The effects of ICS 205-930, a 5-HT antagonist, on arrhythmias and catecholamine release during canine myocardial ischaemia and reperfusion

Magnolol reduces myocardial ischemia/reperfusion injury via neutrophil inhibition in rats

The accumulation of oxygen-free radicals and activation of neutrophils are strongly implicated as important pathophysiological mechanisms mediating myocardial ischemia/reperfusion injury. It has been proven that various antioxidants have cardioprotective effects. Magnolol, an active component extracted from the Chinese medicinal herb Magnolia officinalis, possesses potent antioxidant and free radical scavenging activities. In this study, the cardioprotective activity of magnolol was evaluated in an open-chest anesthetized rat model of myocardial ischemia/reperfusion injury. The results demonstrated that pretreatment with magnolol (0.2 and 0.5 microg/kg, i.v. bolus) at 10 min before 45 min of left coronary artery occlusion, significantly suppressed the incidence of ventricular fibrillation and mortality when compared with the control group. Magnolol (0.2 and 0.5 microg/kg) also significantly reduced the total duration of ventricular tachycardia and ventricular fibrillation. After 1 h of reperfusion, pretreatment with magnolol (0.2 and 0.5 microg/kg) caused a significant reduction in infarct size. In addition, magnolol (0.2 microg/kg) significantly reduced superoxide anion production and myeloperoxidase activity, an index of neutrophil infiltration in the ischemic myocardium. In addition, pretreatment with magnolol (0.2 and 0.5 microg/kg) suppressed ventricular arrhythmias elicited by reperfusion following 5 min of ischemia. In vitro studies of magnolol (5, 20 and 50 microM) significantly suppressed N-formylmethionyl-leucyl-phenylalanine (fMLP; 25 nM)-activated human neutrophil migration in a concentration-dependent manner. It is concluded that magnolol suppresses ischemia- and reperfusion-induced ventricular arrhythmias and reduces the size of the infarct resulting from ischemia/reperfusion injury. This pronounced cardioprotective activity of magnolol may be mediated by its antioxidant activity and by its capacity for neutrophil inhibition in myocardial ischemia/reperfusion.

Potent antifibrillatory effect of combined blockade of calcium channels and 5-HT2 receptors with nexopamil during myocardial ischemia and reperfusion in dogs: comparison to diltiazem

The need for novel antifibrillatory therapy is underscored by clinical trials indicating that the incidence of sudden cardiac death is increased by sodium or potassium channel blockade and is only partially reduced by beta-blockade. We examined the efficacy of nexopamil, which possesses the unique combination of calcium channel and 5-HT2 receptor blockade, in preventing ventricular tachycardia (VT) and fibrillation (VF) and reducing T-wave alternans magnitude during coronary artery occlusion and abrupt reperfusion in dogs. The results were compared with L-type calcium channel blockade alone with diltiazem. The effect of nexopamil was tested during a 10-min period of left anterior descending (LAD) coronary artery occlusion and release in chloralose-anesthetized dogs. T-wave alternans magnitude was assessed by complex demodulation. The drug reduced the incidence of VT during occlusion (from 5 of 6 to 0 of 6, p < 0.03) and VT/VF during abrupt reperfusion (from 5 of 6 to 0 of 6, p < 0.03) and suppressed the T-wave alternans magnitude increase induced by occlusion (from 14.62 +/- 3.96 to 1.39 +/- 0.34 mV x ms, p < 0.01) and reperfusion (from 17.33 +/- 4.67 to 2.34 +/- 0.77 mV x ms, p < 0.01). When 30-s left stellate ganglion stimulation (10 V, 5-ms pulses, 10 Hz) was superimposed on occlusion, nexopamil reduced the VT/VF incidence (from 8 of 11 to 4 of 11, p < 0.05) and T-wave alternans magnitude (from 24.80 +/- 5.05 to 15.81 +/- 5.09 mV x ms, p < 0.05). Calcium channel blockade alone with diltiazem decreased the incidence of ventricular tachyarrhythmias (from 5 of 10 to 1 of 10, p < 0.05) and T-wave alternans magnitude (from 16.75 +/- 3.06 to 2.87 +/- 1.23 mV x ms, p < 0.05) during coronary artery occlusion. During reperfusion, diltiazem's reduction in arrhythmia incidence (from 5 of 8 to 2 of 8) was not statistically significant, although the decrease in T-wave alternans (from 28.60 +/- 3.43 to 8.27 +/- 3.73 mV x ms, p < 0.05) was significant. Therefore, nexopamil was superior to diltiazem in protecting against reperfusion-induced arrhythmias. Nexopamil's significant antifibrillatory effect during both coronary artery occlusion and abrupt reperfusion is reliably tracked by T-wave alternans magnitude. Because the major component of the protection could be reproduced by blockade of the L-type calcium channel with diltiazem, nexopamil's antiarrhythmic action appears to be due mainly to blockade of this channel. Nexopamil's antiplatelet action through blockade of 5-HT2 receptors may confer additional protection against reperfusion arrhythmias.

Platelet activation during myocardial ischaemia: a contributory arrhythmogenic mechanism

Experimental and clinical observations of the involvement of platelets in the pathophysiology of myocardial ischaemia indicate the importance of interactions between these formed elements and the heart. The aim of this review is to outline evidence linking platelet activation, myocardial ischaemia and infarction, and to present evidence for a link between platelet activation, arrhythmogenesis and sudden death. A brief review of platelet physiology and pharmacology is provided, with a review of the cardiac electrophysiological effects of ischaemia and the electrophysiological effects of platelet-derived substances. The concept that platelet activation during myocardial ischaemia is a contributory arrhythmogenic mechanism is discussed.

Block of the delayed rectifier current (IK) by the 5-HT3 antagonists ondansetron and granisetron in feline ventricular myocytes

1. We investigated the effects of two 5-HT3 antagonists, ondansetron and granisetron, on the action potential duration (APD) and the delayed rectifier current (IK) of feline isolated ventricular myocytes. Whole-cell current and action potential recordings were performed at 37 degrees C with the patch clamp technique. 2. Ondansetron and granisetron blocked IK with a KD of 1.7 +/- 1.0 and 4.3 +/- 1.7 microM, respectively. At a higher concentration (30 microM), both drugs blocked the inward rectifier (IKl). 3. The block of IK was dependent on channel activation. Both drugs slowed the decay of IK tail currents and produced a crossover with the pre-drug current trace. These results are consistent with block and unblock from the open state of the channel. 4. Granisetron showed an intrinsic voltage-dependence as the block increased with depolarization. The equivalent voltage-dependency of block (delta) was 0.10 +/- 0.04, suggesting that granisetron blocks from the intracellular side at a binding site located 10% across the transmembrane electrical field. 5. Ondansetron (1 microM) and granisetron (3 microM) prolonged APD by about 30% at 0.5 Hz. The prolongation of APD by ondansetron was abolished at faster frequencies (3 Hz) showing reverse rate dependence. 6. In conclusion, the 5-HT3 antagonists, ondansetron and granisetron, are open state blockers of the ventricular delayed rectifier and show a clear class III action.

Pathophysiology of myocardial ischemia: importance of platelet-vessel wall interactions

Clinical and experimental evidence for important interactions between platelets and vascular endothelium under conditions of myocardial ischemia is briefly summarized. The clinical evidence for a role of platelets in myocardial ischemia includes studies indicating alterations in platelet behavior and the therapeutic benefit of some antiplatelet drugs. Experimental evidence suggesting the importance of platelets in ischemia includes reductions in transmyocardial platelet number under these conditions, the cyclical reductions in coronary blood flow that result from intermittent intravascular platelet aggregation, and the relationship between induced platelet aggregation and cardiac arrhythmias. Evidence strongly suggesting the importance of platelet emboli and thromboxane generation as a major cause of reperfusion-induced ventricular arrhythmias is outlined.

Evidence for inhibition by ICS 205-930 and stimulation by BRL 34915 of K+ conductance in cardiac muscle

Electrophysiological effects of the 5-HT3 receptor antagonist ICS 205-930 [(3 alpha-tropanyl)-1H-indole-3-carboxylic acid ester] were investigated in guinea pig isolated heart preparations. ICS 205-930 prolonged the functional refractory period and decreased the force of contraction in left driven atria. It decreased spontaneous beating rate in right atria. These effects were concentration dependent between 3 X 10(-6) and 10(-4) mol/l of ICS 205-930. In fast action potentials of papillary muscles ICS 205-930 concentration-dependently depressed Vmax and prolonged the action potential duration (APD) between 10(-6) and 10(-5) mol/l. The action potential amplitude (APA) and the resting membrane potential (RMP) remained unchanged. In papillary muscles partially depolarized by high K+ (22 mmol/l) and reactivated by high voltage stimulation, slow response APs were prolonged by ICS 205-930 10(-6) to 3 X 10(-5) mol. Vmax, APA and RMP were not affected. Similar effects on APD were obtained with sotalol (3 X 10(-5) mol/l), an inhibitor of outward K+ current. The slow-APD prolongation induced by ICS 205-930 as well as by sotalol was reversed by BRL 34915 (6-cyano-3,4-dihydro-2,2-dimethyltrans-4-(2-oxo-1-pyrrolidyl )-2H-benzo[b]pyran-3-ol), known to open K+ channels. BRL 34915 alone reduced slow-APD stereoselectively. The results suggest that ICS 205-930 may inhibit and BRL 34915 may stimulate the K+ conductance of guinea pig myocardial cell membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

Ischaemia-induced release of noradrenaline and creatine kinase in the isolated, working rat heart

In order to examine an early ischaemia-induced local release of myocardial noradrenaline (NA), the left coronary artery of the isolated working rat heart was ligated for periods varying between 7.5 min and 30 min, followed by reperfusion for 5 min. As perfusion substrate, glucose, lactate or both were used. In part of the experiments, hearts were pre-labelled with [3H]NA. Already after 7.5 min of ischaemia, an increased efflux of endogenous NA was observed in the perfusate at reperfusion, concomitant with a decrease in tissue NA content. This effect was most pronounced with lactate as substrate. Qualitatively similar effects were seen on [3H]NA efflux from labelled hearts. The combination of glucose and lactate as substrate markedly reduced (compared with lactate alone) the efflux of NA, whereas no such reduction was observed on the efflux of creatine kinase (CK). It is concluded that ischaemia is associated with an early local release of NA. Furthermore, ischaemia may induce different effects on the metabolic processes of the myocyte as compared with the adrenergic nerve ending.