Ischemic Regions Of Myocardium Research Articles

Melatonin Treatment Does Not Modify Ectopic Activity During Ischemia and Reperfusion in Rats

Introduction Ventricular tachycardia and/or ventricular fibrillation (VT/VF), a life-threatening complication of acute myocardial ischemia are based on the interaction between a vulnerable arrhythmogenic substrate and an arrhythmic trigger. Melatonin was proposed as a potential antiarrhythmic medication and was shown to ameliorate the arrhythmogenic substrate. Also, melatonin provides a sympatholytic effect and thereby can attenuate ectopic activity, which provides the triggers for VT/VF. However, little is known about a melatonin effect on the arrhythmic triggering. In the present study, we aimed at evaluation of the melatonin effects on catecholamine synthesis in normal and ischemic myocardium and heart rhythm disturbances during an episode of ischemia and reperfusion. Methods Experiments were done in 30 control and 32 melatonin-treated (10 mg/kg, daily, for 7 days) male rats. The hearts from 10 control and 10 treated animals were taken for histochemical analysis of sympathetic fibers (glyoxylic acid-induced fluorescence). Continuous ECG recording was performed in 20 control and 22 treated anesthetized animals before and during an ischemia-reperfusion episode induced by reversible 5-min occlusion of the left anterior descending coronary artery. Tyrosine hydroxylase expression in normal and ischemic myocardial regions was assessed by Western blotting analysis in 12 control and 12 melatonin-treated animals. Results No differences in the state of sympathetic innervation (density and fluorescence intensity) were observed in histochemical analysis. However, Western blotting demonstrated that melatonin treatment suppressed tyrosine hydroxylase expression in normal (p<0.05 vs control) but not in ischemic regions of myocardium. Melatonin-treated animals had longer RR-intervals in the baseline state, but this difference progressively disappeared during the period of ischemia due to tachycardic response to ischemia in the treated group (Figure). No differences in the number of extrasystoles were found between the control and treated groups during ischemia and reperfusion periods (Figure). Conclusion Chronic melatonin treatment led to the peripheral sympatholytic effect in myocardium, which was associated with decreased heart rate in preischemic conditions. However, this effect attenuated during ischemia and no differences in heart rate or extrasystolic burden were found between the control and treated animals during ischemia and reperfusion.

Melatonin pretreatment does not modify extrasystolic burden in the rat ischemia-reperfusion model.

The mechanism of reentrant ventricular tachyarrhythmias complicating acute myocardial ischemia is largely based on the interaction between an arrhythmogenic substrate and triggers. Melatonin was proposed as an antiarrhythmic medication and was shown to ameliorate the arrhythmogenic substrate. Also, melatonin provides a sympatholytic effect in different settings and might attenuate ectopic activity, which provides reentry triggers. In the present study, we aimed at evaluating the melatonin effects on cardiac sympathetic activity and the incidence of premature ventricular beats during the episode of ischemia-reperfusion. Experiments were done in a total of 26 control and 28 melatonin-treated (10 mg/kg, daily, for 7 days) male rats. Sympathetic fibers density was assessed by glyoxylic acid-induced fluorescence. Continuous electrocardiograms recording was performed during ischemia-reperfusion episodes (5 min/5 min, respectively) induced by reversible coronary occlusion. Myocardial expression of tyrosine hydroxylase, a rate-limiting enzyme of catecholamine biosynthesis was assessed by Western blotting. No differences in the state of sympathetic innervation were observed in histochemical analysis. However, Western blotting analysis demonstrated that melatonin treatment suppressed tyrosine hydroxylase expression in the non-ischemic (p < 0.05 versus control) but not ischemic regions of myocardium. The melatonin-treated animals had longer RR-intervals in the baseline state than the control animals (264 ± 48 ms versus 237 ± 33 ms, p = 0.044, respectively), but this difference decayed during the period of ischemia due to the increase of heart rate in the treated group. The number of premature ventricular beats did not differ between the control and treated groups during the ischemic and reperfusion periods. One-week melatonin pretreatment caused a slight peripheral sympatholytic effect that attenuated during ischemia and completely disappeared by the onset of reperfusion. The slight expression of sympathetic downregulation was associated with the lack of any effect of melatonin on extrasystolic burden. Collectively, the data suggest that melatonin cannot target the triggers of reentrant arrhythmias.

Abstract 12911: Metformin Preserves ATP Concentration, Attenuates Action Potential Shortening, and Prevents Ventricular Fibrillation During Low-flow Ischemia in Normal Hearts

Introduction: Metformin (MET) is the most common drug used to treat type 2 diabetes, in part because it is believed to reduce cardiovascular mortality. However, it is unknown whether MET provides protection from ischemic ventricular fibrillation (VF) in the normal heart. In a region of ischemic myocardium, reduced energy charge causes ATP-sensitive potassium channels to open, shortening the action potential (AP). The resulting dispersion of AP duration and refractoriness may predispose to VF. We determined effects of MET on myocardial ATP concentration, AP, and VF during regional ischemia in metabolically normal pigs. Methods: Normal domestic pigs were treated with a clinically relevant dose of MET (30 mg/kg/d, n=25) for 2-3 weeks and compared with untreated controls (CTL, n=23). Under anesthesized, open-chest conditions, phospho-AMP activated protein kinase (pAMPK, Thr172) was measured in myocardial biopsies taken before ischemia (n=8 each group). Low-flow ischemia was induced by 50% LAD flow reduction. [ATP] was measured in myocardial biopsies at 15 min ischemia (MET n=8, CTL n=7). Monophasic AP was recorded at baseline and at 15 min ischemia. Survival free of VF was observed during 90 min of ischemia and 45 min reperfusion. Results: pAMPK was 85% greater in MET than in CTL (p&lt;0.001), establishing a myocardial effect of MET prior to ischemia. MET had no effect on non-ischemic AP duration. At 15 min ischemia, [ATP] was higher in MET than CTL (4.1 vs 3.5 μmol/g, p=0.039), with less AP shortening from baseline in MET (4±2%) than in CTL (13±2%, p&lt;0.001). Survival free of VF was greater in MET pigs than CTL (Figure). Conclusions: Chronic treatment with a clinically relevant dose of MET activates AMPK, preserves [ATP] and prevents AP shortening during ischemia, and diminishes the propensity for ischemic VF in metabolically normal pigs. Based on these findings, MET may have therapeutic utility beyond type 2 diabetes, extending to ischemic protection of the normal heart.

Use of an intracoronary Doppler guidewire for evaluation of coronary hemodynamics in the porcine model of acute hibernating myocardium during dobutamine stress tests

To determine the coronary hemodynamic characteristics of acute hibernating myocardium (AHM), evaluate the changes in coronary hemodynamics during dobutamine infusion, and investigate the mechanisms by which dobutamine stress echocardiography (DSE) detects AHM. The porcine model of acute hibernating myocardium was created in 10 animals, all of which underwent DSE with doses of 0-40 μg/kg/min. Myocardial segments abnormality was used as a DSE criterion for evaluating AHM. An intracoronary Doppler guidewire was used to measure the coronary hemodynamics; electrocardiography and systemic hemodynamics were recorded simultaneously. The ischemic regions of myocardium were reperfused, and all variables were recorded. Finally, the animals were euthanized and pathologic changes in the heart tissue were documented. There was no myocardium necrosis. There were 55 myocardial segments with abnormal DSE responses after stenosis, among which 41 segments were judged as AHM because of a biphasic response. Average peak velocity (APV) and coronary flow velocity reserve were improved during DSE. Coronary and systemic hemodynamics were increased during dobutamine infusion. There were significant differences for APV at all evaluated doses and for heart rate blood pressure product at higher doses. The difference between APV resting values and the values for peak dosage (ΔAPV) correlated with the amount of AHM during DSE. Coronary volume blood flow and coronary flow velocity reserve decreased after stenosis. ΔAPV may reflect the number of hibernating segments. The relative imbalance between blood supply and oxygen consumption in regional myocardium may be one of the mechanisms by which DSE detects AHM.

Biologic bypass with the use of adenovirus-mediated gene transfer of the complementary deoxyribonucleic acid for vascular endothelial growth factor 121 improves myocardial perfusion and function in the ischemic porcine heart

Objectives: Vascular endothelial growth factor (VEGF), a potent angiogenic mediator, can be delivered to targeted tissues by means of a replication-deficient adenovirus (Ad) vector. We hypothesized that direct administration of Ad vector expressing the VEGF 121 complementary deoxyribonucleic acid (Ad GVVEGF121.10) into regions of ischemic myocardium would enhance collateral vessel formation and improve regional perfusion and function. Methods: Yorkshire swine underwent thoracotomy and placement of an Ameroid constrictor (Research Instruments & MFG, Corvallis, Ore.) on the circumflex coronary artery. Three weeks later, myocardial perfusion and function were assessed by single photon emission computed tomography imaging (SPECT) with 99 mTc-labeled sestamibi and by echocardiography during rest and stress. Ad GVVEGF121.10 ( n = 7) or the control vector, AdNull ( n = 8), was administered directly into the myocardium at 10 sites in the circumflex distribution (10 8 pfu/site). Four weeks later, these studies were repeated and ex vivo angiography was performed. Results: SPECT imaging 4 weeks after vector administration demonstrated significant reduction in the ischemic area at stress in Ad GVVEFG121.10-treated animals compared with AdNull control animals ( p = 0.005). Stress echocardiography at the same time demonstrated improved segmental wall thickening in Ad GVVEGF121.10 animals compared with AdNull control animals ( p = 0.03), with Ad GVVEGF121.10 animals showing nearly normalized function in the circumflex distribution. Collateral vessel development assessed by angiography was also significantly greater in Ad GVVEGF121.10 animals than in AdNull control animals ( p = 0.04), with almost complete reconstitution of circumflex filling in Ad GVVEGF121.10 animals. Conclusions: An Ad vector expressing the VEGF 121 cDNA induces collateral vessel development in ischemic myocardium and results in significant improvement in both myocardial perfusion and function. Such a strategy may be useful in patients with ischemic heart disease in whom complete revascularization is not possible. (J Thorac Cardiovasc Surg 1998;115:168–77)

Expression of E-selectin on coronary endothelium after myocardial ischemia and reperfusion.

Neutrophils localize in ischemic or infarcted myocardium and thus are implicated in playing a role in myocardial reperfusion injury and stunning. We studied one of the mechanisms by which neutrophils are recruited into the region of ischemic myocardium. Anesthetized adult rhesus monkeys (n = 2) underwent ligation of one of the obtuse marginal coronary arteries for 90 minutes followed by 5 hours of reperfusion. Tissues from the normal perfused area of the heart and from the ischemic area were preserved in methacarn fixative after sacrificing the animal at the end of the reperfusion period. Immunohistochemical staining showed that E-selectin is not constitutively expressed on coronary endothelium in vivo. E-selectin, however, was upregulated in selective endothelial cells located in the postcapillary coronary venules in response to ischemia and reperfusion. The presence of E-selectin in this setting suggests that it may have a similar role as in the recruitment of neutrophils into other inflamed or damaged tissues.

Inhibition of adenosine-mediated coronary vasodilation exacerbates myocardial ischemia during exercise

Persisting coronary vasoconstrictor tone that is responsive to exogenous adenosine administration has been demonstrated during myocardial ischemia. Therefore, the role and extent of endogenous adenosine-mediated coronary vasodilation in opposing coronary vasoconstriction within regions of ischemic myocardium was investigated in 10 chronically instrumented exercising dogs. Studies were performed on dogs with left circumflex coronary artery stenosis during treadmill exercise (6.5 km/h, 6% grade), while myocardial blood flow was measured with radioactive microspheres. Blood flow was measured before and again after inhibition of the effects of endogenously produced adenosine through combined inactivation of adenosine and adenosine receptor antagonism by the administration of intracoronary adenosine deaminase (ADA) (5 micrograms.kg-1 x min-1 x 10 min) plus 8-phenyltheophylline (8-PT) (5 mg/kg i.v.), respectively. Coronary perfusion pressure was held equal during both conditions at approximately 41 mmHg with a hydraulic occluder. During exercise in the presence of a coronary stenosis, blood flow was reduced in all layers of myocardium in regions supplied by the stenosed left circumflex coronary artery compared with blood flow in regions of myocardium supplied by the nonstenotic left anterior descending coronary artery. After ADA plus 8-PT, myocardial blood flow (in ml.min-1 x g-1) was further reduced in all layers of myocardium in regions supplied by the stenotic left circumflex coronary artery compared with baseline (subendocardial layer 0.44 +/- 0.09 vs. 0.67 +/- 0.13 ml.min-1 x g-1, mean transmural flow 0.92 +/- 0.13 vs. 1.25 +/- 0.2 ml.min-1 x g-1, both P < 0.05). Blood flow in regions of myocardium supplied by the nonstenotic left anterior descending coronary artery were unchanged following ADA plus 8-PT.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of photoproducts, free radicals, and particulate debris generated during in vivo argon laser myoplasty

Catheter-guided laser myoplasty in a closed ventricle has been advocated for the treatment of hypertrophic cardiomyopathy, ablation of arrhythmogenic foci, and transmyocardial laser revascularization of ungraftable regions of ischemic myocardium. The purpose of this study was to evaluate the nature of particulate debris and photoproducts generated in vivo. Accordingly, cardiopulmonary bypass was established in four dogs without active cooling and an apical left ventricular vent was placed. In two dogs, a laser catheter was inserted into the cardioplegia-arrested left ventricle through the left atrial appendage and across the mitral valve. In the other two dogs the laser catheter was inserted into a perfused, fibrillating left ventricle retrograde across the aortic valve. Myocardial ablation of the ventricular septum was performed using continuous argon-ion laser irradiation (8-10 watts, 4-11 minutes) and blood samples were drawn through the ventricular vent. The final ablation in each dog was done during a separate ventricular flush with a chemical "spin trap" capable of detecting free radicals. Analysis of ventricular effluent revealed levels of carboxyhemoglobin and hydrogen cyanide which were well below accepted clinical toxic limits. Microparticulate debris (2-300 femtoliter) was not detected. Evidence of trace free radical generation was suggested in one of four dogs. In conclusion, closed ventricular argon laser myoplasty did not generate clinically significant levels of the toxic photoproducts we measured, or microparticulate debris. These in vivo findings suggest that the clinical application of catheter-based laser myoplasty will not be limited by the generation of these toxic photoproducts or microembolic debris.

Selective accumulation of the first component of complement and leukocytes in ischemic canine heart muscle. A possible initiator of an extra myocardial mechanism of ischemic injury.

Myocardial concentrations of C1q, a subunit of the first component of complement, were measured 5-120 minutes after ligation of a coronary artery in dogs injected with 125I-labeled human C1q and 131I-labeled human albumin. The 131I-labeled human serum albumin was used as a plasma protein marker. Ischemic regions of myocardium were defined by measuring regional myocardial blood flow by the reference sample method at intervals after coronary artery occlusion. Significant accumulations of 125I-C1q were demonstrated in the ischemic myocardium after coronary artery occlusions lasting 45 minutes. Some localization of C1q in ischemic myocardium was observed after a 15-minute occlusion, but the accumulations of C1q achieved in this case were not statistically significant. After coronary artery occlusions lasting greater than or equal to 45 minutes, left ventricular concentrations of C1q correlated reciprocally with regional myocardial blood flow. Moreover, high concentrations of C1q persisted in formerly ischemic segments after reperfusion. Radiolabeled neutrophils also accumulated selectively in ischemic segments relatively rich in C1q. It is suggested that complement activation may initiate the neutrophil-dependent portion of ischemic injury, delineated in recent years, that is associated with free radical release by phagocytic cells.

Gated magnetic resonance imaging of acute myocardial ischemia in dogs: Application of multiecho techniques and contrast enhancement with Gd DTPA

ECG gated magnetic resonance images were obtained in six canines prior to and immediately following occlusion of either the LAD or circumflex coronary artery using a surgically placed snare. Multiecho and single-echo acquisition techniques were utilized. 0.25 mmol/kg Gd DTPA was injected as an IV bolus 1 hr following coronary artery ligation. In two animals, the region of ischemic myocardium was clearly visualized on multiecho technique without the use of intravenous contrast. The ischemic zone could be best identified on images with a long TE of 120 msec. Contrast enhancement with Gd DTPA enabled visualization of the ischemic myocardium in all six canines. Administration of Gd DTPA, a perfusion agent, improved both detectability and definition of the myocardial lesions.

Evidence for a flow-independent contribution to the phenomenon of thallium redistribution

Although thallium-201 is known to redistribute slowly into regions of ischemic myocardium after restoration of blood flow, it is not clear to what extent normalization of flow is an essential requirement for the redistribution process. In a search for a flow-independent component of thallium redistribution, 12 dogs with stenosis of the circumflex coronary artery underwent atrial pacing for either 20 minutes (group I, 6 dogs) or 2 hours (group II, 6 dogs). Radioactive thallium and radioactive microspheres, 7 to 10 μ, were injected after 10 minutes of atrial pacing in both groups. Pacing resulted in a 40 percent reduction in subendocardial blood flow to the circumflex-perfused myocardium in both groups I and II. This relative reduction in flow was maintained at a stable level over the 2 hour pacing period in group II. Thallium activity in the relatively ischemic zone was significantly greater in dogs with 2 hours of pacing (group II) than in those with 10 minutes of pacing (group I). Redistribution of thallium occurred despite the continued presence of reduced flow in circumflex-perfused endocardial tissue. These data suggest that a significant component of thallium redistribution may be flow-independent.

Inhibition of endo- and epicardial glycogenolysis by propranolol in ischemic hearts

The effect of coronary artery ligation on myocardial glyocogenolysis was studied in the endo- and epicardial layers of the left ventricle in dogs pretreated with saline or 1 mg/kg of propranolol. Coronary artery ligation was performed by ligating one of the small branches of the left anterior descending coronary artery. Even after coronary artery ligation, neither increase in phosphorylase activity nor breakdown of glycogen was observed in both layers of ischemic region of myocardium in propranolol-pretreated dogs. These results indicate that pretreatment with propranolol inhibits the increase in glycogenolysis being caused by coronary artery ligation. Propranolol howefer, did not inhibit completely the coronary artery ligation-induced increase in glucose 6-phosphate and lactate and decrease in phosphocreatine in the myocardium, especially in the endocardial layers.

Isosorbide dinitrate during acute coronary occlusion. Effects in the baboon.

THE USE OF coronary vasodilators in the treatment of acute myocardial infarction has been considered by some authors to be physiologically unsound, since hypotension frequently accompanies their use. 1 Seemingly, if a specific coronary vasodilator were available to adequately dilate coronary vessels without an associated systemic hypotension, such a drug would be both beneficial and useful. The supply of oxygenated blood provided to a region of ischemic myocardium through collaterals probably could be increased, and the danger of ventricular fibrillation lessened. In spite of the paradoxical lowering of blood pressure with the use of nitrates, a few authors continue to advocate their use in humans during acute myocardial infarction. Isosorbide dinitrate (Isordil) is claimed to be a potent coronary vasodilator, and, because of its rapid absorption and prolonged action appears potentially more useful than some other nitrates. 2,3 Hedges et al 4 recently suggested possible benefits with isosorbide dinitrate in