Cellular lipid accumulation in different regions of myocardial infarcts in cats during beta adrenergic blockade with timolol

The effects of non-selective beta adrenergic blockade on intracellular lipid accumulation in hearts with acute ischaemia were studied by electron microscopy of myocardial biopsy specimens using quantitative stereological techniques. Pentobarbital anaesthetised cats with coronary ligation were divided into eight controls and eight cats treated with timolol intravenously just before ligation. Biopsy specimens were collected from ischaemic, borderline, and normally perfused myocardium, defined by an in vivo injection of fluorescein and verified by regional myocardial blood flow measurements with 15 microns radiolabelled microspheres. During a 3 h occlusion period timolol treated cats had a lower heart rate, left ventricular dP/dt, and plasma free fatty acid concentration. In control cats the cytosolic volume fraction of lipids was 0.71 X 10(-3) in non-ischaemic myocardium, 2.63 X 10(-3) in central ischaemic tissue, and 6.53 X 10(-3) in borderline tissue. Timolol reduced the appreciable lipid accumulation in borderline tissue by 24% (to 4.97 X 10(-3)) compared with controls, whereas accumulation in central ischaemic tissue was not affected. Thus timolol diminished lipid accumulation in borderline myocardial tissue. The mechanism is most likely related to reduced ischaemic intensity and better preserved metabolic function.     

Is reduced cardiac performance the only mechanism for myocardial infarct size reduction during beta adrenergic blockade?

Equal reductions in heart rate (44 beats X min-1) were obtained in cats by treatment with either the beta blocking agent timolol or alinidine, an agent claimed to cause bradycardia without interfering with beta adrenoceptor function. Infarct size was measured by staining with triphenyltetrazolium-chloride after 5 h of coronary occlusion and related to the area of hypoperfused myocardium as measured by autoradiography. Regional myocardial blood flow was measured by 15 micron radiolabelled microspheres. Compared with the control cats, in whom 87.4 (SEM 2.2)% of hypoperfused myocardium developed into necrosis, timolol reduced infarct size to 65.8 (SEM 2.6)% (p less than 0.001) and alinidine to 76.2 (SEM 3.1)% (p less than 0.01) of the hypoperfused area. Timolol reduced infarct size more than did alinidine (p less than 0.01). Necrosis was more extensive in the endocardium than in the epicardium in all groups. In the subendocardium timolol and alinidine reduced infarct size to the same extent, whereas timolol reduced infarct size more than alinidine in the subepicardium. Although heart rate proved to be the dominant haemodynamic predictor of infarct size, this study indicates that mechanisms other than reduced oxygen demand associated with bradycardia and cardiodepression are operating in the ischaemic myocardium during beta adrenergic blockade.     

Effect of prostacyclin on the severity of ischaemic injury in rabbit hearts subjected to coronary ligation

The hypothesis that prostacyclin (PGI2) might have a direct cytoprotective action in ischaemic cardiac tissue was investigated. Myocardial ischaemia was induced in perfused rabbit hearts by ligating the left main coronary artery. Coronary flow, oxygen uptake, and turnover of lactate and purines were measured before and up to 120 min after coronary occlusion. After this, ischaemic tissue was separated from perfused myocardium, and levels of lactate, adenine nucleotides and creatine phosphate were determined in specimens from non ischaemic, ischaemic and border zones. PGI2 (final conc. 10(-7) M) was infused before or 30 min after ligation and the results were compared to those in control hearts. Coronary ligation reduced coronary flow and oxygen consumption by about 50%. The fractional extraction of lactate decreased from 20% to close to zero and purine release increased 5-fold. In the non-ischaemic area the tissue levels of ATP and creatine phosphate were high, with a low content of lactate, but in the ischaemic area the levels of ATP and creatine phosphate were considerably reduced and the content of lactate was high. Although coronary flow and oxygen uptake were elevated after treatment with PGI2, no change in lactate or purine turnover was observed. Neither the weight of the non-perfused myocardium nor the tissue levels of the adenine nucleotides, creatine phosphate and lactate were affected by PGI2 treatment. The data indicate that in this model, in which effects on cardiac work, collateral flow and platelets are eliminated, PGI2 does not limit ischaemic myocardial injury. Hence, the hypothesis of a direct cytoprotective action of PGI2 in ischaemic myocardial tissue was not supported.     

Metabolic and flow correlates of myocardial ischaemia

Seven hundred and twenty three biopsies were obtained from 20 dogs after coronary artery ligation for 5, 30, 45, 60 or 120 min (n = 4 dogs for each group). Paired values for blood flow (radioactive microspheres) and tissue ATP content were obtained for each biopsy and related to the duration of ischaemia. Three states of ischaemic injury could be recognised. In the first, designated as "tolerable" ischaemia, coronary flow was reduced by up to 50%. In this flow band, ATP depletion was relatively small and time-independent. If flow was reduced by 60 to 80%, a state of "critical" ischaemia was identified where ATP depletion was both flow- and time-dependent and, in this relatively narrow range, small changes in flow or duration could result in major changes in ATP depletion. With severe flow reductions of greater than 80%, designated as "lethal" ischaemia, a complex pattern emerged such that with up to 30 min of ischaemia, ATP fell progressively with increasing time and flow deprivation. Between 30 and 45 min ATP depletion accelerated and beyond 45 min the time-dependency disappeared with tissue ATP content remaining relatively constant at a severely depressed level for several hours. All of these results are discussed in the light of earlier proposals (disputed) that tissue injury as expressed by ATP depletion can be predicted by the product of ischaemic duration and flow deprivation.     

The effect of coronary artery occlusion and reperfusion on the activities of triglyceride lipase and glycerol 3-phosphate acyl transferase in the isolated perfused rat heart

Glycerol 3-phosphate acyltransferase (GPAT) and Triglyceride lipase (TGL) were measured in homogenates from non-ischaemic and ischaemic tissue from the isolated perfused rat heart. Ischaemia was produced by occlusion of the left descending coronary artery for 10 min. Compared to activities measured in tissue from normally perfused hearts, GPAT activity measured in tissue from the ischaemic area was considerably reduced. TGL activity in the ischaemic area was markedly increased compared to activity measured in normally perfused hearts. No change was seen in GPAT or TGL activity measured in tissue from the non-ischaemic area. The change in activities produced by ischaemia were prevented by pre-perfusion with the cardio-selective beta-antagonist Atenolol. Reperfusion of the ischaemic area resulted in TGL activity returning to the value measured in tissue from normally perfused hearts. However, GPAT activity, after 1 min of reperfusion, fell to a value lower than after 10 min ischaemia. The reperfusion-induced fall in GPAT activity was prevented by pre-perfusion with the alpha 1 antagonist Doxasozin. Pre-perfusion of the alpha 2 antagonist Yohimbine resulted in a prolongation of the increased TGL activity in the ischaemic area during reperfusion. All changes in enzyme activities were prevented by injection of 6 OH-dopamine 24 h before hearts were removed. These changes in enzyme activities show that during ischaemia there is an increased beta-adrenergic drive. On reperfusion the beta-adrenergic drive is removed but an alpha 1 adrenergic drive becomes apparent.     

Chronic oral pretreatment with the angiotensin converting enzyme inhibitor, trandolapril decreases ventricular fibrillation in acute ischaemia and reperfusion

The effect of chronic angiotensin converting enzyme (ACE) inhibitionon the incidence of fatal ventricular fibrillation during regionalmyocardial ischaemia and reperfusion is not known. A reductionin cardiac angiotensin II and/or a vagomimetic response mayresult in antiarrhythmic activity. Pigs pre-treated with theACE inhibitor trandolapril 0.3 mg. kg^–1 or placebo orallyfor 10 days were subjected to thoracotomy. The left anteriordescending coronary artery was ligated (CAL) for 20 min andreperfused for 45 min. Trandolapril decreased cardiac ACE activityand prevented the fall in the ventricular fibrillation threshold(VFT) during ischaemia: after 5 min of ischaemia the VET inthe trandolapril group was 25.5 ± 4.6 mA (mean ±SEM) vs 13.1 ± 2.2 mA in the placebo group (P<0.05).Trandolapril also decreased the incidence of spontaneous ventricularfibrillation during reperfusion to 116 vs 617 in the placebogroup (P<0.05). Heart rate in the trandolapril group fell.During ischaemia, trandolapril increased blood flow in the non-ischaemiczone, but decreased blood flow in the central ischaemic zoneof the left ventricle. Similarly it decreased tissue levelsof phosphocreatine in this zone. During reperfusion trandolaprilincreased blood flow both in non-ischaemic and central ischaemiczones. Chronic oral pre-treatment with trandolapril resultedin marked antifibrillatory effects which were associated withinhibition of cardiac ACE activity and a decreased heart rate.     

Locally targeted cytoprotection with dextran sulfate attenuates experimental porcine myocardial ischaemia/reperfusion injury.

AIMS: Intravascular inflammatory events during ischaemia/reperfusion injury following coronary angioplasty alter and denudate the endothelium of its natural anticoagulant heparan sulfate proteoglycan (HSPG) layer, contributing to myocardial tissue damage. We propose that locally targeted cytoprotection of ischaemic myocardium with the glycosaminoglycan analogue dextran sulfate (DXS, MW 5000) may protect damaged tissue from reperfusion injury by functional restoration of HSPG. METHODS AND RESULTS: In a closed chest porcine model of acute myocardial ischaemia/reperfusion injury (60 min ischaemia, 120 min reperfusion), DXS was administered intracoronarily into the area at risk 5 min prior to reperfusion. Despite similar areas at risk in both groups (39+/-8% and 42+/-9% of left ventricular mass), DXS significantly decreased myocardial infarct size from 61+/-12% of the area at risk for vehicle controls to 39+/-14%. Cardioprotection correlated with reduced cardiac enzyme release creatine kinase (CK-MB, troponin-I). DXS abrogated myocardial complement deposition and substantially decreased vascular expression of pro-coagulant tissue factor in ischaemic myocardium. DXS binding, detected using fluorescein-labelled agent, localized to ischaemically damaged blood vessels/myocardium and correlated with reduced vascular staining of HSPG. CONCLUSION: The significant cardioprotection obtained through targeted cytoprotection of ischaemic tissue prior to reperfusion in this model of acute myocardial infarction suggests a possible role for the local modulation of vascular inflammation by glycosaminoglycan analogues as a novel therapy to reduce reperfusion injury.     

Continuous monitoring of energy metabolites using microdialysis during myocardial ischaemia in the pig

A diagonal branch of the left anterior descending artery wasoccluded for 80 min. One microdialysis probe was inserted inthe ischaemic area and two in the non-iscliaemic areas. In sixanimals radiolabelled microspheres were injected before andduring ischaemia and after reperfusion to monitor flow. Arterialand pulmonary artery pressures, pulmonary capillary wedge pressureand cardiac output were followed-up. Radiolabelled microspheredistribution confirmed the reduction of myocardial blood flowduring coronary artery occlusion, with flow returning to normalvalues upon reperfusion. Rate pressure product as an index of heart work was stable throughoutthe 80 min ischaemic period, but increased at reperfusion. Dialysatesfrom non-iscltaemic tissue showed stable baseline values. Inischaemic tissue, lactate increased by 300% and pyruvate levelsdecreased by 50% within 15 min. Adenosine increased rapidlyfive-fold, but started to decline after 15 min of ischaemia,Inosine showed a slower but a marked 20-fold increase, decreasingat the end of the ischaemic period. Hypoxanthine increased five-foldduring the first 30 min. Finally, guanosine showed a slow increaseto about four times above basic values. Microdialysis was an excellent tool for the continuous monitoringof myocardial metabolites during ischaemia. Adenosine appearedto be a more sensitive marker of early iscliaemia than lactate,although interestingly, adenosine levels had decreased alreadyafter 15 min, most probably due to intracellular accumulationof protons and Pi.     

Ischaemic preconditioning and contractile function: studies with normothermic and hypothermic global ischaemia.

A significant reduction in the extent of cell necrosis or the incidence of reperfusion-induced arrhythmias can be achieved with ischaemic preconditioning. If preconditioning was also found to be effective in protecting against global ischaemia, then this may have significant implications for the preservation of the heart during cardiac surgery. We therefore investigated this phenomenon in relation to recovery of contractile function after global ischaemia in the isolated rat heart. Isolated working rat hearts (n = 6 per group) were perfused aerobically at 37 degrees C for 20 min and contractile function recorded. This was followed by 10 min of aerobic Langendorff perfusion (control hearts) or 5 min global ischaemia (37 degrees C) + 5 min Langendorff reperfusion (preconditioned hearts). The hearts were then subjected to 10, 15, 20 or 25 min of global ischaemia (37 degrees C) and reperfusion (15 min Langendorff + 20 min working) after which function was again assessed. Preconditioning improved functional recovery after all durations of ischaemia. Thus aortic flow after 10, 15, 20 and 25 min of ischaemia and 35 min of reperfusion recovered to 84, 58, 16 and 5%, respectively, in controls and 88, 74, 55 and 20%, respectively, in the preconditioned groups. To assess whether preconditioning was effective in a surgically relevant model of hypothermic ischaemia, the experiments were repeated with longer periods (45, 70, 90, 115, 135 and 160 min) of ischaemia at 20 degrees C. Under these conditions, normothermic preconditioning increase the post-ischaemic recovery of aortic flow after 115, 135 and 160 min of ischaemic (from 36, 20 and 10%, respectively, in controls to 57, 39 and 26%, respectively, in preconditioned hearts). There was no consistent correlation between tissue high energy phosphate content and enhanced post-ischaemic recovery. Thus, we have demonstrated that ischaemic preconditioning can improve contractile function after global ischaemia in the isolated rat heart, we have defined the duration of ischaemia for which it is operative, and we have shown that this protection is additive to that of hypothermia-induced protection during ischaemia. This may have clinical implications for cardiac surgery.     

Comparison of the effect of ildamen and nonachlazin on the blood supply and activities of the heart

Instravenous infusion of ildamen (0.8-1 mg/kg) into anesthesized cats caused an increase in coronary blood flow and oxygen consumption by the heart to an approximately equal degree; the resistance of the coronary vessels diminished. The values of cardiac activity (cardiac output, contractions, pulse rate) increased simultaneously with increase of the coronary blood flow. In distinction to ildamen, nonachlasin (6 mg/kg) reduced the arterio-venous difference in oxygen and increased the content of oxyhemoglobin in the draining coronary blood. In anesthesized animals cardiac activity altered in two stages. Since beta-adrenergic blocking agents prevented the increase in coronary blood flow and the intensification of cardiac contractions induced by both drugs it is assumed that their effect is caused through stimulation of the beta-adrenergic structures. Besides, ildamen and nonachlasin possess the property of inhibiting cardiac activity. The combination of these two opposite effects evidently plays an important role in the realization of the antianginal effect of the substances studied.     

Dynamics of myocardial metabolism in the preconditioned porcine heart studied using continuous microdialysis

A microdialysis technique was used to measure myocardial catabolismof adenine nucleotides during preconditioning. Following thoracotomy in domestic pigs, a probe was insertedinto the ischaemic myocardium and two probes into normal myocardium.Preconditioning was instituted by occluding a diagonal branchof the left anterior descending coronary artery four times overa 10 min period (interspersed with 20 min of reperfusion eachtime), followed by index ischaemia for 40 min and terminatedby 20 min of reperfusion. Nonpreconditioned animals, which hadbeen subjected to 40 min ischaemia and 20 min of reperfusionwere used as controls. Sham operated animals were additionalcontrols. Following equilibration, the dialysis fluid was continuouslypumped and the microdialysate analysed at 5 min intervals. Inthe dialysate, the amount of lactate, pyruvate, adenosine, inosine,hypoxanthine and guanosine was measured Circulatory parameterswere: rate pressure product, pulmonary artery pressures, cardiacoutput and pulmonary wedge pressure. There was no significant statistical difference in cardiac workbetween preconditioned pigs and controls. A sharp rise in adenosinewas observed during the first short ischaemic period with attenuationduring the following three short ischaemic periods. In the microdialysate,lactate, adenosine, inosine, hypoxanthine and guanosine weresignificantly lower during index ischaemia in the preconditionedgroup than in the non-preconditioned group. Pyruvate levels decreased during preconditioning and rose toa higher level after each brief ischaemic period, followed bya decrease, which was proportionally similar in the preconditionedand unpreconditioned animals during index ischaemia. The effect of preconditioning on lactate and catabolite levelsindicated a powerful inhibition of adenine nucleotide catabolicprocesses due to ischaemia.     

Involvement of neutrophils in postischaemic damage to the small intestine.

Haemorrhagic mucosal lesions are produced during intestinal ischaemia and after reperfusion probably mediated by oxygen radicals. Oxygen radicals react with cell membrane lipids and induce cell damage by peroxidation and induce accumulation of polymorphonuclear leucocytes in the tissue. The aim of the study was to elucidate the involvement of polymorphonuclear leucocytes in post-ischaemic intestinal damage. Intestinal ischaemia was induced in cats by partial occlusion of the superior mesenteric artery. Samples from the small intestine were excised before and at the end of the two hour hypotensive period as well as 10 minutes and 60 minutes after reperfusion. Conjugated dienes, myeloperoxidase, and the purine metabolites were determined in the samples. The tissue was also examined histologically. Seven cats were treated before reperfusion with a monoclonal antibody (IB4) which inhibits leucocyte adherence to endothelial cells and its subsequent activation. After reperfusion myeloperoxidase activity increased and the ischaemic mucosal lesions worsened significantly. IB4 treatment prevented an increase in post-hypotensive myeloperoxidase activity and attenuated the normally observed severe mucosal lesions. We conclude that the severe post-ischaemic lesions are induced by polymorphonuclear leucocytes. Such mucosal injury may be appreciably reduced by blocking leucocyte adherence with IB4.     

Intrathoracic ganglionic beta-adrenergic receptors involved in efferent sympathetic regulation of the canine heart

The enhancement of cardiac inotropism induced by electrical stimulation of acutely decentralized efferent preganglionic sympathetic axons was reduced, but still present, following intravenous hexamethonium administration (10 mg/kg). This residual enhancement was eliminated following subsequent administration of timolol (0.1 mg in 0.1 mL normal saline) into the ipsilateral stellate and middle cervical ganglia. Similar results were obtained when the order of administration of these agents was reversed. When the beta-adrenergic agonist isoproterenol (1, 2 or 5 micrograms in 1 microL of normal saline) was administered into specific loci of an acutely decentralized stellate or middle cervical ganglion, cardiac chronotropism and/or inotropism were increased; when similar injections were made into adjacent ganglionic sites cardiac responses usually were not elicited. Following hexamethonium administration similar results were obtained indicating that these effects were not primarily dependent upon nicotinic cholinergic synapses. In contrast, repeat injections of isoproterenol into the site that initiated cardiac responses when injected with isoproterenol failed to elicit any cardiac responses following local administration of timolol into the same site indicating that the effects of isoproterenol could be blocked by a beta-adrenergic receptor blocking agent. Injections of 1 to 5 micrograms of isoproterenol into a ganglion which was surgically disconnected from the heart failed to elicit cardiac responses, demonstrating that at these doses detectable activation of cardiac myocyte beta-adrenergic receptors via circulating isoproterenol which had leaked out of the ganglion did not occur. It is concluded that beta-adrenergic receptors in intrathoracic ganglia are involved in the efferent sympathetic regulation of the heart and that such receptors are associated with neurons in specific ganglionic loci.     

Release of cardiac troponin I after temporally graded acute coronary ischaemia with electrocardiographic ST depression

BACKGROUND: Elevation of cardiac biochemical markers and ST segment depression in the electrocardiogram have important roles in the risk stratification of unstable coronary syndromes. We assessed graded duration of acute coronary ischaemia with ST depression versus release of cardiac troponin I (cTnI) and conventional cardiac markers in 15 ischaemic pigs and 11 controls. METHODS: Coronary ischaemia was induced via percutaneous technique by semiinflating an angioplasty balloon in the left circumflex artery. Blood velocity monitored by Doppler was reduced until ST depression > or =0.1 mV was obtained. Among 26 pigs, six controls had jugular vein sheath introduced only, five controls jugular vein and bilateral femoral sheaths, and 15 pigs were divided into three equal groups (n=5) in which ischaemia was maintained for 10, 20 and 30 min, respectively. RESULTS: Mean blood flow velocity (cm/s) at baseline was 16.3+/-6.5 and was reduced to 4.1+/-3.2 (25% of normal, range 20-29%) during ischaemia. cTnI (microg/l) did not increase in controls but increased from 0.05 to 0.52 (P<0.05) and 0.76 (P<0.05) with 10 and 20 min of ischaemia, and to 30.77 (P<0.05) with 30 min of ischaemia. A rise of myoglobin and conventional cardiac enzymes did not distinguish controls with arterial cut-down from the ischaemia groups. CONCLUSION: Release of cTnI depends on the duration of ST depression ischaemia. The critical time for a major release seems to be between 20 and 30 min. Thus, very early intervention in patients with prolonged ST depression ischemia should be focused on in future clinical trials.     

Ischaemic endothelial dysfunction after single or multidose cardioplegia.

The effect of ischaemia, either alone or after a single or multidose infusion of St Thomas' Hospital cardioplegia solution (ST), on endothelial function was studied by examining its influence on 5-Hydroxytryptamine- (5-HT-) and nitroglycerine- (GTN-) induced coronary vasodilation in the isolated rat heart. Eighty rat hearts were perfused on a modified Langendorff preparation. Thirty minutes of unprotected ischaemia (N = 16) abolished the vasodilatory response to 5-HT but maintained the response to GTN (expressed as % of the control value 5-HT, pre 45.0 +/- 3.5, post 2.3 +/- 3.2, GTN, pre 42.3 +/- 4.4, post 39.8 +/- 4.0). The increase in coronary flow induced by either agent was unaltered following 30 or 60 min of ischaemia protected by a single dose of ST (N = 16 in each). Ninety minutes of ischaemia preceded by a single infusion of ST (N = 16) markedly reduced the 5-HT response but maintained the vasodilation induced by GTN (5-HT, pre 37 +/- 2.6, post 6.9 +/- 2.6, GTN, pre 40.1 +/- 2.5, post 37 +/- 2.8). During 90 min of ischaemia, infusion of an additional dose of ST after 45 min (N = 8) or two extra doses every 30 min (N = 8) maintained the 5-HT-induced increase in coronary flow (pre 39.5 +/- 3.9, post 36.4 +/- 3.5). It can be concluded that coronary vascular endothelium is more susceptible to ischaemic damage when compared with vascular smooth muscle. This results in early loss of endothelium-dependent vasodilation. Multidose infusion of St Thomas' Hospital cardioplegia solution is superior to a single infusion in protecting endothelial function following prolonged cardiac ischaemia.     

Effects of regional ischaemia, with or without reperfusion, on endothelium dependent coronary relaxation in the dog.

OBJECTIVE: The aim was to establish whether the duration of coronary ischaemia and coronary ischaemia with reperfusion selectively reduced the magnitude of relaxation mediated by endothelium dependent relaxing factor (EDRF) in response to thrombin, compared with relaxation produced by acetylcholine and calcimycin. METHODS: Adult male dogs, anaesthetised with sodium pentobarbitone (30 mg.kg-1 intravenously) were used. Coronary artery occlusions were maintained for either 15 or 45 min; in half the dogs from each timepoint, occlusion was followed by 60 min reperfusion. At the end of each in situ period, coronary arteries were removed from both ischaemic and non-ischaemic regions, cut into rings, and hung in isolated organ baths. Dose-response relationships to the EDRF dependent vasodilators thrombin, acetylcholine, and calcimycin, and to the EDRF independent vasodilator isoprenaline, were then established. RESULTS: Thrombin (0.003-0.3 units.ml-1) caused dose dependent relaxation in all tissues. Relaxant responses (E(max)) in the non-ischaemic vessels from both 15 and 45 min treatment groups were used as control data for the responses in ischaemic vessels. Maximum responses were not different in the non-ischaemic groups from either 15 or 45 min studies, at 82.7 (SEM 3.7)% after 15 min, and 82.1(2.4)% after 45 min. There was a small but significant reduction in E(max) after 15 min and 45 min ischaemia, to 74.4(3.2)% and 74.4(3.0)% respectively. Sixty minutes reperfusion provoked a further reduction in E(max) to 64.9(3.8)% after 45 min ischaemia, but not after 15 min ischaemia [70.3(4.2)%]. Neither 15 nor 45 min interventions altered E(max) of relaxation to acetylcholine or calcimycin (greater than 88.0% in each group). Similarly there were no significant differences between groups to the relaxation stimulated by isoprenaline (E(max) greater than 90.0%). CONCLUSIONS: The data suggest that loss of EDRF dependent relaxation to thrombin is more sensitive to ischaemia than the relaxation produced by either acetylcholine or calcimycin, and appears to be manifested early in the onset of ischaemic injury.     

Preservation of myocardial blood flow by calcium antagonists does not prevent attenuation of regional myocardial function after repetitive brief periods of myocardial ischaemia in the rat heart

The aim of this study was to assess the effect of two differentcalcium channel blockers on myocardial blood flow and functionin a rat model of myocardial ‘stunning’ by repeatedshort episodes of ischaemia (‘repetitive ischaemia’).In an open chest rat model, the left anterior descending coronaryartery was ligatedfor 10 mm followed by 15 min reperfusion.In total, five periods of ischaemia and reperfusion were performed.Myocardial blood flow was assessed by the hydrogen clearancetechnique and systolic thickening fraction by pulsed Doppler.After five episodes of ischaemia, myocardial blood flow andmyocardial thickening in the ischaemic area were reduced by60±8% and 52±7% (n=9), respectively, as comparedto baseline. Continuous intravenous infusion of the calciumchannel blockers nifedipine (n=6) and gallopamil (n=6), started20 min prior to onset of ischaemia, attenuated the ischaemia-induceddecrease of myocardial perfusion. Nifedipine was the most effectivewith only 5±2% reduction in blood flow after five ischaemicepisodes, whereas reduction of myocardial blood flow was 30±4%in the presence of gallopamil. However, neither nifedipine norgallopamil were able to prevent regional ventricular dysfunctioninduced by repetitive ischaemia. Despite the preservation ofmyocardial blood flow following repetitive ischaemia, calciumchannel blockers do not prevent ischaemia-induced reductionof myocardial function in the ischaemic area.     

The contribution of ischaemia to the development of microvascular incompetence in the myocardium

STUDY OBJECTIVE--The aim was to determine the contribution of ischaemia per se to the development of microvascular incompetence in the myocardium. DESIGN--Isolated, buffer perfused rat hearts were made globally ischaemic for 0-60 min, then fixed with nitrogen bubbled glutaraldehyde and perfused with nuclear track emulsion to identify and quantify competent blood vessels in scanning and transmission electron micrographs. SUBJECTS--Adult Male Wistar rats weighing between 275 and 350 g were used. MEASUREMENTS AND MAIN RESULTS--Thirty or more min ischaemia significantly (p less than 0.05) reduced the density of competent capillaries in the subendocardial third of the myocardium, as did 45 or more min in the subepicardial third and 60 min in the middle third. Following 60 min ischaemia virtually all vessels in the subendocardial third were not perfusable. Severely ischaemic myocardium showed relatively normal, open, unobstructed capillaries and an absence of the endothelial, myocyte and mitochondrial swelling which have previously been attributed to ischaemia. CONCLUSIONS--In severely ischaemic myocardium microvascular incompetence shows a transmural gradient in severity. It develops progressively, starting near the endocardium. These findings suggest that postischaemic reoxygenation may accelerate the development of microvascular incompetence.     

Prevention of ventricular fibrillation by metoprolol in a pig model of acute myocardial ischaemia: absence of a major arrhythmogenic role for cyclic AMP

Absence of a Major Arrhythmogenic Role for Cyclic AMP. Journal of Molecular and Cellular Cardiology (1986) 18, 375-387. We examined the mechanism whereby beta-adrenoceptor antagonism exerts an antiarrhythmic effect in early myocardial ischaemia. Ligation of the anterior descending coronary artery in the anaesthetized open-chest pig resulted in severe transmural anteroseptal ischaemia. Blood flow in the mid-ischaemic zone 20 min after ligation was decreased to 5.7 +/- 0.7% of the preligation control value. Epicardial ST-segment deflections of 6.7 +/- 0.4 mV were recorded over this zone. A distinct phase of ventricular arrhythmias was evident about 10 to 30 min after ligation. A high incidence of ventricular fibrillation (14/16 pigs) was associated with a circumstantial increase in levels of cyclic AMP in ischaemic tissue. Twenty minute values were: 1.10 +/- 0.06, P less than 0.05 v. the non-ischaemic tissue level of 0.86 +/- 0.05 nmol/g. Propranolol 3 mg/kg IV, metoprolol 20 mg/kg IV or sotalol 10 mg/kg IV were given between 30 min prior to and 10 min after ligation. Adequate beta-adrenoceptor antagonism by each agent could be proven. Metoprolol decreased the incidence of ventricular fibrillation (2/13, P less than 0.0005 v. control group), while propranolol or sotalol did not. All three beta-antagonists decreased tissue levels of cyclic AMP prior to ligation. However, the temporary increase in ischaemic tissue after ligation could not be prevented. Furthermore, cyclic AMP in ischaemic tissue 20 min after ligation was higher in the metoprolol group than in the propranolol or sotalol group (0.94 +/- 0.04 v. 0.81 +/- 0.02 P less than 0.05, and 0.79 +/- 0.03 nmol/g P less than 0.01, respectively). Blood flow in the mid-ischaemic zone of the metoprolol group was increased to 8.6 +/- 0.6% of preligation control value (P less than 0.0001 v. control group). In contrast, blood flow in the mid-ischaemic zone of the propranolol or sotalol group was decreased. Metoprolol also reduced epicardial ST-segment deflections over the mid-ischaemic zone to 3.5 +/- 0.2 mV (P less than 0.0001 v. control group). ST-segment deflections in the propranolol group were increased. The mechanism whereby metoprolol prevented ventricular fibrillation may be explained by a decrease in the severity of ischaemia but not in terms of changes of tissue levels of cyclic AMP.     

Identification of the growth arrest and DNA damage protein GADD34 in the normal human heart and demonstration of alterations in expression following myocardial ischaemia

Growth arrest and DNA damage protein 34 (GADD34) is a multifunctional protein upregulated in response to cellular stress and is believed to mediate DNA repair and restore protein synthesis. In the present study we have examined GADD34 immunoreactivity in human myocardial tissue at defined survival times following cardiac arrest and determined alterations in expression following ischaemia. In the normal human heart, GADD34 immunoreactivity was generally intense and present within most cells. GADD34 immunoreactivity was downregulated in tissue displaying ischaemic damage and remained intense in adjacent non-infarcted tissue. Unlike brain, GADD34 was not found to be upregulated in the peri-infarct zone. Cells displaying apoptotic changes were located in regions displaying reduced GADD34 immunoreactivity. In the brain, it is thought that GADD34 supports re-initiation of protein synthesis following ischaemia. Similarly, GADD34 may perform important functions in cardiac tissue in response to ischaemia.