Allan S. Manning

SR 33589, a new amiodarone-like antiarrhythmic agent: anti-adrenoceptor activity in anaesthetized and conscious dogs

We have assessed the ability of amiodarone and the new amiodarone-like antiarrhythmic agent, SR 33589 (N,N-dibutyl-3-[4-((2-butyl-5-methylsulphonamido)benzofuran-3-yl-c arbonyl) phenoxy]propylamine), to inhibit the effects of adrenoceptor stimulation in anaesthetized and conscious dogs. In anaesthetized, atropinized dogs, adrenoceptor stimulation was achieved (i) by i.v. administration of adrenaline and measurement of increased blood pressure (ii) by i.v. administration of isoprenaline and measurement of increased heart rate and decreased blood pressure. In conscious dogs, adrenoceptor stimulation was achieved by i.v. administration of isoprenaline and measurement of increased heart rate. In anaesthetized, atropinized dogs, both amiodarone and SR 33589 inhibited to similar extents, alpha-adrenoceptor stimulation (as indicated by attenuation of adrenaline-induced increases in blood pressure). The beta 1-adrenoceptor inhibitory activity of SR 33589 (as demonstrated by blockade of isoprenaline-induced increases in heart rate) was significant, but less marked than amiodarone (heart rate elevation reduced by 39%, P < 0.001 with 10 mg/kg SR 33589 and by 52%, P < 0.01 with 10 mg/kg amiodarone). In contrast, its beta 2-adrenoceptor antagonistic activity (as demonstrated by blockade of isoprenaline-induced reduction in blood pressure) was more marked (mean blood pressure decrease reduced by 69%, P < 0.01 with 10 mg/kg SR 33589 and by 31%, P < 0.05 with 10 mg/kg amiodarone). In conscious dogs, both SR 33589 and amiodarone (12.5, 25 and 50 mg/kg p.o.) inhibited isoprenaline-induced increases in heart rate by approximately the same amount for varying durations depending on the dose.(ABSTRACT TRUNCATED AT 250 WORDS)

SR 33589, a new amiodarone-like antiarrhythmic agent: electrophysiological effects in anesthetized dogs.

Summary We compared the electrophysiological effects of a new amiodarone-like agent, SR 33589, with those of amiodarone. Mongrel dogs were anesthetized with chloralose, and electrodes were implanted in right atrium and ventricle for electrical stimulation and regional ECG measurement. Sinus cycle length (CL), AH interval, Wenckebach CL (WCL), atrial, atrioventricular node, and ventricular effective refractory periods (AERP, AVNERP, VERP), and parameters calculated from surface ECG were measured. SR 33589 was administered intravenously (i.v.) at 1, 2.5, or 5 mg/kg followed 60 min later by a second similar dose. The same protocol was followed with amiodarone 5 mg/kg, which reduced heart rate (HR) by 19% (p < 0.05), increased WCL by 31% (p < 0.01), AH interval by 14% (p < 0.05) and AERP, AVNERP, and VERP by 13% (p < 0.05), 19% (p < 0.05), and 11% (p < 0.01) respectively. No effect was observed on HV or PQ intervals. A second administration of 5 mg/kg changed these indexes further. SR 33589 (2.5 mg/kg) reduced HR by 21% (p < 0.001), increased WCL by 44% (p < 0.001), AH interval by 24% (p < 0.01), and AERP, AVNERP, and VERP by 17% (p < 0.001), 63% (p < 0.01), and 15% (p < 0.01) respectively, with an 18% increase in PQ interval (p < 0.05) but no significant effect on HV interval. Higher doses (5 mg/kg) and/or administration of a second dose both resulted in greater changes. Both amiodarone and SR 33589 prolonged VERP more at longer CL than at shorter CL, but the degree of reduction at shorter CL was less with SR 33589 than with amiodarone. Results suggest that acute administration of SR 33589 results in electro-physiological actions similar to those produced by amiodarone.

SR 33589, a new amiodarone-like agent : effect on ischemia- and reperfusion-induced arrhythmias in anesthetized rats

Summary: We have assessed the ability of the new amiodarone-like antiarrhythmic agent, SR 33589, to reduce the incidence of ischemia- and reperfusion-induced arrhythmias, in comparison to amiodarone, D-sotalol, and lignocaine. Rats were anesthetized, artificially ventilated, and the thorax opened by a left thoracotomy. Ischemia was induced by left coronary artery ligation, and reperfusion was achieved (after a 5-min period of ischemia) in a separate group of rats by removing the ligature. Agents were given intravenously 5 min before occlusion or orally 4 h before study. During a 20-min period of ischemia, SR 33589 reduced significantly the incidence of ventricular fibrillation (VF) from 80 to 30% (p < 0.05) at 3 mg/kg i.v. and eliminated VF and mortality at 10 mg/kg i.v. In contrast, amiodarone at 10 mg/kg i.v. reduced significantly only the incidence of mortality during ischemia (from 60 to 0%, p < 0.01), while having no significant effect on 3 mg/kg i.v. On reperfusion (after a 5-min period of ischemia), SR 33589 reduced significantly the incidence of mortality (from 90 to 20%, p < 0.01) at 1 mg/kg and eliminated VF and mortality when administered at 3 and 10 mg/kg. Against both ischemia- and reperfusion-induced arrhythmias, ∼20% of animals showed AV block at the highest dose of SR 33589 tested. This was not observed with lower doses. Amiodarone (10 mg/kg i.v.) eliminated completely reperfusion-induced VF and mortality while having no significant effect at 1 and 3 mg/kg. D-Sotalol, at doses that did not cause large reductions in heart rate (3 mg/kg), did not offer any significant protection; however, at high doses (30 mg/kg i.v.), D-sotalol completely sup-pressed reperfusion-induced mortality (p < 0.01) and significantly reduced the incidence of VF (from 70 to 10%, p < 0.01). Lignocaine at 1 or 3 mg/kg i.v. did not reduce significantly the incidence of reperfusion-induced arrhythmias. When given intravenously, all agents investigated produced dose-dependent reductions in heart rate that could not be correlated to reduced rhythm disturbances. When given orally, SR 33589 reduced significantly (p < 0.05) resultant mortality when given at 10 mg/kg and ventricular fibrillation and mortality at 30 mg/kg (p < 0.001). Amiodarone did not show a significant reduction <30 mg/kg. Thus, when given intravenously, SR 33589 demonstrated significant antiarrhythmic actions at lower doses than amiodarone or D-sotalol and was also antiarrhythmic after oral administration.

Effects of a new amiodarone-like agent, SR 33589, in comparison to amiodarone, D,L-sotalol, and lignocaine, on ischemia-induced ventricular arrhythmias in anesthetized pigs.

We compared the ability of a new amiodarone-like agent, SR 33589, with that of amiodarone, D,L-sotalol, and lignocaine to reduce the incidence of ventricular fibrillation (VF) and associated arrhythmias caused by acute coronary artery occlusion in anesthetized pigs. Ischemia was induced by occlusion of the left coronary descending artery (LAD) for 30 min. Premature ventricular complexes (PVCs), ventricular tachycardia (VT), and ventricular fibrillation (VF) were recorded during coronary occlusion. SR 33589 (1.25, 2.50, and 5 mg/kg intravenously, i.v.) markedly reduced the occurrence of ventricular arrhythmias during ischemia. The incidence of VF was reduced from 90% in the control group to 30% (p < 0.05) with 1.25 mg/kg, to 10% (p < 0.001) with 2.50 mg/kg, and to 20% (p < 0.01) with 5 mg/kg. In addition, SR 33589, especially at the two higher doses, caused a sustained reduction in both the incidence of VT and the number of PVCs per minute. In comparison, amiodarone 10 and 20 mg/kg i.v. reduced the incidence of VF (40 and 50%, respectively), but these reductions never reached a level of statistical significance. The incidence of VT and the number of PVCs per minute were also decreased significantly by amiodarone. D,L-sotalol 3 mg/kg i.v. exerted significant anti-arrhythmic activity; the incidence of VF was reduced 20% (p < 0.01), and both the incidence of VT and number of PVC per minute were also reduced. In contrast, lignocaine given as a 2-mg/kg bolus followed by an infusion at 70 micrograms/kg/min had no antiarrhythmic or antifibrillatory activity in this preparation.(ABSTRACT TRUNCATED AT 250 WORDS)

Superoxide dismutase and the reduction of reperfusion-induced arrhythmias: In vivo dose-response studies in the rat

SummaryUsing anesthetized rats we have investigated the dose-response characteristics for the ability of superoxide dismutase (SOD) to reduce the vulnerability of the rat heart to reperfusion-induced arrhythmias in vivo. Hearts (n=15 in each group) were subjected to 7 min of regional ischemia followed by 10 min of reperfusion. In the control group (saline), 73% (11/15) of the hearts fibrillated during reperfusion, 20% (3/15) had atrioventricular block and 47% (7/15) died as a result of ventricular arrhythmias. Superoxide dismutase, administered as an intravenous bolus2 min prior to reperfusion exerted a marked protective effect. At its most effective dose (10 mg/kg body wt i.e. 27,000 IU/kg body wt) reperfusion-induced ventricular fibrillation was reduced to 33% (5/15). Reperfusion-induced atrioventricular block was eliminated (0/15) and mortality was reduced to 7% (1/15, p<0.05). The protective effects were however very dose-dependent and at higher doses SOD exhibited no antiarrhythmic actions during reperfusion. These results, together with our previous findings in vitro, lend further support to our proposition that oxygen-derived free radicals may play a role in the induction of potentially lethal cardiac arrhythmias and that antifree radical interventions, even when givenafter the onset of ischemia, can be highly protective.

Ischemia- and reperfusion-induced arrhythmias: beneficial actions of nifedipine.

The effects of nifedipine against ischemia- and reperfusion-induced arrhythmias were investigated using anesthetized rats with transient coronary artery occlusion. Nifedipine (5 μg/kg i.v.) administered 10 min prior to occlusion significantly decreased the incidence of arrhythmias occurring during 20-min coronary occlusion. The incidence and duration of reperfusion-induced ventricular fibrillation and subsequent mortality following 5-min coronary occlusion were also significantly reduced by this intervention. However, administration of nifedipine 1 min prior to reperfusion afforded no protection against reperfusion arrhythmias. To investigate whether nifedipine possesses a true antiarrhythmic action or merely extends the ischemic duration prior to reperfusion resulting in maximal rhythm disturbances, reperfusion was initiated after 3, 5, 7, 10, 20, and 30 min of ischemia. Nifedipine reduced the incidence of reperfusion-induced ventricular fibrillation after all ischemic intervals, with no change in the time of peak vulnerability to reperfusion arrhythmias. Measurements of coronary flow with 153Gadolinium microspheres indicated that flow within ischemic tissue relative to that in normal tissue was significantly increased by nifedipine. Thus, administration of nifedipine prior to occlusion affords a protective effect against ischemia- and reperfusion-induced arrhythmias, and this action is not due to extension of the ischemic duration prior to reperfusion resulting in maximal rhythm disturbances.

Creatine phosphate and protection against reperfusion-induced arrhythmias in the rat heart

An isolated perfused working rat heart preparation was used to assess the effect of including creatine phosphate (10 mmol/l) in the perfusion fluid of hearts subjected to aerobic perfusion (20 min), regional ischaemia (15 min) and reperfusion (2 min). Creatine phosphate had no detectable effect upon pre-ischaemic, ischaemic or post-ischaemic contractile function, it also had no statistically significant effect upon myocardial tissue ATP content. However, creatine phosphate was found to afford striking protection against reperfusion-induced arrhythmias. The incidence of ventricular fibrillation was reduced from over 80% (13/16) in the control group to 10% in the creatine phosphate-treated group (P less than 0.001). Possible mechanisms underlying the anti-arrhythmic effects of creatine phosphate were investigated using isolated rat papillary muscles superfused with or without added creatine phosphate (10 mmol/l). During aerobic superfusion at 37 degrees C creatine phosphate did not cause any statistically significant changes in contractile (developed tension) or electrophysiological (dV/dtmax and action potential duration) indices. Creatine phosphate did however influence the extent to which hypoxia (10 min) and reoxygenation (10 min) altered tension and electrophysiological characteristics. It accelerated the hypoxia-induced decline in developed tension and also the reoxygenation-induced recovery of developed tension. Relatively small changes in dV/dtmax and action potential duration were observed during hypoxia and these rapidly normalized during reoxygenation. In general creatine phosphate acted to exacerbate any changes during hypoxia and accelerate the recovery during reoxygenation. While some of the electrophysiological changes observed would indicate an anti-arrhythmic effect, they were relatively small and perhaps insufficient to explain fully the potent anti-arrhythmic properties of creatine phosphate.

In vitro characterization of a novel Ca2+ entry blocker: SR 33805.

In this study, SR 33805 was shown to inhibit competitively [3H]fantofarone binding to cardiac sarcolemmal membranes. In contrast, SR 33805 was shown to inhibit allosterically [3H](+)-PN200-110, [3H](-)-D888 and cis-(+)-[3H]diltiazem binding. In isolated rabbit atrial preparations, SR 33805 was shown to be the least potent of fantofarone, nifedipine, verapamil and diltiazem in terms of both negative chronotropic and inotropic responses (IC50s 6 and 12 microM, respectively). In superfused rat aortic strips, SR 33805 like other Ca2+ channel antagonists, caused a significant inhibition of both K(+)-induced 45Ca2+ influx and contractile responses. In addition this agent was shown to antagonize Ca(2+)-induced contractions in K(+)-depolarized aorta with a pA2, value of 8.39 +/- 0.02. In femoral, renal and basilar arteries, SR 33805 was equiactive to the other Ca2+ channel antagonists studied in antagonizing K(+)-induced contractions (IC50 approximately 40 nM), but unlike the reference Ca2+ channel antagonists, was equiactive in antagonizing serotonin-induced contractions (IC50 approximately 250 nM). This suggests that the effects of SR 33805 depend mainly on membrane potential. In conclusion, SR 33805 is a potent Ca2+ channel antagonist which, unlike fantofarone, verapamil and diltiazem, is highly selective for vascular smooth muscle and devoid of any potent negative inotropic actions.

Abrupt withdrawal of chronic beta-blockade: Adaptive changes in cyclic AMP and contractility

Abstract The effect of the abrupt withdrawal of chronic beta-blockade on the sensitivity of the myocardium to catecholamine challenge has been investigated. Oxprenolol and metoprolol (200 mg/kg body wt/day) were administered orally to rats for 3 weeks. This dosage produced effective beta-blockade together with peak plasma drug levels within the normal clinical range. Hearts were perfused as isolated Langendorff preparations on various days following the abrupt termination of drug treatment. Response to perfusion with 10 −8 M isoprenaline was measured in terms of changes in dP d t max , heart rate and tissue cAMP content. On the last day of drug administration (day 0), at a time when plasma drug levels were high, hearts from neither the oxprenolol treated group nor the metoprolol treated group exhibited a significant blockade of isoprenaline induced increases in contractility or cAMP when compared with hearts from untreated animals. However, at 2 and 3 days after the termination of drug administration, there was a significant decline in the myocardial response to isoprenaline challenge. Thus, hearts from the drug-treated groups, perfused on day 3, showed a similar degree of betablockade as that shown by hearts obtained from rats having received only a single dose of either beta-blocker. After day 3, both chronically treated groups developed an increasing sensitivity to isoprenaline challenge characterized by a marked hypersensitive response (in d P d t max , and cAMP levels) which peaked 7 to 9 days after the withdrawal of the drugs. There followed a decline in sensitivity which in turn was followed by a second period of hypersensitivity. Under the conditions of this study, chronic beta-blockade induces an oscillatory, hypersensitive response to catecholamine challenge which is manifest in terms of d P d t max and cAMP levels. This adaptation, with its concomitant periods of increased and decreased sensitivity, may suggest a possible mechanism for the clinically observed propranolol “rebound” phenomenon.

Vascular calcium overload produced by vitamin D3, in rats. Effect of treatment with SR 33805, a novel calcium entry blocker

The effect of SR 33805, a novel calcium entry blocker, on calcium overload was studied in six vascular beds in rat. Fantofarone, a parent compound, and verapamil were used as reference calcium entry blockers. Calcium overload induced with a single dose (300,000 IU, i.m. of vitamin D3) was measured by means of atomic absorption and histological techniques. From the time-course of calcium overload, a treatment period of 3 days was selected to determine the effects of drug treatment. The compounds were given orally twice a day in the following dose ranges: SR 33805 2-100 mg/kg, fantofarone 10-300 mg/kg, verapamil 100 mg/kg. SR 33805 significantly decreased the calcium content beginning at the dose of 2 mg/kg in the thoracic aorta, 5 mg/kg in the mesenteric artery and 30 mg/kg in the heart. Fantofarone and verapamil had the same effect at the dose of 100 mg/kg. Histological assessment of the heart revealed that lesions appearing in the tissue adjacent to the arteries were significantly diminished by treatment with SR 33805 at a dose which produced a significant decrease in the arterial calcium content. Thus, SR 33805 can inhibit both calcium overload and its deleterious consequences and its actions are evident at doses as low as 2 mg/kg.