Lack of involvement of thromboxane A2 in postischemic recovery of stunned canine myocardium

Mild myocardial stunning affects platelet aggregation and certain hemostatic factors in swine

Myocardial stunning is characterized by transient contractile dysfunction occurring subsequent to an episode of ischemia followed by reperfusion. Platelet activation and hemostatic abnormalities have been described in patients with unstable angina and acute myocardial infarction, however, their role in the pathogenesis of myocardial stunning is unknown. The purpose of this study was to determine if platelet aggregation and certain hemostatic factors change during myocardial stunning following brief coronary arterial occlusion. Nine Yorkshire swine underwent left anterior descending coronary artery occlusion for 8 minutes followed by 90 minutes of reperfusion. Blood samples were obtained at baseline, at 4 and 8 minutes of occlusion, and at 60 and 90 minutes of reperfusion. Platelet aggregability and concentrations of antithrombin III, protein C, protein S, fibronectin, endothelin 1, and the stable metabolites of thromboxane (TxB2) and prostacyclin (6-keto-PGF1a) were measured in systemic circulation. The occlusion phase was associated with a decline of endothelin 1 (-13.6%), and TxB2 (-19.6%), and elevation of antithrombin III (+40.2%) and protein C (+22.9%). Mild myocardial stunning was associated with a significant increase in platelet aggregation (+33.7%), endothelin-1 (+24.7%), 6-keto-PGF1a (+41.5%), TxB2 (+11.9%), and protein C (+42.3%) during the reperfusion phase. There were no changes in plasma fibronectin and total protein S. Thus, mild myocardial stunning following brief coronary artery occlusion is associated with substantial dynamic changes in platelet aggregability and certain hemostatic factors. These results may be relevant to understanding the mechanisms determining myocardial stunning and coronary arterial patency following reperfusion.

Serial changes of the plasma prostanoids during myocardial ischemia-reperfusion in swine. Effects of magnesium, diltiazem, and a novel Mac-1 inhibitor

The key role of prostanoids has been recognized in patients with ischemic heart disease. However, serial changes of thromboxane and prostacyclin during both brief and prolonged ischemia-reperfusion are poorly known. These plasma prostanoids were measured during myocardial stunning (MS) and acute myocardial infarction (AMI). The effects of magnesium (Mg), diltiazem, and a Mac-1 inhibitor on the level of the stable metabolites of thromboxane (TXB2) and prostacyclin (6-keto-PGF1 alpha) were elucidated. Forty-nine swine underwent brief (8 min) or prolonged (50 min) coronary artery occlusion followed by reperfusion. The occlusion phase was associated with a decline of plasma prostanoids, followed by a significant increase during reperfusion. Mg and diltiazem similarly affected plasma prostanoids by reducing TXB2 release at 1 h of reperfusion. There was, however, no effect on plasma 6-keto-PGF1 alpha. The Mac-1 inhibition was associated with stabilization of both antagonistic prostanoids as well. Ability of Mg, diltiazem, and leumedins to favorably modulate plasma prostanoid levels have direct clinical implications for the use of these agents in patients with coronary artery disease.

Role of aspirin in modulating myocardial ischemic reperfusion injury

The role of low-dose aspirin (3 mg/kg, i.v.) in attenuating ischemic reperfusion injury was studied in a canine model. Regional ischemia for 40 min was produced by temporary occlusion of the left anterior descending coronary artery and thereafter reperfusion instituted for 3 h. Mean arterial pressure (MAP), heart rate (HR), left ventricular end diastolic pressure (LVEDP), positive (+) LV dP/dtmax and negative (-) LV dP/dtmax were monitored along with myocardial adenosine triphosphate (ATP), creatine phosphate (CP), glycogen and lactate. Following reperfusion, there was a significant fall in (i) MAP, (ii) (+) LV dP/dtmax and (iii) (-) LV dP/dtmax. LVEDP was corrected after about 2 h of reperfusion. Replenishment of only myocardial CP occurred, without any change in ATP and glycogen, although lactate accumulation was corrected. Aspirin administered 15 min before reperfusion (post-treatment) caused normalisation of LVEDP within 15 min and prevented any deterioration in (-) LV dP/dtmax, although it had no effect on MAP and (+) LV dP/dtmax. After 3 h of reperfusion (post-treatment), myocardial ATP, CP, glycogen and lactate contents became normal. The number of premature ventricular complexes was significantly reduced after aspirin treatment. The present study indicates that low-dose aspirin post-treatment can ameliorate at least some of the deleterious consequences of reperfusion injury of the myocardium.

Can myocardial stunning contribute to myocardial infarction during coronary artery bypass surgery?

Myocardial stunning commonly occurs after cardiopulmonary bypass (CPB). Myocardial stunning can be cardioprotective under some conditions, but in other situations may actually contribute to myocardial infarction (MI). Vascular endothelial stunning may be one of the mechanisms by which myocardial stunning can cause MI. It has been found that 15 minutes of reversible ischemia is enough to cause elevations in vascular resistance and impairment of vasodilator responsiveness. However, no correlation between contractile dysfunction and microvascular stunning has been observed. Transduction defects (increased oxygen extraction and consumption despite normal regional oxygen blood flow and delivery) may be another mechanism by which stunning predisposes to MI, indicating uncoupling of substrate utilization from energy production. In addition, abnormalities in wall motion, oxygen free radical toxicity, hypotension, use of inotropic agents (leading to increased oxygen consumption, high heart rates, and arrhythmias) increase the risk of cellular necrosis. Following CPB, flow limitations due to diffuse atherosclerosis in some areas may result in poor contractility, and newly grafted areas have a high probability of becoming ischemic and stunned. These areas are likely to contribute to MI.

Angiotensin converting enzyme inhibitors improve contractile function of stunned myocardium by different mechanisms of action

Angiotensin converting enzyme (ACE) inhibitors enhance contractile function of myocardium "stunned" by a brief episode of coronary artery occlusion, yet their mechanism(s) of action remain unresolved. In addition to possible hemodynamic effects, ACE inhibitors may stimulate the synthesis of cardioprotective prostaglandins. Furthermore, the beneficial effects of ACE inhibitors that contain a sulfhydryl group may be due in part to the ability of thiol compounds to act as nonspecific antioxidants or direct scavengers of cytotoxic oxygen-derived free radicals. To investigate this question we compared the effects of (1) the sulfhydryl-containing ACE inhibitor zofenopril, (2) the sulfhydryl-containing stereoisomer of captopril (SQ 14,534) with essentially no ACE inhibitor properties, (3) the nonsulfhydryl-containing ACE inhibitor enalaprilat, and (4) solvent alone, given at the time of reperfusion, on recovery of contractile function after 15 minutes of coronary occlusion in the anesthetized open-chest dog. Segment shortening in control animals remained depressed or "stunned" after reperfusion, recovering to only -5 +/- 12% of baseline preocclusion values at 3 hours after reperfusion. In contrast, all three treatment agents attenuated postischemic dysfunction: segment shortening was restored to 33 +/- 12%, 54 +/- 6%, and 83 +/- 5% of baseline values at 3 hours after reflow in dogs treated with SQ 14,534 (p less than 0.05), zofenopril (p less than 0.01), and enalaprilat (p less than 0.01), respectively (all vs control value). These improvements in segment shortening did not appear to be the result of altered oxygen supply or demand after reperfusion, inasmuch as no significant differences in systemic hemodynamic parameters or myocardial blood flow were observed among the groups. In the second phase of the study, we found that the improved contractile function associated with enalaprilat treatment could largely be reversed by infusion of the potent cyclooxygenase inhibitor indomethacin: segment shortening was reduced from 69 +/- 12% at 2 hours after treatment/reperfusion to 38 +/- 12% at 2 hours after indomethacin infusion (p less than 0.01 vs 2 hours after reperfusion). Infusion of indomethacin had no effect, however, on the improved contractile function associated with zofenopril treatment. We therefore conclude that sulfhydryl- versus nonsulfhydryl-containing agents enhance contractile function of stunned myocardium by different mechanisms of action: enalaprilat attenuates postischemic dysfunction at least in part by a prostaglandin-mediated mechanism, whereas the salutary effects of zofenopril and SQ 14,534 may be due in part to the antioxidant properties of the sulfhydryl moiety.

The cardioprotective effects of the thromboxane receptor antagonist SQ 30,741 are not reversed by aspirin

A previous study indicated that 5 mg/kg aspirin can reverse the cardioprotective effects of thromboxane A2 synthetase inhibitors. We determined in the present study if this dose of aspirin can also reverse the protective effects of the thromboxane A2/PGH2 receptor antagonist SQ 30,741 in the same model of coronary occlusion and reperfusion. Anesthetized dogs were subjected to 90 min of coronary occlusion and 5 h of reperfusion and were treated with vehicle or SQ 30,741 (1 mg/kg + 1 mg/kg/h) 10 min after the onset of coronary occlusion. SQ 30,741 was given to dogs pretreated with aspirin (5 mg/kg, 24 h presurgery) or vehicle. SQ 30,741 significantly reduced infarct size compared to vehicle treatment (58% vs 35% of the left ventricular area at risk for vehicle and SQ 30,741 groups respectively) and aspirin did not reverse this. These anti-ischemic effects occurred despite a lack of change in collateral flow. Thus, important differences in mechanism of action between thromboxane synthesis inhibitors and receptor antagonists seem to exist and further work in this area is warranted.

Effects of amlodipine on myocardial ischemia-reperfusion injury in dogs

The effects of the dihydropyridine calcium channel blocker amlodipine on subendocardial segment shortening (%SS), regional myocardial blood flow, myocardial high-energy phosphate levels and tissue water content were compared with those of a saline-treated group of barbital-anesthetized dogs subjected to a 45-minute coronary artery occlusion followed by 60 minutes of reperfusion. Saline or amlodipine (200 micrograms/kg administered intravenously) was given 15 minutes before coronary occlusion. There were no significant differences between groups in ischemic bed size or hemodynamics although dP/dt was higher after amlodipine administration. Subepicardial collateral blood flow was higher in the amlodipine group during coronary occlusion. After occlusion, %SS in the ischemic region was markedly decreased in both series and passive systolic lengthening resulted. Despite similar decreases in %SS during occlusion, the amlodipine-treated dogs showed a marked improvement in myocardial segment function of the ischemic reperfused region throughout 60 minutes of reperfusion compared with saline-treated dogs. In addition, amlodipine prevented the rebound increase in phosphocreatine and attenuated the loss of adenine nucleotides and increase in tissue water in the ischemic reperfused area at 60 minutes of reperfusion. These results suggest that amlodipine has a favorable effect on the functional and metabolic recovery of the ischemic reperfused myocardium and may have potential as a therapeutic agent for the treatment of coronary artery disease. The mechanism of action of amlodipine in this model is unknown but may be partially related to a drug-induced increase in coronary collateral blood flow or a decrease in afterload.

Effect of amlodipine on myocardial functional and metabolic recovery following coronary occlusion and reperfusion in dogs

The effects of the dihydropyridine calcium-channel blocker, amlodipine, on subendocardial segment shortening (%SS), regional myocardial blood flow, myocardial high-energy phosphate levels and tissue water content were compared to those of a saline-treated group of barbital-anesthetized dogs subjected to a 45-minute coronary artery occlusion followed by 60 minutes of reperfusion. Saline or amlodipine (200 micrograms/kg, IV) were administered 15 minutes prior to coronary occlusion. There were no significant differences between groups in ischemic bed size or hemodynamics, although dP/dt was higher following amlodipine. Subepicardial collateral blood flow was higher in the amlodipine group during coronary occlusion. Following occlusion, %SS in the ischemic region was markedly decreased in both series and passive systolic lengthening resulted. In spite of similar decreases in %SS during occlusion, the amlodipine- treated dogs showed a marked improvement in myocardial segment function (%SS) of the ischemic-reperfused region throughout 60 minutes of reperfusion as compared to saline-treated animals. In addition, amlodipine prevented the rebound increase in phosphocreatine and attenuated the loss of adenine nucleotides and the increase in tissue water in the ischemic-reperfused area at 60 minutes of reperfusion. These results suggest that amlodipine has a favorable effect on the functional and metabolic recovery of the ischemic-reperfused myocardium, and may have potential as a therapeutic agent for the treatment of coronary artery disease. The mechanism of action of amlodipine in this model is unknown but may be partially related to a drug-induced increase in coronary collateral blood flow.

Prostaglandin E1 attenuates postischemic contractile dysfunction after brief coronary occlusion and reperfusion

We have previously demonstrated that administration of the prostacyclin analogue iloprost improved postischemic functional recovery in reversibly injured ischemic-reperfused myocardium. The present study investigated the effects of administering an endogenous vasodilator prostanoid, prostaglandin E1 (PGE1), in the stunned myocardium (15 minutes of coronary artery occlusion and 3 hours of reperfusion) of anesthetized dogs. The percentage of regional myocardial segment shortening (%SS) after administration of PGE1 by two routes, intravenously (1 microgram/kg/min) or intraatrially (0.1 microgram/kg/min), to avoid pulmonary metabolism, 15 minutes before and throughout the period of occlusion, was compared to %SS in a control group treated with saline solution. Nearly equivalent reductions in mean arterial pressure during occlusion compared to pretreatment control (PTC) values were produced by intravenous (33%) or intraatrial (25%) PGE1. There was no difference in transmural myocardial blood flow (radioactive microsphere technique) in the ischemic region between the PGE1-treated and control groups at any time. Although there were no differences in %SS in the nonischemic region between groups throughout the experiment, postischemic recovery of segment function in the ischemic-reperfused area was significantly improved (p less than 0.05) at all times during reperfusion by intravenous PGE1 (%SS of PTC: 30 minutes = 65 +/- 8; 3 hours = 58 +/- 7) or intraatrial PGE1 (%SS of PTC: 30 minutes = 57 +/- 12; 3 hours = 50 +/- 4) compared to the control group (%SS of PTC: 30 minutes = 25 +/- 13; 3 hours = 10 +/- 13). Thus treatment with PGE1 attenuates postischemic contractile dysfunction in the stunned myocardium.2+ both.

Selective thromboxane A2 receptor blockade in experimental exercise-induced myocardial ischaemia in dogs

Thromboxane A2 receptor stimulation induces blood platelet aggregation and vasoconstriction, both potential causes of impaired perfusion of ischaemic myocardium. To study the potential role of thromboxane A2 receptor blockade in exercise-induced myocardial ischaemia and post-exercise myocardial dysfunction, nine conscious chronically instrumented dogs with single-vessel coronary artery stenosis (ameroid constrictor) were studied before, during and after steady-state treadmill runs which induced regional myocardial ischaemia. Three hours after a control run, the dogs were exercised again after the infusion of a selective thromboxane A2 receptor blocker: BM 13.177 (10 mg kg-1 i.v.). In the control run, systolic wall thickening (WTh, sonomicrometer) in the post-stenotic myocardium decreased from 22.1 +/- 9.1% at rest to 8.8 +/- 5.2% (mean +/- SD). Subendocardial blood flow (microspheres) in the ischaemic area decreased from 0.75 +/- 0.25 to 0.45 +/- 0.27 (ml min-1 g). The WTh in the ischaemic region remained depressed at 20 min after the run. BM 13.177 reduced peak left ventricular (+) dP/dt (micromanometer) and WTh in both control and post-stenotic myocardium at rest, during and after the run. WTh in the ischaemic area was reduced to approximately the same levels during running with BM 13.177 (not significantly different from control exercise) and remained depressed for at least 30 min after the run. Regional myocardial blood flow was not affected by BM 13.177. Thus, selective thromboxane A2 receptor blockade with BM 13.177 had a modest negative inotropic effect and did not improve regional function or blood flow in post-stenotic ischaemic subendocardium.