Ágnes Végh

Preconditioning of the ischaemic myocardium; Involvement of the L-arginine nitric oxide pathway

1 Short periods of coronary artery occlusion protect the heart against the effects of a subsequent prolonged period of ischaemia. This phenomenon is known as preconditioning of the ischaemic myocardium. 2 In mongrel, chloralose‐urethane anaesthetized open‐chest dogs, within a restricted body weight range, two 5 min periods of occlusion of the anterior descending branch of the left coronary artery markedly reduced the severity of the early ischaemic arrhythmias resulting from a prolonged (25 min) occlusion of the same coronary artery starting 20 min later. Thus, the number of ventricular premature beats (VPBs) was reduced from 528 ± 140 in controls to 78 ± 27 in preconditioned dogs, the incidence of ventricular fibrillation (VF) was reduced from 47% to 0% and the incidence of ventricular tachycardia (VT) from 100% to 20%. ST‐segment elevation recorded from electrodes within the ischaemic area, and the degree of inhomogeneity of conduction within the ischaemic area were markedly reduced in these preconditioned dogs. 3 The incidence of VF following reperfusion of the ischaemic myocardium at the end of the 25 min occlusion period was reduced in the preconditioned dogs from 100% to 60%; there was thus a 40% survival from the combined ischaemia‐reperfusion insult compared with 0% in the controls. 4 NG‐nitro‐l‐arginine methyl ester (l‐NAME) an inhibitor of the l‐arginine nitric oxide pathway, given in a dose of 10 mg kg−1 intravenously on two occasions, both before the initial preconditioning occlusion and then again before the prolonged occlusion, partially attenuated the protective effects of preconditioning. There were more VPBs (220 ± 75), a higher incidence of VT (60%) and more episodes of VT (11.5 ± 6.0 compared to 0.7 ± 0.3 episodes in the preconditioned dogs not given l‐NAME); none of the animals survived reperfusion (incidence of VF 100%). The improvement in the severity of the degree of inhomogeneity which resulted from preconditioning was abolished by l‐NAME administration. 5 l‐NAME itself elevated blood pressure (from 96 ± 5 mmHg diastolic to 119 ± 7 mmHg), reduced heart rate (from 155 ± 7 to 144 ±4 beats min−1) but did not change LVEDP, LVdP/dtmax, coronary blood flow, ST‐segment elevation or the degree of inhomogeneity of conduction. When given 10 min before the prolonged coronary artery occlusion in dogs not subjected to preconditioning, l‐NAME had no significant effect on the severity of arrhythmias except for more periods of VT (a mean of 11.7 ± 4.7 episodes per dog). 6 It is concluded from these studies that the generation of nitric oxide contributes to the marked antiarrhythmic effects of preconditioning in the canine myocardium, probably through elevation of cyclic GMP.

Attenuation of the antiarrhythmic effects of ischaemic preconditioning by blockade of bradykinin B2 receptors

1 The possibility that bradykinin is involved in the pronounced antiarrhythmic effects of ischaemic preconditioning in anaesthetized mongrel dogs was examined with the use of the selective B2 antagonist, icatibant (Hoe‐140). 2 Preconditioning, achieved by two 5 min occlusions of the left anterior descending coronary artery, followed 20 min later by a 25 min occlusion of the same artery resulted, during this prolonged occlusion, in less severe arrhythmias (VF 0% versus 47% in control non‐preconditioned dogs), reductions in the incidence and number of episodes of ventricular tachycardia (VT) and in the number of ventricular premature beats and increased survival following reperfusion (50% versus 0% in the controls). 3 Hoe‐140 was given in a dose of 300μg kg−1 either before the preconditioning procedure or after preconditioning but before the prolonged occlusion. This dose of Hoe‐140 had insignificant haemodynamic effects and failed to modify the increase in coronary blood flow that occurred in the circumflex coronary artery when the anterior descending branch was occluded. 4 It was difficult to precondition dogs in the presence of Hoe–140. There were more ventricular arrhythmias during the initial 5 min occlusion (44 ± 12 versus 10 ± 3) and a higher incidence of ventricular fibrillation (50% versus 21%) during the preconditioning procedure. There was also a more pronounced ST‐elevation (recorded from epicardial electrodes) during the preconditioning occlusions in those dogs given Hoe‐140. 5 In those dogs that survived to the long (25 min) occlusion, Hoe‐140 prevented the antiarrhythmic effects of preconditioning (reduction in ventricular premature beats and in VT) although all the dogs survived the occlusion period. However on reperfusion, survival in the preconditioned dogs given Hoe‐140 was less than in those dogs preconditioned without the B2 antagonist. 6 Changes in the degree of inhomogeneity of conduction within the ischaemic area, which were markedly suppressed by preconditioning, were attenuated in those dogs preconditioned in the presence of Hoe–140. 7 These results suggest that bradykinin acts as both a ‘trigger’ for preconditioning and as one of the mediator protective (antiarrhythmic) substances generated by the myocardium under these conditions. Since the protection afforded both by preconditioning and by local intracoronary infusions of bradykinin is markedly attenuated by an inhibitor of the 1‐arginine nitric oxide pathway, we suggest that much of the protection afforded by ischaemic preconditioning results from the generation of nitric oxide, and that bradykinin, released early during ischaemia, acts as a stimulant for this generation.

Local intracoronary infusions of bradykinin profoundly reduce the severity of ischaemia-induced arrhythmias in anaesthetized dogs.

Bradykinin in a dose (25 ng kg−1min−1) which did not alter coronary flow, or saline, were infused into a small branch of the left anterior descending coronary artery in dogs anaesthetized with chloralose and urethane, for 10 min prior to coronary artery occlusion and throughout the 25 min occlusion period. The degree of inhomogeneity of conduction and epicardial ST‐segment changes were measured in the ischaemic zone with a composite electrode. In control dogs, coronary artery occlusion led to severe arrhythmias with an incidence of ventricular fibrillation of 47% and tachycardia of 80% and with a mean of 528 ± 140 ventricular premature beats. In marked contrast, those dogs administered bradykinin had no ventricular fibrillation or tachycardia and the number of premature beats was significantly less (53 ± 19). ST‐segment changes were also much less in these dogs. These results raise the possibility that bradykinin might contribute to the protective effects of preconditioning and acts as an ‘endogenous myocardial protective substance’.

Prevention by dexamethasone of the marked antiarrhythmic effects of preconditioning induced 20 h after rapid cardiac pacing

Dogs were paced, via a pacing electrode in the right ventricle, for four 5 min periods at a rate of 220 beats min−1. On the following day they were reanaesthetized, thoracotomized and the left anterior descending coronary artery occluded for 25 min. Pacing markedly reduced the severity of ischaemia‐induced arrhythmias (e.g. reduction in VF from 45% in unpaced dogs to 10% in paced dogs; P < 0.05), an effect reversed by dexamethasone (4 mg kg−1 i.v., 45 min prior to pacing). This protection may be due to the induction of nitric oxide synthase or cyclo‐oxygenase.

Prevention by an inhibitor of the L-arginine-nitric oxide pathway of the antiarrhythmic effects of bradykinin in anaesthetized dogs.

The intracoronary administration of bradykinin (25 ng kg−1 min−1) markedly reduces the severity of arrhythmias that occur during a 25 min occlusion of the left anterior descending coronary artery in chloralose, urethane anaesthetized dogs. This protection was abolished by the prior administration, by the same route, of NG‐nitro‐l‐arginine methyl ester (l‐NAME), an inhibitor of the l‐arginine‐nitric oxide pathway. The protective effect of bradykinin on reperfusion‐induced VF was not affected by l‐NAME. These results strongly suggest that the antiarrhythmic effect of bradykinin in this model is mediated by nitric oxide release. It also supports the concept that bradykinin might be a ‘primary mediator’ of the protective, antiarrhythmic effects of ischaemic preconditioning.

The local intracoronary administration of methylene blue prevents the pronounced antiarrhythmic effect of ischaemic preconditioning

Short periods of coronary artery occlusion (2 × 5 min) markedly reduce the severity of arrhythmias and the changes in ST‐segment elevation and in the degree of inhomogeneity of conduction during a subsequent 25 min occlusion of the left anterior descending coronary artery in anaesthetized dogs. These changes were completely reversed if methylene blue (5 mg min−1) was infused into a side branch of the coronary artery throughout both the preconditioning and prolonged occlusions. These results suggest that the pronounced antiarrhythmic effects of preconditioning result from activation of guanylyl cyclase and result in increased levels of guanosine 3′:5′‐cyclic monophosphate.

Cardiovascular effects of the calcium sensitizer, levosimendan, in heart failure induced by rapid pacing in the presence of aortic constriction.

1 The haemodynamic effects of a novel cardiotonic drug, levosimendan, which has both calcium‐sensitizing and phosphodiesterase III (PDE III) inhibitory properties, were studied in conscious dogs in which heart failure had been induced by prolonged cardiac pacing in the presence of aortic constriction. These effects were compared with those in sham‐operated dogs with essentially normal cardiac function. 2 Eighteen mongrel dogs were instrumented for the measurement of left ventricular pressure (LVSP, LVEDP) and contractile function (dP/dt; dP/dt/P). In twelve dogs a balloon catheter, positioned in the thoracic aorta, was inflated producing an approximate 60% reduction in effective aortic diameter. Twenty min later rapid ventricular pacing (240 beats mean−1) was commenced and maintained for 48 h by means of a bipolar pacing electrode introduced into the right ventricle. This electrode served also for recording changes in the endocardial electrogram in the absence of pacing. Six of these dogs were used to evaluate the haemodynamic changes of pacing‐induced heart failure; a further six of these dogs the haemodynamic changes elicited by levosimendan under these conditions. Six sham‐operated dogs (group 2) served as controls. 3 In six dogs (group 1) the haemodynamic alterations were assessed after the development of heart failure. In the presence of aortic constriction, 48 h continuous rapid cardiac pacing resulted in a marked deterioration in left ventricular function which remained stable for at least 48 h after cessation of pacing. Thus, there was a marked reduction in LVSP (15%), + dP/dtmax (35%), ‐dP/dtmax (36%) and also in dP/dt/P (29%), whereas LVEDP was increased considerably (from 6.4 ± 1.4 to 20.0 ± 2.2 mmHg). A marked elevation occurred in endocardial ST‐segment (138%), lasting for 20 min. 4 Levosimendan was administered intravenously in doses of 0.005, 0.01 and 0.03 μmol kg−1 to 2 groups of conscious dogs. In the sham‐operated dogs (group 2), only the higher dose (0.03 μmol kg−1) produced significant increases in LVSP (19%), + dP/dtmax (37%), and in dP/dt/P (32%). In dogs with heart failure (group 3) doses of 0.005, 0.01 and 0.03 μmol kg−1 levosimendan resulted in an improvement in + dP/dtmax (26%, 38% and 49%), ‐dP/dtmax (20%, 25% and 38%) and in dP/dt/P (19%, 34% and 50%) and reduction in the elevated LVEDP (from 20 ± 2.2 mmHg to 16 ±1.0, 10 ± 1.3 and 9± 1.0 mmHg, respectively). 5 Levosimendan proved to be a potent cardiotonic drug at the doses used, and was approximately three times more effective under conditions of impaired left ventricular function than in normal hearts.

Sildenafil (Viagra) reduces arrhythmia severity during ischaemia 24 h after oral administration in dogs

Sildenafil (Viagra) prolongs repolarisation in cardiac muscle, an effect that could lead to ventricular fibrillation (VF). Sildenafil (2 mg kg−1) was given by mouth to 12 mongrel dogs and, 24 h later, these dogs were anaesthetised, thoracotomised and subjected to a 25 min occlusion of the anterior descending coronary artery. Haemodynamic parameters were similar in this and the control group, but there were fewer and less serious ventricular arrhythmias during occlusion in the sildenafil group (VF 17 vs 60%; ventricular premature beats 140±52 vs 437±127% and episodes of ventricular tachycardia 4.0±3.2 vs 19.3±7.7%, all P<0.05). However, reperfusion VF and indices of ischaemia severity (epicardial ST‐segment mapping, inhomogeneity) were not modified by the drug. Sildenafil increased the QT interval, especially during ischaemia. Our conclusion is that ischaemia‐induced ventricular arrhythmias are reduced by sildenafil, but this protection is less pronounced than that following cardiac pacing or exercise.

Hemodynamic effects of calcium-sensitizing agents.

Over the last few years the development of calcium-sensitizing agents has led to a new approach to the treatment of congestive heart failure (CHF). These novel inotropic agents act directly on the contractile protein systems and correct the reduced responsiveness of the myofilaments to calcium. The resultant favorable inotropic activity of this new class of cardiotonic agents may represent an important pharmacologic approach to the future treatment of acute and chronic CHF. This brief review summarizes the cardiotonic effects of calcium sensitizers, with particular emphasis on the hemodynamic actions of these drugs in various in vitro and in vivo heart preparations. We also attempt to draw conclusions for the therapeutic exploitation of these agents and delineate their possible role in the therapy of CHF.

Pacing-induced delayed protection against arrhythmias is attenuated by aminoguanidine, an inhibitor of nitric oxide synthase

Cardiac pacing, in anaesthetized dogs, protects against ischaemia and reperfusion‐induced ventricular arrhythmias when this is initiated 24 h after the pacing stimulus. Now we have examined whether this delayed cardioprotection afforded by cardiac pacing is mediated through nitric oxide. Twenty‐two dogs were paced (4×5 min periods at 220 beats min−1) by way of the right ventricle, 24 h prior to a 25 min period of coronary artery occlusion. Nine of these dogs were given the inhibitor of induced nitric oxide synthase, aminoguanidine (50 mg kg−1 i.v.), 0.5 h prior to coronary artery occlusion. Sham‐operated non‐paced dogs with and without aminoguanidine treatment served as controls. Pacing markedly (P<0.05) reduced arrhythmia severity (ventricular fibrillation, VF, during occlusion 15%; survival from the combined ischaemia‐reperfusion insult 62%) compared to control, sham‐operated, unpaced dogs (VF during occlusion 58%; survival 17%). This protection was attenuated by the administration of aminoguanidine prior to coronary artery occlusion (survival from the combined ischaemia‐reperfusion insult 11%, which was significantly (P<0.05) less than in the paced dogs not given aminoguanidine and similar to the controls). Aminoguanidine had no significant effects on coronary artery occlusion when given to dogs that had not been paced. In the dose used aminoguanadine transiently elevated systemic arterial pressure by a mean of 20 mmHg and reduced heart rate by a mean of 22 beats min−1. These results suggest that nitric oxide, probably derived from induced nitric oxide synthase, contributes significantly to the delayed cardioprotection afforded by cardiac pacing.