Marked activation delay caused by ischemia initiated after regional K+ elevation in in situ pig hearts

Effects of Verapamil and Pinacidil on Extracellular K+, pH, and the Incidence of Ventricular Fibrillation during 60 Minutes of Ischemia

Ca⁺⁺-channel antagonist verapamil and ATP-sensitive K⁺-channel opener pinacidil are known to decrease the rise in extracellular K⁺ ([K⁺]_e) level and pH (pH_e) that occurs during reversible acute myocardial ischemia and to lessen the accompanying activation delay. Verapamil is also known to decrease the incidence of ventricular tachycardia (VT)/fibrillation (VF) during acute myocardial ischemia; however, the effects of ATP-sensitive K⁺-channel opener on the incidence of VT/VF are controversial. We studied, in an in vivo pig model, the effects of verapamil and pinacidil on the changes in [K⁺]_e level and pH_e, local activation, and the incidence of VT/VF during 60 minutes of ischemia. Thirty-one pigs were divided into 2 groups: a verapamil group (9 control pigs and 8 verapamil-treated pigs) and pinacidil group (5 control pigs and 9 pinacidil-treated pigs). In the verapamil group, VF developed in 1 of the 9 control pigs, whereas no VF developed in 8 verapamil-treated pigs. In the pinacidil group, VF developed in 3 of the 5 control pigs and all 9 pinacidil-treated pigs. Under verapamil treatment (versus the control condition), onset of the second rise in [K⁺]_e level was delayed, and the maximum rise in [K⁺]_e level was decreased. Under pinacidil treatment (versus the control condition), time to the onset of VT/VF was shorter than that under the control condition, and VT/VF developed at lower [K⁺]_e level and higher pH_e. In conclusion, VF may develop at a lesser [K⁺]_e rise and pH_e fall in the presence of pinacidil during acute myocardial ischemia.

Regulation of gap junction conductance by calcineurin through Cx43 phosphorylation: implications for action potential conduction

Cardiac arrhythmias are associated with raised intracellular [Ca²⁺] and slowed action potential conduction caused by reduced gap junction (GJ) electrical conductance (Gj). Ventricular GJs are composed of connexin proteins (Cx43), with Gj determined by Cx43 phosphorylation status. Connexin phosphorylation is an interplay between protein kinases and phosphatases but the precise pathways are unknown. We aimed to identify key Ca²⁺-dependent phosphorylation sites on Cx43 that regulate cardiac gap junction conductance and action potential conduction velocity. We investigated the role of the Ca²⁺-dependent phosphatase, calcineurin. Intracellular [Ca²⁺] was raised in guinea-pig myocardium by a low-Na solution or increased stimulation. Conduction velocity and Gj were measured in multicellular strips. Phosphorylation of Cx43 serine residues (S365 and S368) and of the intermediary regulator I1 at threonine35 was measured by Western blot. Measurements were made in the presence and absence of inhibitors to calcineurin, I1 or protein phosphatase-1 and phosphatase-2.Raised [Ca²⁺]_i decreased Gj, reduced Cx43 phosphorylation at S365 and increased it at S368; these changes were reversed by calcineurin inhibitors. Cx43-S368 phosphorylation was reversed by the protein kinase C inhibitor chelerythrine. Raised [Ca²⁺]_i also decreased I1 phosphorylation, also prevented by calcineurin inhibitors, to increase activity of the Ca²⁺-independent phosphatase, PPI. The PP1 inhibitor, tautomycin, prevented Cx43-365 dephosphorylation, Cx43-S368 phosphorylation and Gj reduction in raised [Ca²⁺]_i. PP2A had no role. Conduction velocity was reduced by raised [Ca²⁺]_i and reversed by calcineurin inhibitors. Reduced action potential conduction and Gj in raised [Ca²⁺] are regulated by calcineurin-dependent Cx43-S365 phosphorylation, leading to Cx43-S368 dephosphorylation. The calcineurin action is indirect, via I1 dephosphorylation and subsequent activation of PP1.

Initial and Secondary ST-T Alternans During Acute Myocardial Ischemia in the In-Situ Pig Heart

The factors responsible for the ST-T wave alternans (STTA) and associated arrhythmias during acute ischemia have not been clarified.In acutely ischemic porcine myocardium, we recorded transmural unipolar and bipolar electrocardiograms and mid-myocardial extracellular K(+) ([K(+)]e) from the center of the ischemic zone during 8-minute episodes of ischemia. Two different STTAs occurred. The initial STTA, which occurred at 4 minutes 15 seconds ± 12 seconds of ischemia during sinus rhythm, was most prominent in the subendocardium, independent of [K(+)]e and activation block, and heart rate dependent. It occurred in 13/19 (68%) occlusions at heart rates ≤ 100 bpm and in 22/23 (96%) at > 100 bpm. The second STTA was more obvious and greatest in the subepicardium. It began in the later phase of ischemia and was also heart rate dependent (5/19 [26%] occlusions at heart rates ≤ 100 bpm and 10/23 [44%] at > 100 bpm). This STTA was consistently associated with 2:1 change in the bipolar electrogram morphology, possibly due to 2:1 conduction block. Ventricular fibrillation (VF) occurred only at > 100 bpm.The initial STTA may be independent of conduction abnormalities and represent primary repolarization alternans. The second STTA may be secondary to and indicative of 2:1 activation block or marked alternans of the action potential amplitude/duration. The associated VF most likely reflects the underlying conduction abnormality.

Ventricular fibrillation: dynamics and ion channel determinants

Ventricular fibrillation (VF) is the leading cause of sudden cardiac death. This brief review addresses issues relevant to the dynamics of the rotors responsible for functional reentry and VF. It also makes an attempt to summarize present-day knowledge of the manner in which the dynamic interplay between inward and outward transmembrane currents and the heterogeneous cardiac structure establish a substrate for the initiation and maintenance of rotors and VF. The fragmentary nature of our current understanding of ionic VF mechanisms does not even allow an approach toward a "Theory of VF". Yet some hope is provided by recently obtained insight into the roles played in VF by some of the sarcolemmal ion channels that control the excitation-recovery process. For example, strong evidence supports the idea that the interplay between the rapid-inward sodium current and the inward-rectifier potassium current controls rotor formation, as well as rotor stability and frequency. Solid evidence also exists for an involvement of L-type calcium current in the control of rotor frequency and in determining VF-to-ventricular tachycardia conversion. Less clear, however, is whether or not time dependent outward currents through voltage-gated potassium channels affect the fibrillatory process. Hopefully, taking advantage of currently available approaches of structural, molecular and cellular biology, together with computational and imaging techniques, will afford us the opportunity to further advance knowledge on VF mechanisms.

Mechanistic investigation into the arrhythmogenic role of transmural heterogeneities in regional ischaemia phase 1A

AIMS

Studies of arrhythmogenesis during ischemia have focused primarily on reentrant mechanisms manifested on the epicardial surface. The goal of this study was to use a physiologically-accurate model of acute regional ischemia phase 1A to determine the contribution of ischaemia-induced transmural electrophysiological heterogeneities to arrhythmogenesis following left anterior descending artery occlusion.

METHODS AND RESULTS

A slice through a geometrical model of the rabbit ventricles was extracted and a model of regional ischaemia developed. The model included a central ischaemic zone incorporating transmural gradients of I(K(ATP)) activation and [K+]o, surrounded by ischaemic border zones (BZs), with the degree of ischaemic effects varied to represent progression of ischaemia 2-10 min post-occlusion. Premature stimulation was applied over a range of coupling intervals to induce re-entry. The presence of ischaemic BZs and a transmural gradient in I(K(ATP)) activation provided the substrate for re-entrant arrhythmias. Increased dispersion of refractoriness and conduction velocity in the BZs with time post-occlusion led to a progressive increase in arrhythmogenesis. In the absence of a transmural gradient of I(K(ATP)) activation, re-entry was rarely sustained.

CONCLUSION

Knowledge of the mechanism by which specific electrophysiological heterogeneities underlie arrhythmogenesis during acute ischaemia could be useful in developing preventative treatments for patients at risk of coronary vascular disease.

Epoxygenases and peroxisome proliferator-activated receptors in mammalian vascular biology

Epoxygenases, particularly of the CYP2C and CYP2J families, are important lipid-metabolizing enzymes. Epoxygenases are found throughout the cardiovascular system where their lipid products, particularly the epoxyeicosatrienoic acids (EETs), which are arachidonic acid metabolites, have the potential to regulate vascular tone, cellular proliferation, migration, inflammation and cardiac function. The receptors for EETs are, however, poorly understood. The peroxisome proliferator-activated receptors (PPARs) are a family of three (alpha, beta/delta and gamma) nuclear receptors that are activated by lipid metabolites. Activation of PPAR alpha and PPAR gamma, similar to the longer term effects of EETs, causes the inhibition of vascular cell proliferation, migration and inflammation. Interestingly, EETs and their metabolites have recently been found to active both PPAR alpha and PPAR gamma. The epoxygenase-EET-PPAR pathway may therefore represent a novel endogenous protective pathway by which short-lived lipid mediators control vascular cell activation.

Reduction of ST elevation in repeated coronary occlusion model depends on both altered metabolic response and conduction property

The aim of this study was to elucidate the mechanisms of altered electrical response to ischemia in repeated coronary occlusion model. To test its dependence on metabolic response, extracellular K+ concentration (eKC), myocardial pH and PCO2 were simultaneously measured with epicardial ECG during three consecutive 4 min of left anterior descending coronary artery (LAD) occlusion separated by 15 min of reperfusion in canine hearts. ECG changes induced by infusion of high K+-buffer (10 mM) into the coronary arterial bed via carotid artery-LAD bypass (referred to as high K+-challenges: HKC) were also tested prior to (the first HKC), and during each reperfusion period (the second to the fourth HKC). ST elevation was significantly reduced in subsequent occlusions (3.14 +/- 0.48 and 2.98 +/- 0.47 mV in the second and third occlusion, both P<0.05, compared to 4.91 +/- 0.78 mV in the first). This was accompanied by significant attenuation of the changes in eKC, tissue pH and PCO2. ST elevation induced by HKC also significantly reduced after repeated occlusion (4.09 +/- 0.79 mV in the fourth HKC vs. 5.64 +/- 0.68 mV in the first, P<0.05) in spite of the identical changes in eKC during HKC. This progressive decrease in ST changes by HKC was rather consistent with augmented conduction delay (86.4 +/- 7.1% increase in activation time in the fourth vs. 54.3 +/- 3.4% in the first, P<0.01). These findings indicate that repeated ischemia induces altered electrical response to subsequent ischemia based on both attenuated metabolic response and altered conduction property.

Effects of acute ischemia, early extrabeats and propafenone on complex activation patterns in intact and ischemic canine hearts

Although, sodium channel blockers have the ability to suppress nonsustained ventricular arrhythmias, an excessive drug-associated arrhythmic death rate has been reported in patients with coronary heart disease (CHD). Sodium channel blockers should prevent initiation of reentry activation by reducing directional differences in cardiac conduction (anisotropy). However, in vitro data demonstrated, that reduction of membrane excitability, e.g. by lowering the inward Na+ current, increases the risk for conduction failure and associated reentry arrhythmias. In 11 dogs the effects of myocardial ischemia, premature epicardial stimulation (PES) and propafenone on anisotropic conduction properties were tested using three-dimensional mapping techniques. The epicardial (longitudinal and transverse to fiber orientation) and transmural (oblique and straight) spread of activation was reconstructed during constant and PES. At baseline, conduction velocities (CV) were higher along (1.20 +/- 0.41 m/s) than across (0.91 +/- 0.19 m/s; p < 0.05) epicardial muscle fibers as well as along oblique (1.77 +/- 0.75 m/s) compared to straight (0.39 +/- 0.09 m/s, p < 0.05) transmural pathways. Acute ischemia did not significantly reduce tissue anisotropy. PES and additional administration of propafenone epicardially eliminated and transmurally profoundly reduced tissue anisotropy (longitudinal 0.58 +/- 0.09 m/s, transverse 0.69 +/- 0.08 m/s, oblique 0.69 +/- 0.28 m/s, straight 0.27 +/- 0.07 m/s). However, reduced anisotropy was associated with a higher probability for conduction block along myocardial fibers in the epicardium and along oblique transmural pathways. Our data show, that propafenone exhibits both potential pro- and antiarrhythmic effects in dogs with acute myocardial ischemia. These results possibly provide more insights in mechanisms underlying the excessive drug-associated arrhythmic death rate in patients with CHD.

Stimulation of cardiac L-type calcium channels by extracellular ATP

The co-release of ATP with norepinephrine from sympathetic nerve terminals in the heart may augment adrenergic stimulation of cardiac Ca(2+) channel activity. To test for a possible direct effect of extracellular ATP on L-type Ca(2+) channels, single channels were reconstituted from porcine sarcolemma into planar lipid bilayers so that intracellular signaling pathways could be controlled. Extracellular ATP (2-100 microM) increased the open probability of the reconstituted channels, with a maximal increase of approximately 2.6-fold and an EC(50) of 3.9 microM. The increase in open probability was due to an increase in channel availability and a decrease in channel inactivation rate. Other nucleotides displayed a rank order of effectiveness of ATP > alpha,beta-methylene-ATP > 2-methylthio-ATP > UTP > adenosine 5'-O-(3-thiotriphosphate) >> ADP; adenosine had no effect. Several antagonists of P2 receptors had no impact on the ATP-dependent increase in open probability, indicating that receptor activation was not required. These results suggest that extracellular ATP and other nucleotides can stimulate the activity of cardiac L-type Ca(2+) channels via a direct interaction with the channels.

Understanding conduction of electrical impulses in the mouse heart using high-resolution video imaging technology

The conduction of electrical impulses in the heart depends on the ability to efficiently transfer excitatory current between individual myocytes. Several recent studies have focused on the use of optical mapping techniques to determine the electrophysiological consequences and the proarrhythmic effects of reducing intercellular coupling in newly developed connexin knockout mice. This work has begun to unravel important questions regarding the role of connexins in intercellular coupling and propagation of electrical impulses in the heart. The purpose of this review is to discuss the techniques and unique issues involved in imaging electrical wave propagation in the heart. In addition, we will review recent experimental studies that address the role of intercellular communication in the development of cardiac arrhythmias.

Effect of rates of perfusion on dominant frequency and defibrillation energy in isolated fibrillating hearts

This study assessed the influence of rates of reperfusion on excitability of the myocardium using dominant frequency (DF) (in Hz) of VF and the relationship of DF to the minimum defibrillation energy (MDE) (in J). Our hypothesis was that increasing flow during reperfusion increases DF that raises MDE. Initially, six Langendorff perfused swine hearts were serially fibrillated and perfusion arrested for 4 minutes followed by reperfusion and defibrillation to establish reproducibility of the model. The epicardial ECG was analyzed for DF. In subsequent studies (n = 8), no flow VF was followed by 1-minute reperfusion at normal flow or 10% flow (low flow) and shocked with increasing energy via epicardial pads until defibrillation. The DF at onset of no flow VF was 9.5 +/- 1.4 and decreased to 3.6 +/- 1.4 after 4 minutes. Reperfusion at normal flow increased the DF of VF compared to low flow after 1 minute (10.8 +/- 1.1 vs 4.5 +/- 1.1 Hz, P = 0.0002) and was associated with increased defibrillation energy requirements (13.5 +/- 5.0 vs 7.3 +/- 6.2 J, P = 0.047). In summary, defibrillation energy requirements are lower when myocardial excitability is reduced during low flow reperfusion.

Connexins and impulse propagation in the mouse heart

Gap junction channels are essential for normal cardiac impulse propagation. Three gap junction proteins, known as connexins, are expressed in the heart: Cx40, Cx43, and Cx45. Each of these proteins forms channels with unique biophysical and electrophysiologic properties, as well as spatial distribution of expression throughout the heart. However, the specific functional role of the individual connexins in normal and abnormal propagation is unknown. The availability of genetically engineered mouse models, together with new developments in optical mapping technology, makes it possible to integrate knowledge about molecular mechanisms of intercellular communication and its regulation with our growing understanding of the microscopic and global dynamics of electrical impulse propagation during normal and abnormal cardiac rhythms. This article reviews knowledge on the mechanisms of cardiac impulse propagation, with particular focus on the role of cardiac connexins in electrical communication between cells. It summarizes results of recent studies on the electrophysiologic consequences of defects in the functional expression of specific gap junction channels in mice lacking either the Cx43 or Cx40 gene. It also reviews data obtained in a transgenic mouse model in which cell loss and remodeling of gap junction distribution leads to increased susceptibility to arrhythmias and sudden cardiac death. Overall, the results demonstrate that these are potentially powerful strategies for studying fundamental mechanisms of cardiac electrical activity and for testing the hypothesis that certain cardiac arrhythmias involve gap junction or other membrane channel dysfunction. These new approaches, which permit one to manipulate electrical wave propagation at the molecular level, should provide new insight into the detailed mechanisms of initiation, maintenance, and termination of cardiac arrhythmias, and may lead to more effective means to treat arrhythmias and prevent sudden cardiac death.

Characterization of conduction in the ventricles of normal and heterozygous Cx43 knockout mice using optical mapping

INTRODUCTION

Gap junction channels are important determinants of conduction in the heart and may play a central role in the development of lethal cardiac arrhythmias. The recent development of a Cx43-deficient mouse has raised fundamental questions about the role of specific connexin isoforms in intercellular communication in the heart. Although a homozygous null mutation of the Cx43 gene (Cx43-/-) is lethal, the heterozygous (Cx43+/-) animals survive to adulthood. Reports on the cardiac electrophysiologic phenotype of the Cx43+/- mice are contradictory. Thus, the effects of a null mutation of a single Cx43 allele require reevaluation.

METHODS AND RESULTS

High-resolution video mapping techniques were used to study propagation in hearts from Cx43+/- and littermate control (Cx43+/+) mice. Local conduction velocities (CVs) and conduction patterns were quantitatively measured by determining conduction vectors. We undertook the characterization of ECG parameters and epicardial CVs of normal and Cx43+/- mouse hearts. ECG measurements obtained from 12 Cx43+/+ and 6 Cx43+/- age matched mice did not show differences in any parameter, including QRS duration (14.5 +/- 0.9 and 15.7 +/- 2.3 msec for Cx43+/+ and Cx43+/-, respectively). In addition, using a sensitive method of detecting changes in local CV, video images of epicardial wave propagation revealed similar activation patterns and velocities in both groups of mice.

CONCLUSION

A sensitive method that accurately measures local CVs throughout the ventricles revealed no changes in Cx43+/- mice, which is consistent with the demonstration that ECG parameter values in the heterozygous mice are the same as those in wild-type mice.

Action potentials that mimic fibrillation activate sodium current

Ventricular fibrillation (VF) has brief action potentials (50-70 ms) with short diastolic intervals (10-30 ms). Under these conditions ion channel activity may be grossly different to normal sinus rhythm (NSR). In particular, sodium channel activation may not contribute to the generation and propagation of action potentials during VF. This study determined if sodium channels can be activated when action potentials mimic VF. Isolated chick ventricular myocytes (n=7) were voltage-clamped to quantitate fast inward sodium current. The voltage clamp protocol simulated VF with a 10 pulse train at 10 Hz (100 ms cycle length (CL)) and depolarization interval (action potential duration) ranging from 90 to 20 ms. After each train a test pulse was delivered from holding (-80 mV) in 10-ms steps. The train preceded each step pulse. Peak sodium current for control and each VF protocol occurred at a membrane potential (V(m)) of -10 mV. Sodium current was evident during brief resting intervals as short as 20 ms, albeit 10-20% of baseline. Resting intervals less than 60 ms shifted the sodium conductance activation curve from Vm(0.5)-30 mV to -22 mV membrane potential. Similar findings occurred when resting potential was at -65 mV, although there was less sodium current with all tested protocols. There was significantly less inactivation of sodium current when the prepulse was shorter (100 v 1000 ms). There was approximately 20% greater sodium current when the test pulse followed a short v long depolarized (>-80 mV) prepulse. Although the longer depolarization pulses produce approximately 20% greater sodium current at membrane potentials more negative than -80 mV. Lastly the time for half recovery of sodium current from activation was significantly less when the inactivating prepulse was short v long (45.9+/-9 v 118+/-20 ms, P<0.05). In conclusion, sodium current is evident when the diastolic rest interval is as brief as 10-20 ms. Rest interval, length of membrane depolarization and membrane potential interact to affect sodium channel activation, inactivation and recovery from inactivation. These data demonstrate that the brief action potentials at more depolarized membrane potentials seen during VF allow for inward sodium current upon depolarization, less sodium channel inactivation, and a faster recovery from inactivation, thereby compensating for a short diastolic rest interval. Therefore, it is likely that the inward sodium channel contributes to wave front propagation during ventricular fibrillation.

Mapping propagation of mechanical activation in the paced heart with MRI tagging

The temporal evolution of three-dimensional (3-D) strain maps derived from magnetic resonance imaging (MRI) tagging were used to noninvasively evaluate mechanical activation in the left ventricle (LV) while seven canine hearts were paced in situ from three different sites: the base of the LV free wall (LVb), the right ventricular apex (RVa), and the right atrium (RA). Strain maps plotted against time showed the evolution of shortening over the entire LV midwall and were used to generate mechanical activation maps showing the onset of circumferential shortening. RA pacing showed rapid synchronous shortening; LVb pacing showed a wave front of mechanical activation propagating slowly and steadily from the pacing site, whereas RVa pacing showed regions of rapid and slower propagation. The mechanical (M) activation times correlated linearly with the electrical (E) activation (M = 1.06E + 8.4 ms, R = 0.95). The time for 90% activation of the LV was 63.1 +/- 24.3 ms for RA pacing, 130.2 +/- 9.8 ms for LVb pacing, and 121.3 +/- 17.9 ms for RVa pacing. The velocity of mechanical activation was calculated for LVb and RVa pacing and was similar to values reported for electrical conduction in myocardium. The propagation of mechanical activation for RVa pacing showed regional variations, whereas LVb pacing did not.

Inhibition of cardiac L-type calcium channels by epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs), products of the cytochrome P-450 monooxygenase metabolism of arachidonic acid, can regulate the activity of ion channels. We examined the effects of EETs on cardiac L-type Ca2+ channels that play important roles in regulating cardiac contractility, controlling heart rate, and mediating slow conduction in normal nodal cells and ischemic myocardium. Our experimental approach was to reconstitute porcine L-type Ca2+ channels into planar lipid bilayers where we could control the aqueous and lipid environments of the channels and the regulatory pathways that change channel properties. We found that 20 to 125 nM EETs inhibited the open probability of reconstituted L-type Ca2+ channels, accelerated the inactivation of the channels, and reduced the unitary current amplitude of open channels. There was no selectivity among different EET regioisomers or stereoisomers. When 11,12-EET was esterified to the sn-2 position of phosphatidylcholine, restricting it to the hydrophobic phase of the planar lipid bilayer, the reconstituted channels were similarly inhibited, suggesting that the EET interacts directly with Ca2+ channels through the lipid phase. The inhibitory effects of EET persisted in the presence of microcystin, an inhibitor of protein phosphatases 1 and 2A, suggesting that dephosphorylation was not the mechanism through which these eicosanoids down-regulate channel activity. This inhibition may be an important protective mechanism in the setting of cardiac ischemia where arachidonic acid levels are dramatically increased and EETs have been shown to manifest preconditioning-like effects.

Comparison of the effects of regional ischemia and hyperkalemia on the membrane action potentials of the in situ pig heart. Experimental Cardiology Group, University of North Carolina at Chapel Hill

INTRODUCTION

This study was designed to determine the role of increased extracellular potassium [K+]e on action potential duration (APD) in the in situ porcine heart during acute regional no-flow ischemia.

METHODS AND RESULTS

In open chest, anesthetized swine, an arterial shunt from the carotid artery to the mid-left anterior descending coronary artery was created through which a solution of KCl was infused to raise [K+]e. Myocardial [K+]e was determined by potassium-sensitive electrodes, and transmembrane action potential was recorded by floating glass microelectrode. During the first 2 minutes of ischemia, APD at 90% repolarization (APD90) lengthened by 31.2 +/- 1.1 msec (P < 0.05). The comparable increase in [K+]e alone shortened APD90. During the next 6 minutes of ischemia, [K+]e rose to 11.3 +/- 0.3 mM and APD90 showed a decrease. However, the comparable increase in [K+]e by infusion of KCl caused further shortening of APD90 at similar levels of [K+]e.

CONCLUSIONS

Acutely ischemic myocardium showed a brief increase in APD90 during the first 2 minutes of ischemia, followed by a fall in APD90 after 2 minutes of ischemia, but the shortening is less than anticipated by the rise in [K+]e. Thus, we hypothesize that other component(s) of ischemia may inhibit action potential repolarization.