The effects of RO-22-4679, a thromboxane synthetase inhibitor, on ventricular fibrillation induced by coronary artery occlusion in conscious dogs

Neutrophil mediated myocardial injury

1. Activated polymorphonuclear neutrophils (PMN) were shown to exacerbate ischemic myocardial injury and their activation is modulated by complement system, platelet activating factor, arachidonic acid metabolites, adenosine and nitric oxide. 2. Mechanisms of injurious PMN effect on ischemic myocardium are related to both mechanical and biochemical processes. 3. Activated PMN aggregate and adhere to endothelium that results in capillary plugging and subsequent impairment of coronary blood flow as well as participating in the development of endothelial cell edema. 4. PMN-related biochemical damage of ischemic myocardium is a result of the release of cytotoxic free oxygen radicals and proteolytic enzymes as well as vasoconstrictor leukotriene B4 and leukotoxin.

A novel loop diuretic, torasemide, inhibits thromboxane A2-induced contraction in the isolated canine coronary artery

We examined the effect of a novel antihypertensive diuretic, torasemide, on the vasoconstriction induced by TXA2 in the isolated canine coronary artery. Carbocyclic thromboxane A2 (CTA2), a stable analogue of the potent coronary vasoconstrictor thromboxane A2, exhibited a slow onset and progressive contraction of isolated canine coronary arteries at 2 x 10(-8) M. Torasemide (10(-7) approximately 10(-4) M) elicited a dose-dependent vasodilating action in the isolated canine coronary arteries contracted by CTA2, whereas indapamide or furosemide had little effect on this preparation. The maximum vasodilating response to torasemide was 45 +/- 12% of vasodilating effect induced by 10(-4) M papaverine. These results suggest that torasemide is a promising antihypertensive agent with a coronary protective effect.

The effect of thromboxane inhibition on vulnerability to ventricular fibrillation in the acute and chronic feline infarction models

Inhibition of the enzyme that synthesizes thromboxanes may protect against the development of ventricular fibrillation (VF) during acute myocardial ischemia. This study was carried out to test this hypothesis with a new thromboxane synthetase inhibitor, and to extend the studies to alternative animal models of myocardial infarction. In a series of acute experiments, 19 cats were pretreated with 10 mg/kg of U-63557A (a dose that produced greater than 75% reduction in thromboxane B2 [TxB2] levels) or saline before abrupt left anterior descending coronary artery occlusion. Seven of the nine control animals suffered spontaneous VF associated with a 77% fall in VF threshold compared with the treated animals, of which 2 of 10 had spontaneous VF and in which VF threshold fell by only 45% (p less than 0.025). Despite a similar extent of TxB2 inhibition in another set of nine animals, U-63557A failed to protect against a fall in VF threshold during coronary reperfusion. Finally, chronic changes in VF threshold and inducibility of sustained ventricular tachycardia by programmed stimulation were assessed in a group of eight animals. The lowering of VF threshold and inducibility of ventricular tachycardia seen in the control state were not influenced by treatment with U-63557A. Thus protection against infarct-related VF by TxB2 inhibition is a property shared by more than one pharmacologic agent. Arrhythmias generated by reperfusion or induced in a more chronic setting may not be thromboxane-dependent. These results have important implications for the planning of studies designed to assess the antiarrhythmic potential of drugs that inhibit thromboxane synthesis.

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.

Thromboxane synthetase inhibitors reduce infarct size by a platelet-dependent, aspirin-sensitive mechanism

Platelets are suggested to exacerbate ischemia-induced myocardial injury, which has led to the study of various antiplatelet therapies including thromboxane synthetase inhibitors (TXSI). Two such agents, benzylimidazole and OKY-046, reduce infarct size commensurate with a diminution in serum thromboxane B2 formation in anesthetized dogs subjected to 90 minutes of coronary artery occlusion followed by 5 hours of reperfusion. In contrast, platelet depletion with specific antiserum does not reduce infarct size but prevents the cardioprotection afforded by the TXSI. Platelet-derived prostaglandin endoperoxides (PGG2 and PGH2), which cannot be converted to thromboxane A2 in the inhibited platelet, can be transformed to PGE2 and PGD2 in plasma and to PGI2 by the blood vessel wall. These prostaglandins are considered "cardioprotective." Consequently, a low dose of aspirin (3-5 mg/kg) given 24 hours before coronary occlusion was used to selectively block the platelet cyclooxygenase enzyme. Aspirin, by itself, does not reduce infarct size, but it suppresses the myocardial salvage induced by OKY-046. Thus, TXSI reduce infarct size by platelet-dependent, aspirin-sensitive mechanism that depends on the redirection of platelet-derived PGG2 and PGH2 to protective metabolites, rather than inhibition of thromboxane A2 per se. Moreover, myocardial salvage induced by the TXSI is accompanied by a reduction in neutrophil accumulation in the myocardium, as indicated by the levels of the neutrophil-specific myeloperoxidase enzyme. Platelet depletion or pretreatment with aspirin prevents the TXSI-induced suppression of neutrophil accumulation. Consequently, it is proposed that the prostaglandin-mediated protective effects of TXSI can be resolved, at least in part, in terms of a braking action on neutrophil activation to prevent leukocyte-dependent tissue injury.

Antiarrhythmic agents inhibit the in vitro formation of thromboxane (TX) in platelets from spontaneously hypertensive rats (SHR)

The occurrence of early life threatening arrhythmias in the ischemic myocardium has been associated with local generation of thromboxane (TX). Antiarrhythmic agents are typically classified according to the fundamental mechanism involved in restoring normal rhythm but identifying those agents also capable of suppressing TX formation offers a means of improving the rationale of antiarrhythmic therapy. Accordingly, representative antiarrhythmic agents from classes I-IV were evaluated in the present study for their ability to suppress TX formation from intact rat platelets, in vitro. Agents from class I, II and IV achieved significant reductions in TX levels as compared to a reference TX inhibitor, dazoxiben. The IC-50 value for dazoxiben was 1.5 X 10(-6) M. Nicardipine and flecainide, being the most active of the antiarrhythmic agents tested, had IC-50's of 5 X 10(-6) and 1 X 10(-5) M respectively. Verapamil and propranolol had values of 1 X 10(-5) and 5 X 10(-5) M respectively; labetalol and quinidine were 5 X 10(-5) M, and phenytoin and diltiazem were approximately 1 X 10(-4) M. These data suggest a subsidiary antiarrhythmic property of these particular agents as related to their ability to suppress TX generation in the ischemic myocardium and implies these agents may be preferred in the treatment of early life threatening arrhythmias resulting as an initial response to myocardial ischemia.

Antiarrhythmic effects of the thromboxane antagonist BM 13.177

The effects of the thromboxane antagonist BM 13.177 (5 mg kg-1 + 0.15 mg kg-1 min-1) was investigated on the ventricular arrhythmias that result from coronary artery occlusion and reperfusion in anaesthetised open-chest greyhounds. BM 13.177 markedly reduced the severity and incidence of arrhythmias resulting from ischaemia; the number of ventricular ectopic beats was reduced from 1,084 +/- 159 (in controls) to 544 +/- 179 (in dogs given BM 13.177) and the incidences of ventricular tachycardia (VT) and ventricular fibrillation (VF) were reduced from 86 to 22% and from 30 to 10% respectively. Following reperfusion the incidence of VF was 86% in controls and 44% in dogs given BM 13.177. Thus the total incidence of VF during the combined ischaemia-reperfusion insult was significantly reduced by treatment with BM 13.177 from 90% (in control dogs) to 50%. These results lend further support to the hypothesis that thromboxane is involved in the genesis of arrhythmias and that blockade of thromboxane receptors may be a suitable approach to antiarrhythmic therapy under conditions of ischaemia and reperfusion.

Ginger: inhibition of thromboxane synthetase and stimulation of prostacyclin: relevance for medicine and psychiatry

Ginger, the common spice, has recently been found to act as a potent inhibitor of thromboxane synthetase, raising levels of prostacyclin, without a concomitant rise in PGE2 or PGF2 alpha. Indications for use of ginger to replace either thromboxane inhibitors having serious side effects or prostacyclin are given.

Effect of thromboxane synthetase inhibition on vulnerability to ventricular arrhythmia following coronary occlusion

Release of thromboxane (TXA2) during acute myocardial infarction may be an important contributing factor in the genesis of ventricular fibrillation (VF). We assessed the effect of selective TXA2 inhibition on vulnerability to VF after total occlusion of the anterior descending coronary artery in chloralose-anesthetized cats. Animals were pretreated with vehicle or with CGS-13080, a TXA2 synthetase inhibitor, 3.0 or 9.0 mg/kg intravenously. There was an apparent dose-dependent protective effect following CGS-13080 administration, in which the decrease in VF threshold following coronary occlusion was attenuated. Also, the incidence of spontaneous ventricular arrhythmia in the first 30 minutes after occlusion was reduced by two thirds in the 9.0 mg/kg CGS-13080 group compared to the vehicle-treated animals. This protective effect does not appear to be due to a change in hemodynamics, effective refractory periods, or extent of ischemia. TXA2 released during coronary occlusion appears to be arrhythmogenic, and inhibiting its synthesis may be protective.

Interaction of platelets with the vessel wall in the pathophysiology of sudden cardiac death

The occurrence of ventricular fibrillation after occlusion of the circumflex coronary artery in the conscious dog was examined as one preparation for sudden cardiac death arising from coronary artery disease. The incidence of ventricular fibrillation after after circumflex occlusion was reduced from 53% to 6% (p less than .01) by the infusion of prostacyclin in doses that do not alter cardiac output or peripheral resistance. This effect of prostacyclin led to the evaluation of two structurally different inhibitors of thromboxane synthase in this preparation; RO 22-4679 and U-63557A reduced ventricular fibrillation by 82% and 100%, respectively (p less than .05). The inhibition of ventricular fibrillation by U-63557A was abrogated by pretreatment with indomethacin, suggesting that a metabolite of the endoperoxide that accumulated during thromboxane synthase inhibition was responsible for the protection from ventricular fibrillation.

The effects of metoprolol and dazmegrel, alone and in combination, on arrhythmias induced by coronary artery occlusion in conscious rats

The effects of metoprolol and the thromboxane synthetase inhibitor dazmegrel, alone and in combination, were examined in a model of coronary artery occlusion in conscious rats. In a dose (2 mg kg-1), intravenously, that resulted in a marked bradycardia (of 50-80 beats min -1) metoprolol did not influence the incidence or severity of the ventricular arrhythmias that occur in the first 20 min following occlusion, nor did it improve survival (assessed at both 20 min and 16 h). In a dose (5 mg kg-1), intravenously, that in another conscious rat model involving tissue hypoperfusion inhibited thromboxane production, dazmegrel also did not modify ischaemic arrhythmias or survival. In contrast, metoprolol and dazmegrel (2 mg kg-1 and 5 mg kg-1 i.v.) when given together prior to coronary artery occlusion, produced a significant reduction in mortality both at 20 min and 16 h (e.g. from 60-75% in the control, metoprolol alone and dazmegrel alone groups and only 25% in the combined-treatment group). This was due to a decrease in the incidence of terminal ventricular fibrillation. The results suggest that a combination of beta-adrenoceptor blocking drug with a drug that inhibits thromboxane synthesis may offer more protection against ischaemia-induced ventricular fibrillation than either drug used alone. They suggest a role for both catecholamines and thromboxane in the genesis of ischaemia-induced ventricular fibrillation.

Platelet depletion in experimental myocardial infarction

Accumulation of platelets in the microvasculature after acute myocardial ischemia may exacerbate tissue injury through the formation of microthrombi and by the release of vasoactive substances. To assess the role of platelets in myocardial ischemic injury and infarction, circulating platelets were reduced by 94 +/- 2% (mean +/- S.E.M.) with sheep antiserum to canine platelets. Regional myocardial ischemia was produced by occlusion of the left circumflex coronary artery (LCCA) for 90 min followed by reperfusion for 5 hours. Infarct size did not differ significantly between antiplatelet serum and nonimmune serum groups: 36 +/- 8 vs. 43 +/- 4% of the area at risk, determined by a post-mortem dual staining technique (p greater than 0.05). A second occlusion-reperfusion control group, sacrificed at 24 hours, did not differ from 5 hr reperfused groups with regard to infarct size. Coronary sinus thromboxane B2 (TXB2) concentrations were not altered significantly by platelet depletion. Histopathologic examination confirmed the presence of necrosis in the infarcted myocardium and revealed substantial leukocytic infiltration in both groups. The results suggest that circulating platelets are not required for the full expression of myocardial ischemic injury resulting from temporary coronary artery occlusion followed by reperfusion.

In vivo inhibition of thromboxane synthetase in infarcted canine myocardium

The in vivo effectiveness of the thromboxane synthetase inhibitor OKY-1581 was tested in normal and infarcted canine myocardium. A rapid in vitro assay was developed which permits an accurate assessment of the status of the tissue thromboxane synthetase at the time of sacrifice. Reperfused infarcts were created by two hours of coronary artery occlusion followed by release of occlusion and three days of recovery. OKY-1581 was infused at 100 micrograms/kg/min for 15 minutes, a dose previously found to cause an 85% inhibition of canine platelet thromboxane synthetase in vivo. The heart was rapidly excised and transmural tissue plugs of infarcted and normal areas were obtained. These were incubated for 5 minutes with prostaglandin endoperoxide (PGH2) in phosphate buffer. Thromboxane production was inhibited from 16 +/- 1 ng TxB2 per tissue plug to 5 +/- 1 in normal myocardium and from 27 +/- 5 to 6 +/- 1 in infarcted areas of myocardium. Control incubations showed no further inhibition with the in vitro addition of 20 micrograms/ml OKY-1581, confirming the completeness of in vivo inhibition. Thus significant inhibition of thromboxane synthetase by intravenous OKY-1581 occurs even in a reperfused zone of infarction.