Effects of verapamil on ischemia-induced changes in extracellular K+, pH, and local activation in the pig

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.

Effects of Propranolol and Verapamil on Changes in TQ and ST Segment Potentials During Graded Coronary Flow Reduction in a Porcine Myocardial Ischemia Model

Acute myocardial ischemia causes TQ depression and ST elevation. However, the effects of cardioprotective drugs such as β-blockers and Ca⁺⁺-antagonists on the extent of TQ depression, ST elevation, and myocardial ischemic injury are not fully understood.We created a carotid-coronary shunt in 30 pigs, and extracellular K⁺ ([K⁺]_e), TQ, and ST segments were recorded simultaneously with K⁺-selective plunge electrodes placed in the left anterior descending artery (LAD) distribution during graded LAD flow reduction before and after administration of propranolol or verapamil. Unipolar DC-coupled electrograms were recorded from the reference pole of the K⁺-selective plunge electrodes. The microvolt readings from the K⁺-selective electrodes were converted to [K⁺]_e and then to the changes in potassium equilibrium potential (ΔE_K). The shunted LAD flow was reduced in a stepwise fashion at 5-minute intervals.segment depression at the similar ΔE_K was not affected by propranolol or verapamil. However, ST segment elevation was reduced by propranolol but exacerbated by verapamil at the similar ΔE_K.TQ-ST changes recorded by AC coupled ECG are not a reliable index of ischemia and therefore cannot be used to evaluate the effects of drugs that might affect the electrophysiologic properties of ischemic myocardium.

Efficacy of Verapamil against Ventricular Arrhythmias Induced by Programmed Electrical Stimulation in the Late Myocardial Infarction Phase in Dogs

The aim of the present study was to investigate the antiarrhythmic potential of verapamil in the late myocardial infarction period in conscious dogs. Verapamil was administered in cumulative doses (0·3 + 0·3 mg kg−1). The drug significantly lowered systolic and diastolic blood pressure after both doses. ECG signals showed short-lasting significant decrease in RR and QT intervals together with an increase in QTc interval. The parameters of the atrioventricular conduction system (PQ interval, 2:1 AV-conduction point) were significantly prolonged over the entire observation period. Ventricular effective refractory periods remained unaltered. In contrast to results obtained during acute ischaemia and in the first week thereafter, the present study demonstrates that verapamil moderately increases intraventricular conduction time 14 days after acute myocardial infarction. Verapamil prevented the induction of arrhythmias by programmed electrical stimulation (PES) in only 11% of all induction attempts. The lack of lengthening of refractory periods in the presence of a prolongation of intraventricular conduction time may be responsible for the poor antiarrhythmic efficacy. We conclude that verapamil is only of negligible value for the management of PES-induced ventricular arrhythmias in the late myocardial infarction period.

Nonplatelet effects of aspirin during acute coronary occlusion: electrophysiologic and cation alterations in ischemic myocardium

BACKGROUND

Mortality after acute myocardial ischemia has been reduced by aspirin (ASA) but mechanisms other than the antiplatelet effect have not been established. This article evaluates an antiarrhythmic action during sympathetic stimulation in the intact anesthetized dog with and without ischemia.

METHODS AND RESULTS

The ventricular fibrillation threshold (VFT) was examined before and after epinephrine (E) in normals (group I). A VFT reduction during E was normalized after 1 week of ASA (P<.01). Regional myocardial ischemia for 1 hour resulted in similar hypoperfusion in controls of group II and after ASA. Action potential responses in isolated superfused ischemic tissue showed prolonged repolarization (APD90) in response to E, which was normalized after ASA (P<.01). To assess the antiarrhythmic role of the anion in group III, Na salicylate was given. During 1 hour of ischemia, the VF incidence was reduced and cation abnormalities diminished in ischemic myocardium compared with untreated ischemia.

CONCLUSIONS

ASA antagonizes the reduction of the VFT induced by catecholamine in normals as well as the repolarization abnormality elicited by E during acute ischemia. The salicylate anion appears to be the active component in view of the efficacy in preventing VF during the early ischemic period.

Effects of blockade of fast and slow inward current channels on ventricular fibrillation in the pig heart

OBJECTIVE

To determine the contribution of fast and slow inward channels to the electrocardiogram (ECG) of ventricular fibrillation.

METHODS

Ventricular fibrillation was induced by endocardial electrical stimulation in pigs anaesthetised with pentobarbitone sodium (30 mg/kg intravenously). ECGs simultaneously recorded from the body surface (lead II) and from the endocardium were studied by power spectrum analysis (0-40 Hz).

RESULTS

The mean (SEM) dominant frequency of fibrillation (9.0 (1.1) Hz in lead II at 0-40 s) did not change significantly with time in pigs given intravenous saline. However, the dominant frequency was significantly reduced by intravenous pretreatment with the class I antiarrhythmic drugs, lignocaine (3 mg/kg, 6.5 (0.5) Hz; 10 mg/kg, 4.2 (0.6) Hz), mexiletine (3 mg/kg, 6.2 (0.4) Hz; 10 mg/kg, 5.5 (0.4) Hz), and disopyramide (2.5 mg/kg, 5.4 (0.6) Hz). After flecainide (3 mg/kg, 6.9 (0.5) Hz) the reduction in frequency was not significant. Similar data were obtained with endocardial recordings. In contrast pre-treatment with verapamil (0.2 mg/kg, 11.7 (0.8) Hz; and 1.0 mg/kg, 12.9 (1.6) Hz) produced a significantly higher dominant frequency of fibrillation than saline and widened the bandwidth of frequencies around the dominant frequency.

CONCLUSIONS

These results indicate that voltage-dependent sodium channel currents contribute to the rapid frequencies of ventricular fibrillation. Blockade of L-type inward calcium channel activity increases the fibrillation frequency and fractionates the frequencies of the fibrillation wavefronts.

Multiparameter monitoring and analysis of in vivo ischemic and hypoxic heart

We describe a unique in vivo technique which addresses the multifactorial function of the heart, i.e., simultaneous measurement of myocardial ion transport (two mini-electrode systems to measure K+e and Ca2+e), energy metabolism (NADH fluorescence to measure NADH redox state), and coronary flow (laser-Doppler perfusion) using a multiprobe assembly (MPA) which contains transducers for all measurements. The MPA (which is 6 mm in diameter) was applied to the external surface of the heart in an open chest dog model. To test MPA function, myocardial ischemia was produced by application of a balloon occluder to the left anterior descending coronary (LAD) artery, and hypoxia was produced by changing the inspired O2-N2 ratio until the PaO2 was 20-30 torr. The MPA simultaneously monitored changes in ion flux, heart metabolism, and tissue perfusion during pathophysiological intervention.

Failure of impulse propagation in a mathematically simulated ischemic border zone: influence of direction of propagation and cell-to-cell electrical coupling

INTRODUCTION

It is suggested that heterogeneous extracellular potassium concentration, cell-to-cell coupling, and geometric nonuniformities of the ischemic border zone contribute to the incidence of unidirectional block and subsequent development of lethal ventricular arrhythmias.

METHOD AND RESULTS

A discrete electrical network was used to model a single cardiac fiber with a [K+]e gradient characteristic of an ischemic border zone. Directional differences in propagation were evaluated by creating discrete regions with increased gap junctional resistance within the [K+]e gradient. Furthermore, the effect of homogeneity/heterogeneity of call length on impulse propagation through the [K+]e gradient in the presence of increased gap junctional resistance was evaluated. The results indicate that failure of impulse propagation occurs at the junction between partially uncoupled and normally coupled cells. Furthermore, propagation failure was more likely to occur as the impulse propagated from a region of high [K+]e to low [K+]e. Heterogeneity in cell length contributes to the variability in the occurrence of unidirectional and bidirectional block.

CONCLUSIONS

The onset of cellular uncoupling in an ischemic border zone may interact with the inherent [K+]e gradient leading to unidirectional conduction block. This mechanism may be important for the generation of reentrant arrhythmias at the ischemic border zone.

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.

Anti-arrhythmic efficacy of beta-adrenergic blockade during acute ischemia in myocardium with scar

Ventricular arrhythmia production in the ischemic heart is considered to be influenced by prior infarction. Although beta-adrenergic blockade is known to have beneficial effects during acute ischemia, its anti-arrhythmic efficacy during post-infarction ischemia is not known. To explore this question, we have used a model with a relatively high incidence of ischemic arrhythmias. Mongrel dogs 2 to 3 years of age were studied intact under anesthesia. An irreversible injury of the infero-posterior myocardium was produced with an electrode catheter 1 week earlier. The arrhythmic response to acute ischemia was assessed using serial, transient 15-minute occlusions of the left-anterior descending coronary artery with a balloon catheter. During ischemia alone, the incidence of ventricular fibrillation in animals who underwent all phases of the study was 6 of 9; with atenolol (0.2 mg/kg intravenously) and ischemia, 1 of 9 (p < 0.05). To assess the role of the bradycardic response, the latter was repeated 1 week subsequently during atrial pacing at the heart rate that existed before ischemia. Fibrillation occurred in 8 of 9, a significant reversal of the therapeutic effect. To exclude the potential artifact of a fixed intervention protocol, a study was undertaken with the short-acting esmolol, in which three ischemic periods were alternated at 1-hour intervals: (A) ischemia without treatment, (B) ischemia with continuous infusion of 150 micrograms/kg/min esmolol, and (C) same as B except that heart rate was maintained by atrial pacing.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic regulation of in vivo myocardial contractile function: multiparameter analysis

To gain insight into the mechanisms of myocardial regulation as it relates to the interaction of mechanical and metabolic function and perfusion, intact animal models were instrumented for routine physiological measurements of mechanical function and for measurements of metabolism (31P NMR, NADH fluorescence (redox state)) and perfusion (2H NMR and Laser doppler techniques). These techniques were applied to canine and cat models of volume and/or pressure loading, hypoxia, ischemia and cardiomyopathic states. Data generated using these techniques indicate that myocardial bioenergetic function is quite stable under most loading conditions as long as the heart is not ischemic. In addition, these data indicate that there is no universal regulator and that different biochemical regulators appear to mediate stable function under different physiological and pathophysiological conditions: for example; during hypoxia, NADH redox state appears to play a regulatory role; and in pressure loading, ADP, phosphorylation potential and free energy of ATP hydrolysis as well as NADH redox state appear to be regulatory.

The effects of calcium antagonists on extracellular potassium accumulation during global ischaemia in isolated perfused rat hearts

The effects of equipotent concentrations of diltiazem, verapamil, and nifedipine upon the accumulation of extracellular potassium [K+]out and the left ventricular pressure (LVP) were studied during global ischemia in isolated perfused rat hearts. Measurement of [K+]out and LVP were performed in two series of experiments. Diltiazem (2 x 10(-6), 3 x 10(-6), and 10(-5) M), verapamil (3 x 10(-8), 10(-7), and 3 x 10(-7) M), and nifedipine (3 x 10(-8), 10(-7), and 1.5 x 10(-7) M) were able to slow, in a concentration-dependent manner, the initial rate of rise of [K+]out without affecting the final plateau value of [K+]out reached at t = 5 to t = 10 minutes. Notably, at the lowest concentrations, which slightly influenced LVP diltiazem, verapamil, and to a lesser degree nifedipine, were still able to slow the rise in [K+]out. In addition, after preperfusion with low-calcium media [( Ca2+] from 1.8 to 1.3 or 0.9 mM), inducing similar negative inotropic effects as those of the calcium antagonists, the rise in [K+]out was not significantly influenced. Our data indicate that the ability to slow the rise in [K+]out is a specific characteristic of calcium antagonists that is independent of their negative inotropic effects.

Pharmacological analysis of established ventricular fibrillation

1. The effects of anti-arrhythmic drugs on the power spectrum of established ventricular fibrillation induced by endocardial electrical stimulation, have been studied in greyhounds anaesthetized with sodium pentobarbitone (35 mg kg-1, i.v.). 2. In dogs receiving no drug, initial recording of ventricular fibrillation showed a dominant frequency of 9.9 +/- 0.7 Hz (lead II) and 10.0 +/- 0.6 Hz (endocardium). After 3.3 min the frequency had fallen to 4.0 +/- 0.4 Hz in lead II, but remained high in the endocardium (10.7 +/- 0.5 Hz). 3. Lignocaine significantly reduced the dominant frequency for fibrillation recorded from lead II at (0-80 s), and for endocardial fibrillation at (0-200 s). 4. Pretreatment with propranolol or bretylium had little effect on the time course of the dominant frequency of fibrillation in lead II or the endocardium. 5. Verapamil prevented the fall in frequency seen in lead II after 80 s in the no drug group. A significantly higher frequency was maintained in both lead II (14.7 +/- 0.9 Hz) and the endocardium (14.8 +/- 0.9 Hz) for 3.3 min, compared with the no drug group (P less than 0.01). 6. Activation of fast sodium channels may determine the rapid frequency of the initial stages of ventricular fibrillation. The rapid fall in dominant frequency in lead II after fibrillation for 80 s can be prevented by calcium channel blockade and may be due to intracellular accumulation of calcium.

Extracellular potassium ion dynamics and ventricular arrhythmias in the canine heart

The relation between extracellular potassium ion activity [( K+]o) and ventricular tachyarrhythmias was studied in an open chest canine model with the use of two protocols. In Protocol I, potassium chloride was administered into the proximal left anterior descending coronary artery at a rate of 0.125 mEq/min for either 20 min or until [K+]o = 20 mEq/liter, whichever came first. In Protocol II, the proximal left anterior descending coronary artery was occluded in one step and was reperfused 20 min later. Fifteen dogs were subjected to Protocol I, nine of which were also subjected to Protocol II. In the latter group, a recovery period of greater than or equal to 1 h separated the two protocols. Local K+ and intramyocardial activities were recorded with use of bifunctional ion-sensitive plunge electrodes at multiple sites located in the region of the left ventricle perfused by the left anterior descending artery and at one site outside of this region. The following variables were recorded and analyzed: Lead II electrocardiogram, heart rate, systemic arterial blood pressure, local [K+]o and its time derivative (dK+/dt), local electrograms and ventricular arrhythmias. Maximal [K+]o and dK+/dt were 23 +/- 3 mEq/liter and 9 +/- 1 mEq/liter per min in Protocol I and 14 +/- 1 mEq/liter and 3 +/- 1 mEq/liter per min in Protocol II, respectively. In both protocols, the occurrence of ventricular arrhythmias correlated with [K+]o (p less than 0.02) as well as with dK+/dt (p less than 0.05). Ventricular arrhythmias were more frequent and more severe in Protocol II than in Protocol I (p less than 0.05). Therefore, whereas K+ dynamics were more pronounced in Protocol I, ventricular arrhythmias were more severe in Protocol II. This occurrence was apparently due, at least in part, to less heterogeneous changes in K+ gradients during constant K+ infusion. It was concluded that, in addition to the magnitude of [K+]o, the rate of change of this variable (that is, dK+/dt) apparently plays an important role in the genesis of ischemic ventricular arrhythmias.

Effects of gallopamil, diltiazem and nifedipine on the loss of K+ from ischaemic pig hearts

K+ release into the extracellular space was investigated during repeated 6-min coronary occlusions before and after the intravenous administration of cardiovascular active doses of gallopamil (0.02; 0.05 mg/kg), diltiazem (1.0; 2.0 mg/kg) or nifedipine (0.01; 0.05 mg/kg) to anaesthetized pigs. [K+]e was measured epicardially using silver valinomycin electrodes calibrated in vivo. During control occlusions [K+]e- rose steeply in all groups, from a pre-ischaemic baseline value of about 3.5 mmol/l reaching a plateau value within the ischaemic period. This response was reproducible in an untreated control group. Gallopamil reduced the ischaemic K+ efflux dose dependently and significantly 10 min after injection; the higher dose also did 60 min after injection. Diltiazem had less effect on K+ efflux 10 min after administration and an effect was no longer detectable after 60 min. Nifedipine did not significantly inhibit the ischaemic K+ loss. Besides these differences in the direct protection of the ischaemic myocardium, the Ca2+ antagonists also had the following effects on the haemodynamic profile. Diltiazem and gallopamil significantly prolonged PQ intervals whereas nifedipine caused a shortening accompanied by a significant increase in heart rate. Blood pressure and LV dP/dtmax were significantly reduced by all compounds, but to a different degree. Diltiazem reduced blood pressure to a greater extent than did nifedipine and gallopamil. LV dP/dtmax was comparably reduced by gallopamil and diltiazem, while nifedipine had less effect. Thus, gallopamil exerted pronounced protective effects on the ischaemic pig heart.

Protection of rat atrial myocardium against electrical, mechanical and structural aspects of injury caused by exposure in vitro to conditions of simulated ischaemia

1. Rat isolated and superfused atria were exposed for varying periods to a solution simulating the composition of extracellular fluid during myocardial ischaemia (SI). 2. Atria subjected to SI showed a loss of systolic contractile tension, a rise in diastolic tension, a shortening of electrical refractory periods, a slowing of action potential conduction velocity and disruption of the mitochondrial ultrastructure. All these features were reversible when the muscle was returned to normal superfusate. 3. Atria pretreated with a superfusate containing a calcium channel antagonist, a calmodulin inhibitor or an intracellular calcium antagonist showed fewer features of the response to SI than did controls. 4. Atria pretreated with a superfusate containing various non-steroidal anti-inflammatory agents did not show identical responses to SI. Sulphinpyrazone protected against all features of the response to SI but ibuprofen, flurbiprofen and GP25671 (a metabolite of sulphinpyrazone) had little effect. Flufenamate, phenylbutazone and salicylate enhanced the responses to SI.

Extracellular potassium dynamics in the border zone during acute myocardial ischemia in a canine model

Bifunctional intramyocardial potassium ion (K+)-sensitive and bipolar wire electrodes were used to evaluate extracellular K+ dynamics and electrophysiologic changes during acute myocardial ischemia in the border zone, ischemic zone (5 to 7 mm from the border), central ischemic zone (15 to 25 mm from the border) and normal myocardium in 11 open chest dogs during a 30 min ligation of the left anterior descending coronary artery. At the end of this period, the hearts were injected with rhodamine dye and quickly frozen. Ultraviolet NADH (nicotinamide adenine dinucleotide) rhodamine fluorescence photography was used to localize the border between normally perfused and ischemic tissue and determine the site of electrodes in relation to this border. Before coronary ligation, extracellular K+ ranged from 4.0 +/- 0.3 to 4.3 +/- 0.3 mM in these four zones. After ligation, extracellular K+ accumulated in the ischemic and central ischemic zones in a pattern characterized by an initial rapid increase for approximately 5 min, followed by a slowly rising plateau phase, reaching maximal levels of 9.8 +/- 2.0 and 14.4 +/- 4.4 mM, respectively. In contrast, K+ dynamics in the border zone showed a biphasic response, with an initial rapid increase to a maximal level of 7.5 +/- 2.4 mM at approximately 9 min after coronary ligation, followed by a gradual decrease to a level of 5.3 +/- 1.2 mM by the end of the 30 min ligation period. No significant changes in K+ occurred in the normal zone throughout the ischemic period. The correlation of K+ electrode, electrophysiologic and postmortem NADH-rhodamine fluorescence data indicated the existence of a well defined border zone.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium antagonists and acute myocardial ischemia: comparative effects of gallopamil and nifedipine on ischemia-induced and reperfusion-induced ventricular arrhythmias, epicardial conduction times, and ventricular fibrillation thresholds

The comparative effects of the calcium-antagonists gallopamil and nifedipine on ischemia-induced and reperfusion-induced ventricular arrhythmias, particularly ventricular fibrillation (VF), were assessed in a total of 40 mongrel dogs in two experimental preparations. In part I of the study, changes in the time course of spontaneous ventricular arrhythmias and VF parallel to changes in epicardial conduction following acute coronary artery occlusion lasting 20 minutes and followed by subsequent reperfusion were determined. In part II, repeated coronary artery occlusions (20 min) followed by reperfusion (60 min) were performed, and changes in ventricular fibrillation threshold (VFT) were assessed. Gallopamil proved to be highly effective in preventing ventricular arrhythmias and VF following coronary delay was reduced. The ischemia-induced fall in conduction delay was reduced. The ischemia-induced fall in VFT occurring during the first few minutes after occlusion (phase Ia) was significantly reduced. In contrast, nifedipine failed to influence the incidence of ventricular arrhythmias and VF. Following reperfusion, neither drug reduced the incidence of VF nor the associated fall in VFT at the onset of reperfusion. The time course of recovery of epicardial conduction was not affected by either drug. However, the increase in the VFT during the early postreperfusion period was significantly enhanced by both agents. The effects of gallopamil were more pronounced than those of nifedipine. Delayed reperfusion ventricular arrhythmias arising 5 to 10 minutes after release of coronary artery obstruction were significantly reduced by gallopamil whereas nifedipine proved ineffective. The results show that calcium antagonists display direct antiarrhythmic and cardioprotective actions in acute transient myocardial ischemia. The different effectiveness of gallopamil compared to nifedipine can be explained by differences in electrophysiological properties of the drugs. Enhanced ventricular vulnerability following acute transient coronary artery occlusion and subsequent release of coronary artery obstruction, first described by Tennant and Wiggers, has been extensively investigated over the past decade in a variety of experimental and clinical settings. However, the basic mechanisms underlying ischemia- and reperfusion-induced ventricular arrhythmias and ventricular fibrillation (VF) have not yet been fully elucidated. Furthermore, the results of pharmacological approaches to prevent ventricular arrhythmic activity are conflicting. The present study aimed to evaluate the antiarrhythmic efficacy of calcium antagonists in acute myocardial ischemia and reperfusion. We have examined the effects of gallopamil and nifedipine on the time course of ventricular arrhythmias during the first 20 minutes after acute coronary artery occlusion and subsequent reperfusion. We have studied the underlying mechanisms by mapping epicardial conduction and by assessing the electrically induced ventricular fibrillation threshold (VFT) both within and outside ischemic areas.

Effects of the calcium antagonist gallopamil on the increase of myocardial extracellular potassium activity during LAD occlusion in dogs

It has been implied that the increase of myocardial extracellular potassium activity [( K+]e) in the early stage of acute myocardial ischemia is a major cause of the increased likelihood of arrhythmia after acute coronary artery occlusion. There is also experimental evidence that some calcium antagonists reduce the occurrence of ischemia-induced early ventricular arrhythmias. In order to clarify the antiarrhythmic effect of gallopamil during the early phase of acute LAD occlusion, the influence of this calcium antagonist on the time course of [K+]e during acute ischemia was measured in open-chest anesthetized dogs using a K+-selective surface multielectrode. The regional myocardial blood flow was determined with 9 micron radioactive tracer microspheres. After application of gallopamil (bolus 25 micrograms/kg and infusion 2.5 micrograms/kg.min for 30 min) the maximal and mean rate of rise of [K+]e as well as the plateau of [K+]e reached during ischemia were significantly diminished compared with the control occlusions. 90 min after gallopamil, the rate of rise of [K+]e as well as the plateau of [K+]e reached were still significantly reduced, but 180 min after the gallopamil application, no significant differences between the time course of [K+]e and that of the two control occlusions could be found. Gallopamil significantly elevated myocardial blood flow in the non-ischemic area, but did not influence blood flow in the ischemic region. While collateral perfusion remains unchanged, the slowed and reduced increase of myocardial [K+]e during acute coronary artery occlusion may be an important component of the antiarrhythmic effect of gallopamil during early ischemia.