Pre‐conditioning activates adenosine utilization in a cost‐effective way during myocardial ischaemia

During pre‐conditioning the interstitial concentration of adenosine, in contrast to lactate, presents a die‐away curve‐pattern for every successive episode of ischaemia. This die‐away pattern might not necessarily be attributed to diminished adenosine production. The present study was undertaken to investigate whether pre‐conditioning alters the metabolic turnover of adenosine as observed by the lactate production during ischaemia. Interstitial levels of metabolites in pre‐conditioned (n=21) and non‐preconditioned (n=21) porcine hearts were monitored with microdialysis probes inserted in both ischaemic and non‐ischaemic tissue in an open chest heart model. Three subgroups perturbated with either plain microdialysis buffer (control), buffer containing adenosine (375 μM ), or buffer containing deoxyadenosine (375 μM ) were studied. All animals were subjected to 90 min of equilibrium microdialysis before 40 min of regional myocardial ischaemia and 120 min of reperfusion. Pre‐conditioning consisted of four repetitive episodes of 10 min of ischaemia and 20 min of reperfusion. Significantly higher levels of inosine and lactate were found in the ischaemic tissue of the pre‐conditioned subgroup receiving adenosine (P < 0.05) compared with the other two subgroups receiving deoxyadenosine and plain buffer, respectively. This difference was only valid for pre‐conditioned ischaemic myocardium, and hence equal amounts of inosine and lactate were produced in the non‐preconditioned ischaemic myocardium regardless of the presence of adenosine or deoxyadenosine. In the non‐ischaemic myocardium baseline levels of metabolites were measured in all subgroups. Pre‐conditioning favoured degradation of exogenous adenosine to inosine successively ending up in enhanced lactate production. This was probably because of the involvement of the hexose monophosphate pathway in the pre‐conditioned ischaemic myocardium. This route may therefore be supplementary in energy metabolism as a metabolic flow can be started by adenosine ending up in lactate without initial adenosine 5′‐triphosphate (ATP) investment. Utilization of adenosine in this way may also explain the successive die‐away pattern of adenosine seen in consecutive pre‐conditioning cycles.     

Cardiodepressant mediators are released after myocardial ischaemia: modulation by catecholamines and adenosine.

The interaction of recently characterized cardiodepressant mediators with catecholamines and adenosine after myocardial ischaemia was investigated using a model of sequential perfusion of two isolated guinea-pig hearts. Sequential perfusion was initiated after 10, 20, and 30 min (group I, II, and III) of global ischaemia in the first heart. At the onset of sequential perfusion LVdP/dtmax and min of Heart II decreased by 46 and 44% in group I, by 28 and 34% in group II, and increased by 60 and 24% in group III. Infusion of the beta1-receptor antagonist metoprolol (2.8 micromol L(-1)) into Heart II did not modulate contractile changes after 10 min of ischaemia in Heart I, prevented the attenuation of the cardiodepressant effect after 20 min of ischaemia, and completely reversed the positive inotropic effect after 30 min of ischaemia. The A1- and A2-receptor antagonists DPCPX (2 micromol L(-1)) and DMPX (20 micromol L(-1)) enhanced the positive inotropic and lusitropic effects in Heart II (LVdP/dtmax +154%, LVdP/dtmin +71%) during sequential perfusion after 30 min of ischaemia in Heart I. It is concluded that the effects of cardiodepressant mediators released after myocardial ischaemia are counteracted by a time-dependent release of catecholamines. Endogenous cardiac adenosine, in turn, attenuates the modulatory effects of catecholamines.     

Transmural distribution of myocardial adenosine content during coronary constriction.

The transmural distribution of tissue adenosine content was determined in samples of the left ventricle of anesthetized open-chest dogs. Transmural tissue samples were obtained and quickly freeze-clamped during either normal coronary perfusion or partial constriction of the inflow tubing to the left common coronary artery. Inner, middle, and outer segments of the samples were assayed for adenosine, inosine, ATP, creatine phosphate, and lactate. In control experiments, there were no significant differences among the tissue contents of the substances in the three segments. However, during a 50.2 +/- 4.7% reduction of coronary inflow, adenosine content of inner, middle, and outer segments was 0.41 +/- 0.007, 0.030 +/- 0.006, and 0.016 +/- 0.003 nmol/mg protein, respectively. Outer segment content was significantly less than either middle or inner content. Lactate distribution was similar to that of adenosine, whereas ATP and creatine phosphate were lowest in the inner segment. Increases of the inner/outer adenosine ratio occurred when the coronary-ventricular pressure index was lower than 1.2. Thus, selective underperfusion of the subendocardium during reduced coronary inflow can result in greater accumulation of adenosine in that region.     

Mediation by adenosine of bradycardia in rat heart during graded global ischaemia

The role of adenosine as a mediator of the bradycardia associated with graded global ischaemia in rat heart was examined. Hearts were perfused at 37°C in the isovolumic mode with Krebs-bicarbonate medium at 12.0 ml/min/g. After equilibration, the coronary flow was reduced to 0.5, 2.5, or 5.0 ml/min/g for 20 min. Effluent was collected and assayed for adenosine and inosine by HPLC. Heart rate was measured and bipolar electrograms were obtained in severely ischaemic hearts. Basal adenosine release was 124±15 pmol/min/g. Adenosine release increased by approximately 50% in hearts perfused at 5.0 ml/min/g. In hearts perfused at 2.5 and 0.5 ml/min/g, adenosine release increased by approximately 1300 and 2300% respectively. The pattern of adenosine release at 0.5 and 2.5 ml/min/g was phasic, with adenosine release rate increasing to a maximum after about 10 min then dropping to values slightly higher than initial values. Ischaemia produced significant bradycardia and first degree AV block. Adenosine antagonism with 5 m 8-phenyltheophylline blocked up to 25% of this bradycardia and significantly reduced the conduction delay. Adenosine release rate correlated closely with that component of heart rate slowing which was inhibited by 8-phenyltheophylline. It is concluded that adenosine released during graded global ischaemia mediates up to a quarter of the associated bradycardia. The effect of adenosine is phasic. Adenosine acts primarily to depress the sinus pacemaker. First degree AV block also occurs. These effects were only apparent at coronary flow rates below 5.0 ml/min/g.     

Cardiodepressant mediators are released after myocardial ischaemia: modulation by catecholamines and adenosine

The interaction of recently characterized cardiodepressant mediators with catecholamines and adenosine after myocardial ischaemia was investigated using a model of sequential perfusion of two isolated guinea-pig hearts. Sequential perfusion was initiated after 10, 20, and 30 min (group I, II, and III) of global ischaemia in the first heart. At the onset of sequential perfusion LVdP/dt_max and _min of Heart II decreased by 46 and 44% in group I, by 28 and 34% in group II, and increased by 60 and 24% in group III. Infusion of the β₁-receptor antagonist metoprolol (2.8 μmol L^–1) into Heart II did not modulate contractile changes after 10 min of ischaemia in Heart I, prevented the attenuation of the cardiodepressant effect after 20 min of ischaemia, and completely reversed the positive inotropic effect after 30 min of ischaemia. The A1- and A2-receptor antagonists DPCPX (2 μmol L^–1) and DMPX (20 μmol L^–1) enhanced the positive inotropic and lusitropic effects in Heart II (LVdP/dt_max +154%, LVdP/dt_min +71%) during sequential perfusion after 30 min of ischaemia in Heart I. It is concluded that the effects of cardiodepressant mediators released after myocardial ischaemia are counteracted by a time-dependent release of catecholamines. Endogenous cardiac adenosine, in turn, attenuates the modulatory effects of catecholamines.     

Biochemical assessment of acute myocardial ischaemia.

AIMS--To evaluate the efficacy of biochemical parameters in different fluids in the diagnosis of myocardial infarction of different causes, analysed after death. METHODS--The myoglobin concentration and total creatine kinase (CK) and creatine kinase MB isoenzyme (CK-MB) activities were measured in serum, pericardial fluid, and vitreous humour from seven diagnostic groups of cadavers classified according to the severity of myocardial ischaemia and cause of death. Lactate dehydrogenase (LDH) and myosin were measured only in serum and pericardial fluid, and cathepsin D only in pericardial fluid. Routine haematoxylin and eosin and acridine orange staining were used for microscopy studies of heart tissue. RESULTS--In pericardial fluid there were substantial differences between the different groups with respect to CK, CK-MB, and LDH activities and myosin concentrations. The highest values were found in cases with morphological evidence of myocardial ischaemia. CONCLUSIONS--Biochemical parameters, which reach the pericardial fluid via passive diffusion and ultrafiltration due to a pressure gradient, were thus detectable in this fluid earlier than in serum in cases with myocardial ischaemia. These biochemical parameters may be of use for ruling out myocardial ischaemia in those controversial cases in which reliable morphological findings are lacking.     

Ventricular histamine concentrations and arrhythmias during acute myocardial ischaemia in rats

The relation between ventricular histamine concentrations and the occurrence of early ventricular arrhythmias during acute myocardial ischaemia was investigated in pentobarbitone-anaesthetized rats. There was significant decrease in the left, but not the right, ventricular histamine level at 5 min following acute left coronary artery ligation. Pretreatment with rhodanine caused remarkable reduction in ventricular histamine concentrations as well as significantly lower incidence and slower onset of ventricular tachycardia and fibrillation resulting from acute myocardial ischaemia. On the contrary, aminoguanidine pretreatment did not significantly alter ventricular histamine levels nor did it influence the occurrence of early ventricular arrhythmias induced by coronary artery ligation. The responses of blood pressure and heart rate to acute coronary artery ligation were not noticeably affected by rhodanine or aminoguanidine pretreatment. These findings support the hypothesis that histamine release from cardiac tissues may contribute to the genesis of early ventricular arrhythmias, but not to the changes in blood pressure and heart rate, during acute myocardial ischaemia.     

Wavelet transform analysis of heart rate variability during myocardial ischaemia

Analysis of heart rate variability (HRV) is a valuable, non-invasive method for quantifying autonomic cardiac control in humans. Frequency-domain analysis of HRV involving myocardial ischaemic episodes should take into account its non-stationary behaviour. The wavelet transform is an alternative tool for the analysis of non-stationary signals. Fourteen patients have been analysed, ranging from 40 to 64 years old and selected from the European Electrocardiographic ST-T Database (ESDB). These records contain 33 ST episodes, according to the notation of the ESDB, with durations of between 40s and 12min. A method for analysing HRV signals using the wavelet transform was applied to obtain a time-scale representation for very low-frequency (VLF), low-frequency (LF) and high-frequency (HF) bands using the orthogonal multiresolution pyramidal algorithm. The design and implementation using fast algorithms included a specially adapted decomposition quadrature mirror filter bank for the frequency bands of interest. Comparing a normality zone against the ischaemic episode in the same record, increases in LF (0.0112±0.0101 against 0.0175±0.0208s²Hz⁻¹; p<0.1) and HF (0.0011±0.0008 against 0.0017±0.0020s²Hz⁻¹; p<0.05) were obtained. The possibility of using these indexes to develop an ischaemic-episode classifier was also tested. Results suggest that wavelet analysis provides useful information for the assessment of dynamic changes and patterns of HRV during myocardial ischaemia.     

Pulmonary gas exchange during acute myocardial ischaemia a study in the closed chest anaesthetized dog

The occurrence and the causes of arterial hypoxaemia were studied in spontaneously breathing dogs in which myocardial ischaemia was induced during anaesthesia. Changes in several circulatory and ventilatory variables and in parameters of gas exchange were assessed in twelve dogs in the two 1st h after the intervention; these included changes in the distribution of ventilation-perfusion ratios determined with an inert tracer gas method.Eight out of 12 dogs developed haemodynamic signs of an acute myocardial infarction after occlusion of a branch of the left coronary artery; the circulatory changes were moderate in 6 and severe in 2 animals. A drop in occurred in 5 dogs. In one without significant haemodynamic changes it was largely due to a decrease in alveolar ventilation. The ventilation-perfusion ratio distribution in the other 4 dogs did not change in a consistent way; appreciable shunt circulation (6.6%) developed in one dog. Neither ventilation-perfusion mismatch nor shunt circulation contributed much to the hypoxaemia. The most consistent finding in the 5 dogs in whom fell, was a drop in (mean 1.3 kPa); it accounted for 86% of the drop in .Supported by Grant 2^b of the Interuniversity Cardiological Institute and 13-22-17 of the Foundation for Medical Research FUNGO     

Excitation of afferent cardiac sympathetic nerve fibres during myocardial ischaemia

1. Occlusion of the main left coronary artery of lightly anaesthetized cats provoked a pseudaffective reaction. The afferent pathway was in the cardiac sympathetic nerves.2. The compound action potential evoked in the inferior or middle cardiac nerves by stimulation of the thoracic sympathetic trunk contained two elevations, a small Adelta wave and a much larger sC wave. Occasionally a B wave was present.3. During coronary occlusion, the Adelta elevation was reduced by 35-55%, indicating afferent activity in these fibres. Multifibre preparations also showed increased afferent discharge during occlusion.4. It seems probable that the afferent activity in these fibres was mainly responsible for signalling the pseudaffective response elicited by coronary occlusion.5. Myocardial ischaemia produced by coronary occlusion was probably the stimulus for the increased activity.     

Ventricular histamine concentrations in naive and morphine-treated rats during acute myocardial ischaemia

The ventricular histamine concentrations of naive and morphine-treated rats subjected to acute left coronary artery ligation were examined. In naive animals, there was a significant increase in the right ventricular histamine level at 5 min following ligation, but not at 3 or 10 min. Left ventricular histamine concentrations tended to decrease, but the changes were not statistically significant. In shamoperated rats, neither acute nor chronic morphine treatment significantly altered either right or left ventricular histamine levels. Acute morphine treatment also did not significantly affect the ventricular histamine content at 5 min following coronary artery ligation. However, both right and left ventricular histamine concentrations were found to be significantly lower in chronic morphine-treated rats than in the naive animals when they were subjected to acute myocardial ischaemia. If the hypothesis that histamine release may contribute to the genesis of early ventricular arrhythmias resulting from acute myocardial ischaemia is accepted, the present findings suggest that the previously reported decreased incidence and delayed onset of early ventricular arrhythmias induced by acute left coronary artery ligation in chronic morphine-treated rats may be attributed to the reduced ventricular histamine concentrations.     

Apoptosis in myocardial ischaemia and infarction

Recent studies indicate that, in addition to necrosis, apoptosis also plays a role in the process of tissue damage after myocardial infarction, which has pathological and therapeutic implications. This review article will discuss studies in which the role and mechanisms of apoptosis in myocardial infarction were analysed in vivo and in vitro in humans and in animals.     

Plasma nitrotyrosine in reversible myocardial ischaemia

BACKGROUND: Nitric oxide (NO) plays a vital role in vascular homeostasis and in the pathophysiology of coronary heart disease. Its metabolites, nitrite and nitrate, have vasculoprotective properties, whereas peroxynitrite, an oxidant metabolite of NO, is cytotoxic and can aggravate myocardial damage during ischaemic reperfusion injury. Peroxynitrite nitrates free and protein bound tyrosine residues to produce nitrotyrosine. The measurement of nitrotyrosine provides an indirect estimation of plasma peroxynitrite concentrations.AIMS: To measure plasma nitrotyrosine concentrations to see whether peroxynitrite could contribute to myocardial dysfunction during myocardial ischaemia induced by an exercise tolerance test (ETT). MATERIALS/METHODS: Plasma free nitrotyrosine concentrations were compared before and after exercise in 29 subjects with a positive ETT and 34 subjects with a negative ETT. RESULTS: Plasma nitrotyrosine concentrations were similar in patients with exercise induced myocardial ischaemia and controls. CONCLUSION: Peroxynitrite does not contribute to the myocardial dysfunction in reversible myocardial ischaemia.     

Cardiovascular effects of ranitidine and cimetidine during acute myocardial ischaemia in anaesthetized dogs

The effects of ranitidine and cimetidine on ventricular fibrillation threshold and haemodynamics were studied in pentobarbitone-anaesthetized dogs subjected to acute coronary artery ligation. These drugs did not significantly change the ventricular fibrillation threshold nor haemodynamics before coronary artery ligation, except for remarkable haemodynamic depression by ranitidine 1 mg/kg. Ligation of the left anterior descending coronary artery reduced the ventricular fibrillation threshold, decreased systemic and left ventricular pressure and myocardial contractility, and slightly increased heart rate. Pretreatment with ranitidine 0.25 or 1 mg/kg, or with, cimetidine 2 mg/kg, significantly abolished the reductions in ventricular fibrillation threshold, but did not noticeably alter the haemodynamic changes. These findings further support the hypothesis that histamine release may contribute to the increased ventricular vulnerability resulting from acute myocardial ischaemia. However, the role of histamine in the haemodynamic responses to coronary artery ligation remains obscure.     

Capillary ATPase inactivation in early myocardial ischaemia

The Wachstein-Meisel ATPase reaction can be used in formalin-fixed, postmortem material to demonstrate the microvasculature in the human myocardium. In cases of sudden or rapid cardiac death, the disappearance of enzymatic activity in capillary walls was observed earlier than other light-microscopic signs of damage. This was especially marked in the subendocardial regions. The staining of capillaries was found to be positive again in the varying stages of organization. Fibrotic scars were devoid of staining. The findings besides being of practical value in the diagnosis of early myocardial necrosis, support the theory that loss of capillary metabolic and functional integrity occurs early in ischaemia; it promotes the 'no reflow' phenomenon and may contribute towards myofibre necrosis.     

B-type natriuretic peptide in reversible myocardial ischaemia

BACKGROUND: Coronary heart disease is associated with increased B-type natriuretic peptides (BNPs), and, although controversial, may cause exaggerated exercise-induced BNP secretion. We investigated BNP in relation to reversible myocardial ischaemia. Materials and methods: Serum N-terminal proBNP (NT-proBNP) was measured before and after an exercise electrocardiogram test (ETT) in 14 patients with and 45 patients without exercise-induced myocardial ischaemia. Statistical analysis was carried out on logarithmically transformed data. Results, however, are pre-transformed data. RESULTS: NT-proBNP increased with exercise both in ETT-positive patients (mean (SD) 71.4 (41.2) v 76.8 (44.0) ng/l; p<0.001) and ETT-negative patients (54.0 (61.2) v 60.1 (69.0) ng/l; p<0.001). Pre-exercise and post-exercise NT-proBNP were higher (p<0.05) in ETT-positive than in ETT-negative patients. Incremental NT-proBNP was similar in ETT-positive (4.7 (4.2) ng/l) and ETT-negative (6.2 (8.6) ng/l) patients. CONCLUSION: Serum NT-proBNP concentrations are higher in patients with exercise-induced myocardial ischaemia than in those without. Exercise-induced electrocardiographic myocardial ischaemia, however, is not associated with exaggerated BNP secretion.     

Regional myocardial tissue blood flow during beta-adrenergic blockade in cat hearts with acute ischaemia

Selective beta 1- or beta 2-adrenergic blockade was achieved by practolol or IPS 339, respectively, in cats with acute ligation of a coronary artery. During blockade, heart rate was kept constant by atrial pacing and blood pressure reduction was prevented by aortic clamping. Regional myocardial blood flow was measured by the distribution of 15 micron labelled microspheres. Practolol slightly reduced epicardial blood flow in ischaemic myocardium, while blood flow in border and normally perfused myocardium remained unchanged. Following IPS 339, myocardial tissue flow increased in normally perfused myocardium, on average by 37% in the endocardium and 30% in the epicardium. No changes occurred in the other regions. The flow changes brought about by IPS 339 were unrelated to haemodynamic changes, and the coronary vascular resistance was reduced. These results are indicative of coronary vasodilation related to beta 2-adrenergic receptor blockade and was confined to well-oxygenated areas surrounding the acutely ischaemic zone.     

Cardiac accumulation of citrate during brief myocardial ischaemia and reperfusion in the pig in vivo

Citrate is a key intermediate in energy metabolism and an inhibitor of phosphofructokinase of the glycolytic pathway. During myocardial ischaemia glycolysis is the main source of cardiac ATP. The aim of the present study was to determine if myocardial ischaemia and reperfusion alter cardiac tissue levels of citrate. Open-chest, anaesthetized pigs were subjected to 10 min of regional myocardial ischaemia by occlusion of the left anterior descending coronary artery, with and without reperfusion, and to 10 min of global ischaemia by circulatory arrest. Citrate, amino acids, glucose and NH₃ were measured in biopsies. Ischaemia, whether regional or global, caused a 60–70% increase in tissue levels of citrate. During 1 min of reperfusion following regional ischaemia the level of citrate increased 460%, to ≈600 nmol g^?1 wet weight. The level of glutamate decreased by 20–33% (corresponding to 1300–2200 nmol g^?1 wet weight), indicating net consumption of this amino acid during ischaemia. The level of aspartate decreased 50% indicating conversion of aspartate to oxaloacetate for the synthesis of citrate. Theoretically, the accumulation of myocardial citrate during brief ischaemia and early reperfusion is large enough to significantly inhibit phosphofructokinase activity and could therefore affect the ability of the myocardium to increase the glycolytic rate in response to ischaemia. This could, however, be partly compensated by the metabolism of myocardial glutamate.     

Exogenous adenosine enhances caspase-3 activity in warm renal ischaemia

Renal injury due to ischaemia/reperfusion (I/R) leads to impaired renal function. One of the essential pathological changes thereby is cell death due to apoptosis. This study investigated the effect of adenosine administration on caspase-3 (C3) activity and expression during warm renal ischaemia in rat kidney and the role of nitric oxide (NO) as a mediator of the adenosine-induced effect. The following experimental groups were studied: control, ischaemia, ischaemia with adenosine administration, ischaemia with adenosine and N-nitro-l-arginine methyl ester (L-NAME) treatment and ischaemia with NO donor administration. C3 activity was measured and its protein expression determined by Western blot analysis. Supplementation of adenosine or NO during ischaemia increased C3 activity and protein expression but the effect of adenosine was reversed in rats treated with L-NAME. We conclude that adenosine increases C3 activity through an NO-dependent mechanism.