Experimental therapy of genetic arrhythmias: disease-specific pharmacology.

The integration between molecular biology and clinical practice requires the achievement of fundamental steps to link basic science to diagnosis and management of patients. In the last decade, the study of genetic bases of human diseases has achieved several milestones, and it is now possible to apply the knowledge that stems from the identification of the genetic substrate of diseases to clinical practice. The first step along the process of linking molecular biology to clinical medicine is the identification of the genetic bases of inherited diseases. After this important goal is achieved, it becomes possible to extend research to understand the functional impairments of mutant protein(s) and to link them to clinical manifestations (genotype-phenotype correlation). In genetically heterogeneous diseases, it may be possible to identify locus-specific risk stratification and management algorithms. Finally, the most ambitious step in the study of genetic disease is to discover a novel pharmacological therapy targeted at correcting the inborn defect (locus-specific therapy) or even to "cure" the DNA abnormality by replacing the defective gene with gene therapy. At present, this curative goal has been successful only for very few diseases. In the field of inherited arrhythmogenic diseases, several genes have been discovered, and genetics is now emerging as a source of information contributing not only to a better diagnosis but also to risk stratification and management of patients. The functional characterization of mutant proteins has opened new perspectives about the possibility of performing gene-specific or mutation-specific therapy. In this chapter, we will briefly summarize the genetic bases of inherited arrhythmogenic conditions and we will point out how the information derived from molecular genetics has influenced the "optimal use of traditional therapies" and has paved the way to the development of gene-specific therapy.

Electrocardiographic prediction of abnormal genotype in congenital long QT syndrome: experience in 101 related family members.

INTRODUCTION: Previous studies showed that diagnosing congenital long QT syndrome (LQTS) is difficult due to variable penetrance and genetic heterogeneity, especially when subjects from multiple families with diverse mutations are combined. We hypothesized that a combination of clinical and ECG techniques could identify gene carriers within a single family with congenital LQTS. METHODS AND RESULTS: One hundred one genotyped members of a family with LQTS, including 26 carriers of a HERG mutation, underwent history and ECG analysis. Forty-eight family members also underwent exercise testing with QT and T wave alternans (TWA) analysis and 24-hour Holter monitoring with QT and heart rate variability analysis. A logistic regression model, which included age, gender, QTc, and QTc by age, provided the best prediction of gene carrier status, although there was substantial overlap (78%) of QTc among subjects with and without the mutation. QTc was not helpful as a discriminator in children < or = 13 years. TWA (observed infrequently) did not add significantly to the model's ability to predict abnormal genotype. CONCLUSION: Even in this homogeneous LQTS population, the phenotype was so variable that clinical and detailed ECG analyses did not permit an accurate diagnosis of gene carrier status, especially in children. Sustained microvolt TWA was a specific (100%) but insensitive (18%) marker for LQTS. Its ability to predict risk of arrhythmia in this population remains to be determined. Genetic testing serves an essential role in screening for carriers of LQTS.

[Long QT syndrome and Brugada syndrome: 2 aspects of the same disease?]. / Sindrome del QT lungo e sindrome di Brugada: due aspetti della stessa malattia?

In clinical cardiology, resort has recently been made to molecular genetics in order to explain some mechanisms that underlie sudden cardiac death in young people with structurally normal hearts. It has become evident that genetic mutations regarding cardiac ion channels may disrupt the delicate balance of currents in the action potential, thus inducing malignant ventricular tachyarrhythmias. The cardiac sodium channel gene, SCN5A, is involved in two of such arrhythmogenic diseases, the Brugada syndrome and one form of the long QT syndrome (LQT3). It is believed that these syndromes result from opposite molecular effects: Brugada syndrome mutations cause a reduced sodium current, while LQT3 mutations are associated with a gain of function. The effects of class I antiarrhythmic drugs have been used to differentiate these diseases. Intravenous flecainide is used as a highly specific test to unmask the electrocardiographic phenotype of the Brugada syndrome. On the other hand, on the basis of experimental and clinical studies, the possibility that the same drugs act as a gene-specific therapy in this disorder by contrasting the effect of mutations in LQT3 has been explored. Recent evidence shows that phenotypic overlap may exist between the Brugada syndrome and LQT3. One large family with a SCN5A mutation and a "mixed" electrocardiographic pattern (prolonged QT interval and ST-segment elevation) has been reported. Moreover, our recent data showed that flecainide challenge may elicit ST-segment elevation in some LQT3 patients. The presence of "intermediate" phenotypes highlights a remarkable heterogeneity suggesting that clinical features may depend upon the single mutation. Only deepened understanding of the genotype-phenotype correlation will allow the definition of the individual patient's risk and the development of guidelines for clinical management.

Novel characteristics of a misprocessed mutant HERG channel linked to hereditary long QT syndrome.

Hereditary long QT syndrome (hLQTS) is a heterogeneous genetic disease characterized by prolonged QT interval in the electrocardiogram, recurrent syncope, and sudden cardiac death. Mutations in the cardiac potassium channel HERG (KCNH2) are the second most common form of hLQTS and reduce the delayed rectifier K(+) currents, thereby prolonging repolarization. We studied a novel COOH-terminal missense mutation, HERG R752W, which segregated with the disease in a family of 101 genotyped individuals. When the mutant cRNA was expressed in Xenopus oocytes it produced enhanced rather than reduced currents. Simulations using the Luo-Rudy model predicted minimal shortening rather than prolongation of the cardiac action potential. Consequently, a normal or shortened QT interval would be expected in contrast to the long QT observed clinically. This anomaly was resolved by our observation that the mutant protein was not delivered to the plasma membrane of mammalian cells but was retained intracellularly. We found that this trafficking defect was corrected at lower incubation temperatures and that functional channels were now delivered to the plasma membrane. However, trafficking could not be restored by chemical chaperones or E-4031, a specific blocker of HERG channels. Therefore, HERG R752W represents a new class of trafficking mutants in hLQTS. The occurrence of different classes of misprocessed channels suggests that a unified therapeutic approach for altering HERG trafficking will not be possible and that different treatment modalities will have to be matched to the different classes of trafficking mutants.

Cellular dysfunction of LQT5-minK mutants: abnormalities of IKs, IKr and trafficking in long QT syndrome.

Mutations in the minK gene KCNE1 have been linked to the LQT5 variant of human long QT syndrome. MinK assembles with KvLQT1 to produce the slow delayed rectifier K+ current IKs and may assemble with HERG to modulate the rapid delayed rectifier IKr. We used electrophysiological and immunocytochemical methods to compare the cellular phenotypes of wild-type minK and four LQT5 mutants co-expressed with KvLQT1 in Xenopus oocytes and HERG in HEK293 cells. We found that three mutants, V47F, W87R and D76N, were expressed at the cell surface, while one mutant, L51H, was not. Co-expression of V47F and W87R with KvLQT1 produced IKs currents having altered gating and reduced amplitudes compared with WT-minK, co-expression with L51H produced KvLQT1 current rather than IKs and co-expression with D76N suppressed KvLQT1 current. V47F increased HERG current but to a lesser extent than WT-minK, while L51H and W87R had no effect and D76N suppressed HERG current markedly. Thus, V47F interacts with both KvLQT1 and HERG, W87R interacts functionally with KvLQT1 but not with HERG, D76N suppresses both KvLQT1 and HERG, and L51H is processed improperly and interacts with neither channel. We conclude that minK is a co-factor in the expression of both IKs and IKr and propose that clinical manifestations of LQT5 may be complicated by differing effects of minK mutations on KvLQT1 and HERG.

Molecular biology of the long QT syndrome: impact on management.

The long QT syndrome (LQTS) is a familial disease characterized by prolonged ventricular repolarization and high incidence of malignant ventricular tachyarrhythmias often occurring in conditions of adrenergic activation. Recently, the genes for the LQTS inked to chromosomes 3 (LQT3), 7 (LQT2), and 11 (LQT1) were identified as SCN5A, the cardiac sodium channel gene and as HERG and KvLQT1 potassium channel genes. These discoveries have paved the way for the development of gene-specific therapy for these three forms of LQTS. In order to test specific interventions potentially beneficial in the molecular variants of LQTS, we developed a cellular model to mimic the electrophysiological abnormalities of LQT3 and LQT2. Isolated guinea pig ventricular myocytes were exposed to anthopleurin and dofetilide in order to mimic LQT3 and LQT2, respectively. This model has been used to study the effect of sodium channel blockade and of rapid pacing showing a pronounced action potential shortening in response to Na+ channel blockade with mexiletine and during rapid pacing only in anthopleurin-treated cells but not in dofetilide-treated cells. Based on these results we tested the hypothesis that QT interval would shorten more in LQT3 patients in response to mexiletine and to increases in heart rate. Mexiletine shortened significantly the QT interval among LQT3 patients but not among LQT2 patients. LQT3 patients shortened their QT interval in response to increases in heart rate much more than LQT2 patients and healthy controls. These findings suggest that LQT3 patients are more likely to benefit from Na+ channel blockers and from cardiac pacing because they are at higher arrhythmic risk at slow heart rates. Conversely, LQT2 patients are at higher risk to develop syncope under stressful conditions, because of the combined arrhythmogenic effect of catecholamines with the insufficient adaptation of their QT interval. Along the same line of development of gene-specific therapy, recent data demonstrated that an increase in the extracellular concentration of potassium shortens the QT interval in LQT2 patients suggesting that intervention aimed at increasing potassium plasma levels may represent a specific treatment for LQT2. The molecular findings on LQTS suggest the possibility of developing therapeutic interventions targeted to specific genetic defects. Until definitive data become available, antiadrenergic therapy remains the mainstay in the management of LQTS patients, however it may be soon worth considering the addition of a Na+ channel blocker such as mexiletine for LQT3 patients and of interventions such as K+ channel openers or increases in the extracellular concentration of potassium for LQT1 and LQT2 patients.

The long QT syndrome: new diagnostic and therapeutic approach in the era of molecular biology.

The idiopathic long QT syndrome is a congenital disease characterized by prolongation of the QT interval and by stress-induced syncopal episodes caused by the development of "torsades de pointes". Over the last decade, the great advances in the field of molecular biology have made it possible to elucidate the genetic causes of the disease. In particular, three genes have been implicated in the pathogenesis of the disease: SCN5A (LQT3), encoding for the cardiac sodium channel and located on chromosome 3, HERG (LQT2), encoding for a cardiac potassium channel (Ikr) and located on chromosome 7 and KVLQT1 (LQT1), located on chromosome 11 and encoding for a cardiac potassium channel whose electrophysiologic profile is still undefined. Within each of these genes several different mutations have been identified and subsequently expressed to determine the electrophysiological changes induced by the mutation in the normal function of the channels. These studies have suggested that LQT3 is caused by alterations in the inactivation of cardiac sodium channels while LQT2 is caused by a reduction in the delayed rectifier potassium current. Based on this evidence, we developed the first cellular model for LQTS in order to provide a mean of assessing the effect of different interventions in two different forms of disease, LQT2 and LQT3. We exposed guinea pig ventricular myocytes to anthopleurin, a toxin that interferes with the inactivation of INa, and to dofetilide, a selective blocker of Ikr, obtaining a prolongation of cellular repolarization with both drugs. We then exposed cells to a Na+ channel blocker, mexiletine, which significantly reduced APD in cells treated with anthopleurin while it did not modify the prolongation induced by dofetilide. In addition, anthopleurin-treated cells demonstrated a greater shortening of APD to rapid pacing than both control and dofetilide-treated cells. Based on this experimental evidence, we tested the same therapeutic interventions, mexiletine and pacing, in fifteen genetically characterized LQTS patients. Mexiletine significantly shortened the QT interval in LQT3 patients but not in LQT2 patients. When we examined the response to an increase in heart rate, we found that LQT3 patients had a more shortened QT interval in response to heart rate changes than LQT2 patients and than healthy controls.

A molecular basis for the therapy of the long QT syndrome.

The long QT syndrome (LQTS) is a familial disease characterized by abnormally prolonged ventricular repolarization and high incidence of malignant ventricular tachyarrhythmia. Recently, molecular biology studies brought major advancements in the understanding of the pathophysiologic mechanisms of this disease. The genes for the LQTS linked to chromosomes 3 (LQT3). 7 (LQT2) and 11 (LQT1) were identified as SCNSA, the cardiac sodium channel gene and as HERG and KVLQT1 potassium channel genes. We developed a cellular model in which ventricular myocytes were exposed to anthopleurin and dofetilide in order to mimic LQT3 and LQT2, respectively. The effects of sodium channel blockade and rapid pacing were then studied showing a pronounced action potential shortening in response to mexlietine and during rapid pacing only in antopleurin-treated cells but no in dofetilide-treated cells. On this experimental basis, we tested the hypothesis that QT interval would behave differently during similar intervention in LQT3 and LQT2 patients. Results showed that 1) mexiletine shortened significantly the QT interval among LQT3 patients but not among LQT2 patients and 2) LQT3 patients shortened their QT interal in response to increases in heart rate much more than LQT2 patients and healthy controls. These findings demonstrate differential responses of LQTS patients to interventions targeted to their specific genetic defect. They also suggest that LQT3 patients are more likely to benefit from Na+ channel blockers and from cardiac pacing. Conversely, LQT2 patients are at higher risk to develop syncope under stressful conditions, because of the combined arrhythmogenic effect of cathecolamines with the insufficient adaptation of their QT interval when heart rate increases.

Differential response to Na+ channel blockade, beta-adrenergic stimulation, and rapid pacing in a cellular model mimicking the SCN5A and HERG defects present in the long-QT syndrome.

The long-QT syndrome (LQTS) is a hereditary disorder characterized by an abnormally prolonged QT interval and by life-threatening arrhythmias. Recently, two of the genes responsible for LQTS have been identified: SCN5A, a voltage-dependent Na+ channel on chromosome 3 (LQT3), and HERG, responsible for the rapid component of the delayed rectifier current (IKr), on chromosome 7 (LQT2). We developed an in vitro model to attempt reproduction of the expected alterations in LQT3 and LQT2 patients. Guinea pig ventricular myocytes were exposed to anthopleura toxin A (anthopleurin), an inhibitor of the inactivation of the Na+ current, and to dofetilide, a selective blocker of IKr. Both interventions significantly prolonged action potential duration (APD), by 54 +/- 13 and 62 +/- 16 ms, respectively. Cells pretreated with anthopleurin significantly shortened APD in response to mexiletine, isoproterenol, and rapid pacing (from 264 +/- 38 to 226 +/- 32 ms after mexiletine, P < .001). On the contrary, cells exposed to dofetilide did not shorten the APD after mexiletine and even prolonged it after initial exposure to isoproterenol (from 280 +/- 25 to 313 +/- 20 ms, P < .001); during rapid pacing, APD was shortened but less (38 +/- 9 versus 60 +/- 11 ms, P < .05) than in anthopleurin-treated cells. This study shows that a cellular model for LQTS, based on the recent advances in molecular genetics, can provide adequate "phenotypes" of prolonged repolarization amenable to the testing of interventions of potential clinical relevance. We found differential responses to Na+ channel blockade, to beta-adrenergic stimulation, and to rapid pacing according to specific pretreatment with either anthopleurin (to mimic LQT3) or dofetilide (to mimic LQT2). These different responses in myocytes bear striking similarities with the differential response to analogous interventions in LQTS patients with mutations on the SCN5A and HERG genes.

Sympathetic activation, ventricular repolarization and Ikr blockade: implications for the antifibrillatory efficacy of potassium channel blocking agents.

OBJECTIVES: The aim of the present study was to test, in vivo and in vitro, the influence of adrenergic activation on action potential prolongation induced by the potassium channel blocking agent d-sotalol. BACKGROUND: d-Sotalol is not effective against myocardial ischemia-dependent ventricular fibrillation in the presence of elevated sympathetic activity. Most potassium channel blockers, such as d-sotalol, affect only one of the two components of Ik (Ikr) but not the other (Iks). Iks is activated by isoproterenol. An unopposed activation of Iks might account for the loss of anti-fibrillatory effect by d-sotalol in conditions of high sympathetic activity. METHODS: In nine anesthetized dogs we tested at constant heart rate (160 to 220 beats/min) the influences of left stellate ganglion stimulation on the monophasic action potential prolongation induced by d-sotalol. In two groups of isolated guinea pig ventricular myocytes we tested the effect of isoproterenol (10(-9) mol/liter) on the action potential duration at five pacing rates (from 0.5 to 2.5 Hz) in the absence (n = 6) and in the presence (n = 8) of d-sotalol. RESULTS: In control conditions, both in vivo and in vitro, adrenergic stimulation did not significantly change action potential duration. d-Sotalol prolonged both monophasic action potential duration in dogs and action potential duration of guinea pig ventricular myocytes by 19% to 24%. Adrenergic activation, either left stellate ganglion stimulation in vivo or isoproterenol in vitro, reduced by 40% to 60% the prolongation of action potential duration produced by d-sotalol. CONCLUSIONS: Sympathetic activation counteracts the effects of potassium channel blockers on the duration of repolarization and may impair their primary antifibrillatory mechanism. An intriguing clinical implication is that potassium channel blockers may not offer effective protection from malignant ischemic arrhythmias that occur in a setting of elevated sympathetic activity.

[Idiopathic ventricular fibrillation: from a collection of clinical cases to a prospective evaluation. The U-CARE Steering Committee. Unexplained Cardiac Arrest Registry of Europe]. / Fibrillazione ventricolare idiopatica: da una raccolta di casi clinica alla valutazione prospettica. Steering Committee di U-CARE.

BACKGROUND: The primary aim of U-CARE (Unexplained Cardiac Arrest Registry of Europe) is to collect clinical information on survivors about a documented episode of idiopathic ventricular fibrillation (IVF) and to follow these patients (pts) prospectively to acquire information on 1) recurrence of malignant arrhythmias or cardiac arrest, 2) development of a previously non obvious organic heart disease, 3) potential difference in outcome in pts treated with different drugs or devices. METHODS AND RESULTS: Within April 15th, 1994, eighty-six pts have been enrolled, 65 males and 21 females. The mean age at the time of the first cardiac arrest was 35 +/- 15 years. Clinical evaluation revealed "minor" functional or anatomical abnormalities in 14 subjects and they were excluded from the analysis. In the remaining 72 pts, no abnormalities were found at echocardiogram, Holter, angiography, exercise stress test. At the electrophysiologic study 35/68 pts were inducible. Thirty-eight pts received pharmacologic therapy, 28 an implantable defibrillator (ICD), three pts received both an ICD and drug therapy and three were left untreated. Follow-up data are available for 37 pts with a mean follow-up of 4.4 +/- 2.6 years. No patient had evidence of structural heart disease. Twenty-three pts remained asymptomatic, 12 (32%) had a recurrence of syncope or cardiac arrest: three died suddenly and 2 were defibrillated by the ICD. This study that represents the largest experience in IVF, shows: 1) all patients remained free from any organic heart disease at follow-up, 2) they have a high risk of recurrence of major arrhythmic events. CONCLUSIONS: An ICD implant would be appropriate in this population, at least until data on the efficacy of the pharmacologic therapy will be available.

Dispersion of the QT interval. A marker of therapeutic efficacy in the idiopathic long QT syndrome.

BACKGROUND: QT interval dispersion, measured as interlead variability of QT, is a marker of dispersion of ventricular repolarization and, hence, of cardiac electrical instability. We tested the hypothesis that dispersion of ventricular repolarization may be differently affected by interventions destined to provide complete or incomplete protection against malignant arrhythmias in patients with long QT syndrome (LQTS). Twenty-eight patients affected by the Romano Ward form of LQTS entered the study and were divided into three groups: LQTS patients before institution of therapy, patients who did respond to beta-blocker therapy, and patients who continued to have syncope and cardiac arrest despite beta-blockade and who underwent left cardiac sympathetic denervation. A group of 15 healthy volunteers served as control subjects. METHODS AND RESULTS: Dispersion of QT and QTc were calculated using two indexes: the difference between the longest and the shortest value measured in each of the 12 ECG leads (QTmax-QTmin, QTcmax-QTcmin) and the relative dispersion of QT and QTc (standard deviation of QT/QT average x100, standard deviation of QTc/QTc average x100). Both indexes of dispersion of repolarization were higher in the LQTS patients than in control subjects; also, patients not responding to beta-blockers had a significantly higher dispersion of repolarization than responders. A cutoff value of 100 milliseconds for QTmax-QTmin had an 80% sensitivity and 82% specificity in discriminating between responders and nonresponders. A cutoff value of 6 for QT relative dispersion yielded similar results. The LQTS patients who did not respond to beta-blockade underwent left cardiac sympathetic denervation and thereafter remained asymptomatic (mean follow-up, 5 +/- 4 years). In this group, dispersion of repolarization was significantly reduced by the surgical denervation to values similar to that of the responders to beta-blockade. CONCLUSIONS: These data indicate that QT dispersion is a useful clinical tool to predict efficacy of antiadrenergic therapy in LQTS patients.

Cardiac receptor activation and arrhythmogenesis.

New evidence has been accumulated allowing a better understanding of the physiology and pathophysiology of receptors in the heart. Three major receptor systems influence electrophysiological characteristics of myocardial cells and are critical in the development and prevention of cardiac arrhythmias: the adrenergic, the muscarinic and the adenosine systems. Although it has long been recognized that beta adrenergic stimulation is arrhythmogenic, only recently have the mechanisms of this arrhythmogenic effect been clarified. In addition, the contribution to arrhythmogenesis of alpha receptor stimulation, which has been overlooked for many years, has been recognized as an important accompaniment to the beta component, especially during hypoxia-ischaemia. On the other hand, it has been demonstrated that although direct electrophysiological effects of acetylcholine on the ventricle remain controversial, the antagonism of sympathetic activation by cholinergic stimulation may be important in preventing arrhythmias induced by a high sympathetic tone in the presence of myocardial ischaemia. More recently, the importance of the adenosine system has been better appreciated. Experimental studies have shown that adenosine receptor activation inhibits the adenylyl cyclase system by activating the Gi regulatory proteins. Activation of this pathway inhibits the development of adrenergic-dependent triggered activity in isolated cells and these have also been confirmed in man. It is likely that this effect is specific against triggered rhythms induced by adrenergic activation. Even though further research is needed to clarify fully the interaction of these three systems at the subcellular level, the pharmacological modulation of these cardiac receptors appears as a rational approach for refining the treatment of arrhythmias.

Ventricular fibrillation induced by the interaction between acute myocardial ischemia and sympathetic hyperactivity: effect of nifedipine.

Sympathetic hyperactivity plays a major role in the genesis of malignant arrhythmias during acute myocardial ischemia. An experimental model in which life-threatening arrhythmias are specifically and consistently induced by the interaction between acute myocardial ischemia and left stellate ganglion stimulation has been developed in alpha-chloralose anesthetized cats. In this preparation, drugs that share antiischemic, antiadrenergic, and specific electrophysiologic effects, such as verapamil, diltiazem, and amiodarone, were most effective. To evaluate the relative role of these different properties in mediating the effect of antiarrhythmic drugs, we used this same model to test nifedipine, a calcium channel blocker that is able to counteract the consequences of sympathetic stimulation on coronary circulation but has no electrophysiologic properties at concentrations relevant in the clinical setting. Nifedipine (15 micrograms/kg) prevented the occurrence of ventricular fibrillation in 10 of 13 animals (77%). Its efficacy was independent of changes in the peripheral hemodynamics. Plasma concentrations of nifedipine were within the therapeutic range in humans. To evaluate if this rather striking protective effect was specifically related to the prevention of the deleterious consequences of sympathetic stimulation, the effect of nifedipine on ventricular fibrillation threshold was studied in an additional group of 13 cats in the nonischemic state, during acute myocardial ischemia and during ischemia plus sympathetic stimulation. Nifedipine did not modify ventricular fibrillation threshold in nonischemic or in ischemic conditions. However, nifedipine specifically prevented the further reduction in ventricular fibrillation threshold occurring when sympathetic stimulation was superimposed on acute ischemia. These data suggest that the extension of ischemic damage by sympathetic stimulation is an important progenitor of arrhythmogenic action during acute ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficacy of diltiazem in two experimental feline models of sudden cardiac death.

The potential role of calcium entry blockers in the prevention of life-threatening arrhythmias associated with acute myocardial ischemia and reperfusion is still controversial. In 98 anesthetized cats, the effect of diltiazem was examined in two experimental models. In protocol I, ventricular tachycardia or fibrillation was consistently induced by the interaction between a 2 minute coronary artery occlusion and a 30 second left stellate ganglion stimulation. After three trials under control conditions, if the same pattern of arrhythmia was induced, the drug under study was administered and three additional trials were performed. In 16 animals the administration of saline solution did not modify the pattern of arrhythmias. In contrast, diltiazem (0.1 mg/kg body weight plus 0.2 mg/kg per h) abolished both ventricular tachycardia and fibrillation that had occurred in 64 and 36%, respectively, of the cats in the control state. In protocol II, a 20 minute coronary artery occlusion was released in three groups; one served as the control group, one received diltiazem 15 minutes before occlusion and one received diltiazem 3 minutes before reperfusion. The incidence of reperfusion ventricular fibrillation was 62% (16 of 26) in the control group. It was significantly (p less than 0.05) reduced by diltiazem administered before the occlusion to 25% (4 of 16), whereas it was not affected when diltiazem was administered just before reperfusion (7 [47%] of 15). These results indicate that diltiazem exerts a striking protective effect against the malignant arrhythmias induced by the combination of acute myocardial ischemia and sympathetic hyperactivity. Diltiazem was also effective in reducing the incidence of life-threatening reperfusion arrhythmias.(ABSTRACT TRUNCATED AT 250 WORDS)