Ionic currents during hypoxia in voltage-clamped cat ventricular muscle

KATP channels are regulators of programmed cell death and targets for the creation of novel drugs against ischemia/reperfusion cardiac injury

BACKGROUND

The use of percutaneous coronary intervention (PCI) in patients with ST-segment elevation myocardial infarction (STEMI) is associated with a mortality rate of 5%-7%. It is clear that there is an urgent need to develop new drugs that can effectively prevent cardiac reperfusion injury. ATP-sensitive K+ (KATP ) channel openers (KCOs) can be classified as such drugs.

RESULTS

KCOs prevent irreversible ischemia and reperfusion injury of the heart. KATP channel opening promotes inhibition of apoptosis, necroptosis, pyroptosis, and stimulation of autophagy. KCOs prevent the development of cardiac adverse remodeling and improve cardiac contractility in reperfusion. KCOs exhibit antiarrhythmic properties and prevent the appearance of the no-reflow phenomenon in animals with coronary artery occlusion and reperfusion. Diabetes mellitus and a cholesterol-enriched diet abolish the cardioprotective effect of KCOs. Nicorandil, a KCO, attenuates major adverse cardiovascular event and the no-reflow phenomenon, reduces infarct size, and decreases the incidence of ventricular arrhythmias in patients with acute myocardial infarction.

CONCLUSION

The cardioprotective effect of KCOs is mediated by the opening of mitochondrial KATP (mitoKATP ) and sarcolemmal KATP (sarcKATP ) channels, triggered free radicals' production, and kinase activation.

Subcellular trafficking and endocytic recycling of KATP channels

Sarcolemmal/plasmalemmal ATP-sensitive K+ (KATP) channels have key roles in many cell types and tissues. Hundreds of studies have described how the KATP channel activity and ATP sensitivity can be regulated by changes in the cellular metabolic state, by receptor signaling pathways and by pharmacological interventions. These alterations in channel activity directly translate to alterations in cell or tissue function, that can range from modulating secretory responses, such as insulin release from pancreatic β-cells or neurotransmitters from neurons, to modulating contractile behavior of smooth muscle or cardiac cells to elicit alterations in blood flow or cardiac contractility. It is increasingly becoming apparent, however, that KATP channels are regulated beyond changes in their activity. Recent studies have highlighted that KATP channel surface expression is a tightly regulated process with similar implications in health and disease. The surface expression of KATP channels is finely balanced by several trafficking steps including synthesis, assembly, anterograde trafficking, membrane anchoring, endocytosis, endocytic recycling, and degradation. This review aims to summarize the physiological and pathophysiological implications of KATP channel trafficking and mechanisms that regulate KATP channel trafficking. A better understanding of this topic has potential to identify new approaches to develop therapeutically useful drugs to treat KATP channel-related diseases.

The Spiked Helmet Sign Predicting a Poor Outcome in a Patient with Non-Myocardial Infarction ST-Segment Elevation

Spiked helmet sign is a novel electrocardiogram marker that reflects a poor prognosis, and may mimic myocardial infarction, especially in patients with an acute alteration of mental status or out-of-hospital cardiac arrest. In cases where a spiked helmet sign is missed, there may be a delay in surgical intervention for the underlying conditions because of unnecessary cardiac catheterization. In addition, antiplatelet agents for acute coronary syndrome in such cases can lead to catastrophic complications. Therefore, early recognition of spiked helmet sign is useful for timely correction of the underlying disease and prevention of poor outcomes. Herein, we describe a rare case of a patient with internal bleeding and subarachnoid hemorrhage presenting with spiked helmet sign on an electrocardiogram.

Ibutilide for the control of refractory ventricular tachycardia and ventricular fibrillation in patients with myocardial ischemia and hemodynamic instability

INTRODUCTION

Recurrent ventricular tachycardia (VT) and ventricular fibrillation (VF) in patients with myocardial ischemia requiring hemodynamic support can be refractory to available antiarrhythmic agents and even to cardioversion and defibrillation. The purpose of this study was to report the effect of intravenous ibutilide in patients with a VT and/or VF storm in the presence of incomplete revascularization requiring hemodynamic support.

METHODS AND RESULTS

Standard continuous telemetry and frequent 12-lead electrocardiograms were obtained to determine the effect of intravenous Ibutilide in these patients. We studied six consecutive patients (age 60 ± 12 years; five males) with incomplete revascularization and mechanical support (extracorporeal membrane of oxygenation = 2; left ventricular assist device = 4) with VT/VF refractory to lidocaine and amiodarone. Intravenous ibutilide was given as a last resort for management of their ventricular arrhythmias. Intravenous ibutilide (1-2 mg) allowed restoration of sinus rhythm in three patients with persistent VF that were refractory to multiple defibrillation shocks. When the 24-hour period before and after ibutilide administration was compared, this drug markedly reduced the number of required cardioversions/defibrillations in all patients from 20 ± 9 to 0.7 ± 0.8 shocks ( P = 0.036).

CONCLUSIONS

In patients with myocardial ischemia requiring hemodynamic support, intravenous Ibutilide demonstrates a potent antiarrhythmic effect and can facilitate defibrillation in patients with refractory VF.

Fosphenytoin for control of electrical storm in acute myocardial infarction and Purkinje fiber-mediated arrhythmias

BACKGROUND

Purkinje fiber-mediated arrhythmias in the setting of acute myocardial infarction are poorly responsive to conventional antiarrhythmic therapy, increases overall mortality and often requires radiofrequency ablation (RFA) for control. In this study, we report the use of intravenous Fosphenytoin for the control of arrhythmic storm in patients with acute myocardial infarction.

METHODS AND RESULTS

Six patients with acute myocardial infarction (5 AW/1 LW) and Purkinje-triggered ventricular arrhythmias refractory to conventional antiarrhythmics were treated with intravenous Fosphenytoin before considering RFA. Arrhythmia control was obtained in all patients after the initial bolus dose. Breakthrough episodes were seen in 5/6 within 24-36 hours of the initial bolus, necessitating a second bolus. Complete arrhythmia control was obtained in all patients within 72 hours and 5/6 patients were successfully discharged from the hospital. One patient succumbed to sepsis in hospital while another patient succumbed to Sub Dural Hematoma after 3 months.

CONCLUSIONS

Intravenous Fosphenytoin should be considered before RFA for control of Purkinje fiber-mediated refractory arrhythmias in acute myocardial infarction patients.

Shensong Yangxin capsules prevent ischemic arrhythmias by prolonging action potentials and alleviating Ca2+ overload

Shensong Yangxin capsules (SSYX) are an effective traditional Chinese medicine that has been used to treat coronary heart disease clinically. The present study aimed to establish whether SSYX prevent ischemic arrhythmias in rats, and to explore the underlying mechanisms. Male rats were pretreated with distilled water, SSYX and amiodarone for one week. Acute myocardial ischemia (AMI) was performed to induce ischemic arrhythmias. The incidence and severity of ischemic arrhythmias were evaluated. The action potential, transient outward K+ current (Ito) and inward rectifier K+ current (IK1) of rat cardiomyocytes were measured using the patch‑clamp technique. The intracellular Ca2+ concentration of the cardiomyocytes was measured using a laser scanning confocal microscope. The results revealed that SSYX lowered the incidence of arrhythmia markedly during AMI. Furthermore, SSYX delayed the appearance, and reduced the severity, of ischemic arrhythmias compared with the control. In addition, SSYX markedly decreased the ratio of the myocardial infarction region to the whole heart. In an in vitro study, SSYX prolonged the action potential duration of rat cardiomyocytes, and inhibited Ito and IK1 markedly. Additionally, SSYX inhibited Ca2+ elevation induced by KCl in cardiomyocytes. These results suggested that SSYX prevents ischemic arrhythmia, and the underlying mechanism responsible for this process may include prolonging the action potential and alleviating Ca2+ overload.

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.

Effects of prolonged anoxia on electrical activity of the heart in Crucian carp (Carassius carassius)

The effects of sustained anoxia on cardiac electrical excitability were examined in the anoxia-tolerant Crucian carp (Carassius carassius). The electrocardiogram (ECG) and expression of excitation-contraction coupling genes were studied in fish acclimatised to normoxia in summer (+18°C) or winter (+2°C), and in winter fish after 1, 3 and 6 weeks of anoxia. Anoxia induced a sustained bradycardia from a heart rate of 10.3±0.77 to 4.1±0.29 bpm (P<0.05) after 5 weeks, and heart rate slowly recovered to control levels when oxygen was restored. Heart rate variability greatly increased under anoxia, and completely recovered under re-oxygenation. The RT interval increased from 2.8±0.34 s in normoxia to 5.8±0.44 s under anoxia (P<0.05), which reflects a doubling of the ventricular action potential (AP) duration. Acclimatisation to winter induced extensive changes in gene expression relative to summer-acclimatised fish, including depression in those coding for the sarcoplasmic reticulum calcium pump (Serca2-q2) and ATP-sensitive K+ channels (Kir6.2) (P<0.05). Genes of delayed rectifier K+ (kcnh6) and Ca2+ channels (cacna1c) were up-regulated in winter fish (P<0.05). In contrast, the additional challenge of anoxia caused only minor changes in gene expression, e.g. depressed expression of Kir2.2b K+ channel gene (kcnj12b), whereas expression of Ca2+ (cacna1a, -c and –g) and Na+ channel genes (scn4a and scn5a) were not affected. These data suggest that low temperature pre-conditions the Crucian carp heart for winter anoxia, whereas sustained anoxic bradycardia and prolongation of AP duration are directly induced by oxygen shortage without major changes in gene expression.

Acute myocardial ischemia: cellular mechanisms underlying ST segment elevation

The electrocardiogram (ECG) is an essential tool for the diagnosis of acute myocardial ischemia in the emergency department, as well as for that of an evolving acute myocardial infarction (AMI). Changes in the surface ECG in leads whose positive poles face the ischemic region are known to be related to injury currents flowing across the boundaries between the ischemic and the surrounding normal myocardium. Although experimental studies have also shown an endocardium to epicardium differential sensitivity to the effect of acute ischemia, the important contribution of this transmural heterogeneous response to the changes observed in the surface ECG is less appreciated by the clinical cardiologist. This review briefly discusses our current knowledge regarding the electrophysiology of the ischemic myocardium focusing primarily on the electrophysiologic changes underlying the ECG alterations observed at the onset of a transmural AMI.

Ischemic ventricular arrhythmias: experimental models and their clinical relevance

In the United States, sudden cardiac death accounts for an estimated 300,000 to 350,000 cases each year, with 80,000 presenting as the first manifestation of a preexisting, sometimes unrecognized, coronary artery disease. Acute myocardial infarction (AMI)-induced ventricular fibrillation frequently occurs without warning, often leading to death within minutes in patients who do not receive prompt medical attention. Identification of patients at risk for AMI-induced lethal ventricular arrhythmias remains an unmet medical need. Recent studies suggest that a genetic predisposition may significantly contribute to the vulnerability of the ischemic myocardium to ventricular tachycardia/ventricular fibrillation. Numerous experimental models have been developed for the purpose of advancing our understanding of the mechanisms responsible for the development of cardiac arrhythmias in the setting of ischemia and with the aim of identifying antiarrhythmic therapies that could be of clinical benefit. While our understanding of the mechanisms underlying AMI-induced ventricular arrhythmias is coming into better focus, the risk stratification of patients with AMI remains a major challenge. This review briefly discusses our current state of knowledge regarding the mechanisms of ischemic ventricular arrhythmias and their temporal distribution as revealed by available experimental models, how these correlate with the clinical syndromes, as well as prospective prophylactic therapies for the prevention and treatment of ischemia-induced life-threatening arrhythmias.

Repetitive endocardial focal discharges during ventricular fibrillation with prolonged global ischemia in isolated rabbit hearts

BACKGROUND

Ventricular fibrillation (VF) during prolonged (>5 min) global ischemia (GI) could be due to repetitive endocardial focal discharges (REFDs). This hypothesis was tested in isolated rabbit hearts.

METHODS AND RESULTS

With optical mapping, simultaneous endocardial (left ventricle, LV) and epicardial (both ventricles) activations during VF with prolonged GI were studied (protocol I, 8 hearts). Lugol solution was applied to the LV endocardium in additional 5 hearts after 5-min GI (protocol II). During prolonged GI, sustained VF (>30 s) was successfully induced in 7 protocol I hearts. The dominant frequency of summed optical signals at the LV endocardium was higher than at the epicardium (P<0.05). Mapping data showed that after 5-min GI, REFDs were present in >90% for recording time. There were 18 windows of optical recording showing spontaneous VF termination. In 10, once REFDs ceased, the VF episode terminated immediately. Electrical defibrillation was also performed on 3 hearts. Eight shocks showed early VF recurrence after successful defibrillation. REFDs were consistently involved in the initiation period of recurrence. In protocol II, Lugol subendocardial ablation diminished REFD genesis during re-induced VF. These VF episodes were all non-sustained.

CONCLUSIONS

REFDs at the LV endocardium were important for both VF maintenance and post-shock recurrence during prolonged GI in this model.

Intramural foci during long duration fibrillation in the pig ventricle

For more than 50 years, it has been assumed that ventricular fibrillation (VF) is maintained solely by reentry in the working myocardium. This hypothesis has never been tested by recording VF with electrodes spaced sufficiently close to map activation sequences in 3D. We recorded the first 10 minutes of electrically induced VF from the anterior left ventricular (LV) free wall near the insertion of the anterior papillary muscle in 6 pigs. A 3D transmural unipolar electrode array consisting of a 9x9 array of needles with 2-mm spacing and 6 electrodes 2 mm apart on each needle was used for recordings. Automatic analyses were performed to recognize 3D reentry and foci. Our results showed that intramural reentry is present early but not late during VF in the mapped region. The incidence of reentry in working myocardium decreases almost to 0 after 3 minutes of VF. In contrast, intramural foci are present during early VF and, as VF continues, increase in incidence, so that by 10 minutes of VF, 27% of wavefronts arise from intramural foci. These results suggest that, particularly after the first 3 minutes of VF, mechanisms other than local reentry in the working myocardium maintain VF in the anterior LV free wall near the root of the anterior papillary muscle. Intramural foci may play an important role in later VF maintenance. It remains to be determined if these foci arise from Purkinje fibers attributable to abnormal automaticity, afterdepolarizations, or reentry.

Ventricular fibrillation during no-flow global ischemia in isolated rabbit hearts

INTRODUCTION

The dominant frequency (DF) during ventricular fibrillation (VF) in Langendorff-perfused guinea pig hearts is higher in left ventricle (LV) than in right ventricle (RV). However, the onset of VF invariably leads to global ischemia. Whether or not a high DF source exists in LV during global ischemia is unknown.

METHODS AND RESULTS

By using a two-camera optical mapping system, epicardial activation patterns of VF were studied in 12 isolated rabbit hearts during baseline, no-flow global ischemia, and reperfusion. Simultaneous endocardial electrode recording was performed in 4 of the 12 hearts. Optical mapping showed type 1 VF at baseline, with multiple wandering and short-lived wavelets. After the onset of global ischemia, VF showed progressively increased spatiotemporal periodicity. The majority (65%) of VF recorded after 7 minutes of global ischemia showed type 2 VF, containing a single epicardial site with stable (> or = 3.85 seconds in duration) repetitive activities. Among the 33 sites with these activities, 24 were located near the interventricular septum, and 27 showed an epicardial breakthrough pattern with centrifugal propagation and wavebreaks distant from the focal site. After 10 minutes of global ischemia, the DF was lower on LV epicardium (5.0 +/- 1.4 Hz) than on RV epicardium (8.6 +/- 2.5 Hz, P < 0.001). However, there was no DF gradient between RV and LV endocardium (9.7 +/- 1.0 vs 9.6 +/- 0.9 Hz).

CONCLUSIONS

VF during prolonged global ischemia is consistent with type 2 VF with a single subepicardial source of rapid activation, mostly near the interventricular septum. The DF in LV is not higher than in RV.

Effects of the blockade of cardiac sarcolemmal ATP-sensitive potassium channels on arrhythmias and coronary flow in ischemia–reperfusion model in isolated rat hearts

Activation of ATP-sensitive K+ channels (K ATP) during ischemia leads to arrhythmias and blockade of these channels exert antiarrhythmic action. In this study, we investigated the effects of HMR1098, a sarcolemmal K ATP channel blocker and 5-hydroxydeconoate (5-HD), a mitochondrial K ATP channel blocker on cardiac function and arrhythmias in isolated rat hearts. The hearts were subjected to 30 min coronary occlusion, followed by 30 min reperfusion. In the preischemic period, both HMR 1098 and 5-HD slightly increased coronary perfusion pressure. Coronary occlusion increased the perfusion pressure and decreased the left ventricular developed pressure (LVDP) in both control and drug-treated hearts. However, inhibition of LVDP was greater and recovery of the perfusion pressure was lower in 30 micromol/l HMR1098 and 100 micromol/l 5-HD-treated hearts compared to control (P < 0.05). HMR1098, at 3 micromol/l, but not at 30 micromol/l, significantly reduced the ratio of bigeminis, couplets and salvos (P < 0.05). Ventricular tachycardia and ventricular fibrillation were not prevented by HMR1098, at both concentrations, and with 5-HD (100 micromol/l). These results suggest that blockade of sarcK ATP and mitoK ATP channels exert weak antiarrhythmic action, but reduce the recovery of coronary perfusion and contractile force, implying that both types of K(ATP) channels have beneficial role in the recovery of ischemic rat myocardium.

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.

Evolution of activation patterns during long-duration ventricular fibrillation in dogs

Although resuscitation for sudden cardiac arrest attempts are frequently not instituted for several minutes after the onset of ventricular fibrillation (VF), previous mapping studies have examined only the first 40 s of VF or have involved isolated perfused hearts that did not become ischemic during VF. We applied quantitative pattern analysis to mapping data throughout the first 10 min of VF acquired from a 21 x 24 unipolar electrode array located on the ventricular epicardium of six open-chest dogs. The following twelve descriptors were continuously quantified: 1) number of wavefronts, 2) incidence of reentry, 3) wavefront propagation velocity, 4) incidence of breakthrough/focus, 5) incidence of block, 6) mean area activated by the wavefronts, 7) wavefront fractionations, 8) wavefront collisions, 9) multiplicity index, 10) repeatability, 11) negative peak rate of voltage change, and 12) peak frequency of activation. Cluster analysis of these descriptors divided VF into five stages (stages i-v). The values of most descriptors (except block and breakthrough incidence) increased during stage i (1-11 s after VF induction) and maintained high values with rapid dynamic fluctuations during stage ii (12-62 s). Descriptors changed quickly to values indicating greater organization during stage iii (63-86 s), decreased steadily during stage iv (87-310 s), and approached zero during stage v (311-600 s). There was a high incidence of reentry just before, during, and after stage iii. In conclusion, during the first 10 min, VF can be divided into five stages according to the evolution of electrophysiological characteristics. All of the parameters show a rapid deterioration during VF, except for a temporary reversal approximately 1 min after induction when activation briefly became more organized. Thus a quantitative description of activation does not uniformly decrease as VF progresses, but undergo rapid changes and exhibit a brief interval of increased organization after approximately 1 min of VF. Further studies are warranted to determine whether these changes, particularly the increased organization of stage iii, have clinical consequences, such as an alteration in defibrillation efficacy.

Ischemic shortening of action potential duration as a result of KATP channel opening attenuates myocardial stunning by reducing calcium influx

Action potential duration (APD) shortening due to opening of sarcolemmal ATP-dependent potassium (KATP) channels has been postulated to protect the myocardium against postischemic damage by reducing Ca2+ influx. This hypothesis was assessed, assuming that increased postischemic stunning due to KATP channel inhibition with glibenclamide could be reverted by the addition of the Ca2+ channel blocker diltiazem. Percent wall thickening fraction (%WTh, conscious sheep) and APD (open-chest sheep) were obtained from the following groups: control: 12 min ischemia by anterior descending coronary artery occlusion followed by 2 h reperfusion; glibenclamide: same as control, with glibenclamide (0.4 mg/kg) infused 30 min before ischemia; diltiazem: same as control, with diltiazem (100 microg/kg) administered prior to ischemia; glibenclamide+diltiazem: both drugs infused as in glibenclamide and diltiazem groups. APD was reduced in control ischemia. Conversely, KATP-channel blockade by glibenclamide lengthened APD and increased postischemic stunning (p < 0.01 vs. control); glibenclamide+diltiazem did not shorten APD but enhanced functional recovery (p < 0.01 vs. glibenclamide). Ca2+ channel blockade improvement of increased stunning provoked by KATP channel inhibition supports the hypothesis that APD shortening due to opening of KATP channels protects against postischemic stunning by limiting Ca2+ influx.

Hypoxia influences generation and propagation of electrical activity in embryonic cardiomyocyte clusters

The influence of tissue hypoxia on the generation and propagation of excitation was studied in spontaneously beating embryonic cardiomyocyte clusters grown in eight 9-12 days old embryoid bodies. Within the embryoid bodies one to three separately active clusters of cardiomyocytes were found, each having its own pacemaker cell. Lowering of tissue PO(2) caused bradycardia as well as arrhythmia in all embryoid bodies investigated. The mean frequency of the extracellularly recorded action potentials decreased under conditions of pronounced hypoxia from a mean of 1.4-1.8 Hz to below 0.8 Hz. In three embryoid bodies hypoxia-sensitive as well as hypoxia-tolerant cardiomyocyte clusters were found. The hypoxia-insensitive cardiomyocytes showed a low frequency of spontaneous activity. In addition to the observed changes in the generation of excitation, tissue hypoxia caused an approximately 60% reduction in the velocity of conduction within the cardiomyocyte clusters. Moreover, in at least one of the eight experiments propagation failure with an incomplete block in spread of excitation was observed. All hypoxia-induced effects on generation and propagation of embryonic cardiomyocyte excitation were completely reversible after reoxygenation.

Effects of tilisolol, a nonselective beta-adrenergic blocker, on the membrane currents of isolated guinea pig ventricular myocytes

The effects of tilisolol, a nonselective beta-adrenoceptor blocker, on transmembrane ionic currents were studied in single guinea pig ventricular myocytes by using the whole-cell voltage clamp technique. In the absence of beta-adrenergic stimulation, 10 microM tilisolol, a concentration higher than that used in the clinical therapeutic regimen, did not affect the L-type Ca2+ current (ICa), the inwardly rectifying K+ current (IK1), or the delayed rectifying K+ current (IK). In addition, it did not induce currents through the adenosine triphosphate (ATP)-sensitive K+ channels. However, under the nonselective beta-adrenergic stimulation with 1 microM isoproterenol, 1 microM tilisolol almost completely reversed the agonist-induced increase of IK. The increase of ICa by isoproterenol was blocked only by approximately 30% with tilisolol. We concluded that, at therapeutic concentrations (0.01-0.15 microM), tilisolol is a pure beta-adrenoceptor antagonist that has no direct effects on the transmembrane ionic currents of mammalian ventricular myocytes, such as ICa, IK1, or IK. Comparison of the dose-dependent effects of tilisolol on ICa and IK suggested that tilisolol may selectively inhibit catecholamine-induced increase of IK at the therapeutic concentrations. The virtually selective inhibition of IK, leaving ICa intact, may be favorable to prevent the catecholamine-induced arrhythmia without inhibiting contraction.

Which cardiac potassium channel subtype is the preferable target for suppression of ventricular arrhythmias?

Prolongation of the cardiac action potential duration is the hallmark of Class III antiarrhythmic activity. Action potential duration prolongation may be achieved by several means: enhancement of inward current and, more commonly, blockade of one or more of the many outward currents that are carried by K+. However, it is far from clear whether blockade of one particular K+ channel is more efficacious than blockade of another. The objective of this review is to consider this question with particular reference to ischaemic heart disease, a condition for which effective prevention of ventricular arrhythmias continues to be sought.

Altered pharmacologic responsiveness of reduced L-type calcium currents in myocytes surviving in the infarcted heart

The pharmacologic responses of macroscopic L-type calcium channel currents to the dihydropyridine agonist, Bay K 8644, and beta-adrenergic receptor stimulation by isoproterenol were studied in myocytes enzymatically dissociated from the epicardial border zone of the arrhythmic 5-day infarcted canine heart (IZs). Calcium currents were recorded at 36 degrees to 37 degrees C using the whole cell, patch clamp method and elicited by applying step depolarizations from a holding potential of -40 mV to various test potentials for 250-msec duration at 8-second intervals. A Cs+ -rich and 10 mM EGTA-containing pipette solution and a Na+ -and K+ -free external solutions were used to isolate calcium currents from other contaminating currents. During control, peak ICa,L density was found to be significantly less in IZs (4.0 +/- 1.1 pA/pF) than in myocytes dispersed from the epicardium of the normal noninfarcted heart (NZs; 6.5 +/- 1.8 pA/pF). Bay K 8644 (1 micro M) significantly increased peak ICa,L density 3.5-fold above control levels in both NZs (to 22.5 +/- 6.2 pA/pF; n = 7) and IZs (to 12.8 +/- 3.0 pA/pF; n = 5), yet peak ICa,L density in the presence of drug was significantly less in IZs than NZs. The effects of Bay K 8644 on kinetics of current decay and steady-state inactivation relations of peak ICa,L were similar in the two cell types. In contrast, the response of peak L-type current density to isoproterenol (1 micro M) was significantly diminished in IZs compared to NZs regardless of whether Ba2+ or Ca2+ ions carried the current. Thus, these results indicate an altered responsiveness to beta-adrenergic stimulation in cells that survive in the infarcted heart. Furthermore, application of forskolin (1 micro M and 10 micro M) or intracellular cAMP (200 micro M), agents known to act downstream of the beta-receptor, also produced a smaller increase in peak IBa density in IZs versus NZs, suggesting that multiple defects exist in the beta-adrenergic signaling pathway of IZs. In conclusion, these studies illustrate that reduced macroscopic calcium currents of cells in the infarcted heart exhibit an altered pharmacologic profile that has important implications in the development of drugs for the diseased heart.

Electrophysiologic changes in ischemic ventricular myocardium: I. Influence of ionic, metabolic, and energetic changes

Myocardial ischemia leads to significant changes in the intracellular and extracellular ionic milieu, high-energy phosphate compounds, and accumulation of metabolic by-products. Changes are measured in extracellular pH and K+, and intracellular pH, Ca2+, Na+, Mg2+, ATP, ADP, and inorganic phosphate. Alterations of membrane currents occur as a consequence of these ionic changes, adrenergic receptor stimulation, and accumulation of lactate, amphipathic compounds, and adenosine. Changes in the volume of the extracellular and intracellular spaces contribute further to the ultimate perturbations of active and passive membrane properties that underlie alterations in excitability, abnormal automaticity, refractoriness, and conduction. These characteristic changes of electrophysiologic properties culminate in loss of excitability and failure of impulse propagation and form the substrate for ventricular arrhythmias mediated through abnormal impulse formation and reentry. The ability to detail the changes in ions, metabolites, and high-energy phosphate compounds in both the extracellular and intracellular spaces and to correlate them directly with the simultaneously occurring electrophysiologic changes have greatly enhanced our understanding of the electrical events that characterize the ischemic process and hold promise for permitting studies aimed at developing interventions that may lessen the lethal consequences of ischemia.

Distinct modes of blockade in cardiac ATP-sensitive K+ channels suggest multiple targets for inhibitory drug molecules

Elementary K+ currents were recorded at 19 degrees C in inside-out patches from cultured neonatal rat cardiocytes to elucidate the block phenomenology in cardiac ATP-sensitive K+ channels when inhibitory drug molecules, such as the sulfonylurea glibenclamide, the phenylalkylamine verapamil or sulfonamide derivatives (HE 93 and sotalol), are interacting in an attempt to stress the hypothesis of multiple channel-associated drug targets. Similar to their adult relatives, neonatal cardiac K(ATP) channels are characterized by very individual open state kinetics, even in cytoplasmically well-controlled, cell-free conditions; at -7 mV, tau open(1) ranged from 0.7 to 4.9 msec in more than 200 patches and tau open(2) from 10 to 64 msec--an argument for a heterogeneous channel population. Nevertheless, a common response to drugs was observed. Glibenclamide and the other inhibitory molecules caused long-lasting interruptions of channel activity, after cytoplasmic application, as if drug occupancy trapped cardiac K(ATP) channels in a very stable, nonconducting configuration. The resultant NPo depression was strongest with glibenclamide (apparent IC50 13 nmol/liter) and much weaker with verapamil (apparent IC50 9 mumol/liter), HE 93 (apparent IC50 29 mumol/liter) and sotalol (apparent IC50 43 mumol/liter) and may have resulted from the occupancy of a single site with drug-specific affinity or of two sites, the high affinity glibenclamide target and a distinct nonglibenclamide, low affinity target. Changes in open state kinetics, particularly in the transition between the O1 state and the O2 state, are other manifestations of drug occupancy of the channel. Any inhibitory drug molecule reduced the likelihood of attaining the O2 state, consistent with a critical reduction of the forward rate constant governing the O1-O2 transition. But only HE 93 (10 mumol/liter) associated (with an apparent association rate constant of 2.3 x 10(6) mol-1 sec-1) to shorten significantly tau open(2) to 60.6 +/- 6% of the pre-drug value, not the expected result when the entrance in and the exit from the O2 state would be drug-unspecifically influenced. Sotalol found yet another and definitely distinctly located binding site to interfere with K+ permeation; both enantiomers associated with a rate close to 5 x 10(5) mol-1 sec-1 with the open pore thereby flicker-blocking cardiac K(ATP) channels. Clearly, these channels accommodate more than one drug-binding domain.

Contractile failure in early myocardial ischemia: models and mechanisms

In early myocardial ischemia we find a number of salient electrical and ionic alterations. This article reviews action potential shortening, K accumulation, and contractile failure. Enhanced K efflux during early myocardial ischemia has been attributed to a number of mechanisms, including: the inhibition of active K uptake, osmotic changes, efflux of K ions linked to anion extrusion, cation exchange, altered cellular energy levels, in particular, the opening of ATP-dependent K channels, the involvement of other ion channels, a H/K-ion exchanger, and a catecholamine-dependent pathway. The different mechanisms are discussed. Action potential shortening was described as a salient characteristic of myocardial ischemia in 1954 by Trautwein and Dudel, and was attributed to enhanced outward current. Recently it has been shown by several authors that ATP-dependent potassium channels play a key role in this context. Contractile failure in early myocardial ischemia has been explained by shortening of the action potential duration, reduced cytoplasmic free calcium levels, intracellular acidification, and a rise in inorganic phosphate and Mg. In summary, it is concluded that ATP-dependent K channels may be involved in each of these three phenomena.

Is oxygen supply sufficient to induce normoxic conditions in isolated rat heart?

The aim of this study is to assess whether oxygen supply is sufficient to induce normoxic conditions in isolated rat hearts. Hearts are perfused with a Krebs medium supplemented with 11 mM glucose, 0.6 mM lactate, 0.06 mM pyruvate, non delipidated albumin (0.1 mM fatty acids), and either 1.78 mM or 0.76 mM free calcium, at 10 ml.min-1. Graded hypoxia is induced by a stepwise decrease of partial pressure of oxygen (PO2) from 660 to 52 mmHg. Contractile performance, oxygen uptake and lactate plus pyruvate balance are assessed. With high calcium, left ventricular developed pressure, dP/dt max and oxygen uptake increase linearly with PO2 up to 660 mmHg; heart rate increases with PO2 up to 250 mmHg and then tends to stabilize. With low calcium, all parameters reach a plateau over 400 mmHg. Lactate plus pyruvate production suggests a stimulation of glycolysis with high calcium, even at 660 mmHg; conversely, there is no lactate plus pyruvate production with low calcium over 250 mmHg. In conclusion, our results demonstrate that, under a high level of calcium at a constant flow of 10 ml.min-1, cardiac function is always limited by O2 supply, except for heart rate. This raises the question as to the definition of a normoxic state. The better preservation of heart rate during hypoxia, compared to other dynamic parameters, could be explained by a contribution of glycolytic ATP.

Transient outward current in human ventricular myocytes of subepicardial and subendocardial origin

In various mammalian species, shapes of action potentials vary within the cardiac wall because of differences in transient outward current (Ito). A prominent Ito exists in human ventricular myocytes, but cells have not been separated according to their original localization. Human ventricular myocytes were isolated from separated subepicardial and subendocardial tissue, and regional variations in Ito were studied. Ito was larger in subepicardial than subendocardial cells. Current density at +60 mV was 7.9 +/- 0.7 pA/pF (n = 28) in subepicardial cells and 2.3 +/- 0.3 pA/pF (n = 16) in subendocardial cells. When cells from explanted failing and nonfailing donor hearts were compared, Ito was not different in subepicardial cells; however, it was larger in subendocardial cells from nonfailing hearts. The potential of half-maximal activation (V0.5) was more positive in subendocardial cells (+25.6 +/- 3.5 mV, n = 15) than in subepicardial cells (+9.2 +/- 1.8 mV, n = 28). There was no difference in V0.5 between cells from failing and nonfailing hearts. Ito inactivation was similar in all cell types and independent of membrane depolarization (time constant [tau] = approximately 60 milliseconds at 22 degrees C). The potential of half-maximal steady-state inactivation was similar in all cell types. Recovery from inactivation of Ito was fast in subepicardial cells at -100 mV (tau = 24 +/- 4 milliseconds, n = 6), exceeding control values transiently (overshoot), and slow at -40 mV without overshoot (tau = 638 +/- 91 milliseconds, n = 6). In subendocardial cells, Ito recovered at -100 mV with a fast phase (tau = 25 milliseconds) and a slow phase (tau = 328 milliseconds), and recovery was not complete after 6 seconds at -100 mV. In conclusion, regional differences in Ito between subepicardial and subendocardial cells may have clinical implications with respect to rhythmic disturbance during heart failure.

Reduced calcium currents in subendocardial Purkinje myocytes that survive in the 24- and 48-hour infarcted heart

BACKGROUND

The abnormal transmembrane action potentials of subendocardial Purkinje fibers that survive 24 to 48 hours after coronary artery occlusion can be a source of the multiform ventricular tachycardias that occur during this time. A change in the density or function of either or both the T-type and L-type cardiac Ca2+ channels may contribute to the altered electrical activity of these Purkinje myocytes.

METHODS AND RESULTS

The purpose of this study was to determine the function of the T- and L-type Ca2+ currents (iCat and iCaL, respectively) in Purkinje myocytes dispersed from the subendocardium of the left ventricle 24 and 48 hours after coronary artery occlusion (IZPC24 and IZPC48, respectively). To do this we compared whole-cell Ca2+ currents from Purkinje myocytes enzymatically dispersed from free-running fiber bundles (SPCs), from the subendocardium of the noninfarcted canine heart (NZPCs), and from IZPC24 and IZPC48. ICaL and iCat were recorded with Cs(+)- and EGTA-rich pipettes and in Na(+)-K(+)-free external solutions to eliminate overlapping currents. ICaL density was significantly reduced in IZPC48 compared with NZPC or IZPC24. This was not accompanied by a shift in the current-voltage relation or by a change in the time course of decay of iCaL. Replacement of Ca2+ with equimolar Ba2+ increased iCaL density in all cell types, but peak iBaL of IZPC48 remained reduced compared with control iBaL values. T-type Ca2+ currents were recorded in all SPCs and NZPCs. In IZPC24 and IZPC48 there was a reduction in peak iCat amplitudes and densities. This was not accompanied by a shift in the current-voltage relation or by a change in the time course of decay of peak iCat. However, there was a hyperpolarizing shift in the steady-state availability relations in both IZPC24 and IZPC48. In addition, the maximally available iCat in IZPC24 was not different from control, whereas it was significantly reduced in IZPC48.

CONCLUSIONS

The L-type ICa density in subendocardial Purkinje myocytes that survive in the infarcted heart is significantly decreased by 48 hours after the time of coronary artery occlusion. The peak T-type ICa density is decreased in subendocardial Purkinje myocytes that survive in the infarcted heart at 24 hours, but further reduction occurs in these myocytes by 48 hours. This loss in Ca2+ channel function could contribute to the abnormal transmembrane potentials of these myocytes surviving in the infarcted heart.

Differential effects of hypoxia on electrical and mechanical activities of isolated ventricular muscles from fetal and adult guinea-pigs

1. Effects of hypoxia (95% N2 + 5% CO2) and cromakalim (30 microM) on mechanical and electrical activities of isolated ventricular muscles were examined in fetal and adult guinea-pigs. 2. Hypoxia markedly reduced both the action potential duration (APD) and the contractile force (CF) in the adult, which was only partially restored by 10 microM glibenclamide, while it only slightly reduced CF with little affecting APD in the fetus. 3. Cromakalim markedly reduced both APD and CF in both age groups similarly. 4. Thus, we demonstrated that APD and CF of fetal ventricular muscles are resistant to hypoxia as compared with those of adult. This may be explained by difference in metabolic pathways rather than by lack of ATP-sensitive potassium channels.

Differences in the electrophysiological response of canine ventricular epicardium and endocardium to ischemia. Role of the transient outward current

BACKGROUND

Acute ischemia is known to produce more severe electrophysiological disturbances in canine ventricular epicardium than endocardium, although the mechanism for the differential sensitivity is still unresolved. Recent studies have demonstrated the presence of a prominent transient outward current (Ito) in ventricular epicardium but not endocardium. The present study was designed to test the hypothesis that the differential sensitivity of these two tissues to ischemia results, at least in part, from a more prominent Ito in epicardium than in endocardium.

METHODS AND RESULTS

Isolated canine ventricular epicardial and endocardial tissues and myocytes were studied by standard microelectrode techniques. Simulated ischemia (hyperkalemia, hypoxia, and acidosis) abolished the action potential plateau and caused a 50% to 60% shortening of action potential duration in epicardium but only a 10% to 20% shortening in endocardium. 4-Aminopyridine, an Ito inhibitor, restored the plateau in epicardium and reduced the dispersion of action potential duration between epicardium and endocardium. Stimulation protocols that minimized the contribution of Ito, such as acceleration of the stimulation rate or introduction of early premature beats, produced a paradoxical prolongation of the epicardial response caused by restoration of the action potential dome. Thus, ischemia-induced dispersion of repolarization was greatly diminished at rapid rates and after premature beats. Similar results were obtained in tissues and myocytes obtained from the same myocardial layers, suggesting that the differential sensitivities of epicardium and endocardium to ischemia are largely a result of inherent differences in cellular properties.

CONCLUSIONS

Our data suggest that the presence of a prominent Ito in epicardium but not endocardium contributes importantly to the selective electrical depression of epicardium by simulated ischemia. The repolarizing influence of Ito serves to amplify the ischemia-induced changes in inward (ICa and INa) and outward (calcium-activated) currents. By facilitating loss of the dome in epicardium, Ito contributes to the development of a marked dispersion of repolarization between normal and ischemic epicardium and between epicardium and endocardium, thereby providing the electrophysiological substrate for the genesis of reentrant arrhythmias.

Pinacidil-induced electrical heterogeneity and extrasystolic activity in canine ventricular tissues. Does activation of ATP-regulated potassium current promote phase 2 reentry?

BACKGROUND

Pinacidil is known to augment a time-independent outward current in cardiac tissues by activating the ATP-regulated potassium channels. Activation of this current, IK-ATP, is thought to be responsible for increased potassium permeability in ischemia. The contribution of IK-ATP activation to arrhythmogenesis and the role of activation of this current in suppression of arrhythmias are areas of great interest and debate. Because electrical depression attending myocardial ischemia is more accentuated in ventricular epicardium than in endocardium, we endeavored to contrast the effects of pinacidil-induced IK-ATP activation on the electrophysiology of canine ventricular epicardium and endocardium.

METHODS AND RESULTS

Standard microelectrode techniques were used. Pinacidil (1 to 5 mumol/L) produced a marked dispersion of repolarization and refractoriness in isolated canine ventricular epicardium as well as between epicardium and endocardium. In endocardium, pinacidil abbreviated action potential duration (APD90) and refractoriness by 8.0 +/- 2.3%. In epicardium, the effects of pinacidil were nonhomogeneous. At some sites, pinacidil induced an all-or-none repolarization at the end of phase 1 of the action potential, resulting in 55.5 +/- 8.7% abbreviation of APD90 and refractoriness. Adjacent to these were sites at which the dome was maintained with only minor changes in APD and refractoriness. Extrasystolic activity displaying features of reentry was observed in isolated sheets of epicardium (63.2%) after exposure to pinacidil (1 to 5 mumol/L) but never in its absence. Dispersion of repolarization and ectopic activity was most readily induced in epicardium by a slowing of the stimulation rate in the presence of pinacidil. Electrical homogeneity was restored and arrhythmias abolished after washout of pinacidil or addition of either a transient outward current blocker, 4-aminopyridine, or a blocker of the ATP-regulated potassium channels, glybenclamide.

CONCLUSIONS

Our data suggest that the activation of IK-ATP can produce a marked dispersion of repolarization and refractoriness in epicardium as well as between epicardium and endocardium, leading to the development of extrasystolic activity via a mechanism that we have called phase 2 reentry. The available data also suggest that blockade of the transient outward current and/or the ATP-regulated potassium channels may be useful antiarrhythmic interventions under ischemic or "ATP depleted" conditions.

Simulation of ECG from two pairs of action potentials

A simple model for simulation of ECG is presented with the purpose to mimic some common ECG configurations and to generate a hypothesis regarding their electrophysiological background. Action potentials (AP) were simulated on a personal computer from ion currents as described previously (Wohlfart & Arlock, 1993). The difference between two APs of a pair was used to create an electrogram (EG). The subendocardial AP of the pair was triggered by means of a simulated current injection and the subepicardial Ap was triggered from the first AP due to electric coupling within the pair. The subendocardial APs were of longer duration than the epicardial AP because of differences in background currents. A second pair of APs representing another ventricular site was simulated in an analogous way and this pair was activated somewhat later in time. ECG was calculated as the difference between the two EGs. Right-bundle branch block could be imitated by reducing the coupling between the APs representing the right ventricle. Left-bundle branch block was generated in an analogous way. ECG in acute myocardial infarction was created after making one of the epicardial APs ischaemic in appearance (reduced amplitude, short duration). Status-post infarction ECG (Q-wave and negative T) was produced by reducing the influence from the EG of the infarcted area. Negative T and increased R-wave as in hypertrophy could also be reproduced. Down sloping ST-segment and negative T as in subendocardial ischaemia was also possible to imitate. The simulations showed that biphasic T-waves or T and U-waves follows when the two EGs are separated properly in time.

Effects of beraprost on the transmembrane potentials of guinea-pig ventricular muscles during normoxia and hypoxia-reoxygenation

1. The present study was performed to determine whether beraprost, a new stable analogue of prostacyclin, may exert beneficial effects on the transmembrane action potentials during normoxia and hypoxia-reoxygenation in isolated right ventricular muscles of the guinea-pig. 2. Under normal oxygenation, beraprost (0.01-100 mumol-1) had no effects on the electrophysiological parameters. 3. Hypoxic conditions induced a decrease in action potential duration (APD) without affecting other action potential parameters. Beraprost inhibited this hypoxia-induced decrease in APD. However, beraprost had no effect on the decrease in contractile force induced by hypoxia, whereas it significantly improved the recovery of contractile force after reoxygenation. 4. Pinacidil-induced shortening of APD was not antagonized by beraprost. 5. Hypoxia significantly decreased the myocardial adenosine triphosphate (ATP) level, which was also prevented by beraprost. 6. These results suggested that beraprost may inhibit the hypoxia-induced shortening of APD by some mechanisms which contribute to the maintenance of muscle ATP level.

Dissociation between cellular K+ loss, reduction in repolarization time, and tissue ATP levels during myocardial hypoxia and ischemia

The mechanisms underlying the marked increase in [K+]o in response to ischemia are not fully understood. Accordingly, the present study was performed to assess the contribution of ATP-regulated K+ channels by using simultaneous measurements of cellular K+ efflux, [K+]o, transmembrane action potentials, and tissue ATP, ADP, phosphocreatine, and creatine content in a unique isolated, blood-perfused papillary muscle preparation during hypoxia compared with ischemia. During 15 minutes of hypoxic perfusion (PO2, 6.1 +/- 0.9 mm Hg) with normal [K+]o of 4.1 +/- 0.1 mM, action potential duration (APD) was not altered even though tissue ATP levels decreased markedly from 33.5 +/- 1.8 to 14.7 +/- 2.0 nmol.mg protein-1 (p < 0.01). Net cellular K+ efflux, based on measured differences of [K+] between the venous effluent and the perfusate, was 13.23 +/- 0.79 mumol.g wet wt-1 during hypoxia. In contrast, after 15 minutes of zero-flow ischemia, APD at 80% of repolarization (APD80) decreased by 47% from 171 +/- 5 to 92 +/- 5 msec (p < 0.01), but integrated net cellular K+ efflux over 15 minutes of ischemia was 8.4-fold less (1.57 +/- 0.13 mumol.g wet wt-1) than during hypoxia. Tissue ATP levels, however, decreased by only 35.2% to 21.7 +/- 2.1 nmol.mg protein-1, which was significantly less than that induced by 15 minutes of hypoxia. Perfusion with hypoxic blood containing high [K+]o of 10.3 +/- 0.3 mM resulted in APD shortening similar to that observed during ischemia. Cellular K+ loss, however, was inhibited markedly by high [K+]o perfusion (only 4.51 +/- 0.28 mumol.g wet wt-1). Pretreatment with glibenclamide (5 microM), a drug that has been reported to inhibit ATP-regulated K+ channels and accelerate glycolysis in normoxic tissue, partially inhibited cellular K+ efflux during hypoxic perfusion with normal [K+]o (7.35 +/- 0.71 versus 13.23 +/- 0.79 mumol.g wet wt-1, p < 0.01) but had no significant influence on repolarization time or tissue ATP levels. Although glibenclamide partially prevented action potential shortening induced by hypoxic perfusion in the presence of elevated [K+]o, the proportion of cellular K+ efflux reduced by glibenclamide was less (23%) than that observed with glibenclamide in hypoxic perfusion with normal [K+]o (44%).(ABSTRACT TRUNCATED AT 400 WORDS)

Nucleotide diphosphates activate the ATP-sensitive potassium channel in mouse skeletal muscle

Patch-clamp techniques were used to study the effects of internal nucleotide diphosphates on the KATP channel in mouse skeletal muscle. In inside-out patches, application of GDP (100 microM) and ADP (100 microM) reversibly increased the channel activity. In the presence of internal Mg2+ (1 mM), low concentrations of ADP (< 300 microM) enhanced channel activity and high concentrations of ADP (> 300 microM) limited channel opening while GDP activated the channel at all concentrations tested. In the absence of internal Mg2+, ADP decreased channel activity at all concentrations tested while GDP had no noticeable effect at submillimolar concentrations and inhibited channel activity at millimolar concentrations. GDP [beta S] (100 microM), which behaved as a weak GDP agonist in the presence of Mg2+, stimulated ADP-evoked activation whereas it inhibited GDP-evoked activation. The K+ channel opener pinacidil was found to activate the KATP channel but only in the presence of internal GDP, ADP and GDP [beta S]. The results are discussed in terms of the existence of multiple nucleotide binding sites, in charge of the regulation of the KATP channel.

Effects of 2,4-dinitrophenol or low [ATP]i on cell excitability and action potential propagation in guinea pig ventricular myocytes

Inhibition of aerobic metabolism leads to a major disruption of cardiac cell homeostasis. The purpose of the present study was twofold: 1) We determined the relative importance of junctional and nonjunctional membrane resistance (Rj and Rm, respectively) in the development of propagation failure during inhibition of aerobic metabolism in guinea pig ventricular cell pairs. 2) We used the patch-action potential clamp technique in single ventricular myocytes to study some of the properties of the membrane channels that are responsible for shortening of action potential duration and eventual failure of cell excitation after metabolic blockade. In most experiments, whole-cell patch pipettes were filled with a solution containing 1 mM EGTA, 5 mM HEPES, and 5 mM ATP. Our results in cell pairs showed that pharmacological inhibition of aerobic metabolism with the mitochondrial uncoupler 2,4-dinitrophenol (DNP) led to a drop in Rm followed by an increase in Rj. The increase in Rj was not sufficient to cause a measurable delay in cell-to-cell propagation, whereas the drop in Rm consistently led to failure of cell excitation. Similar results were obtained in additional experiments in which the EGTA concentration in the pipette was reduced to 50 microM. Similar results were also obtained by loading the recording patch pipettes with a solution containing only 0.1 mM ATP. Our patch-action potential clamp experiments, on the other hand, revealed that DNP induced the opening of time- and voltage-independent membrane channels, with a unitary conductance of 23 pS. The channels allowed for the passage of outward current in the voltage range of the action potential, and the increase in membrane patch conductance correlated with the observed shortening of action potential duration during DNP superfusion. Our experiments provide the first simultaneous recordings of action potentials and DNP-induced channel currents in guinea pig ventricular myocytes. Overall, the data provide new evidence for the understanding of the cellular and subcellular mechanisms involved in the development of slow conduction velocity and propagation block after metabolic blockade.

Anoxia induces time-independent K+ current through KATP channels in isolated heart cells of the guinea-pig

1. Isolated ventricular heart cells of the guinea-pig were exposed to anoxia (PO2 < 0.1 Torr) which induced a time-independent K+ current. This current was studied with the patch clamp technique in the whole-cell and cell-attached configuration. 2. The latency until anoxia-induced changes of whole-cell current developed was distributed exponentially (mean 10.5 min; n = 41). The current was abolished within 2-4 s of reoxygenation. 3. The reversal potential of the anoxia-induced change of whole-cell current at 5.4 and 15 mM [K+]o was -82 and -61 mV, respectively. 4. Analysis of current noise in whole-cell current during the phase of the slow development of the anoxia-induced current yielded a slope conductance of unitary currents of 8.1 pS (5.4 mM [K+]o) which is far below the 30 pS of KATP channels in inside-out patches with no Na+ and Mg2+ in the bath. 5. Reduced unitary current induced by anoxia was recorded in single-channel measurements with 10.4 mM-K+ in the pipette. 6. Using 150 mM-K+ in the pipette, anoxia caused unitary inward currents with a slope conductance of 83 pS. The open probability of the channels (P(o)) reached maximum values between 0.6 and 0.95. The channels closed within 1-3 s of reoxygenation. 7. At voltages between -85 and -45 mV and maximum P(o), open time histograms were dominated by a fast exponential (tau 01 = 0.55 +/- 0.21 ms, mean +/- S.D.) and one or two slow exponentials. 8. Voltage ramp experiments showed that single-channel currents were slightly rectifying in the inward direction. 9. Glibenclamide (1 microM) reversibly blocked the anoxia-induced whole-cell and single-channel currents. 10. It is concluded that during anoxia it is only KATP channels which open by a sufficient decrease of submembrane ATP levels.

ATP-sensitive potassium channels and myocardial ischemia: why do they open?

There is evidence that the "ATP-sensitive" potassium channel opens, at least during the early stages of myocardial ischemia, despite relatively high ATP levels. Thus, channel opening may partially contribute to potassium efflux and accumulation of extracellular potassium, but probably much more profoundly to electrical abnormalities associated with ischemia, including the development of lethal arrhythmias. Several factors are discussed that may promote a significant open-channel probability of the channel, in spite of relatively high levels of ATP. It is argued that, even with a very low open probability, the magnitude of total membrane current carried by these channels may be substantial (comparable to other potassium currents) because of the high density and conductance of the ATP-sensitive potassium channel. Finally, it is shown how the ATP-sensitive potassium channel may play a role in various tissue types, ranging from the physiological to the pathophysiological. This potassium channel is therefore increasingly targeted for drug development and research.

ATP-regulated K+ channels protect the myocardium against ischemia/reperfusion damage

The role of ATP-regulated K+ channels in protecting the myocardium against ischemia/reperfusion damage was explored using glibenclamide and pinacidil to block and activate the channels, respectively. Electrical and mechanical activity of arterially perfused guinea pig right ventricular walls was recorded simultaneously via an intracellular microelectrode and a force transducer. The preparations were subjected to either 1) 20 minutes of no-flow ischemia with or without glibenclamide (1 and 10 microM) followed by reperfusion, or 2) 30 minutes of no-flow ischemia with or without pinacidil (1 and 10 microM) followed by reperfusion. No-flow ischemia for 20 minutes produced changes in electrical and mechanical activity that were completely reversed on reperfusion; resting membrane potential declined by 13 +/- 1.2 mV, action potential duration at 90% repolarization (APD90) decreased by 62%, and developed tension fell by greater than 95%, but resting tension did not change significantly. Glibenclamide (10 microM) had no effect on activity during normal perfusion, but during ischemia, resting membrane potential fell slightly further (17 +/- 1.8 mV) and APD90 declined by only 24%. Developed tension declined more slowly and to a lesser extent, but resting tension rose significantly between 10 and 20 minutes of ischemia. Reperfusion of glibenclamide-treated tissues elicited arrhythmias (extrasystoles and tachycardia), and the preparations failed to recover mechanical function. Glibenclamide at 1 microM produced qualitatively similar effects, albeit less severe. After 30 minutes of no-flow ischemia in untreated tissues, resting tension increased by approximately 130% during the no-flow period. Reperfusion caused arrhythmias (extrasystoles, tachyarrhythmias, and fibrillation) and failed to restore resting or developed tension to preischemic levels. Pinacidil at 1 microM did not affect electrical or contractile function, but at 10 microM it had a negative inotropic effect, decreasing APD90 and developed tension by 5% and 18%, respectively. Both concentrations of the drug caused a faster and greater decline in APD90 during the no-flow period. Resting tension did not change during 30 minutes of no-flow ischemia in the presence of pinacidil, and reperfusion led to 85% and complete recovery of electrical and mechanical activity at 1 and 10 microM, respectively. The data indicate that glibenclamide enhances whereas pinacidil reduces myocardial damage caused by ischemia/reperfusion. The results are consistent with the hypothesis that activation of ATP-regulated K+ channels during ischemia is an important adaptive mechanism for protecting the myocardium when blood flow to the tissue is compromised.

ATP-dependent potassium channels of muscle cells: their properties, regulation, and possible functions

ATP-dependent potassium channels are present at high density in the membranes of heart, skeletal, and smooth muscle and have a low Popen at physiological [ATP]i. The unitary conductance is 15-20 pS at physiological [K+]o, and the channels are highly selective for K+. Certain sulfonylureas are specific blockers, and some K channel openers may also act through these channels. KATP channels are probably regulated through the binding of ATP, which may in turn be regulated through changes in the ADP/ATP ratio or in pHi. There is some evidence for control through G-proteins. The channels have complex kinetics, with multiple open and close states. The main effect of ATP is to increase occupancy of long-lived close states. The channels may have a role in the control of excitability and probably act as a route for K+ loss from muscle during activity. In arterial smooth muscle they may act as targets for vasodilators.

Effects of ATP-sensitive K+ channel blockers on the action potential shortening in hypoxic and ischaemic myocardium

1. In order to determine whether activation of adenosine triphosphate (ATP)-sensitive K+ channels exclusively explains the hypoxia- and ischaemia-induced action potential shortening, effects of tolbutamide and glibenclamide on changes in action potential duration (APD) during hypoxia, metabolic blockade or experimental ischaemia were examined in guinea-pig and canine isolated myocardium by standard microelectrode techniques. 2. With use of patch clamp techniques, activity of ATP-sensitive K+ channels was recorded from open cell-attached patches of guinea-pig isolated ventricular myocytes. The probability of opening of the K+ channels was decreased by 2 mM tolbutamide and 20 microM glibenclamide to almost the same extent, whereas it was increased by 100 microM pinacidil. 3. In guinea-pig papillary muscles a marked shortening of the action potential produced by 100 microM pinacidil was completely antagonized by 2 mM tolbutamide or 20 microM glibenclamide. 4. In guinea-pig papillary muscles exposed to hypoxic, glucose-free solution or dinitrophenol (10 microM)-containing, glucose-free solution, APD declined gradually and twitch tension decreased. Pretreatment with glibenclamide partially but significantly inhibited the action potential shortening, whereas tolbutamide failed to improve it during hypoxia or metabolic blockade. 5. When in canine isolated myocardium, experimental ischaemia was produced by the cessation of coronary perfusion, APD was gradually shortened. The action potential shortening was partially but not completely inhibited by pretreatment with 20 microM glibenclamide. 6. These results suggest that changes in membrane current(s) other than the outward current through ATP-sensitive K+ channels also contribute to the action potential shortening in hypoxic or ischaemic myocardium.

Mechanism of potassium efflux and action potential shortening during ischaemia in isolated mammalian cardiac muscle

1. Ischaemia was simulated in the isolated sheep cardiac Purkinje fibre and guinea-pig papillary muscle by immersing the preparations in paraffin oil. Ion-selective microelectrodes recorded potassium (Ks+) and pH (pHs) in the thin film of Tyrode solution trapped at the fibre surface while other microelectrodes recorded intracellular pH (pHi), membrane potential and action potentials (AP) (evoked by field stimulation), or membrane current (two-microelectrode voltage clamp in shortened Purkinje fibres). Twitch tension was also monitored. The paraffin oil model reproduced the salient characteristics of myocardial ischaemia, i.e. a decrease of twitch tension; a decrease of pHi and pHs; a rise in Ks+ (by 2-3 mM); a depolarization of diastolic membrane potential; considerable shortening of the AP (up to 30% within 4 min). 2. The sulphonylurea compounds, glibenclamide (200 microM) and tolbutamide (1 mM), known inhibitors of the KATP channel, completely blocked the ischaemic rise of Ks+ and prevented AP shortening. Ischaemic tension decline was notably less pronounced in the presence of sulphonylureas. 3. The ischaemic increase of slope conductance (Purkinje fibre) was prevented by 1 mM-tolbutamide and 200 microM-glibenclamide. 4. Sulphonylureas did not affect resting membrane potential, the AP or the current-voltage relationship under non-ischaemic conditions (this also indicates that ischaemic Ks+ accumulation is not fuelled by the background K+ current [iK1] which was shown, as expected, to be Ba2+ sensitive). 5. In a normally perfused preparation, reducing intracellular ATP by inhibiting glycolysis with 2-deoxyglucose (DOG) produced a similar AP shortening plus a membrane hyperpolarization, both of which were inhibited by tolbutamide or glibenclamide. The AP shortening was not related uniquely to the fall of pHi observed under these conditions since experimentally reducing pHi (by reducing pHo in the absence of DOG) lengthened rather than shortened the AP. 6. The possibility that the ischaemic rise in Ks+ might be the cause of AP shortening was excluded by the observation that, in a normally perfused Purkinje fibre, experimentally reducing pHi (by an amount similar to that seen during ischaemia) completely neutralized the AP-shortening effect of an elevated Ko+ (from 4.5 to 6.5 mM). Furthermore, the sulphonylurea-sensitive AP shortening seen during DOG treatment could not have been associated with a Ks+ rise since, in these particular experiments, the fibres were well perfused and diastolic membrane potential hyperpolarized.(ABSTRACT TRUNCATED AT 400 WORDS)