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These studies examined the opioid and non-opioid in vivo and in vitro actions of PD117,302 (((±)-trans-N-methyl-N-[2-(l-pyrrolidinyl)-cyclohexyl]benzo[b]thiophene-4-acetamide), a kappa (κ)-opioid receptor agonist. PD117,302 selectively labeled the κ-opioid receptor in guinea pig cerebellar membranes and in mice the ED50 for analgesia was 2.3µmol/kg. A non opioid cardiovascular assessment of PD117,302 showed that it dose-dependently increased left-ventricular peak systolic pressure in rat isolated perfused hearts but reduced heart rate and blood pressure in anaesthetized rats. Over the concentration range 0.3-30µM in vitro, and dose-range 0.25-4µmol/kg in vivo, PD117,302 dose-dependently prolonged the P-R interval, QRS width and Q-T interval of the rat heart ECG. Naloxone (either 1µM or 8µmol/kg) did not antagonize the observed ECG effects of PD117,302. Cardiac electrical stimulation studies in anesthetized rats showed that threshold currents for capture and fibrillation were increased and effective refractory period (ERP) prolonged. In rats subject to coronary artery occlusion PD117,302 reduced arrhythmia incidence. Intracellular cardiac action potential studies qualified the ECG changes produced by PD117,302 such that there was a dose-dependent reduction in the maximum rate of depolarization of phase 0 (dV/dtmax) and prolongation of the action potential duration (APD). In isolated cardiac myocytes PD117,302 dose-dependently (1-100µM) reduced peak Na(+) current and produced a hyperpolarizing shift in the inactivation curve. Transient outward and sustained outward K(+) currents were blocked by PD117,302. Thus, the ECG changes and antiarrhythmic effects observed in vivo result from direct blockade of multiple cardiac ion channels., (Copyright © 2015 Elsevier B.V. All rights reserved.)

This study characterized the antiarrhythmic effects of the opioid receptor antagonist naloxone in rats subject to electrically induced and ischemic arrhythmias. Naloxone (2, 8 and 32 μmol/kg/min) was examined on heart rate, blood pressure, and the electrocardiogram (EKG) as well as for effectiveness against arrhythmias produced by occlusion of the left anterior descending coronary artery or electrical stimulation of the left ventricle. Naloxone reduced blood pressure at the highest dose tested while heart rate was dose-dependently reduced. Naloxone dose-dependently prolonged the P-R and QRS intervals and increased the RSh amplitude indicative of effects on cardiac sodium (Na) channels. Naloxone prolonged the Q-T interval suggesting a delay in repolarization. Naloxone effects were comparable to the comparator quinidine. Naloxone (32 μmol/kg/min) reduced ventricular fibrillation (VF) incidence to 38% (from 100% in controls). This same dose significantly increased the threshold for induction of ventricular fibrillation (VFt), prolonged the effective refractory period (ERP) and reduced the maximal following frequency (MFF). The patterns of ECG changes, reduction in ischemic arrhythmia (VF) incidence and changes in electrically induced arrhythmia parameters at high doses of naloxone suggest that it directly blocks cardiac Na and potassium (K) ion channels., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The 'Lambeth Conventions' is a guidance document, written in 1987 (Walker et al., 1988), intended to be of practical value in the investigation of experimental arrhythmias induced by ischaemia, infarction, and reperfusion. This is an update, expanded to include guidance on the study of supraventricular arrhythmias, drug-induced arrhythmias, heritable arrhythmias, and advances in our knowledge in core areas since 1987. We have updated the guidance on the design and execution of experiments and the definition, classification, quantification, and analysis of all types of arrhythmias. Investigators are encouraged to adopt the conventions and test their validity in the hope that this will improve uniformity and interlaboratory comparisons, aid clinical research, facilitate antiarrhythmic drug discovery and safety assessment, and improve antiarrhythmic drug deployment for different cardiac conditions. We note that there is a gap between some definitions proposed here and their conventional clinical counterparts, and encourage the research necessary to bridge that translational gap. A web link offers the chance to vote and comment on the new conventions (https://bscr.wufoo.com/forms/z7x0x5/)., (Copyright © 2013 Elsevier Inc. All rights reserved.)

1. Combinations of the action potential-widening drug tedisamil (Class III antiarrhythmic activity), and the inactivated state sodium channel blocker lidocaine (Class Ib antiarrhythmic activity) were assessed for antiarrhythmic actions in a rat model of ischaemia-induced arrhythmias and for electrophysiological actions in normal rat myocardial tissue. 2. Both tedisamil and lidocaine dose-dependently suppressed ischaemia-induced arrhythmias. The ED(50) values were 3.0+/-1.3 and 4.9+/-0.6 micro mol kg(-1) min(-1), respectively. 3. Combinations of the two drugs acted synergistically such that the ED(50) for tedisamil was reduced to 0.8+/-0.2 micro mol kg(-1) min(-1) in the presence of 2 micro mol kg(-1) min(-1) lidocaine. Similarly, the ED(50) for lidocaine was reduced to 0.7+/-0.2 micro mol kg(-1) min(-1) in the presence of 2 micro mol kg(-1) min(-1) tedisamil (both P<0.05). 4. In a separate series of experiments in which normal ventricular tissue was electrically stimulated, 2 micro mol kg(-1) min(-1) lidocaine produced a leftward shift in the dose-response curve for tedisamil's effect on effective refractory period (P<0.05). This dose of lidocaine had no effect on its own. These data indicate that the synergistic actions of combinations of tedisamil and lidocaine were mediated, at least in part, by extension of effective refractory period in normal myocardial tissue. 5. In contrast to the strategy of developing drugs that are selective for a single electrophysiological mechanism, the results of the present study suggest that effective antiarrhythmic drugs might be developed by optimising the combination of two complimentary electrophysiological mechanisms (i.e., action potential-prolonging activity and inactivated state sodium channel blockade).

A new model of regional ischaemia using blood-perfused isolated rat hearts is reported in this issue. This model has potential value in pharmacology to test the actions of drugs against the arrhythmias that arise early period (0-30 min) after induction of ischaemia. Unfortunately, the severity of arrhythmias in this new model is reduced, when compared to other models, in both the early and the late period (1-4 h) of coronary artery occlusion. This commentary compares the new model with previous models, and comments on the possible mechanisms of arrhythmias induced by ischaemia.

The electrophysiological actions of lidocaine, tedisamil and RSD1019 were assessed on normal and ischaemic cardiac tissue using monophasic action potentials (MAPs) recorded from the epicardium of anaesthetized rabbits. Drug effects on ischaemia-induced arrhythmias were assessed simultaneously in the same rabbits. Lidocaine, infused at 2.5, 5 and 10 micromol kg(-1) min(-1) i.v., accelerated and worsened the electrophysiological derangement caused by ischaemia, had profibrillatory actions and reduced the time to the occurrence of ventricular fibrillation (VF) relative to controls. Tedisamil, infused at 0.063, 0.125 and 0.25 micromol kg(-1) min(-1) i.v., prolonged MAP duration at 90% repolarization (MAPD(90%)) before induction of ischaemia in a dose-related manner; however, this effect was not maintained 5 min after induction of ischaemia. Tedisamil had no significant antiarrhythmic actions over the dose-range tested. RSD1019, infused at 2, 4 and 8 micromol kg(-1) min(-1) i.v., produced a small increase in MAPD(90%) before induction of ischaemia and only at the highest dose tested. In contrast to tedisamil, RSD1019 suppressed ischaemia-induced MAP shortening assessed 5 min after induction of ischaemia. This effect was dose-related. RSD1019 completely prevented ischaemia-induced tachyarrhythmias at the mid and highest infusion levels tested. The results of this study illustrate a pathologically targeted approach for preventing ischaemia-induced arrhythmias. Suppression of ischaemia-induced MAP shortening, demonstrated herein for RSD1019, represents a novel antifibrillatory approach.

Eight novel and three reference antiarrhythmics were investigated in anaesthetised rats for antiarrhythmic actions, as well as for effects on the electrocardiogram (ECG) under normal and "simulated ischaemic" conditions. In rats subjected to coronary artery occlusion lidocaine, (+/-)-trans-[2-(4-morpholinyl)-cyclohexyl]naphthyl-1-acetate, RSD1000 and (+/-)-trans-[2-(4-morpholinyl)-cyclohexyl]-2-(1-naphthyl)propionate, RSD1030, (Group A) produced dose-related and complete antiarrhythmic protection. Group B compounds, such as (+/-)-trans-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-3, 4-dichlorocinnamamide, RSD995, produced complete antiarrhythmic protection but had aberrant dose-response curves. Group C compounds, such as quinidine and flecainide, failed to give full antiarrhythmic protection and had shallow dose-response curves. The potency of Group A compounds, but not Group B or C compounds, for ECG actions indicative of Na(+) channel blockade (prolongation of PR and QRS intervals) were significantly increased under "simulated ischaemic" conditions ([K(+)] 10 mM and pH 6.4) in isolated rat hearts. Thus, compounds with ischaemia-selective actions provided superior protection against ischaemia-induced arrhythmias in rats.

Objective: In many species arrhythmias induced by myocardial ischaemia appear to be in part dependent upon cardiac sympathetic nerves. However, previous experiments in rats did not suggest that myocardial or other catecholamines are involved in ischaemic arrhythmogenesis in this species. The aim of this study was to investigate this further using transplanted hearts., Methods: We transplanted 'donated' hearts onto the abdominal aorta of recipient rats and, at varying periods after transplantation, subjected donated and recipient hearts to occlusion of the left anterior descending (LAD) coronary artery. Donated and recipient hearts were tested at various times after transplantation for responsiveness to drugs acting upon aspects of the autonomic nervous system. The intention of this latter study was to assess the status of innervation and receptors simultaneously in both donated and recipient hearts., Results: Donated (transplanted) hearts showed responses consistent with denervation and receptor supersensitivity. Changes varied with the duration of the transplant. Over the same period recipient hearts did not change in responsiveness to drugs. When subjected to coronary artery occlusion, transplanted hearts responded to occlusion with the same frequency and severity of arrhythmias as recipient and other control hearts, regardless of the duration of transplant, or sensitivity to drugs., Conclusions: The results of these experiments suggest that cardiac innervation is not an important factor in the genesis of ischaemia-induced arrhythmias in rats.

Spiradoline (U-62,066E), a selective kappa (kappa) receptor agonist, was examined for actions on the cardiovascular system and on myocardial ionic currents in rats. We initially characterized cardiac, hemodynamic, and antiarrhythmic actions of spiradoline in isolated perfused rat hearts and pentobarbital-anesthetized rats. Electrophysiologic studies in isolated myocytes were used to elucidate the mechanism for changes observed in vivo in the ECG, as well as for antiarrhythmic actions against electrical and ischemia-induced arrhythmias. In isolated rat hearts, spiradoline reduced heart rate and cardiac contractility and increased the PR interval and QRS width of the ECG in a concentration-dependent manner. In anesthetized rats, spiradoline dose-dependently reduced blood pressure and heart rate and prolonged the PR interval and QRS width. At slightly higher doses, it increased the QaT interval of the ECG. RSh, an index of sodium channel blockade in the rat, also was dose-dependently increased. Electrical stimulation of the left ventricle suggested that spiradoline may exert its antiarrhythmic action by blockade of myocardial sodium currents. The electrophysiologic actions of spiradoline on sodium currents, the transient outward (i(to)) and sustained plateau potassium (ik(sus)) currents were studied in isolated cardiac rat myocytes by whole-cell patch-clamp techniques. Spiradoline (15-500 microM) reduced peak sodium current in a rapid, reversible, and concentration-dependent manner; it also increased the rate of decay of I(to) and reduced the amplitude of Ik(sus). At a concentration of 150 microM, spiradoline produced a 24 +/- 2 mV hyperpolarizing shift in sodium current inactivation kinetics but did not alter activation processes. Spiradoline showed both tonic and frequency-dependent components of sodium current block. Thus spiradoline produced its antiarrhythmic actions via sodium channel blockade in myocardial tissue, although higher doses also block potassium currents. This combined ion channel-blocking property may be of added clinical benefit in the setting of myocardial ischemia.

The possible ischemia-selective Class III anti-arrhythmic action (selective action potential widening in ischemia) of the IKATP blocker glibenclamide was assessed in anesthetized rabbits during ischemia induced by complete occlusion of a coronary artery. Coronary artery occlusion caused an initial prolongation in monophasic action potential (MAP) duration at 90% repolarization from 145 +/- 2.8 ms (mean +/- S.E.M., n = 14) to 162 +/- 4.5 ms (P < 0.05) 1 min after ischemia. This was followed by a rapid and sustained shortening to 104 +/- 4.9 ms 5 min after the onset of ischemia (P < 0.05 from both values). Glibenclamide (3, 6, 12 or 24 mg/kg, i.v.) caused a statistically significant, dose-related reduction in the rate of MAP shortening induced by ischemia, whereas 0.3 mg/kg was without effect. The effective dose for a 50% maximal effect (ED50) was 13 +/- 0.8 mg/kg (n = 28). Despite this, there was no effect on the final magnitude of MAP shortening. Five min after induction of ischemia, there were no longer any detectable effects of glibenclamide on MAP duration. Glibenclamide significantly reduced the incidence of ventricular fibrillation, although the effect was not dose related. No differences were found in the latency to ventricular fibrillation between groups. Ventricular fibrillation occurred 10.6 +/- 1.1 min (n = 19) after the start of ischemia. In a similar experiment, 0.3 mg/kg glibenclamide i.v. did not affect the rate of MAP shortening, the final magnitude of MAP shortening or the occurrence of arrhythmias caused by ischemia. Since the action potential widening effects of glibenclamide in ischemic tissue were not observed at the time when arrhythmias occurred, it is unlikely that an ischemia-selective Class III anti-arrhythmic action contributes to the limited antiarrhythmic actions of glibenclamide.

A method of recording epicardial monophasic action potentials (MAPs) and ischemia-induced arrhythmias following coronary artery ligation in intact rabbits is described. It is expected that this model will have utility in analyzing drug effects and mechanisms of ischemic arrhythmogenesis. Rabbits were found to have two arrhythmic phases following coronary artery occlusion which correspond to phase Ia and Ib arrhythmias in other species. Epicardial MAPs recorded from ischemic tissue allowed electrophysiological effects to be correlated with these phases. Phase Ia arrhythmias occurred within the first 2 min of coronary artery occlusion and were associated with a reduction in the maximum upstroke velocity of MAPs and changes in MAP duration, including the occurrence of alternans in duration. Phase Ib arrhythmias occurred between 8 and 15 min after coronary artery occlusion. These arrhythmias were associated with a decrease in MAP duration and amplitude, alternans in MAP duration as well as conduction block. Coronary artery occlusion reliably induced arrhythmias in rabbits if the left branch of the coronary artery and the left anterior descending artery were occluded. There was a 95% incidence of premature ventricular contractions, 38% of ventricular tachycardia, and 48% of ventricular fibrillation (n = 21). The results of this study show that epicardial MAPs can be used to aid in the characterization of the electrophysiological mechanisms of ischemia-induced arrhythmias in vivo.

KC8851, a structural analogue of tedisamil, has previously been found to exhibit mixed blockade of K+ and Na+ currents in isolated rat ventricular myocytes. We have now investigated the antiarrhythmic actions of this compound in the anaesthetized rat and isolated rat heart. In the rat, KC8851, at concentrations as low as 0.2 micromol kg(-1) min(-1), widened the QT intervals of the ECG and prolonged the effective refractory period in a dose-dependent manner. Such actions were consistent with blockade of repolarizing K+ currents. At relatively higher doses (above 0.5 micromol kg(-1) min(-1)), KC8851 increased RSh amplitude suggesting blockade of Na+ currents. The compound was found to be effective against occlusion-induced arrhythmias at doses of 0.5 to 2 micromol kg(-1) min(-1). In isolated hearts, the effects of KC8851 on PR and QRS intervals were potentiated by elevated concentrations of K+ and H+. Overall, KC8851 was found to exhibit antiarrhythmic actions consistent with inhibition of both K+ and Na+ currents.

In this study we examined the cardiovascular and possible antiarrhythmic actions of anipamil, a long acting analog of verapamil. Initial dose-response studies for anipamil (0.25-6.0 mg/kg, i.v.) in pentobarbitone-anesthetized pigs (n = 4) were conducted to determine the effects of the drug on EKG and hemodynamic measures. In this initial study anipamil was found to produce a dose-dependent reduction in blood pressure, left-ventricular pressure and its derivative (dP/dtmax), cardiac output, and increase in heart rate. These results were used as a basis from which to choose doses for a second study to assess antiarrhythmic actions of anipamil against arrhythmias induced by regional myocardial ischemia. The antiarrhythmic effects of the two doses were compared with verapamil when the latter was given at a dose producing cardiovascular effects mid-way between those produced by the two doses of anipamil. Anesthetized pigs were randomly assigned to receive one of three drug treatments, or vehicle control, prior to occlusion of the left-anterior descending coronary artery. Antiarrhythmic effectiveness of low (1.0 mg/kg + 0.10 mg/kg/min infusion, n = 8) and high (5.0 mg/kg + 0.50 mg/kg/min infusion, n = 12) dose anipamil was compared to that of verapamil (0.5 mg/kg + 0.60 mg/kg/min infusion, n = 8) in a vehicle controlled study (n = 15). Arrhythmic events (VPB, VT and VF incidence) were monitored and grouped according to their time of occurrence after occlusion. Thus phase 1a arrhythmias occurred 0-5 min after initiation of occlusion, phase 1b, 5-30 min, and phase 2, 0.5-4 hr after occlusion. This study showed that during phase 1a there was a low incidence of arrhythmias in all groups except the one receiving 5 mg/kg anipamil where the group incidence of VT was 58% as compared to 20% in controls (n = 15). Most ventricular arrhythmias occurred in all groups during phase 1b. In this phase verapamil abolished VF and reduced VT, as compared with controls. Anipamil (high and low doses) tended to reduce VT but not VF. In the period 0.5 to 4 hours post occlusion (phase 2) all three drug treatments were associated with fewer arrhythmias but this only reached statistical significance with verapamil. Thus verapamil was more efficacious than anipamil at providing antiarrhythmic protection against both early and late onset arrhythmias. Anipamil may have been proarrhythmic in the early phase of arrhythmias and only moderately antiarrhythmic, if at all, in the later phase.(ABSTRACT TRUNCATED AT 400 WORDS)

The rat has been used extensively for arrhythmia studies, and a large number of models have been developed, which include chemical, electrical, and pathological procedures for including arrhythmias. Such models have been used to investigate mechanisms for inducing arrhythmias as well as the actions of antiarrhythmic drugs. Arrhythmia models in rats are of particular importance in examination of the antiarrhythmic action of various drugs.

The causes of ventricular arrhythmias in the acute setting of coronary artery disease (myocardial ischaemia and reperfusion) may be approached using two paradigms. One, the electrophysiological paradigm (disturbance of ionic homeostasis, electrogenesis, and conduction) has not been addressed in detail here. Instead, we have focused on the concept of a chemical paradigm of arrhythmogenesis. Many endogenous chemical substances (derived from the myocardium, nerves, blood plasma, platelets, leucocytes, and endothelium) accumulate in the ischaemic tissue or are produced during reperfusion and many of these have been suggested to modulate ventricular arrhythmias. Some substances may be arrhythmogenic and others may be antiarrhythmic. Together they determine whether or not arrhythmias occur. Potentially arrhythmogenic substances include potassium, catecholamines, cAMP, histamine, 5-HT, lysophosphatidylcholine, palmitylcarnitine, platelet activating factor, prostaglandins, leukotrienes, thromboxane A2, angiotensin II, endothelin, opioids, protons, calcium, and free radicals. We have considered each of these, with the objective of evaluating which are important in arrhythmogenesis in acute ischaemia and reperfusion. Two alternative models of arrhythmogenesis are possible in the context of the chemical paradigm: a series model (where one substance or its effects determines the arrhythmogenicity of another) and a parallel model (where numerous substances operate independently to cause ventricular arrhythmias). It is not yet clear which model is most appropriate; a combination of the two is possible, so a working prototype has been constructed which accommodates both. A set of criteria (hitherto lacking) for establishing whether a substance is sufficient and necessary for arrhythmogenesis is proposed. Some generalisations are given on approaches to establishment of these criteria for putative arrhythmogenic substances. Finally, we have considered how arrhythmogenic drug development may be influenced by using the chemical paradigm as an alternative to the electrophysiological paradigm of arrhythmogenesis.

The effects of variations in serum concentrations of potassium on the occurrence and severity of ischemia- and reperfusion-induced arrhythmias have been studied in conscious rats. Serum potassium concentrations were modified by maintaining rats on diets which varied in potassium concentration, by treatment with hydrochlorothiazide, amiloride, spironolactone or infusions of potassium chloride. An inverse linear relationship was demonstrated between ischemia-induced arrhythmias and log(e) serum potassium concentration such that a 50% reduction in arrhythmias occur with a 3.8-fold increase in serum potassium concentration. On the other hand, the incidence of reperfusion-induced arrhythmias after 7 min of regional ischemia prior to reperfusion in previously untreated rats were not influenced by elevation of serum potassium concentrations prior to and after reperfusion. Effects on ischemia-induced arrhythmias could not be explained by changes in blood pressure or heart rate. It is speculated that modification of potassium concentration in non-ischemic or border zone ventricular tissue may directly modify arrhythmogenesis due to ischemia but not that due to reperfusion.

The antiarrhythmic actions of low and high doses of U-50,488H, a selective kappa-receptor agonist, were examined in pentobarbitone-anaesthetized rats subjected to occlusion of the left anterior descending coronary artery. At a high dose (16 mumol/kg) U-50,488H reduced blood pressure, heart rate and prolonged the P-R and QRS intervals of the electrocardiogram. This dose reduced the incidence of ventricular arrhythmias produced by occlusion. The blood pressure, heart rate, ECG and antiarrhythmic actions of a high dose of U-50,488H were not antagonized by 8 mumol/kg naloxone, a dose which had no cardiovascular or ECG actions. Naloxone alone reduced arrhythmia incidence but to a lesser extent than U-50,488H. A low dose (0.2 mumol/kg) of U50,488H in the absence or presence of naloxone had no effect on arrhythmias. Thus, U-50,488H had antiarrhythmic actions at a high dose which were independent of opioid receptors.

1. The mechanism underlying the limited antiarrhythmic effects of beta-adrenoceptor blocking agents against occlusion-induced arrhythmias in acutely prepared, pentobarbitone-anaesthetized rats has been investigated. 2. Atenolol, ICI 111,581 and propranolol were given at low, medium and high doses calculated to shift dose-response curves to exogenous agonists by factors of 10-30, 100-300 and 1000-3000, respectively. 3. Arrhythmias, blood pressure, heart rate, ECG changes and serum K+ were measured. 4. Antiarrhythmic activity was seen with beta-blocker treatment. This was minimal with atenolol (0.1, 1 and 10 mg kg-1) and only statistically significant with the highest dose of ICI 111,581 (5 mg kg-1), and propranolol (10 mg kg-1). 5. Treatment with beta-adrenoceptor blockers elevated serum potassium concentrations, as compared with saline controls, especially when measured at 30 min post-occlusion. 6. Only ICI 111,581 (5 mg kg-1) and propranolol (1 and 10 mg kg-1) prolonged P-R interval. 7. In order to evaluate possible mechanisms of antiarrhythmic action, attempts were made to correlate antiarrhythmic activity with beta-blockade, serum potassium concentrations, and/or with changes in the P-R interval of the ECG. 8. Reductions in arrhythmias did not correlate well with presumed beta-blockade. Better correlation was obtained with elevations of serum potassium concentration, and with prolongation of P-R interval (a presumed Class I antiarrhythmic action). 9. These results suggested that antiarrhythmic effects of adrenoceptor blocking agents in acutely-prepared anaesthetized rats, subjected to occlusion of a coronary artery, are unrelated to cardiac beta-blockade. The limited antiarrhythmic effects which were observed could be attributed to elevations in serum potassium concentration (due to peripheral beta-blockade) and/or possible Class I antiarrhythmic actions.

Arrhythmia scores have been used in recent years to facilitate the analysis of arrhythmias, particularly in relation to regional myocardial ischaemia. The recent Lambeth Conventions recommended caution in the use of arrhythmia scores since their use may be misleading. In the present study seven scoring systems were examined in an attempt to validate the use of arrhythmia scores. A strong positive correlation was present between all seven scores. Furthermore, the scores all correlated with the incidences of ventricular fibrillation, ventricular tachycardia, and ventricular premature beats in early myocardial ischaemia. All seven scores successfully detected statistically significant reductions in the incidence of ventricular fibrillation resulting from the administration of two drugs. Some of the scores occasionally showed statistically significant reductions when effects on the raw arrhythmia data were not statistically significant. In this respect, parametric statistical analysis of arrhythmia scores may be a more sensitive method of quantifying arrhythmias than non-parametric analysis of binomially distributed raw data such as the incidence of ventricular fibrillation (in accordance with the power of such tests) indicating that the scores have precision. However, none of the scores incorrectly showed a statistically significant reduction when the raw data expressed a statistically significant or non-significant increase, indicating that the scores have accuracy. In conclusion, it is possible to design many arrhythmia scores that show changes in arrhythmia severity when more conventional analyses show only non-statistically significant trends. When used in conjunction with raw arrhythmia data, comprehensive drug dose ranges, and appropriate parametric statistical tests, arrhythmia scores facilitate the quantification of arrhythmias. It is recommended that arrhythmia scores should be used only for quantifying group data and model building and not for prognostic purposes in individuals.

The actions of intravenous verapamil against arrhythmias induced by occlusion of a coronary artery were investigated in conscious rats. Verapamil (2-20 mg kg-1, i.v. given pre-occlusion) dose-dependently reduced arrhythmias in rats with either large or small occluded zones at an ED50 of 6 mg kg-1. This dose was effective when given immediately post-occlusion. Severe arrhythmias, as opposed to PVC, were preferentially reduced. In conscious, and pentobarbitone-anaesthetized rats, verapamil (6 mg kg-1) had different effects on electrically-induced arrhythmias, and the ECG, from an equi-effective anti-arrhythmic dose of quinidine (20 mg kg-1, i.v.). Quinidine decreased following frequency, but increased threshold current and pulse width, whereas verapamil did not. Both drugs increased P-R interval, but only quinidine increased QRS and Q-T intervals. Thirty minutes post-occlusion, the verapamil content of tissue and blood was determined after a 6 mg kg-1 dose given pre- or post-occlusion. Measurable levels of verapamil were found in both normal and ischaemic myocardium. Plasma and plasma water concentrations were 3.6 +/- 0.8 mumol l-1 and 0.6 +/- 0.1 mumol l-1 (mean +/- s.e. mean), respectively following post-occlusion administration vs. 2.7 +/- 1.2 and 0.24 +/- 0.04 for pre-occlusion administration. Plasma water concentrations were close to IC50 values for inhibition of contractility in rat atria and ventricles. Similar concentrations depressed slow action potentials induced in rat ventricles by raised K+ We suggest that the ability of verapamil to prevent severe ventricular arrhythmias following myocardial ischaemia in the conscious rat is largely due to the calcium antagonist effects of the drug.

The antiarrhythmic activity of felodipine was investigated in conscious chronically prepared rats subjected to coronary artery occlusion. By deliberate high or low placement of an occluder, either large or small occluded zones were produced. In rats with small occluded zones (SOZ), felodipine i.v. (0.2 mumol/kg slow injection + 0.01 mumol/kg per min infusion, and 12.2 mumol/kg slow injection) possessed slight antiarrhythmic activity. In rats with large occluded zones (LOZ), similar doses of felodipine had little antiarrhythmic activity. Felodipine produced a dose-dependent reduction in blood-pressure and delayed the development of ECG signs of ischaemia (ST elevation). The occluded zone size was not reduced by felodipine, although some myocardial salvage (8-17%) may have occurred in SOZ rats. It is concluded that felodipine, at doses producing severe hypotension associated with vasodilatation, possesses little antiarrhythmic activity.

1. Nifedipine and DHM9 (carboxymethyl methyl 1,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)-3,5-pyridinedicarboxylate) were studied for their effects on arrhythmias resulting from regional myocardial ischaemia in conscious rats, and for their effects on left ventricular developed pressure in vitro. 2. Nifedipine possessed antiarrhythmic activity at a high dose of 10 mg kg-1 i.v., but not at 0.5 or 2 mg kg-1. Ventricular fibrillation (VF), tachycardia (VT), and ventricular premature beats (VPB) were all attenuated to a similar degree; nifedipine did not have a selectivity of action for high frequency arrhythmias. 3. Before coronary occlusion, the three doses of nifedipine reduced arterial blood pressure by a similar magnitude, indicating a similar (maximal) degree of systemic vasodilatation. The reductions in blood pressure were accompanied by reflex tachycardia. Heart rate and blood pressure did not correlate with the incidence or severity of arrhythmias. 4. DHM9 had no influence on arrhythmias, haemodynamic variables or the ECG, even at 20 mg kg-1 i.v. 5. Nifedipine concentration-dependently reduced contractility in perfused paced (5 Hz) rat ventricles in vitro. Raising the concentration of K+ in the perfusion fluid from 3 to 10 mequiv.l-1 increased the potency (-log10 EC50) of nifedipine up to four fold, and caused a significant depression in excitability. 6. DHM9 at up to 3 x 10(-5) M had no significant influence on ventricular contractility in vitro. 7. The results provided indirect evidence in support of the hypothesis that calcium antagonists inhibit ischaemia-induced arrhythmias by virtue of inhibition of the slow inward current (Isi) in the ischaemic ventricular myocardium.

The effects of various cardiovascular drugs (prostaglandin E2 (PGE2), propranolol and nitroglycerine) and anaesthetic regimens (halothane, pethidine and pentobarbitone), upon the outcome of coronary artery ligation in acutely prepared rats were determined. Effects upon arrhythmias, blood pressure, heart rate, mortality, ECG and the size of the occluded zone were determined for each drug in the presence of each anaesthetic. PGE2 and nitroglycerine had no statistically significant effects on the outcome of ligation whatever the anaesthetic. Propranolol had limited antiarrhythmic actions. The anaesthetic used had major effects upon the outcome of ligation, regardless of the cardiovascular drugs administered. Pentobarbitone anaesthesia resulted in the highest mortality, and most arrhythmias. Pethidine-N2O anaesthesia was associated with fewer arrhythmias. Halothane-N2O anaesthesia markedly decreased the incidence and severity of arrhythmias, independent of the cardiovascular drug. It was concluded that the anaesthetic used can have a major influence on ligation-induced arrhythmias in acutely prepared anaesthetized rats.

The role of the central nervous system (CNS) in arrhythmogenesis in the 4 h period following occlusion of a coronary artery was investigated in rats by use of CNS ablations and other procedures. Ablations in the CNS included pithing, spinalization and decerebration combined with acute and chronic surgical preparation and noradrenaline/adrenaline infusions. All procedures involving acute surgery reduced the incidence and severity of the arrhythmias induced by occlusion. Such reductions were most marked in the second (0.5-4 h post-occlusion) arrhythmic period. The observed reductions in arrhythmias could not be explained in terms of involvement of the CNS or adrenoceptor activation. When circulating leucocytes, platelets and serum potassium were measured in a group of pithed rats before and after occlusion, reduced levels (20-50%) of both leucocytes and platelets occurred while serum potassium levels rose by 50-100%. Arrhythmias following coronary occlusion may depend in part on factors in the blood such as leucocytes, platelets and serum potassium and these factors may be altered by acute surgery.

In a blind and randomised study, (-), (+/- ), and (+) verapamil were compared for their abilities to reduce mortality induced by occlusion of the left anterior descending coronary artery in conscious rats. In addition, effects on infarct size were also measured. In a separate experiment, the ability of an observer to detect ventricular fibrillation (VF) on the basis of changes in behaviour alone was established; (+/- ) verapamil statistically significantly reduced total mortality compared with controls, while (-) verapamil produced a similar, but non-statistically significant reduction. Both (+/- ) and (-) verapamil reduced mortality caused by VF; (-) verapamil was more effective than (+/- ) verapamil. Neither (-), (+/- ), nor (+) verapamil reduced infarct size in 24-h survivors; (-) verapamil produced the highest incidence of morbidity and associated cardiovascular depression (hypotension, bradycardia, and P-R interval prolongation), whereas (+) verapamil produced the least. Since (-) verapamil is a more potent calcium antagonist than (+) verapamil, the results of this study support the hypothesis that calcium antagonism was responsible for the observed antiarrhythmic actions.

The effect of halothane on arrhythmias induced by ischaemia was investigated in rats, isolated perfused rat hearts, and pigs. Responses to the occlusion of the left anterior descending coronary artery were determined in groups (n = 9) of chronically prepared rats treated with no halothane, 0.5, or 1.0 per cent halothane immediately after occlusion; in isolated rat hearts (n = 10) treated with no halothane, 0.5, 1.0, 2.0, or 4.0 per cent halothane for 15 min before and after occlusion; and 20-25 kg pigs (n = 11) anaesthetised with halothane or pentobarbital. The ECG, arrhythmias, blood pressure (BP), heart rate (HR) and extent of infarction were determined in each model. In pigs, left ventricular pressure, dp/dtmax and cardiac output were also measured. In chronically prepared rats, halothane anaesthesia started after occlusion was antiarrhythmic and decreased the incidence of ventricular fibrillation and resulting mortality. In isolated rat hearts, 0.5 or 1.0 per cent halothane had little effect on occlusion-induced arrhythmias. The highest concentration of halothane increased the incidence of ventricular fibrillation both before and after occlusion. Halothane decreased developed ventricular pressure in a dose-dependent manner. In acutely prepared pigs, halothane pre-treatment had no appreciable effect upon occlusion-induced arrhythmias when compared with pentobarbital anaesthesia. Thus, halothane is antiarrhythmic when treatment is initiated after occlusion in the rat but this action is not seen in isolated hearts or intact pigs. The antiarrhythmic action of halothane is, therefore, species and model dependent.

Prostaglandin E2 and F1alpha infusions have been tested for their ability to reduce the arrhythmias associated with occlusion of the left descending coronary artery in the anaesthetised dog. At 1 microgram/kg/min both PGs reduced the incidence of premature ventricular contractions occurring during 25-min occlusions, while not reducing the incidence of ventricular fibrillation occurring on occlusion release. When infused for 5-min periods at 1 to 16 microgram/kg/min, neither PGE2 nor PGF1alpha effectively reduced the frequency of ventricular arrhythmias occurring 24 hr after a permanent coronary occlusion.

The Lambeth Conventions are guidelines intended to be of practical value in the investigation of arrhythmias induced by ischaemia, infarction, and reperfusion. They cover the design and execution of experiments and the definition, classification, quantification, and analysis of arrhythmias. Investigators are encouraged to adopt the conventions in the hope that this will improve uniformity and interlaboratory comparisons.