Acute Coronary Artery Ligation Research Articles

Protective action of a thromboxane synthetase inhibitor in preventing extension of infarct size in acute myocardial infarction

U-63,557A, a new selective thromboxane synthetase inhibitor, was evaluated for its ability to prevent the extension of myocardial infarct size. Left coronary arteries of male Sprague-Dawley rats (230 – 270 g) were acutely ligated, producing a consistent model of myocardial infarction NO in rats analyzed 48 hours later. Left ventricular free wall (LVFW), creatine kinase (CK) activity,, and amino-nitrogen concentrations were assayed as indices of infarct size. U-63,557A was administered intravenously in two doses (4 and 8 mg/kg) with a split schedule (2 min post-ischemia and either 4 or 24 hrs later). Administration of the thromboxane synthetase inhibitor significantly reduced both myocardial CK and amino-nitrogen loss at a dose of 8 mg/kg, but it was only slightly effective at 4 mg/kg. Drug treatment significantly increased the percent LVFW spared; 27 ± 39% (vehicle) vs 43 ± 796 and 52 ± 796 (8 mg/kg). U-63,557A is an effective agent in myocardial ischemia for limiting the extension of infarct size after acute coronary artery ligation. Previous studies of other thromboxane synthetase inhibitors showed effectiveness in the early stages of MI. This study shows an effect on true infarct size 48 hours post-ligation, and suggests that inhibition of thromboxane A 2 plays an important role in the pathogenesis of ischemic damage in the myocardium.

Functional and morphological changes in the microcirculation of the dog following acute coronary artery ligation

Acute ligation of the anterior descending coronary artery in the dog had little immediate effect on heat clearance but immediately lowered heat production, which fell to its minimum level in 1 2 hr , after which it began to recover; recovery was complete in 6 hr. After 1 2 hr heat clearance also began to fall and reached a minimum level in 1 1 2 hr . Ligation increased oxygen extraction but reduced oxygen consumption markedly though temporarily. Complete recovery followed. After 3 months, there was full recovery in flow, oxygen extraction ratio, and oxygen consumption mVO 2. In morphological studies using Microfil injections soon after ligation, capillaries failed to fill retrogradely, though network vessels were perfused. After 1 hr of acute artery ligation, capillaries still did not perfuse but by 3 hr there was some reperfusion. After 6 hr there was marked capillary reperfusion. In dogs kept for 3 months after ligation the microcirculation was normal. These studies confirm that immediate circulatory failure occurred in capillaries (probably critical closure) which accounts for the immediate fall in venous outflow, heat production, and mVO 2. Flow continued in network vessels which traversed the ischemic zone without perfusing capillaries. These results are contrasted with the findings of some other workers who found a continued functional decline leading to infarction. The present results were accounted for by closure and reopening to capillaries.

The natural history of regional dysfunction in a canine preparation of chronic infarction.

We used a canine preparation of experimental infarction to study the natural course of echocardiographically defined regional wall motion abnormalities in the 6 weeks after acute coronary ligation. Eight dogs underwent serial short-axis echocardiographic evaluation and microsphere blood flow determinations at control, and 30 min, 48 hr, 1 week, 3 weeks, and 6 weeks after acute coronary artery ligation. Wall motion analysis and blood flow calculations were applied to 10 degree radial segments of the left ventricle (short axis) and correlated to the size and extent of infarction as defined histologically at 6 weeks. All animals had at least 50% transmural histologic infarction. The ratio of flow in infarcted tissue vs noninfarcted myocardium fell to 0.40 +/- 0.13 for endocardium and 0.56 +/- 0.13 for epicardium at 30 min after ligation, but recovered to 0.83 +/- 0.15 for endocardium and 1.12 +/- 0.11 for epicardium by 6 weeks. The maximum circumferential extent of abnormal regional wall motion was observed at 48 hr after infarction (mean circumferential extent = 51%), but was resolved to a significant extent by 6 weeks (circumferential extent = 21%, p less than .001). Four animals had virtually normal wall motion by 6 weeks after infarction. Segment-by-segment regional dysfunction correlated highly with the regional transmural reduction in blood flow of 20% )r = .89, p = .0001) for the experiment as a whole, but the echocardiogram tended to underestimate the size of the histologic infarct at 6 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

Preservation of ischemic myocardium by a new converting enzyme inhibitor, enalaprilic acid, in acute myocardial infarction

Enalaprilic acid (MK-422), the biologically active diacid of the converting enzyme inhibitor enalapril, was studied in myocardial ischemia (MI). Acute left coronary artery ligation was produced in 62 male Sprague-Dawley rats, and infarct size was determined by left ventricular free wall (LVFW) creatine kinase (CK) activity. Administration of enalaprilic acid (2 mg/kg) 2 minutes and 24 hours after MI significantly blunted the reduction in LVFW CK activity at 48 hours after ligation, when compared to the MI rats given vehicle (6.4 ± 0.5 vs 4.7 ± 0.2 IU/mg protein, respectively; p < 0.01). The percentage of LVFW spared was significantly ( p < 0.01) increased from 28 ± 2% to 45 ± 5% by MK-422. MK-422 also significantly blunted the loss of LVFW CK activity 48 hours after a coronary ligation (10 minutes) followed by reperfusion, when compared to vehicle (10.1 ± 0.6 vs 8.3 ± 0.6 IU/mg protein, respectively; p < 0.05). This represents a significant increase in the percentage of LVFW spared, 65 ± 5% vs 85 ± 6% ( p < 0.05). These data indicate a significant protective action afforded by MK-422 in two different protocols of ischemic damage to the myocardium and suggest a role for the renin-angiotensin system in the extension of ischemic damage.

Effects of sympathetic stimulation on ventricular refractory periods in cats with acute coronary artery ligation.

Ventricular refractory periods shorten in the ischemic area following acute coronary artery ligation. Subsequent bilateral sympathetic nerve stimulation reduces disparity in refractory periods across normal, border (peri-ischemic) and ischemic areas.

Effects of diltiazem, a calcium channel inhibitor, in retarding cellular damage produced during early myocardial ischemia in pigs: A morphometric and ultrastructural analysis

A protective effect of intravenous diltiazem pretreatment on transmural histologic damage in ischemic hearts of 30 pigs was determined morphometrically. The pig hearts were rendered ischemic by ligation of the distal left anterior descending coronary artery for 20, 40 and 120 minutes in separate experiments. Heart rate, left ventricular systolic pressure and systemic diastolic pressure decreased slightly during 40 minutes of occlusion, and then recovered to control levels. However, there were no significant differences between control and diltiazem-treated groups. Blood flow, measured by the microsphere technique, was uniformly absent in the ischemic areas in both control and diltiazem-treated hearts. In the few cell layers immediately beneath the endocardium and epicardium, little cellular damage was observed in either group regardless of the duration of ischemia. In the subendocardial, mid-myocardial and subepicardial layers, less cellular damage was detected after all periods of ischemia in hearts exposed to diltiazem in comparison with that observed in control hearts. However, even in the drug-treated hearts, after 2 hours of occlusion, the cells remained severely injured. The protective effect of diltiazem on the cellular integrity in the ischemic myocardial regions appears to be independent of blood flow and may be a reflection of a direct effect of diltiazem on myocytes. The precise mechanism is unresolved. Nevertheless, in the ischemic heart without a significant collateral circulation, pretreatment with the calcium channel blocking drug, diltiazem, results in a delayed onset of cellular damage after acute coronary artery ligation and less cellular damage at ischemic periods up to 40 minutes' duration.

Effects of prolonged administration of oxprenolol on severity of ischaemic arrhythmias, enzyme leakage, infarct size, and intracellular cardiac muscle action potentials.

We examined the effects of prolonged oral administration of oxprenolol (twice daily for 6 weeks) to male Sprague-Dawley rats. At two times (1 or 16-18 h) after the last oral dose, the rats were anaesthetised and subjected to acute coronary artery ligation, and the severity of the resulting arrhythmias was assessed. Ischaemic damage was measured histochemically (using frozen section analysis by toluidine blue dye in nitrobluetetrazolium ) and by myocardial enzyme release. Cardiac muscle (atria and papillary muscle) was also removed and the transmembrane action potentials recorded using conventional microelectrode techniques. When coronary artery ligation was performed 1 h after the last oral dose (at which time there was evidence of substantial myocardial beta 1-adrenoceptor blockade), there was significant reduction in the severity of early arrhythmias, but no evidence that the severity of ischaemic damage was reduced or that the intracellular cardiac action potentials were modified. No protection was observed when coronary artery ligation was carried out 16 h after the last oral dose of oxprenolol. These results support our previous studies with acutely administered beta-adrenoceptor blocking drugs that myocardial beta-adrenoceptor blockade is the main factor involved in the protection afforded by such drugs against early ischaemic arrhythmias and that other possible effects, such as membrane stabilisation and action potential prolongation, are relatively unimportant in this model.

Early and late administration of a PGI2-analogue, ZK 36 374 (iloprost): effects on myocardial preservation, collateral blood flow and infarct size.

A number of investigations have reported that prostacyclin or prostacyclin analogues protect the ischaemic myocardium when administered early after myocardial ischaemia. Thus far, there are no reports describing whether these substances exert a cardioprotective effect when administered later than 0.5 h after coronary artery occlusion. Adult cats were subjected to acute coronary artery ligation for 5 h and administered the vehicle or ZK 36 374 (iloprost) (1.19 micrograms X kg-1 X min-1), a prostacyclin analogue, beginning at 0.5, 2 or 4 h. Compared with the MI-vehicle cats, ZK 36 374 prevented a decrease in myocardial creatine kinase specific activity, the loss of free amino nitrogen and the fall in percentage bound cathepsin D in the ischaemic area when infusion was started at 0.5 or 2 h (P less than 0.05). In addition ZK 36 374 started at 4 h still showed a significant protective effect against myocardial creatine kinase specific activity and amino nitrogen concentrations but not against cathepsin D. In a separate group of animals, regional myocardial blood flow and late coronary resistance were determined with radioactive labelled 15 +/- 1 micron microspheres. ZK 36 374 consistently reduced late diastolic coronary vascular resistance and increased coronary blood flow in nonischaemic regions of the myocardium (P less than 0.05) but only attenuated the further increase in late coronary resistance in the ischaemic myocardial regions. The infarcted area (NTB-staining) amounted to 9% of the total left ventricle after 5 h and was not reduced by ZK 36 374 (P greater than 0.05). In conclusion, ZK 36 374 exerted a significant biochemical cardioprotective effect when administered to 0.5, 2 or 4 h. The mechanism of cardioprotection does not appear to be due to increased myocardial perfusion but rather to some direct cellular action whose exact nature has yet to be elucidated.

The effect of beta-adrenoceptor blocking agents, with differing ancillary properties, on the arrhythmias resulting from acute coronary artery ligation in anaesthetized rats.

The effects of several beta-adrenoceptor blocking agents, [+), (-) and (+/-)-oxprenolol, p-oxprenolol, practolol, propranolol and timolol) were investigated on the ventricular arrhythmias occurring within the first 30 min of acutely ligating the main left coronary artery in anaesthetized rats. The degree of cardiac and vascular beta-adrenoceptor blockade was also assessed. All the compounds exhibited antiarrhythmic activity under these conditions. The degree of cardiac beta-adrenoceptor blockade required for this protection was less for the cardioselective agents, p-oxprenolol and practolol, than for the non-selective beta-adrenoceptor blocking agents. A comparison of the two isomers of oxprenolol demonstrated that the (-)-isomer markedly suppressed ischaemic arrhythmias (ventricular ectopic beats, incidence and duration of ventricular tachycardia and duration of ventricular fibrillation) more effectively than the (+)-isomer. Compounds possessing intrinsic sympathomimetic activity (ISA) caused less marked haemodynamic changes (in equivalent beta-blocking doses) than those that did not possess this ancillary property. The membrane stabilizing activity of oxprenolol and p-oxprenolol did not appear to contribute to the antiarrhythmic activity of these agents; however, the membrane stabilizing activity of propranolol may contribute to its effectiveness. In all the drugs studied, the main pharmacological property required to suppress early postischaemic arrhythmias is blockade of cardiac beta-adrenoceptors.

Decreased incidence of ventricular fibrillation after an acute coronary artery ligation in exercised pigs.

Evidence has been presented that regular physical activity may be associated with a decreased incidence of sudden cardiac death. It has been suggested that self-selection of those engaging in regular exercise rather than the physical activity itself is a major factor in explaining these results. We therefore studied the effects of a two-month exercise program on the incidence of ventricular fibrillation after an acute ligation of the left anterior descending (LAD) coronary artery in domestic Yorkshire pigs. At the end of the exercise program, the exercised group (EG, n = 17) had a lower heart rate (10%), a 5 times higher maximum exercise capacity, a 10% larger left ventricular mass and a thicker myocardial wall during end-diastole than a sedentary group (SG, n = 13). After the animals were anesthetized, the LAD artery was occluded at one third of its distal end. Ventricular fibrillation (VF) occurred in 92% of the SG (12 out of 13) against only 30% of the EG (5 out of 17) within 1 hour after occlusion. Percentage of the area at risk was the same (9-10% of total left ventricular mass) in both the EG and SG. Transmural myocardial perfusion after coronary artery ligation was slightly larger in EG than in SG (30 vs 21 ml . min-1 . 100 g-1, p less than 0.05). Although the improvement in perfusion of the ischemic zone of the EG may have contributed to the reduced occurrence of ventricular fibrillation, other mechanisms cannot be excluded.

Contractile performance, mitochondrial function and blood flow distribution in porcine heart with induced coronary collateral circulation.

Progressive stenosis of the proximal left anterior descending coronary artery in young pigs was surgically induced over eight weeks and was accompanied by the development of a coronary collateral circulation originating from the right coronary artery. The antero-apical left ventricular myocardium dependent on this induced collateral blood supply became hypokinetic compared with the other regions of the left ventricle in the same hearts, and compared with the same region in different hearts, where the native circulation exists. Regional myocardial mechanics determined as fractional systolic shortening was 12.5 +/- 1.5% in normal regions, 0.7 +/- 2.4% in collateral-dependent myocardium, and in acutely ischemic myocardium was -2.0 +/- 1.3%, indicative of systolic lengthening. The rate of state 3 respiration of isolated mitochondria was depressed by 20% in collateral regions and by 64% in acutely ischemic regions relative to values obtained in mitochondria from respective normal myocardium (300 natoms of oxygen/min/mg mitochondria protein). Regional myocardial blood flow determined by 15 mu radionuclide labelled spheres revealed subendocardial hypoperfusion of 0.34 +/- 0.11 ml/min/g tissue in the collateral-dependent regions compared to 1.06 +/- 0.26 ml/min/g tissue in the normal regions. Transmural ischemia was observed (less than 0.10 ml/min/g tissue) in regions subjected to acute coronary artery ligation. Light microscopy revealed patchy fibrotic lesions predominately associated with the subendocardial half of the collateral dependent myocardial wall. Accordingly, the hypokinesia of collateral-dependent myocardium is more likely the result of blood flow abnormalities and loss of contractile elements than from mitochondrial dysfunction.

Naloxone inhibits early arrhythmias resulting from acute coronary ligation.

The intravenous administration of naloxone 15 min before acute coronary artery ligation in both anesthetized and conscious male rats markedly reduced the incidence and severity of the ventricular arrhythmias that occur within 30 min of the onset of myocardial ischaemia. The incidence of ventricular fibrillation was especially reduced and, in conscious rats, the survival 16 h after ligation was increased from 27% (in the controls) to 58 and 73% after 2 and 4 mg/kg naloxone respectively. One possible explanation of these results implies a detrimental effect of released endorphin in the early stages of myocardial ischaemia.

Lidoflazine in the early stages of acute myocardial ischaemia.

1 Pretreatment of anaesthetized rats with intravenously administered lidoflazine (an antianginal agent) reduced the incidence and severity of ventricular arrhythmias which resulted from acute coronary artery ligation. Ventricular fibrillation was completely prevented by doses of 50 micrograms/kg and 2 mg/kg and no animal so treated died ( contrast 50% incidence of fibrillation in the controls and 30% mortality). 2 In anaesthetized greyhound dogs, lidoflazine (2 mg/kg) administration resulted in transient reductions in systemic arterial pressure, LV dP/dt max and cardiac output. Coronary sinus Po2 was markedly increased, indicating pronounced coronary vasodilatation. 3 Lidoflazine pretreatment inhibited the increase in epicardial ST-segment elevation which resulted, in dogs, from short (3 min) occlusions of the left anterior descending coronary artery. This effect was especially marked at sites where, in control occlusions, ST-segment elevation was most pronounced. 4 Lidoflazine greatly reduced the number of ventricular ectopic beats which usually resulted from more prolonged (30 min) periods of acute coronary artery occlusion. There was no ventricular fibrillation in these dogs (contrast 25% incidence in the controls). 5 Lidoflazine did not modify the ventricular fibrillation which results from reperfusion of a previously ischaemic area of the left ventricular wall.

Effects of timolol on blood flow distribution in the feline myocardium with acute regional ischaemia during controlled haemodynamic conditions.

The beta-adrenergic blocking agent timolol was given to cats with acute coronary artery ligation under controlled haemodynamic conditions. Regional myocardial tissue flow was measured by the distribution of labelled microspheres. Timolol reduced cardiac contractility and left ventricular end-diastolic pressure rose, whereas heart rate and ventricular systolic pressure were kept constant by atrial pacing and aortic clamping. The systolic period increased following timolol administration under these conditions. Myocardial blood flow remained unchanged in central ischaemic and border areas, whereas flow increased both endocardially and epicardially in normally perfused area following timolol administration. Without pacing there was a significant flow reduction in the epicardium of the normally perfused area, compared with the situation where heart rate was constant. Under controlled haemodynamic conditions, therefore, timolol appears to improve coronary perfusion in normal myocardium, whereas blood flow to ischaemic myocardium remains essentially unchanged.

Factors related to vulnerability to arrhythmias in acute myocardial infarction

Ion-selective electrodes designed in our laboratory have been employed to determine the characteristics of the changes in extracellular K + and pH associated with acute coronary artery ligation and release. These studies have defined major inhomogeneities in the rate and magnitude of change of extracellular K + and pH not only at the lateral margin of the ischemic zone but also between the subepicardium and subendocardium, between the subendocardium and endocardium, and even between closely spaced electrodes throughout the ischemic zone. The inhomogeneities during reperfusion were as great as or greater than those induced by coronary ligation. We have used papillary muscles of guinea pigs to determine the effect of the various combinations of extracellular pH and K + observed at any point in time in an intact heart to study the anticipated changes in conduction and have found both a speeding and a slowing of conduction, which can be expected to occur simultaneously in an intact heart. These results indicate that significant inhomogeneities exist in the ionic and electrical consequences of acute coronary artery ligation and release and that these inhomogeneities can be expected to result in disorders of conduction leading to reentry.

Creatine phosphate suppresses ventricular arrhythmias resulting from coronary artery ligation.

The effects of various doses of creatine phosphate have been examined in a rat model of acute myocardial ischaemia. When given directly into the lumen of the left ventricle in pentobarbitone-anaesthetised male rats, creatine phosphate (50 and 100 mg/kg) markedly reduced the incidence of ventricular ectopic beats, and especially the incidence and duration of ventricular tachycardia and fibrillation which normally resulted from acute coronary artery ligation in this model. This protection was observed even if 1 of 2 h elapsed between creatine phosphate administration and coronary artery ligation. Electrophysiological studies on papillary muscles removed from rats 1 h after administration showed that creatine phosphate both decreased the maximum rate of depolarisation and prolonged the duration of the action potential. These results confirm our previous work in dogs that creatine phosphate is effective against early postinfarction ventricular arrhythmias, at least if given locally. It is suggested that these effects are due, at least in part, to a prolongation of the cardiac muscle action potential. Whether this is the result of maintaining energy production early in myocardial ischaemia is unclear.

Calcium antagonists prevent early post-infarction ventricular fibrillation

The intravenous administration of either nifedipine (in doses of 5–50 μg/kg), niludipine (10 and 50 μg/kg) or nisoldipine (5-1 000 μg/kg) markedly reduced, or abolished, the serious ventricular arrhythmias (tachycardia and fibrillation) that result from acute coronary artery ligation in pentobarbitone-anaesthetized rats. Ventricular ectopic activity, which is pronounced in the first 30 min post-ligation period in this model, was also significantly decreased by nifedipine and niludipine (10 and 50 μg/kg) and by the highest dose (1 mg/kg) of nisoldipine. These results demonstrate the marked efficacy of these three calcium antagonists in this experimental model and raise the possibility that, if given prophylactically, they might reduce the incidence of sudden cardiac death following re-infarction in patients.

The effects of prostaglandins E2, F2 alpha, prostacyclin, flurbiprofen and aspirin on arrhythmias resulting from coronary artery ligation in anaesthetized rats.

1 Various prostaglandins and inhibitors of prostaglandin synthesis were administered prior to acute coronary artery ligation in anaesthetized rats and their effects were assessed on the number and severity of the resulting early arrhythmias (ventricular ectopic activity; incidence and duration of ventricular tachycardia and of ventricular fibrillation). 2 Prostaglandin E2 (PGE2), PGF2 alpha and prostacyclin all showed antiarrhythmic activity; in contrast flurbiprofen increased the incidence of ventricular fibrillation and mortality. 3 Both the number of ventricular ectopic beats and the incidence of ventricular fibrillation were reduced by aspirin. 4 The results suggest that the release of endogenous PGE2, PGF2 alpha and prostacyclin could reduce early post-infarction ventricular arrhythmias whilst the protective effect of aspirin in this model adds further support for the hypothesis that thromboxane release is involved in the genesis of these arrhythmias.

Effect of timolol on blood flow distribution in the myocardium during acute regional ischaemia in cats.

The beta-adrenergic blocking agent timolol was given to cats with acute coronary artery ligation. Regional myocardial tissue flow was measured by the distribution of 15 micrometers labelled microspheres. Timolol reduced heart rate, cardiac contractility and left ventricular systolic pressure, but end-diastolic pressure rose. Ischaemic tissue blood flow remained unchanged following timolol, thus improving the marked imbalance between oxygen demands and delivery. In normal areas of the myocardial tissue flow was reduced, indicating the beta 2-adrenergic blocking effect of timolol. This was also the case for the border area between ischaemic and non-ischaemic tissue, but significantly less flow reduction was found on the endocardial side than on the epicardial side in the border area. Except for this latter observation, timolol does not appear to improve coronary perfusion. The present study, therefore, indicates that beneficial effects of timolol on ischaemic myocardium are mainly related to a reduction of myocardial oxygen demand.

Thromboxane and prostacyclin release from ischaemic myocardium in relation to arrhythmias.

There has been much interest in the possible physiological and pathophysiological roles of myocardial prostaglandins1. One particularly intriguing suggestion2 is that they may be capable of protecting the heart against arrhythmogenic stimuli. There is certainly no doubt that some prostaglandins can protect against a variety of arrhythmias induced either chemically or by coronary artery ligation2–4. For example, PGE2 and prostacyclin are effective against the early post-infarction arrhythmias that result from acute coronary artery ligation in anaesthetized dogs3 and rats4. In a previous study5,6 we could find no evidence that PGE2 (or PGF2α) was released from the canine myocardium at a time when ventricular ectopic activity was pronounced. We now demonstrate that, in contrast, both thromboxane and prostacyclin are released from the acutely ischaemic myocardium and that the balance between thromboxane and prostacyclin release in the period immediately following coronary artery ligation is related to the occurrence of arrhythmias.