Simultaneous assessment of the hemodynamic, cardiomechanical, and electrophysiological effects of terfenadine on the in vivo canine model

Translational Studies on Anti-Atrial Fibrillatory Action of Oseltamivir by its in vivo and in vitro Electropharmacological Analyses

Oseltamivir has been shown to prolong the atrial conduction time and effective refractory period, and to suppress the onset of burst pacing-induced atrial fibrillation in vitro. To better predict its potential clinical benefit as an anti-atrial fibrillatory drug, we performed translational studies by assessing in vivo anti-atrial fibrillatory effect along with in vivo and in vitro electropharmacological analyses. Oseltamivir in intravenous doses of 3 (n = 6) and 30 mg/kg (n = 7) was administered in conscious state to the persistent atrial fibrillation model dogs to confirm its anti-atrial fibrillatory action. The model was prepared by tachypacing to the atria of chronic atrioventricular block dogs for > 6 weeks. Next, oseltamivir in doses of 0.3, 3 and 30 mg/kg was intravenously administered to the halothane-anesthetized intact dogs to analyze its in vivo electrophysiological actions (n = 4). Finally, its in vitro effects of 10–1,000 μM on I_K,ACh, I_Kur, I_Kr, I_Na and I_CaL were analyzed by using cell lines stably expressing Kir3.1/3.4, K_V1.5, hERG, Na_V1.5 or Ca_V1.2, respectively (n = 3 for I_K,ACh and I_Kr or n = 6 for I_Kr, I_Na and I_CaL). Oseltamivir in doses of 3 and 30 mg/kg terminated the atrial fibrillation in 1 out of 6 and in 6 out of 7 atrial fibrillation model dogs, respectively without inducing any lethal ventricular arrhythmia. Its 3 and 30 mg/kg delayed inter-atrial conduction in a frequency-dependent manner, whereas they prolonged atrial effective refractory period in a reverse frequency-dependent manner in the intact dogs. The current assay indicated that IC₅₀ values for I_K,ACh and I_Kr were 160 and 231 μM, respectively, but 1,000 µM inhibited I_Na, I_CaL and I_Kur by 22, 19 and 13%, respectively. The extent of I_Na blockade was enhanced at faster beating rate and more depolarized resting membrane potential. Oseltamivir effectively terminated the persistent atrial fibrillation, which may be largely due to the prolongation of the atrial effective refractory period and inter-atrial conduction time induced by I_K,ACh and I_Kr inhibitions along with I_Na suppression. Thus, oseltamivir can exert a powerful anti-atrial fibrillatory action through its ideal multi-channel blocking property; and oseltamivir would become a promising seed compound for developing efficacious and safe anti-atrial fibrillatory drugs.

Electropharmacological effects of intracellular Ca2+ handling modulator caldaret on the heart assessed in the halothane-anesthetized dogs

We analyzed how the enhancement of net sarcoplasmic reticulum (SR) Ca²⁺ uptake may affect cardiac electrophysiological properties in vivo by using caldaret which can decrease SR diastolic Ca²⁺ leak, enhance SR Ca²⁺ reuptake and inhibit reverse-mode Na⁺/Ca²⁺ exchanger. Caldaret in doses of 0.5, 5 and 50 μg/kg was intravenously administered over 10 min to the halothane-anesthetized beagle dogs (n = 5), attaining pharmacologically active plasma concentration. The low and middle doses of caldaret increased the ventricular contraction, which could be explained by its on-target pharmacological activities. The high dose enhanced the sinus automaticity followed by its suppression in addition to the increase of the total peripheral resistance, which may be unfavorable for treating diastolic heart failure. The low and middle doses enhanced the atrioventricular conduction, which may have some potential for predisposing the atria to the onset of atrial fibrillation via an induction of mitral and/or tricuspid regurgitation. The middle and high doses of caldaret prolonged the ventricular effective refractory period without altering the intraventricular conduction or repolarization period, which may prevent the onset of ventricular arrhythmias. Thus, modulation of intracellular Ca²⁺ handling by caldaret can induce not only inotropic effect, but also various electrophysiological actions on the in situ heart.

Sensitive and reliable proarrhythmia in vivo animal models for predicting drug-induced torsades de pointes in patients with remodelled hearts

As an increasing number of non-cardiac drugs have been reported to cause QT interval prolongation and torsades de pointes (TdP), we extensively studied the utility of atrioventricular (AV) block animals as a model to predict their torsadogenic action in human. The present review highlights such in vivo proarrhythmia models. In the case of the canine model, test substances were administered p.o. at conscious state >4 weeks after the induction of AV block, with subsequent Holter ECG monitoring to evaluate drug effects. Control AV block dogs (no pharmacological treatment) survive for several years without TdP attack. For pharmacologically treated dogs, drugs were identified as high, low or no risk. High-risk drugs induced TdP at 1-3 times the therapeutic dose. Low-risk drugs did not induce TdP at this dose range, but induced it at higher doses. No-risk drugs never induced TdP at any dose tested. Electrophysiological, anatomical histological and biochemical adaptations against persistent bradycardia-induced chronic heart failure were observed in AV block dogs. Recently, we have developed another highly sensitive proarrhythmia model using a chronic AV block cynomolgus monkey, which possesses essentially the same pathophysiological adaptations and drug responses as those demonstrated in the canine model. As a common remodelling process leading to a diminished repolarization reserve may present in patients who experience drug-induced TdP and in the AV block animals, the in vivo proarrhythmia models described in this review may be useful for predicting the risk of pharmacologically induced TdP in humans.

HEMODYNAMIC AND ELECTROPHYSIOLOGICAL EFFECTS OF MITEMCINAL (GM-611), A NOVEL PROKINETIC AGENT DERIVED FROM ERYTHROMYCIN IN A HALOTHANE-ANESTHETIZED CANINE MODEL

Mitemcinal (GM-611) is a novel erythromycin-derived prokinetic agent that acts as an agonist at the motilin receptor. We investigated the QT-prolonging effects of mitemcinal using a halothane-anesthetized canine model. Intravenous administration of mitemcinal at doses of more than 8.3 mg/kg per 10 min significantly prolonged the QT interval corrected by Fridericia's corrections. Mitemcinal exhibited a bradycardiac effect and produced significantly greater prolongation in monophasic action potential duration (MAP(90)) at sinus rhythm compared with MAP(90) at pacing and showed reverse use-dependent prolongation of repolarization, suggesting that the negative chronotropic effect of mitemcinal potentiates the prolongation of the repolarization period. A technique using MAP/pacing electrodes allowed measurements of both MAP(90) and effective refractory period (ERP) simultaneously at the same ventricular site. Although mitemcinal slightly prolonged the MAP(90(CL400)) and ERP in comparison with the control group at the dose of 25 mg/kg per 10 min, the terminal repolarization period, the difference between MAP(90(CL400)) and ERP, did not increase suggesting the absence of a proarrhythmic effect even with a 7000-fold for the therapeutic blood concentration as free level. The electrophysiological results from mitemcinal in this study indicate that the risk of serious arrhythmia such as torsades de pointes, a major clinical concern related to QT interval prolongation, might be low.

Long-term bradycardia caused by atrioventricular block can remodel the canine heart to detect the histamine H1 blocker terfenadine-induced torsades de pointes arrhythmias

Although a second-generation histamine H(1) blocker terfenadine induced torsades de pointes (TdP) arrhythmias in patients via the blockade of a rapid component of delayed rectifier K(+) current (I(Kr)), such action of terfenadine has not been detected in previous animal models. We analysed the potential of the canine persistent atrioventricular block heart, a new in vivo proarrhythmia model, to detect a torsadogenic effect of terfenadine of an oral dose of 3 or 30 mg kg(-1). The doses can provide therapeutic to supra-therapeutic plasma concentrations as an anti-histamine. In 2 weeks of bradycardiac heart model, there were no significant changes in any of the electrocardiogram parameters after the administration of both doses of terfenadine. In 4-6 weeks of bradycardiac heart model, the low dose of terfenadine hardly affected any of the electrocardiogram parameters except that it induced TdP in one out of six animals. The high dose significantly decreased the atrial rate and ventricular rate, prolonged the QT interval, and induced TdP in five out of six animals. Moreover, temporal variability of repolarization increased after the high-dose administration. These results suggest that long-term bradycardia caused by atrioventricular block can remodel the canine heart to detect terfenadine-induced TdP.

A novel approach to data processing of the QT interval response in the conscious telemetered beagle dog

INTRODUCTION

Drug-induced QT interval prolongation may lead to ventricular arrhythmias. The aim of the study was to optimize QT interval data processing to quantify drug-induced QT interval prolongation in the telemetry instrumented conscious dog model.

METHODS

The test substances cisapride, dofetilide, haloperidol, and terfenadine and corresponding vehicles were given to male and female beagle dogs during two consecutive 90-min intravenous infusions. Cardiovascular parameters were recorded for 24 h and exposure to the drugs was measured. The delayed response in the QT interval after an abrupt change in heart rate was investigated. Eight mathematical models to describe the QT interval-heart rate relationship were compared and different sets of covariates were used to quantify the drug-induced effect on the QT interval.

RESULTS

After an abrupt decrease in heart rate, a 75% adaptation of the QT interval was reached after 54+/-9 s. A linear model was preferred to correct the drug-induced effect on the QT interval for heart rate, vehicle effect, serial correlation, plasma concentration and time of day. All test substances significantly prolonged the QT interval.

DISCUSSION

To optimize the processing of QT interval data, the delay in QT interval response after an abrupt change in heart rate should be considered. The QT interval-heart rate relationship and vehicle response were individual-specific and corrections were therefore made individually. When estimating the drug-induced effect on the QT interval it is considered advantageous to use plasma concentration as a covariate, as well as adjusting for vehicle effect and serial correlation in measurements. The conscious dog model detected significant increases in the QT interval for all test substances investigated.

Comparison of Four Rate-Correction Algorisms for the Ventricular Repolarization Period in Assessing Net Effects of IKr Blockers in Dogs

The utility of corrected and uncorrected QT interval changes for assessing net repolarization delay by I(Kr) (a rapid component of delayed rectifier K(+) currents) blockers was assessed in halothane-anesthetized dogs using the electrocardiogram and monophasic action potential (MAP) recordings with electrical ventricular pacing. Intravenous administration of dl-sotalol (0.2 - 2 mg/kg) prolonged the MAP duration and RR interval, while terfenadine (3 mg/kg) increased the MAP duration but transiently shortened RR interval. The order of correlation coefficient between the MAP duration at a pacing cycle length of 400 ms and MAP duration itself or that with arithmetical correction was uncorrected > Van de Water = Matsunaga > Fridericia > Bazett. These results suggest that Matsunaga's and Van de Water's formulae would better predict the net repolarization delay in the in vivo canine model. Also, the risk of drug candidates that may prolong the QT interval should be judged by change in uncorrected QT interval as well as corrected QT interval.

A novel predictive pharmacokinetic/pharmacodynamic model of repolarization prolongation derived from the effects of terfenadine, cisapride and E-4031 in the conscious chronic av node—ablated, His bundle-paced dog

INTRODUCTION

Terfenadine, cisapride, and E-4031, three drugs that prolong ventricular repolarization, were selected to evaluate the sensitivity of the conscious chronic atrioventricular node--ablated, His bundle-paced Dog for defining drug induced cardiac repolarization prolongation. A novel predictive pharmacokinetic/pharmacodynamic model of repolarization prolongation was generated from these data.

METHODS

Three male beagle dogs underwent radiofrequency AV nodal ablation, and placement of a His bundle-pacing lead and programmable pacemaker under anesthesia. Each dog was restrained in a sling for a series of increasing dose infusions of each drug while maintained at a constant heart rate of 80 beats/min. RT interval, a surrogate for QT interval in His bundle-paced dogs, was recorded throughout the experiment.

RESULTS

E-4031 induced a statistically significant RT prolongation at the highest three doses. Cisapride resulted in a dose-dependent increase in RT interval, which was statistically significant at the two highest doses. Terfenadine induced a dose-dependent RT interval prolongation with a statistically significant change occurring only at the highest dose. The relationship between drug concentration and RT interval change was described by a sigmoid E(max) model with an effect site. Maximum RT change (E(max)), free drug concentration at half of the maximum effect (EC(50)), and free drug concentration associated with a 10 ms RT prolongation (EC(10 ms)) were estimated. A linear correlation between EC(10 ms) and HERG IC(50) values was identified.

DISCUSSION

The conscious dog with His bundle-pacing detects delayed cardiac repolarization related to I(Kr) inhibition, and detects repolarization change induced by drugs with activity at multiple ion channels. A clinically relevant sensitivity and a linear correlation with in vitro HERG data make the conscious His bundle-paced dog a valuable tool for detecting repolarization effect of new chemical entities.

Comparison of sensitivity of surrogate markers of drug-induced torsades de pointes in canine hearts

Given a limited information regarding the difference of the sensitivity of surrogate markers of drug-induced torsades de pointes, including early afterdepolarization, ectopic beats, phase 3 repolarization and dispersion of ventricular repolarization, we simultaneously analyzed them in the halothane-anesthetized canine model (n=5). A non-specific IKr channel blocker sparfloxacin, which has been known to induce torsades de pointes in animals and clinical patients, prolonged the repolarization process in a dose-related and reverse use-dependent manner. No significant change was detected in any of the proarrhythmic markers except for the backward parallel shift of phase 3 repolarization in the cardiac cycle with the QT interval prolongation, which would be the most sensitive marker in predicting the potential arrhythmogenic property of sparfloxacin in the "non-remodeled" normal heart.

QT PRODACT: In Vivo QT Assay in Anesthetized Dog for Detecting the Potential for QT Interval Prolongation by Human Pharmaceuticals

The purpose of this study was to assess the utility of the isoflurane-anesthetized dog model for detecting the potential for QT interval prolongation by human pharmaceuticals. The effects of 10 positive compounds with torsadogenic potential, 8 negative compounds with little torsadogenic potential, and dl-sotalol as a common positive compound were evaluated in 5 facilities in accordance with the common protocol approved by QT PRODACT. Each test compound was cumulatively infused into male beagle dogs anesthetized with isoflurane. Surface lead II ECG, blood pressure, and plasma concentrations for the positive compounds were measured. Repeated administration of the vehicle examined in each facility before the start of the experiments resulted in a slight, but not significant, change in corrected QT (QTc) interval, indicating that this model only shows slight experimental variation. Although an inter-facility variability in the extent of dl-sotalol-induced QT interval prolongation was observed, dl-sotalol significantly prolonged QTc interval in all facilities. All positive compounds significantly prolonged QTc interval at plasma levels up to 10 times those in patients who developed prolonged QTc interval or TdP, whereas no negative compounds did so. These data suggest that the in vivo QT assay using the anesthetized dog is a useful model for detecting the potential for QT interval prolongation by human pharmaceuticals.

Electropharmacological effects of a spironolactone derivative, potassium canrenoate, assessed in the halothane-anesthetized canine model

While aldosterone receptor blockers improve survival of patients with congestive heart failure, spironolactone and its derivatives were recently shown to block ether-a-go-go-related gene (HERG) channels and native IKs and IKr currents in guinea pig ventricular myocytes. In this study, we examined in vivo electropharmacological effects of an active derivative of spironolactone, potassium canrenoate, using a halothane-anesthetized canine model. Potassium canrenoate was intravenously administered in three doses of 1, 10, and 100 mg/kg per 10 min with a pause of 20 min between doses (n = 5). The low dose hardly affected any of the cardiovascular parameters. The middle dose, a clinically recommended daily maximum i.v. dose, slightly inhibited the intraventricular conduction. The high dose decreased the heart rate, ventricular contraction and blood pressure, delayed the atrioventricular and intraventricular conduction, and prolonged the ventricular repolarization and refractory period. Increment in the refractoriness by the high dose was greater than that in the repolarization, resulting in the reduction of ventricular electrical vulnerability. This unique electrophysiological profile of potassium canrenoate may in part contribute to the favorable clinical results, whereas caution has to be paid on the cardiohemodynamic actions, particularly for patients with risk of elevated plasma drug concentration.

Electropharmacological and Proarrhythmic Effects of a Class III Antiarrhythmic Drug Nifekalant Hydrochloride Assessed Using the In Vivo Canine Models

Cardiovascular effects of Nifekalant were examined using halothane-anesthetized dogs, and its proarrhythmic potential was estimated with chronic complete atrioventricular block dogs. Nifekalant was intravenously administered to the halothane-anesthetized dogs in three doses of 0.03, 0.3, and 3 mg/kg/10 minutes with a pause of 20 minutes (n = 6). The low dose hardly affected any of the cardiovascular parameters. The middle dose, a clinically recommended antiarrhythmic dose, decreased the total peripheral resistance, increased the cardiac output, and prolonged the ventricular repolarization phase and effective refractory period. The high dose increased the left ventricular contraction, transiently decreased the mean blood pressure, and enhanced the atrioventricular conduction, besides potentiation of the changes induced by the middle dose. Increment in the repolarization phase by the high dose was greater than that in the refractoriness, leading to increase of ventricular electrical vulnerability. To the atrioventricular block animals, clinically relevant antiarrhythmic dose of 3 mg/kg p.o. of Nifekalant and its 10-times-higher dose were administered. The high dose prolonged QT interval leading to torsades de pointes in all animals (n = 5), which was not detected by the clinical dose (n = 5). These results suggest that antiarrhythmic dose of Nifekalant can be used safely; however, caution should be paid for patients complicating bradycardia and/or a risk of elevated plasma drug concentration.

Effects of mexiletine on the canine model of sparfloxacin-induced long QT syndrome

Potential utility of mexiletine for the treatment of sparfloxacin-induced long QT syndrome was assessed using the in vivo halothane-anesthetized canine model. At 30 min after the administration of a supratherapeutic dose of sparfloxacin (30 mg/kg, i.v.), the mean blood pressure and heart rate decreased, whereas repolarization process and effective refractory period of the ventricular muscle were significantly prolonged. Additional administration of a clinically recommended dose of mexiletine (3 mg/kg, i.v.) at this time point increased the mean blood pressure, suppressed ventricular contraction, delayed atrioventricular as well as intraventricular conduction, and shortened repolarization process and effective refractory period. The extent of abbreviation of the repolarization was more prominent than that of the refractoriness, indicating that mexiletine could decrease the electrical vulnerability of the heart during sparfloxacin overdose. Thus, mexiletine may become a promising pharmacological strategy against the drug-induced long QT syndrome.

Evidence-based veterinary dermatology: a systematic review of the pharmacotherapy of canine atopic dermatitis

The efficacy of pharmacological interventions used to treat canine atopic dermatitis, excluding fatty acid supplementation and allergen-specific immunotherapy, was evaluated based on the systematic review of prospective clinical trials published between 1980 and 2002. Studies were compared with regard to design characteristics (randomization generation and concealment, masking, intention-to-treat analyses and quality of enrolment of study subjects), benefit (improvement in skin lesions or pruritus scores) and harm (type, severity and frequency of adverse drug events) of the various interventions. Meta-analysis of pooled results was not possible because of heterogeneity of the drugs evaluated. Forty trials enrolling 1607 dogs were identified. There is good evidence for recommending the use of oral glucocorticoids and cyclosporin for the treatment of canine atopic dermatitis, and fair evidence for using topical triamcinolone spray, topical tacrolimus lotion, oral pentoxifylline or oral misoprostol. Insufficient evidence is available for or against recommending the prescription of oral first- and second-generation type-1 histamine receptor antagonists, tricyclic antidepressants, cyproheptadine, aspirin, Chinese herbal therapy, an homeopathic complex remedy, ascorbic acid, AHR-13268, papaverine, immune-modulating antibiotics or tranilast and topical pramoxine or capsaicin. Finally, there is fair evidence against recommending the use of oral arofylline, leukotriene synthesis inhibitors and cysteinyl leukotriene receptor antagonists.

[Effects of clinically available drugs on the repolarization process of the heart assessed by the in vivo canine models]

The proarrhythmic effects of class III antiarrhythmic agents and non-cardiovascular drugs, which have been shown to prolong QT interval, were assessed using two types of in vivo canine models. First, electrophysiological effects of dofetilide, nifekalant, amiodarone, cisapride, astemizole, sulpiride, haloperidol, and sparfloxacin were assessed using halothane-anesthetized dogs. Each drug prolonged the monophasic action potential (MAP) duration and effective refractory period (ERP) at clinically recommended daily doses. The extent of increase was greater in the refractoriness than in the repolarization only for amiodarone, indicating abbreviation of the terminal repolarization period. The reverse was true for the other drugs. Next, torsadogenic action of sematilide, nifekalant, amiodarone, cisapride, terfenadine, sulpiride, and sparfloxacin was assessed using chronic complete atrioventricular block dogs with Holter ECG monitoring in the conscious state. Oral administration of 1-10 times higher doses than the clinically relevant doses of the drugs induced polymorphic ventricular tachycardia torsades de pointes (TdP), except for amiodarone. These results indicate that the prolongation and backward shift of the terminal repolarization period may be closely related to the drug-induced TdP and suggest that these in vivo models can be used to screen proarrhythmic potential of new drugs.

Famotidine does not induce long QT syndrome: experimental evidence from in vitro and in vivo test systems

The effects of famotidine on the cardiac repolarization process were assessed using four different levels of test systems described in the draft stage guideline ICH S7B. A supratherapeutic concentration of famotidine (10(-5) M), which is >8 times higher than C(max) obtained after its therapeutic dose, neither inhibited human ether-a-go-go-related gene (HERG) K(+) current expressed in human embryonic kidney 293 (HEK293) cells nor affected any of the action potential parameters of guinea pig papillary muscles. Therapeutic (0.3 mg/kg, i.v.) to supratherapeutic doses (3-10 mg/kg, i.v.) of famotidine did not affect the repolarization process of the halothane-anesthetized canine model, while only supratherapeutic doses exerted the positive chronotropic, inotropic and dromotropic effects without affecting the mean blood pressure. Moreover, supratherapeutic doses of famotidine (1-10 mg/kg, i.v.) neither induced torsades de pointes nor prolonged QT interval in the canine chronic atrioventricular conduction block model. These results suggest that famotidine possesses no cardiovascular effects at a therapeutic dose, while it may exert cardiostimulatory actions after drug overdoses that might potentiate the proarrhythmic potential of co-administered cardiotonic agents by increasing the intracellular Ca(2+) concentration.

Proarrhythmic effects of fluoroquinolone antibacterial agents: in vivo effects as physiologic substrate for torsades

Drug-induced prolongation of the QT interval is often associated with the onset of Torsades de Pointes (TdP) resulting in a life-threatening ventricular arrhythmia. The potential of the proarrhythmic effects of the new fluoroquinolone antibacterial agents, levofloxacin and sparfloxacin, was examined in the chronic complete atrioventricular block dogs with stable idioventricular automaticity using Holter ECG monitoring in conscious state (Experiment 1). Next, to better analyze the mechanisms of the proarrhythmic property, the cardiovascular effects of these two drugs were compared in the halothane-anesthetized dogs under the monitoring of ECG, His bundle electrogram, systemic and left ventricular pressure, monophasic action potential, cardiac output, and effective refractory period (Experiment 2). In Experiment 1, oral administration of 6 mg/kg (n = 4) as well as 60 mg/kg (n = 4) of levofloxacin did not induce any ventricular premature depolarization. On the other hand, oral administration of 60 mg/kg of sparfloxacin (n = 4) induced TdP leading to ventricular fibrillation in all animals within 24 h, while 6 mg/kg of sparfloxacin (n = 4) did not induce any ventricular premature depolarization. In Experiment 2, intravenous administration of 0.3 mg/kg as well as 3.0 mg/kg of levofloxacin slightly increased cardiac output, but no significant changes were detected in the other parameters (n = 6). On the other hand, intravenous administration of 0.3 mg/kg of sparfloxacin prolonged the effective refractory period. Additional administration of 3.0 mg/kg of sparfloxacin decreased the heart rate and prolonged the effective refractory period and ventricular repolarization phase in a similar extent, but no significant changes were detected in the other parameters (n = 6). These results suggest that backward shift of the relative repolarization period in a cardiac cycle may be the mechanism responsible for the torsadegenic effect of sparfloxacin.

Effects of antiarrhythmic drugs on the monophasic action potential of the canine isolated, blood-perfused ventricular septum preparation

The present study was designed to combine the monophasic action potential (MAP) recording technique with a well-established canine isolated, blood-perfused ventricular septum preparation for examining, simultaneously, electrical and mechanical drug-induced changes. A MAP catheter was positioned onto the base of a papillary muscle for recording the local MAP, using a manual micromanipulator together with a commercially available catheter sheath to keep the optimal contact pressure against the ventricular wall. The catheter sheath was filled with saline to eliminate the background electrical noise. Tetrodotoxin, disopyramide, lidocaine, and verapamil were used to clarify the potential utility of the preparation. Tetrodotoxin and lidocaine shortened the MAP duration, while disopyramide prolonged it. Verapamil slightly shortened the MAP duration but not significantly. Each drug showed negative inotropic and coronary vasodilator effects. Sodium channel blockers slowed intraventricular conduction and decreased the maximum upstroke velocity of MAP, while verapamil showed no effects. These results suggest that utilization of the blood-perfused ventricular septum preparation together with MAP recording will become a valuable model for evaluating drugs with multiple sites of action on cardiac muscles.