with a volume of 192 pL (picochambers). After a picochamber was loaded with a single cardiomyocyte, P(O2) in the picochamber was equilibrated with that in the headspace, where it was controlled in the critical range between <0.2 and 10 mm Hg. Because the extracellular fluid volume in a picochamber was restricted, these conditions are close to tissue ischemia. Responses of the sarcolemmal K(ATP)-channel current (I(KATP)), the production of reactive oxygen species (ROS), and the mitochondrial membrane potential (delta psi) of single cardiomyocytes to graded ischemia and, in particular, to rapid changes of the ischemic grade by defined oxygen steps were studied. The results show that I(KATP) is readily activated during ischemia and that the grade of ischemia tightly controls the amplitude of I(KATP). Furthermore, maximal ischemia-induced I(KATP) was similar when it followed either reoxygenation or reperfusion, suggesting that there is no major autocrine modulation of maximal I(KATP) during ischemia. A P(O2) staircase from <0.2 to 10 mm Hg increased the ROS signal, starting already at a P(O2) of approximately 0.3 mm Hg. With a similar P(O2) staircase, delta psi first hyperpolarized and then, above 1 mm Hg, depolarized. The depolarizing response of delta psi at a P(O2) of >1 mm Hg could be blocked by increasing the antioxidant defense with glutathione-monoethyl ester. It is concluded that in an ischemic cardiomyocyte I(KATP) is essentially controlled by Po(2) and that at low P(O2) delta psi is balanced by oxygen-induced hyperpolarization and ROS-induced depolarization.

Mechanisms of sudden cardiac death

Despite recent advances in preventing sudden cardiac death (SCD) due to cardiac arrhythmia, its incidence in the population at large has remained unacceptably high. Better understanding of the interaction among various functional, structural, and genetic factors underlying the susceptibility to, and initiation of, fatal arrhythmias is a major goal and will provide new tools for the prediction, prevention, and therapy of SCD. Here, we review the role of aberrant intracellular Ca handling, ionic imbalances associated with acute myocardial ischemia, neurohumoral changes, and genetic predisposition in the pathogenesis of SCD due to cardiac arrhythmia. Therapeutic measures to prevent SCD are also discussed.

Role of ATP-sensitive K+ channels in electrophysiological alterations during myocardial ischemia: a study using Kir6.2-null mice

The role of cardiac ATP-sensitive K(+) (K(ATP)) channels in ischemia-induced electrophysiological alterations has not been thoroughly established. Using mice with homozygous knockout (KO) of Kir6.2 (a pore-forming subunit of cardiac K(ATP) channel) gene, we investigated the potential contribution of K(ATP) channels to electrophysiological alterations and extracellular K(+) accumulation during myocardial ischemia. Coronary-perfused mouse left ventricular muscles were stimulated at 5 Hz and subjected to no-flow ischemia. Transmembrane potential and extracellular K(+) concentration ([K(+)](o)) were measured by using conventional and K(+)-selective microelectrodes, respectively. In wild-type (WT) hearts, action potential duration (APD) at 90% repolarization (APD(90)) was significantly decreased by 70.1 +/- 5.2% after 10 min of ischemia (n = 6, P < 0.05). Such ischemia-induced shortening of APD(90) did not occur in Kir6.2-deficient (Kir6.2 KO) hearts. Resting membrane potential in WT and Kir6.2 KO hearts similarly decreased by 16.8 +/- 5.6 (n = 7, P < 0.05) and 15.0 +/- 1.7 (n = 6, P < 0.05) mV, respectively. The [K(+)](o) in WT hearts increased within the first 5 min of ischemia by 6.9 +/- 2.5 mM (n = 6, P < 0.05) and then reached a plateau. However, the extracellular K(+) accumulation similarly occurred in Kir6.2 KO hearts and the degree of [K(+)](o) increase was comparable to that in WT hearts (by 7.0 +/- 1.7 mM, n = 6, P < 0.05). In Kir6.2 KO hearts, time-dependent slowing of conduction was more pronounced compared with WT hearts. In conclusion, the present study using Kir6.2 KO hearts provides evidence that the activation of K(ATP) channels contributes to the shortening of APD, whereas it is not the primary cause of extracellular K(+) accumulation during early myocardial ischemia.

Na(+) current through KATP channels: consequences for Na(+) and K(+) fluxes during early myocardial ischemia

During early myocardial ischemia, the myocytes are loaded with Na(+), which in turn leads to Ca(2+) overload and cell death. The pathway of the Na(+) influx has not been fully elucidated. The aim of the study was to quantify the Na(+) inward current through sarcolemmal KATP channels (IKATP,Na) in anoxic isolated cardiomyocytes at the actual reversal potential (Vrev) and to estimate the contribution of this current to the Na(+) influx in the ischemic myocardium. IKATP,Na was determined in excised single channel patches of mouse ventricular myocytes and macropatches of Xenopus laevis oocytes expressing SUR2A/Kir6.2 channels. In the presence of K+ ions, the respective permeability ratios for Na(+) to K(+) ions, PNa/PK, were close to 0.01. Only in the presence of Na(+) ions on both sides of the membrane was IKATP,Na similarly large to that calculated from the permeability ratio PNa/PK, indicative of a Na(+) influx that is largely independent of the K+ efflux at Vrev. With the use of a peak KATP channel conductance in anoxic cardiomyocytes of 410 nS, model simulations for a myocyte within the ischemic myocardium showed that the amplitude of the Na(+) influx and K(+) efflux is even larger than the respective fluxes by the Na(+) - K(+) pump and all other background fluxes. These results suggest that during early ischemia the Na(+) influx through KATP channels essentially contributes to the total Na+ influx and that it also balances the K(+) efflux through KATP channels.

Measurement of extracellular pH, K(+), and lactate in ischemic heart

Simultaneous and continuous measurements of extracellular pH, potassium (K(+)), and lactate (L(-)) in ischemic rabbit papillary muscle are presented for the first time. Potentiometric pH and K(+) sensors and an amperometric lactate biosensor were used. These miniature electrodes were previously developed and individually tested for this purpose. The pH sensor was based on an iridium oxide layer electrodeposited on a planar platinum electrode fabricated on a flexible substrate. The potentiometric K(+) sensor was based on a polymeric membrane and valinomycin ionophore. The L(-) biosensor was based on lactate oxidase and an organic conducting salt polarized at 0.15V vs Ag/AgCl reference electrode. The utility of this novel analytical system to cardiovascular research was demonstrated by using the system to study the interrelationship of cellular K(+) and lactate loss in ischemic myocardium, and the role of extracellular pH and buffer capacity on this relationship. The results indicated: (i) sequential brief episodes of ischemia produced reproducible trends of L(-), pH, and K(+) changes during the first three episodes, (ii) extracellular L(-) increased with increasing buffer capacity of extracellular compartment, (iii) the patterns of extracellular L(-) and K(+) changes were not related directly, and (iv) L(-) transport and lactic acid diffusion were not the primary cause of extracellular acidosis during ischemia.

Reduced calcium tolerance in rat cardiomyocytes after myocardial infarction

During ischaemia and reperfusion the intracellular Na+ concentration is elevated in the cardiomyocytes and the cells are depolarized, both favouring reverse mode Na,Ca-exchange loading of the cell with Ca2+. We examined whether cardiomyocytes from rats with congestive heart failure (CHF) and younger rats (HINCX) which both have a high expression of the Na,Ca-exchanger protein (NCX) showed reduced tolerance to extracellular Ca2+. The CHF was induced in Isofluran anaesthetized rats by left coronary artery ligation. Isolated cardiomyocytes were loaded with Fura-2AM and 140 mm Na+ and exposed to 0.05 mm Ca2+. Expression of the Na,Ca-exchanger protein was analysed. Fura-2 340/380 ratio rose more rapidly in HINCX and CHF than in SHAM, and the rise was abolished by Ni2+. Hypercontracture developed more frequently in HINCX and CHF than in SHAM cells. The amount of NCX was 54% higher in HINCX and 76% higher in CHF compared with SHAM. Na+-loaded cardiomyocytes from CHF and HINCX rats are more susceptible to Ca2+ overload than SHAM cells because of the increased capacity for Na,Ca-exchange.

Mechanistic investigation of extracellular K+ accumulation during acute myocardial ischemia: a simulation study

In this study, we have used computer simulations to study the mechanisms of extracellular K+ accumulation during acute ischemia. A modified version of the Luo-Rudy phase II action potential model was used to simulate the electrical behavior of one ventricular myocyte during 14 min of simulated ischemia. Our results show the following: 1) only the integrated effect of activation of ATP-dependent K+ current, an ischemic Na+ inward current, and inhibition of Na(+)-K(+) pump activity in the absence of coronary flow replicates the biphasic time course of extracellular K+ concentration observed during acute ischemia; 2) the time to onset of the plateau phase and the plateau level value are determined by the rate of stimulation and by the rate of alteration of the three mechanisms. However, acidosis and reduction of extracellular volume produce only a slight anticipation of the plateau phase; and 3) cellular K+ loss is mainly due to an increase of K+ efflux via the time-independent K+ current and ATP-dependent K+ current rather than to a decrease of K+ influx.

Intracellular [Na+] and Na+ pump rate in rat and rabbit ventricular myocytes

Intracellular [Na+] ([Na+]i) is centrally involved in regulation of cardiac Ca2+ and contractility via Na+-Ca2+ exchange (NCX) and Na+-H+ exchange (NHX). Previous work has indicated that [Na+]i is higher in rat than rabbit ventricular myocytes. This has major functional consequences, but the reason for the higher [Na+]i in rat is unknown. Here, resting [Na+]i was measured using the fluorescent indicator SBFI, with both traditional calibration and a novel null-point method (which circumvents many limitations of prior methods). In rabbit, resting [Na+]i was 4.5 +/- 0.4 mM (traditional calibration) and 4.4 mM (null-point). Resting [Na+]i in rat was significantly higher using both the traditional calibration (11.1 +/- 0.7 mM) and the null-point approach (11.2 mM). The rate of Na+ transport by the Na+ pump was measured as a function of [Na+]i in intact cells. Rat cells exhibited a higher V(max) than rabbit (7.7 +/- 1.1 vs. 4.0 +/- 0.5 mM x min(-1)) and a higher K(m) (10.2 +/- 1.2 vs. 7.5 +/- 1.1 mM). This results in little difference in pump activity for a given [Na+]i below 10 mM, but at measured resting [Na+]i levels the pump-mediated Na+ efflux is much higher in rat. Thus, Na+ pump rate cannot explain the higher [Na+]i in rat. Resting Na+ influx rate was two to four times higher in rat, and this accounts for the higher resting [Na+]i. Using tetrodotoxin, HOE-642 and Ni2+ to block Na+ channels, NHX and NCX, respectively, we found that all three pathways may contribute to the higher resting Na+ influx in rat (albeit differentially). We conclude that resting [Na+]i is higher in rat than in rabbit, that this is caused by higher resting Na+ influx in rat and that a higher Na+,K+-ATPase pumping rate in rat is a consequence of the higher [Na+]i.

Denervation enhances the physiological effects of the K(ATP) channel during fatigue in EDL and soleus muscle

The objective was to determine whether denervation reduces or enhances the physiological effects of the K(ATP) channel during fatigue in mouse extensor digitorum longus (EDL) and soleus muscle. For this, we measured the effects of 100 microM of pinacidil, a channel opener, and of 10 microM of glibenclamide, a channel blocker, in denervated muscles and compared the data to those observed in innervated muscles from the study of Matar et al. (Matar W, Nosek TM, Wong D, and Renaud JM. Pinacidil suppresses contractility and preserves energy but glibenclamide has no effect during fatigue in skeletal muscle. Am J Physiol Cell Physiol 278: C404-C416, 2000). Pinacidil increased the (86)Rb(+) fractional loss during fatigue, and this effect was 2.6- to 3.4-fold greater in denervated than innervated muscle. Pinacidil also increased the rate of fatigue; for EDL the effect was 2.5-fold greater in denervated than innervated muscle, whereas for soleus the difference was 8.6-fold. A major effect of glibenclamide was an increase in resting tension during fatigue, which was for the EDL and soleus muscle 2.7- and 1.9-fold greater, respectively, in denervated than innervated muscle. A second major effect of glibenclamide was a reduced capacity to recover force after fatigue, an effect observed only in denervated muscle. We therefore suggest that the physiological effects of the K(ATP) channel are enhanced after denervation.

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.

Metabolic inhibition activates a non-selective current through connexin hemichannels in isolated ventricular myocytes

Intracellular Na(+)accumulation and K(+)loss play important roles in the pathogenesis of arrhythmias and injury in the ischemic heart. We investigated the role of metabolically sensitive connexin hemichannels as a potential route for Na(+)influx and K(+)efflux during ischemia, using dye uptake and electrophysiological measurements to assay hemichannel activity in isolated rabbit ventricular myocytes. Consistent with the known size selectivity of connexin hemichannels,;50% of myocytes exposed to either low extracellular Ca(2+)(an established method for opening connexin hemichannels) or to metabolic inhibitors (a recently described method for opening hemichannels) accumulated fluorescent dyes with <1000 MW (propidium iodide and calcein), but excluded a larger dye with 1500-3000 MW (dextran-rhodamine). Using the whole cell patch clamp technique, we found that metabolic inhibitors activated a non-selective current permeant to both small and large cations, and blocked by La(3+), similar to the properties of connexin 43 when overexpressed in human embryonic kidney (HEK) cells. These findings indicate that isolated cardiac myocytes endogenously express metabolically-sensitive connexin hemichannels. If activated during ischemia, these hemichannels could contribute significantly to altered ionic fluxes promoting arrhythmias and myocardial injury.

Activation of the Na(+)/H(+) exchanger is required for reperfusion-induced Ins(1,4,5)P(3) generation

Post-ischemic reperfusion causes a change in inositol phosphate responses to norepinephrine from primary generation of inositol(1,4) bis phosphate (Ins(1,4)P(2)) to generation of inositol(1,4,5) tris phosphate (Ins(1,4,5)P(3)) that is required for the initiation of reperfusion arrhythmias. The current study was undertaken to investigate the role of Na(+)/H(+)exchange in facilitating this transient change in inositol phosphate response. Rat hearts were subjected to 20 min ischemia followed by 2 min reperfusion and Ins(1, 4,5)P(3)content was measured by mass analysis or by anion-exchange HPLC following [(3)H]inositol labeling. Reperfusion caused generation of [(3)H]Ins(1,4,5)P(3)(1732+/-398 to 3103+/-214, cpm/g tissue, mean+/-S.E.M., n=5, P<0.01) and the development of arrhythmias. Inhibition of Na(+)/H(+)exchange, by reperfusing at pH 6.3 or by pretreating with HOE-694 (10 n M-3 microM) or HOE-642 (3 microM) prevented the [(3)H]Ins(1,4,5)P(3)generation, without causing any suppression of norepinephrine release. Increases in Ins(1,4,5)P(3)mass were similarly reduced by inhibition of Na(+)/H(+)exchange. Thus, activation of Na(+)/H(+)exchange is required for the enhanced Ins(1,4,5)P(3)response observed under reperfusion conditions, and prevention of Ins(1,4,5)P(3)generation may be an important contributor to the anti-arrhythmic actions of inhibitors of Na(+)/H(+)exchange.

Mechano- or acid stimulation, two interactive modes of activation of the TREK-1 potassium channel

TREK-1 is a member of the novel structural class of K(+) channels with four transmembrane segments and two pore domains in tandem (1,2). TREK-1 is opened by membrane stretch and arachidonic acid. It is also an important target for volatile anesthetics (2,3). Here we show that internal acidification opens TREK-1. Indeed, lowering pH(i) shifts the pressure-activation relationship toward positive values and leads to channel opening at atmospheric pressure. The pH(i)-sensitive region in the carboxyl terminus of TREK-1 is the same that is critically involved in mechano-gating as well as arachidonic acid activation. A convergence, which is dependent on the carboxyl terminus, occurs between mechanical, fatty acids and acidic stimuli. Intracellular acidosis, which occurs during brain and heart ischemia, will induce TREK-1 opening with subsequent K(+) efflux and hyperpolarization.

Long-term myocardial preservation: effects of hyperkalemia, sodium channel, and Na/K/2Cl cotransport inhibition on extracellular potassium accumulation during hypothermic storage

OBJECTIVES

We previously demonstrated improved myocardial preservation with polarized (tetrodotoxin-induced), compared with depolarized (hyperkalemia-induced), arrest and hypothermic storage. This study was undertaken to determine whether polarized arrest reduced ionic imbalance during ischemic storage and whether this was influenced by Na+/K +/2Cl- cotransport inhibition.

METHODS

We used the isolated crystalloid perfused working rat heart preparation (1) to measure extracellular K+ accumulation (using a K+-sensitive intramyocardial electrode) during ischemic (control), depolarized (K+ 16 mmol/L), and polarized (tetrodotoxin, 22 micromol/L) arrest and hypothermic (7.5 degrees C) storage (5 hours), (2) to determine dose-dependent (0.1, 1.0, 10 and 100 micromol/L) effects of the Na +/K+/2Cl- cotransport inhibitor, furosemide, on extracellular K+ accumulation during polarized arrest and 7.5 degrees C storage, and (3) to correlate extracellular K+ accumulation to postischemic recovery of cardiac function.

RESULTS

Characteristic triphasic profiles of extracellular K+ accumulation were observed in control and depolarized arrested hearts; a significantly attenuated profile with polarized arrested hearts demonstrated reduced extracellular K+ accumulation, correlating with higher postischemic function (recovery of aortic flow was 54% +/-4% [P =.01] compared with 39% +/-3% and 32% +/-3% in depolarized and control hearts, respectively). Furosemide (0.1, 1.0, 10, and 100 micromol/L) modified extracellular K+ accumulation by -18%, -38%, -0.2%, and +9%, respectively, after 30 minutes and by -4%, -27%, +31%, and +42%, respectively, after 5 hours of polarized storage. Recovery of aortic flow was 53% +/-4% (polarized arrest alone), 56% +/-8%, 70% +/-2% (P =.04 vs control), 69% +/-4% (P =.04 vs control), and 65% +/-3% ( P =. 04 vs control), respectively.

CONCLUSIONS

Polarized arrest was associated with a reduced ionic imbalance (demonstrated by reduced extracellular K+ accumulation) and improved recovery of cardiac function. Further attenuation of extracellular K + accumulation (by furosemide) resulted in additional recovery.

Intracellular sodium accumulation during ischemia as the substrate for reperfusion injury

To elucidate the role of intracellular Na+ kinetics during ischemia and reperfusion in postischemic contractile dysfunction, intracellular Na+ concentration ([Na+]i) was measured in isolated perfused rat hearts using 23Na nuclear magnetic resonance spectroscopy. The extension of the ischemic period from 9 minutes to 15, 21, and 27 minutes (at 37 degrees C) increased [Na+]i at the end of ischemia from 270.0+/-10.4% of preischemic level (mean+/-SE, n=5) to 348.4+/-12.0% (n=5), 491.0+/-34.0% (n=7), and 505.3+/-12.1% (n=5), respectively, whereas the recovery of developed pressure worsened with the prolongation of the ischemic period (95.1+/-4.2%, 84.3+/-1. 2%, 52.8+/-13.7%, and 16.9+/-6.4% of preischemic level). The kinetics of [Na+]i recovery during reperfusion was analyzed by the fitting of a monoexponential function. When the hearts were reperfused with low-[Ca]o (0.15 mmol/L) solution, the time constants of the recovery (tau) after 15-minute (8.07+/-0.85 minutes, n=5) and 21-minute ischemia (6.44+/-0.90, n=5) were significantly extended, with better functional recovery (98.5+/-1.4% for 15-minute [P<0.05]; 98.0+/-1.0% for 21-minute [P<0.05]) compared with standard reperfusion ([Ca]o=2.0 mmol/L, tau=3.58+/-0.28 minutes for 15-minute [P<0.0001]; tau=3.02+/-0.20 for 21-minute [P<0.0001]). A selective inhibitor of Na+/Ca2+ exchanger also decelerated the [Na+]i recovery, which suggests that the recovery reflects the Na+/Ca2+ exchange activity. In contrast, high-[Ca]o reperfusion (5 mmol/L) accelerated the [Na+]i recovery after 9-minute ischemia (tau=2.48+/-0.11 minute, n=5 [P<0.0001]) and 15-minute ischemia (tau=2.10+/-0.07, n=6 [P<0. 05]), but functional recovery deteriorated only in the hearts with 15-minute ischemia (29.8+/-9.4% [P<0.05]). [Na+]i recovery after 27-minute ischemia was incomplete and decelerated by low-[Ca]o reperfusion, with limited improvement of functional recovery (42. 5+/-7.9%, n=5 [P<0.05]). These results indicate that intracellular Na+ accumulation during ischemia is the substrate for reperfusion injury and that the [Na+]i kinetics during reperfusion, which is coupled with Ca2+ influx, also determines the degree of injury.

beta2-adrenoceptor agonists reduce the decline of rat diaphragm twitch force during severe hypoxia

The aim of the present study was to investigate the in vitro effects of the short-acting beta2-adrenoceptor agonist salbutamol and the long-acting beta2-adrenoceptor agonist salmeterol on hypoxia-induced rat diaphragm force reduction. In vitro diaphragm twitch force (Pt) and maximal tetanic force (Po) of isolated diaphragm muscle strips were measured for 90 min during hyperoxia (tissue bath PO2 83.8 +/- 0.9 kPa and PCO2 3.9 +/- 0.1 kPa) or severe hypoxia (PO2 7.1 +/- 0.3 kPa and PCO2 3.9 +/- 0.1 kPa) in the presence and absence of 1 microM salbutamol or 1 microM salmeterol. During hyperoxia, salbutamol and salmeterol did not significantly alter the time-related decreases in Pt and Po (to approximately 50% of initial values). Salbutamol had no effects on Po or the Pt-to-Po ratio. Salmeterol treatment significantly reduced Po and increased the Pt-to-Po ratio during hyperoxia (P < 0.05 compared with control value). Hypoxia resulted in a severe decrease in Pt (to approximately 30% of initial value) and Po after 90 min. Both salbutamol and salmeterol significantly reduced the decline in Pt during hypoxia (P < 0.05). The reduction in Po was not prevented. Salbutamol increased Pt rapidly but transiently. Salmeterol had a delayed onset of effect and a longer duration of action. Addition of 1 microM propranolol (a nonselective beta-adrenoceptor antagonist) did not alter Pt, Po, or the Pt-to-Po ratio during hypoxia but completely blocked the inotropic effects of salbutamol and salmeterol, indicating that these effects are dependent on beta2-adrenoceptor agonist-related processes.

Connexin-43 hemichannels opened by metabolic inhibition

The cause of altered ionic homeostasis leading to cell death during ischemia and metabolic inhibition is unclear. Hemichannels, which are precursors to gap junctions, are nonselective ion channels that are permeable to molecules of less than Mr 1000. We show that hemichannels open upon exposure to calcium-free solutions when they are either heterologously overexpressed in HEK293 cells or endogenously expressed in cardiac ventricular myocytes. In the presence of normal extracellular calcium, hemichannels open during metabolic inhibition. During ischemia and other forms of metabolic inhibition, activation of relatively few hemichannels will seriously compromise the cell's ability to maintain ionic homeostasis, which is an essential step promoting cell death.

Lysophosphatidylcholine and Cellular Potassium Loss in Isolated Rabbit Ventricle

BACKGROUND: Lysophospholipids such as lysophosphatidylcholine (LPC) have many direct electrophysiological effects on cardiac muscle and have been implicated as a cause of lethal ventricular arrhythmias during acute myocardial ischemia. Because extracellular K(+) accumulation is also a key arrhythmogenic factor during acute ischemia, we examined the effects of LPC on cellular K(+) balance, including its interaction with adenosine triphosphate-sensitive K(+) (K(ATP)) channels. METHODS AND RESULTS: Isolated rabbit interventricular septa paced at 75 beats/min were loaded with (42)K(+) to measure unidirectional K(+) efflux rate (in (42)K(+) washout experiments) or tissue K(+) content ((42)K(+) uptake experiments) and action potential duration (APD) during exposure to 20 µM LPC for 30 minutes. LPC caused tissue K(+) content to decrease by 15 +/- 2% (n = 4) at a steady rate over 30 minutes, associated with gradual APD shortening and a delayed increase in unidirectional K(+) efflux rate. Pretreatment with 12 µM cromakalim to selectively activate K(ATP) channels shortened APD by 44 +/- 66% and had no effect on net tissue K(+) content during control aerobic perfusion. However, cromakalim increased net K(+) loss during exposure to LPC to 22 +/- 4%, a 47% increase. CONCLUSIONS: LPC induced net K(+) loss in heart, which was potentiated by the K(ATP) channel agonist cromakalim. This ATP finding suggests that if LPC accumulates to similar levels during myocardial ischemia and hypoxia, it may be an important mechanism in net K(+) loss.