NALOXONE ATTENUATES AUGMENTATION OF cAMP LEVELS AND ARRHYTHMIAS FOLLOWING MYOCARDIAL ISCHAEMIA AND REPERFUSION IN THE ISOLATED PERFUSED RAT HEART

The effects of myocardial ischaemia and reperfusion on arrhythmias and cAMP levels were studied using the Langendoff isolated perfused rat heart preparation. Myocardial ischaemia and reperfusion caused arrhythmias and augmentation of cAMP levels concurrently, supporting the suggestion that myocardial cAMP is related to arrhythmias. Pretreatment with naloxone attenuated both arrhythmias and augmentation of cAMP levels to a similar extent. The results suggest that the anti-arrhythmic effect of naloxone may involve myocardial cAMP.     

NALOXONE BLOCKS THE CARDIAC EFFECTS OF MYOCARDIAL ISCHAEMIA AND REPERFUSION IN THE RAT ISOLATED HEART

The effects of naloxone on contractility and cardiac rhythm were studied in the rat isolated perfused heart during myocardial ischaemia and reperfusion. Pretreatment of the rat isolated perfused heart with naloxone abolished the reduction in left ventricular pressures and attenuated greatly the arrhythmias due to myocardial ischaemia and reperfusion. Administration of naloxone into the fibrillating rat isolated heart induced by myocardial ischaemia and reperfusion also attenuated the arrhythmias in a dose-dependent manner. The results indicate a possible involvement of the endogenous opioid peptides in the cardiac effects due to myocardial ischaemia and reperfusion. The antiarrhythmic effect of naloxone has great clinical implications.     

REPERFUSION FOLLOWING MYOCARDIAL ISCHAEMIA ENHANCES INOSITOL PHOSPHATE RELEASE IN THE ISOLATED PERFUSED RAT HEART

1. Global myocariial ischaemia (MI) for periods greater tan 5 min caused an inhibition of phosphatidylinositol specific phospholipase C (PtdIns-PLC) activity. 2. Two min reperfusion following a 20 min MI period, a time point associated with reperfusion-induced arrhythmias, resulted in an activation of PtdIns-PLC activity, dependent on endogenous noradrenaline and mediated via al-adrenoceptors. 3. This 2 min reperfusion response, in contrast to healthy myocardium, resulted in: (i) enhanced PtdIns-PLC activity; (ii) increased sensitivity to endogenous noradrenaline; (iii) rapid increases in inositol(1,4,5)trisphosphate (Ins(1,4,5)P3); and (iv) PLC hydrolysis primarily of PtdIns(4,5)P₂, such that the majority of InsP isomers derive from Ins(1,4,5)P₃. 4. Together, these data suggest a functional role for Ins(1,4,5)P3 under postischaemic reperfusion conditions, and provide a possible link between al-adrenoceptor stimulation of the PtdIns turnover pathway and reperfusion injury.     

PERFUSION PRESSURE AND TRANSMURAL FLOW DISTRIBUTION IN THE LEFT VENTRICLES OF ISOLATED RAT HEARTS

1. The influence of perfusion pressure on flow distribution in the left ventricular wall was investigated with a newly developed microsphere method in Langendorff preparations of the rat heart. 2. Microspheres with a diameter of 10 μm stained with the fluorescent dye yellow-green were infused into the perfusion medium. The hearts were then relaxed with a calcium-chelator, fixed with formaldehyde and cut transversally with a freezing microtome. Photographs were taken of the 52 μm slices with a fluorescence photomacroscope. The ventricular wall was subdivided into four zones, evaluated planimetrically and the beads were counted for calculation of microsphere density. 3. Perfusion of groups of hearts, paced at 5 Hz, with Tyrode's solution at pressures of 30, 50, 80 or 120 cmH₂O for 30 min revealed an increase of left ventricular pressure amplitude, coronary flow, myocardial oxygen consumption and flow redistribution towards the endocardium of the left ventricular wall with increasing perfusion pressure. 4. The quotient of the sphere densities in the inner endocardium over that in the outer epicardium, however, was already maximal at a pressure of 80 cmH₂O and amounted to 1.59 ± 0.15. This quotient was only 0.52 ± 0.11 at a perfusion pressure of 30 cmH₂O, thus indicating massive change in flow distribution upon hypoperfusion. 5. These results indicate that transmural left ventricular flow redistribution in response to hypoperfusion in the isolated rat heart is similar to that in hearts of much larger species.     

EFFECTS OF RESERPINE TREATMENT ON ARRHYTHMOGENESIS DURING ISCHAEMIA AND REPERFUSION IN THE ISOLATED RAT HEART

1. The effects of reserpine treatment on the myocardial contents of catecholamines and enkephalins and the incidence of ventricular arrhythmias during ischaemia and reperfusion in the isolated rat heart were studied. 2. Reserpine treatment almost completely depleted the heart of noradrenaline (NA). It also significantly depleted the heart of adrenaline and dopamines. It did not, however, alter the myocardial contents of enkephalins. 3. Reserpine-treatment attenuated significantly, but did not abolish, cardiac arrhythmias induced by ischaemia and reperfusion in the isolated heart preparation. 4. The results of the present study indicate that myocardial catecholamines especially NA are a contributing factor to arrhythmogenesis during ischaemia and reperfusion.     

ALINIDINE PREVENTS HYPOPERFUSION-INDUCED TRANSMURAL FLOW REDISTRIBUTION IN ISOLATED PACED RAT HEARTS

1. The effect of alinidine on transmural flow redistribution during hypoperfusion was studied in the isolated rat heart. 2. Low flow perfusion of paced (5 Hz) rat Langendorff preparations with Tyrode's solution for 60 min resulted in loss of left ventricular pressure amplitude and contracture. 3. In the presence of 45.6 μmol/L alinidine in the hypoperfusion medium contractility was maintained and contracture was missing. 4. Myocardial sphere densities were assessed by infusion of fluorescent microspheres after 60 min of hypoperfusion. Histomorphometric evaluation of the left ventricular wall indicated marked redistribution of myocardial flow to the epicardium. Endolepicardial quotients of sphere densities were significantly greater in alinidine-treated hearts. 5. Perfusion of hearts with 45.6 μmol/L alinidine for 30 min at a pressure of 80 cmH₂O decreased coronary flow and contractility, but did not change the transmural quotient of sphere densities. 6. These results show that in isolated rat hearts with a ventricular wall of only 3–4 mm thickness severe hypoperfusion induces a shift in transmural flow distribution which is prevented by alinidine.     

THE EFFECTS OF m-AMSA ON RAT ISOLATED HEART

m-AMSA (4'[9-acridinylamino]methansulphon-m-anisidide) is a new cytoxic agent now under clinical trial. We used the rat isolated perfused heart model in order to investigate the cardiac effects of m-AMSA. The results of the dose-response study indicate that m-AMSA has an acute moderate negative inotropic effect. The 90% effect (25% decrease in developed force compared to the control) was observed at drug concentration of 1.5 micrograms/ml. The refractory period (as measured by stimuli of twice diastolic threshold intensity) increased progressively as the drug concentration was increased (up to 2.5 micrograms/ml). Measurements of the strength-duration and strength-interval relationship showed that m-AMSA induced a significant reduction (P less than 0.005) in excitability and prolongation of refactoriness. We suggest that m-AMSA has a membranal cardiotoxic effect in addition to its known intracellular cytotoxic effect.     

巯基化合物对离体大鼠心脏缺血再灌所致心律失常的保护作用

本文研究了半胱氨酸(Cys)及其结构类似物半胱胺(MEA),N-乙酰半胱氨酸(NAC)、胱胺(CSSC),γ-氨丙基甲基异硫脲(APMT),对离体大鼠Langendortff心脏缺血再灌所致心律失常的保护作用.给药(0.1,0.6,3,6μmol/min)10min,结扎LAD 10 min再灌5 min。结果表明含游离巯基的Cys,NAC,MEA在0.6和3.6 μmol/min时,与生理盐水对照组相比可显著降低室颤发生率(P<0.01～0.001),缩短室颤时程(P<0.01～0.001).CSSC和APMT未见明显保护作用。此外,Cys,NAC和MEA还可明显增加冠脉流量(P<0.01),CSSC和APMT则反而使冠脉流量降低。     

POSSIBLE MECHANISM OF CARDIOPROTECTIVE EFFECT OF ISCHAEMIC PRECONDITIONING IN ISOLATED RAT HEART

The present study is designed to investigate the mechanism of the cardioprotective effect of ischaemic preconditioning. Isolated perfused rat heart was subjected to global ischaemia for 30 min followed by reperfusion for 120 min. Coronary effluent was analysed for LDH and CK release to assess the degree of cardiac injury. Myocardial infarct size was estimated macroscopically using TTC staining. Four episodes of ischaemic preconditioning markedly reduced LDH and CK release in the coronary effluent and decreased myocardial infarct size. Administration of prazosin (alpha(1)adrenoceptor antagonist) before global ischaemia reduced the extent of ischaemia-reperfusion induced myocardial injury. The cardioprotective effect of ischaemic preconditioning was abolished by prazosin and colchicine (microtubule disaggregator). On the basis of these results, it may be concluded that the cardioprotective effects of ischaemic preconditioning may be mediated through stimulation of alpha(1)adrenoceptors and translocation of PKC.     

REGIONAL DISTRIBUTION OF THE CARDIAC OUTPUT AND RENAL RESPONSES TO ATRIAL ATRIURETIC PEPTIDE INFUSION IN RABBITS WITH CONGESTIVE HEART FAILURE

1. A biventricular, low-output congestive cardiomyopathy was induced in 19 rabbits by administering adriamycin (16 mg/kg). The effects of alpha-rat atrial natriuretic peptide (ANP) infused at 0.1, 0.2 and 0.4 micrograms/kg per min, were then examined in terms of (i) central haemodynamics (ii) regional blood flow (iii) renal function and (iv) plasma norepinephrine and plasma renin. 2. In this dose range, ANP produced progressive and significant falls in stroke volume, cardiac output and mean arterial pressure, owing to a fall in venous return. The heart rate response to this was blunted. 3. Using radiolabelled microspheres, significant falls in the perfusion of cutaneous, gastrointestinal and musculoskeletal tissues were observed, due to reduced vascular conductances in these beds. These changes were accompanied by activation of the sympathetic nervous system as evidenced by a progressive rise in plasma norepinephrine. A significant increase in plasma renin was only observed with the highest infusion of ANP. 4. Renal blood flow was maintained in the face of a falling mean arterial pressure and cardiac output, but diuretic and natriuretic effects were absent. 5. It was concluded that the dominant influence of ANP infusion in this model of heart failure appeared to be a reduction in cardiac preload with detrimental overall haemodynamic consequences.     

ATENOLOL DEPRESSES POST-ISCHAEMIC RECOVERY IN THE ISOLATED RAT HEART

Metabolic events during ischaemia are probably important in determining post-ischaemic myocardial recovery. The aim of this study was to assess the effects of the beta-blocker atenolol and the high energy demand in an ischaemia-reperfusion model free of neurohormonal and vascular factors. We exposed Langendorff-perfused isolated rat hearts to low-flow ischaemia (30 min) and reflow (20 min). Three groups of hearts were used: control hearts (n =11), hearts that were perfused with 2.5 micrograms l-1atenolol (n =9), and hearts electrically paced during ischaemia to distinguish the effect of heart rate from that of the drug (n =9). The hearts were freeze-clamped at the end of reflow to determine high-energy phosphates and their metabolites. During ischaemia, the pressure-rate product was 2.3+/-0.2, 5.2+/-1.1, and 3.3+/-0.3 mmHg 10(3)min in the control, atenolol and paced hearts, respectively. In addition, the ATP turnover rate, calculated from venous (lactate), oxygen uptake and flow, was higher in atenolol (11.2+/-1.7 micromol min-1) and paced (8.1+/-0.8 micromol min-1) hearts than in control (6.2+/-0.8 micromol min-1). At the end of reflow, the pressurexrate product recovered 75.1+/-6.4% of baseline in control vs 54.1+/-9.1 and 48.8+/-4.4% in atenolol and paced hearts (P<0.05). In addition, the tissue content of ATP was higher in the control hearts (15.8+/-1. 0 micromol g(dw)(-1)) than in atenolol (10.5+/-2.6 micromol g(dw)(-1)) and paced (10.9+/-1.3 micromol g(dw)(-1)) hearts. Thus, by suppressing the protective effects of down-regulation, both atenolol and pacing apparently depress myocardial recovery in this model.     

THE PRODUCTION OF VENTRICULAR ARRHYTHMIAS IN THE GUINEA-PIG ISOLATED HEART USING HYPOXIC PERFUSION FLUIDS CONTAINING ADRENALINE

The effects of hypoxia, adrenaline perfusion, and their combintion on cardiac rhythm were studied in the isolated perfused heart of the guinea-pig. Hypoxia or adrenaline perfusion (5.5 m?mol/1) produced a low incidence of ventricular arrhythmias (26% and 33%, respectively); however, the changes were not statistically significant. A combination of hypoxia and adrenaline perfusion produced ventricular arrhythmias in each of twenty-three preparations: there were frequent ventricular premature contractions in eighteen preparations, and ventricular tachycardia or fibrillation in five preparations. The mean times of onset of these arrhythmias after hypoxia were 33.3 min (s.e.m. = 5.2) and 57.6 min (s.e.m. =4.7), respectively. The responsiveness of the frequent ventricular premature contractions to the antiarrhythmic effects of quinidine and lignocaine was tested in twelve preparations. Both drugs produced dose-dependent reductions in the percentage of ventricular ectopic beats. These results suggest that a combination of hypoxia and adrenaline perfusion is a simple but reliable method of inducing ventricular arrhythmias in the isolated heart of the guinea-pig, and this provides a model that may be useful for the experimental evaluation of antiarrhythmic drugs.     

THE EFFECT OF TRICYCLIC ANTIDEPRESSANT DRUGS ON THE ISOLATED PERFUSED GUINEA-PIG HEART*

1. The effects of tricyclic antidepressants were studied on the isolated perfused guinea-pig heart simultaneously recording myocardial contractile force and cardiac electrogram. 2. Tricyclic antidepressants in a concentration 4 × 10^?-⁵ mol/1 decreased cardiac contractile force and increased cardiac conduction time. 3. Doxepin had significantly greater negative inotropic effect than nortriptyline, protriptyline, desipramine, amitriptyline and imipramine (P < 0.01). 4. There was no significant difference in the increase in P-R interval (P>0.5) and QRS width (P > 0.95) between the tricyclic antidepressants. 5. The isolated perfused guinea-pig heart can be used as a toxicological model for testing and treating cardiac arrhythmias induced by tricyclic antidepressants.     

VALIDATION OF A SUBENDOCARDIAL ISCHAEMIC SHEEP MODEL BY INTRACORONARY FLUORESCENT MICROSPHERES

1. We evaluated the use of non-radioactive fluorescent-labelled microspheres (FM) for the measurement of regional myocardial blood flow (RMBF) in an ischaemic sheep model. 2. Injection of FM directly into the coronary artery was compared with left atrial injection. There was a good correlation in the measurement of RMBF between these two injection methods (r = 0.92; n= 107 data points). Injection into the coronary artery requires less FM (one twentieth of that required by atrial injection) and is more economical. 3. The use of a fluorescent technique without filtering myocardial tissue was investigated. Calibration curves from the fluorescence plus myocardial tissue samples were similar to those of the pure fluorescence samples and both showed a linear relationship between fluorescent intensity and the number of microspheres (r > 0.97). These results indicate that the extraction of six fluorescent dyes (blue-green, yellow-green, green, orange, red and crimson) directly from the aqueous solution using ethyl acetate is effective. 4. A subendocardial ischaemic model was produced by partially occluding the circumflex artery (CxA) with concomitant left atrium (LA) pacing. During ischaemia, the endocardium/epicardium (Endo/Epi) flow ratios in the ischaemic area changed from 1.04 ± 0.12 to 0.47 ± 0.17 (P < 0.05; CxA injection) and from 1.08 ± 0.12 to 0.51 ± 0.05 (P < 0.05; LA injection). The ratio in the non-ischaemic area remained unchanged (1.12 ± 0.26 to 1.01 ± 0.22; not significant). 5. RMBF calculation using coronary inflow as the reference flow was also compared with that using the traditional method. We found that, in this study in which a non-filtering technique was applied, using coronary inflow as the reference flow was superior to the conventional distal sampling method.     

ARRHYTHMOGENIC EFFECT OF FORSKOLIN IN THE ISOLATED PERFUSED RAT HEART: INFLUENCE OF NIFEDIPINE REDUCTION OF EXTERNAL CALCIUM

1. This study investigated first the effects of forskolin on cardiac rhythm, and second the roles of calcium in cardiac arrhythmogenesis by cAMP. 2. Two series of experiments were performed. In the first series, forskolin was administered into the isolated perfused rat heart. In the second series, forskolin administration was preceded by administration of nifedipine, a calcium channel blocker, or infusion of a low concentration calcium solution. In both experiments, the myocardial cAMP level and electrocardiogram were determined. 3. It was found that forskolin increased cAMP level as well as inducing arrhythmia. Pretreatment with nifedipine or a reduction of external calcium, that either maintained or further enhanced the forskolin-induced increase in the cAMP level, abolished the forskolin-induced arrhythmia. 4. The results of the present study support the hypothesis that myocardial cAMP mediates cardiac arrhythmia, and provide evidence that calcium is essential in arrhythmia mediated by cAMP.     

THE MEASUREMENT OF GLOMERULAR BLOOD FLOW IN THE RAT KIDNEY: INFLUENCE OF MICROSPHERE SIZE

1. Radioactively labelled microspheres were used to determine glomerular blood flow in glomerular populations with distinct vascular characteristics. Two batches of microspheres (15 ± 5.0 μm diameter and 7.0–10 μm diameter) were utilized. 2. The results show that the larger microspheres overestimate the superficial glomerular blood flow (414 ± 61 nl/min, mean ± s.e.m.) and underestimate the deep glomerular blood flow (98 ± 10 nl/min), when compared with the data obtained with 7.0–10 μm diameter microspheres (317 ± 30 nl/min and 209 ± 23 nl/min, respectively). 3. The rheological artefact associated with the use of larger microspheres is confirmed by finding an uneven size distribution of microspheres lodged in the glomeruli. In each of three experiments, the mean diameter of 200 microspheres lodged in the superficial glomeruli (16.43 ± 0.27 μm, 15.87 ± 0.23/mi and 16.58 ± 0.27 Jim) was significantly greater than that found in the deep glomeruli (15.36 ± 0.15μm, 15.25 ± 0.21 μm and 15.73 ± 0.24μm; P<0.01, <0.05 and < 0.01, respectively). No such difference was detected when the 7.0–10 μm spheres were used. 4. Glomerular blood flow can be measured at all depths of the rat's cortex and the demonstrated rheological artefact associated with use of the larger spheres is circumvented with the use of 7.0–10 μm microspheres.     

ASSOCIATION OF PRE-ISCHAEMIC DISTURBANCES IN ENERGY METABOLISM WITH POSTISCHAEMIC DYSFUNCTION OF THE RAT ISOLATED WORKING HEART

1. Metabolic and functional effects of two protocols of preconditioning were compared in rat isolated hearts subjected to 20 min global ischaemia (37°C) and reperfusion (30 min Langendorff + 15 min working). Prior to the ischaemic period, hearts were perfused according to Langendorff (control group) or were preconditioned by three 5 min cycles or two 10 min cycles of ischaemia and reperfusion (PC-I and PC-II groups, respectively). 2. There was no difference in the contractile function between the two preconditioned groups at the onset of sustained ischaemia, although the PC-II group showed enhanced release of adenosine (Ado), inosine, hypoxanthine and xanthine into the interstitium accompanied by losses of tissue adenine nucleotides (ΣAN = ATP + ADP + AMP), total creatine (ΣCr = phosphocreatine + creatine) and activation of glycolysis following the preconditioning period. During reperfusion, the PC-I group showed enhanced functional recovery, higher contents of ΣAN and ΣCr, and the smallest lactate dehydrogenase release compared with these indices in the control and PC-II groups. Postischaemic myocardial dysfunction was similar in the control and PC-II groups. 3. Functional recovery of hearts in both preconditioned groups was positively correlated with myocardial contents of ATP, ΣAN and ΣCr at the end of reperfusion, but not with pre-ischaemic Ado release into the interstitium. The results suggest that pre-ischaemic disturbances of energy metabolism, rather than activation of Ado receptors or stunning, may contribute to efficacy of multiple preconditioning in the rat isolated heart.     

蝙蝠葛酚性碱对离体大鼠心肌顿抑的保护作用

目的　探讨蝙蝠葛酚性碱(PAMD)对离体大鼠心肌顿抑的作用。方法　采用缺血10min后再灌注30min造成心肌顿抑模型(S) ,灌流液中加0.5mg·mL^-1 PAMD(P)组同样缺血再灌注。结果　灌注末S组LVSP ,+dp/dt_max和-dp/dt_max分别降至预灌末的49% ,53%和58% ,LVEDP则增至422% ;心肌钙含量为(514±142 ) μg·g^-1 (干重) ;出现可逆性心肌超微结构损伤。而P组再灌注末LVSP ,LVEDP ,+dp/dt_max和-dp/dt_max恢复为预灌末值的70% ,205% ,78%和79% ;心肌钙为(316±84) μg·g^-1 (干重) ;以上变化均有显著差异。结论　PAMD对离体大鼠顿抑心肌有保护作用