Regional redistribution of myocardial perfusion by UL-FS 49, a selective bradycardic agent

Vagal stimulation mimics preconditioning and postconditioning of ischemic myocardium in mice by activating different protection mechanisms

Vagal stimulation (VS) during myocardial ischemia and reperfusion has beneficial effects. However, it is not known whether short-term VS applied before ischemia or at the onset of reperfusion protects the ischemic myocardium. This study was designed to determine whether short-term VS applied before ischemia or at the onset of reperfusion reduces myocardial infarct size (IS), mimicking classic preconditioning and postconditioning. A second objective was to study the participation of muscarinic and nicotinic receptors in the protection of both preischemic and reperfusion stimulation. FVB mice were subjected to 30 min of regional myocardial ischemia followed by 2 h of reperfusion without VS, with 10-min preischemic VS (pVS), or with VS during the first 10 min of reperfusion (rVS). pVS reduced IS, and this effect was abolished by atropine and wortmannin. rVS also reduced IS in a similar manner, and this effect was abolished by the α₇-nicotinic acetylcholine receptor blocker methyllycaconitine. pVS increased Akt and glycogen synthase kinase (GSK)-3β phosphorylation. No changes in Akt and GSK-3β phosphorylation were observed in rVS. Stimulation-mediated IS protection was abolished with the JAK2 blocker AG490. rVS did not modify IL-6 and IL-10 levels in the plasma or myocardium. Splenic denervation and splenectomy did not abolish the protective effect of rVS. In conclusion, pVS and rVS reduced IS by different mechanisms: pVS activated the Akt/GSK-3β muscarinic pathway, whereas rVS activated α₇-nicotinic acetylcholine receptors and JAK2, independently of the cholinergic anti-inflammatory pathway. NEW & NOTEWORTHY Our data suggest, for the first time, that vagal stimulation applied briefly either before ischemia or at the beginning of reperfusion mimics classic preconditioning and postconditioning and reduces myocardial infarction, activating different mechanisms. We also infer an important role of α₇-nicotinic receptors for myocardial protection independent of the cholinergic anti-inflammatory pathway.

Vagal nerve stimulation prevents reperfusion injury through inhibition of opening of mitochondrial permeability transition pore independent of the bradycardiac effect

BACKGROUND

In spite of recent advances in coronary interventional therapy, reperfusion injury is still considered to be a major problem in patients undergoing surgical procedures, such as bypass grafting. Here we demonstrate a novel therapeutic strategy against ischemia-reperfusion injury: vagally mediated prevention of reperfusion-induced opening of mitochondrial permeability transition pore.

METHODS

We investigated the effects of efferent vagal stimulation on myocardial reperfusion injury with ex vivo and in vitro rat models. In the ex vivo model the hearts were perfused with intact vagal innervation, which allowed us to study the effects of the vagal nerve on the heart without other systemic effects.

RESULTS

Compared with sham stimulation, vagal stimulation exerted a marked anti-infarct effect irrespective of the heart rate (34% +/- 6% vs 85% +/- 9% at a heart rate of 300 beats/min, 37% +/- 4% vs 43% +/- 5% at a heart rate of 250 beats/min, and 39% +/- 4% vs 88% +/- 7% at a heart rate of 350 beats/min) after a 30-minute period of global ischemia, activated cell-survival Akt cascade, prevented downregulation of the antiapoptotic protein Bcl-2, and suppressed cytochrome-c release and caspase-3 activation. Furthermore, vagal stimulation-treated hearts exhibited a significant improvement in left ventricular developed pressure (78 +/- 5 vs 45 +/- 8 mm Hg) and a significant attenuation in an incremental change in left ventricular end-diastolic pressure during reperfusion. These beneficial effects of vagal stimulation were abolished by a permeability transition pore opener, atractyloside. In the in vitro study with primary-cultured cardiomyocytes, acetylcholine prevented a reoxygenation-induced collapse in mitochondrial transmembrane potential through inhibition of permeability transition pore opening.

CONCLUSION

Vagal stimulation would be a potential adjuvant therapy for the rescue of ischemic myocardium from reperfusion injury, and the protective effects are independent of its bradycardiac effects.

Comparison of the potassium channel blocker tedisamil with the beta-adrenoceptor blocker esmolol and the calcium antagonist gallopamil in patients with coronary artery disease

BACKGROUND

Tedisamil is a new bradycardic agent proven to exert anti-ischemic and antiarrhythmic effects by blockade of the different cardiac and vascular K+ currents.

HYPOTHESIS

It was the aim of the present study to compare the favorable anti-ischemic effects of tedisamil, with two long established representatives in the treatment of coronary artery disease (CAD), namely, the beta1 blocker esmolol and the Ca2 antagonist gallopamil.

METHODS

The hemodynamic and neurohumoral effects of the new potassium channel blocker tedisamil, an agent with negative chronotropic and class III antiarrhythmic properties, were compared with the ultra-short-acting beta1-selective adrenoceptor blocker esmolol and the calcium antagonist gallopamil. A total of 22 patients with angiographically proven CAD and reproducible ST-segment depression in the exercise electrocardiogram was included in two studies with an almost identical design and inclusion criteria. The investigation was carried out using right heart catheterization and bicycle ergometry. A subgroup of 8 patients receiving 0.3 mg/kg body weight tedisamil intravenously (i.v.) in an open dose-finding study was compared with a group of 14 patients who had received esmolol (i.v. bolus of 500 micrograms/kg, maintenance dose 200 micrograms/kg/min) and gallopamil (initial dose 0.025 mg/kg, maintenance dose 0.0005 mg/kg/h) in a second intraindividual comparison.

RESULTS

Tedisamil and esmolol reduced heart rate at rest by 13% (p < 0.001), and 6% (p < 0.05), and at maximum working levels by 8% (p < 0.01) and 9% (p < 0.05), respectively. Gallopamil increased heart rate at rest by 7% (p < 0.05), with only slight changes occurring during exercise. Corresponding findings for each drug were observed for cardiac output both at rest and during exercise [tedisamil: at rest -10% (NS), max. exercise -8%; esmolol: at rest -14% (NS), max. exercise -18% (NS); gallopamil: no significant changes]. Compared with tedisamil, stroke volume was reduced by esmolol [at rest and max. workload: -9% (NS)] and gallopamil [rest: -6% (NS), max. exercise: -2% (NS)]. Of the indirect parameters of ventricular function, that is, mean capillary wedge pressure (PCWPm) and right ventricular ejection fraction, only PCWPm demonstrated significant differences between tedisamil and gallopamil (+18% and -6% at rest, +17% and -21% during exercise, respectively; p < 0.001). Compared with gallopamil, both tedisamil and esmolol were superior in their effects on rate-pressure product, myocardial oxygen consumption, and ST-segment depression, whereas plasma lactate concentration was more reduced by tedisamil and gallopamil. Tedisamil led to a fall in norepinephrine levels in particular.

CONCLUSION

Tedisamil and esmolol showed almost equipotent anti-ischemic effects at the doses administered. Tedisamil acts mainly by reductions in heart rate, and esmolol, though to a lesser degree, also by reductions in systolic blood pressure. The mechanism of gallopamil is to reduce afterload and to improve coronary perfusion. At the doses applied, however, it has lower antianginal potency compared with tedisamil and esmolol.

Effects of changing heart rate on electrophysiological and hemodynamic function in the dog

Cardiovascular parameters were measured in dogs after RR interval was changed from 0.25 s to 1.2 s with atropine and graded doses of zatebradine, an I(f)-channel blocker. Left ventricular (LV) pre-ejection period (PEP), systemic vascular resistance, tau (an estimate of myocardial stiffness), PQ, QTc, dLVP/dt(max) and dLVP/dt(min), aortic pressure, and right atrial pressure did not change when each parameter was plotted against RR interval (r(2)'s < or = 0.5). LV end-diastolic pressure, stroke volume index, LV ejection time (ET), and QT all increased either linearly or curvilinearly as RR interval prolonged. Cardiac output index and PEP/ET decreased curvilinearly. When heart rate (HR) was fixed by pacing, and graded doses of zatebradine were given, changes in cardiovascular function were minimal. Thus zatebradine affects cardiovascular function principally by changing HR and not by affecting function directly. This study provides data on the effects of changing HR, alone, on cardiovascular parameters measured frequently during pharmacological and toxicological studies. It should prove useful when physiological variables, including HR, change, and there is need to know what change in HR, alone, contributes.

Effects of zatebradine and propranolol on canine ischemia and reperfusion-induced arrhythmias

1,3,4,5-Tetrahydro-7,8-dimethoxy-3[3-[[2-(3, 4-dimethoxyphenyl)-ethyl]methylamino]propyl]-2H-3-benzazepin-2-one -hy drochloride (Zatebradine) is a specific bradycardiac agent, blocking the hyperpolarization-activated pacemaker current (I(f)), and thus has no negative inotropic effect. The purpose of this study was to examine whether zatebradine is effective against ischemia and reperfusion-induced arrhythmias in dogs compared to propranolol. Arrhythmia was induced by ligation of the left anterior descending coronary artery followed by reperfusion. Ischemia-induced biphasic arrhythmias were suppressed in both zatebradine and propranolol groups. During ischemia, fatal ventricular fibrillation occurred in four dogs in the control group, 0 in the zatebradine group, and two dogs in the propranolol group. Of the 31 dogs subjected to reperfusion, mortality rates in the zatebradine, propranolol, and control groups were 56%, 75%, and 86%, respectively, and there were no significant differences. In the heart beating 10 beats/min faster than the predrug heart rate by atrial pacing, both zatebradine and propranolol attenuated ischemia-induced arrhythmias but did not affect reperfusion arrhythmias. Our results suggest that I(f) and/or beta-adrenoceptors rather than the bradycardiac action might be related to the antiarrhythmic effects during ischemia, but that they do not play a role in the generation of the reperfusion-induced ventricular arrhythmias.

Class III antiarrhythmic effects of zatebradine. Time-, state-, use-, and voltage-dependent block of hKv1.5 channels

BACKGROUND

Zatebradine is a bradycardic agent that inhibits the hyperpolarization-activated current (I(f)) in the rabbit sinoatrial node. It also prolongs action potential duration in papillary muscles in guinea pigs and in Purkinje fibers in rabbits. The underlying mechanism by which zatebradine induces this effect has not been explored, but it is likely to involve K+ channel block.

METHODS AND RESULTS

Cloned human cardiac K+ delayed rectifer currents (hKv1.5) were recorded in Ltk- cells transfected with their coding sequence. Zatebradine 10 mumol/L did not modify the initial activation time course of the current but induced a subsequent decline to a lower steady-state current level with a time constant of 109 +/- 16 ms. Zatebradine inhibited hKv1.5 with an apparent KD of 1.86 +/- 0.14 mumol/L. Block was voltage dependent (electrical distance delta = 0.177 +/- 0.003) and accumulated in a use-dependent manner during 0.5- and 1-Hz pulse trains because of slower recovery kinetics in the presence of the drug. Zatebradine reduced the tail current amplitude, recorded at -30 mV, and slowed the deactivation time course, which resulted in a "crossover" phenomenon when control and zatebradine tail currents were superimposed.

CONCLUSIONS

These results indicate that (1) zatebradine is an open-channel blocker of hKv 1.5, (2) binding occurs in the internal mouth of the ion pore, (3) unbinding is required before the channel can close, and (4) zatebradine-induced block is use dependent because of slower recovery kinetics in the presence of the drug. These effects may explain the prolongation of the cardiac action potential and could be clinically relevant.

Effects of the two enantiomers, S-16257-2 and S-16260-2, of a new bradycardic agent on guinea-pig isolated cardiac preparations

1. The electromechanical effects of two enantiomers, S-16257-2 (S57) and S-16260-2 (R60), were studied and compared in guinea-pig isolated atria and ventricular papillary muscles. The possible stereoselectivity of the interaction on the cardiac Na+ channel was analysed by comparing the effects of the two enantiomers on the onset and recovery kinetics of the frequency-dependent Vmax block. 2. In spontaneously beating right atria, S57 and R60 (10(-8)M-10(-4M) exerted a negative chronotropic effect (pIC50 = 5.07 +/- 0.19 and 4.76 +/- 0.18, respectively) and prolonged the sinus node recovery time, this effect being more marked with S57. In electrically driven left atria, S57 decreased (P < 0.05) contractile force only at 10(-4M) and R60 at concentrations > or = 5 x 10(-5M), whereas in papillary muscles the negative inotropic effect appeared at concentrations > 10(-5M). 3. In papillary muscles driven at 1 Hz, S57 and R60 at concentrations higher than 5 x 10(-6M) produced a concentration-dependent decrease in the maximum upstroke velocity (Vmax) and amplitude of the cardiac action potential without altering the resting membrane potential or the action potential duration. S57 and R60 had no effect on the characteristics of the slow action potentials elicited by isoprenaline in ventricular muscle fibres depolarized in high K+ (27 mM) solution. 4. At 5 x 10(-5M), S57 and R60 produced a small tonic Vmax block. However, in muscles driven at rates between 0.5 and 3 Hz both enantiomers produced an exponential decline in Vmax (frequency-dependent Vmax block) which augmented at higher rates of stimulation. The onset and offset rates of the frequency-dependent Vmax block were similar for both drugs. Both S57 and R60 prolonged the recovery time constant from the frequency-dependent block from 20.1 +/- 2.9 ms to 2-3 s.5. At 5 x 10-5 M, S57 and R60 shifted the membrane responsiveness curve in a hyperpolarizing direction.6. It can be concluded that S57 and R60, the two enantiomers of the new bradycardic agent, produced a similar frequency-dependent Vmax block which indicated that the interaction with the Na+ channel was not stereospecific. The analysis of the onset and offset kinetics of the frequency-dependent Vmax block suggested that both enantiomers can be considered as Na+ channel blockers with intermediate kinetics,e.g., class IA antiarrhythmic drugs.

Zatebradine, a specific bradycardic agent, enhances the positive inotropic actions of dobutamine in ischemic myocardium

OBJECTIVES

This investigation determined whether attenuation of the tachycardia produced by dobutamine administration would improve perfusion and function distal to a severe coronary artery stenosis.

BACKGROUND

Tachycardia adversely affects perfusion and function distal to a coronary artery stenosis. It is not known whether a specific bradycardic agent can improve blood flow and function in an ischemic zone during administration of dobutamine.

METHODS

The effects of dobutamine (2, 5 and 10 micrograms/kg body weight per min) alone and in combination with zatebradine (0.5 mg/kg), a specific bradycardic agent, on hemodynamic status, segment shortening (ultrasound length transducers) and myocardial perfusion (microspheres) were studied in anesthetized dogs with severe left circumflex coronary artery stenosis.

RESULTS

A 50% reduction in left circumflex coronary artery blood flow (58 +/- 4 to 29 +/- 2 ml/min [mean value +/- SEM]) produced a decrease in systolic shortening in the ischemic zone. Only a dose of dobutamine that did not elevate heart rate (2 micrograms/kg per min) produced an increase in segment shortening in the ischemic zone. High doses of dobutamine (10 micrograms/kg per min) caused an increase in heart rate without improvement in function and a reduction in the subendocardial/subepicardial flow ratio (0.74 +/- 0.06 to 0.48 +/- 0.05). Zatebradine administered in the presence of dobutamine caused a decrease in heart rate, an increase in subendocardial/subepicardial blood flow ratio (0.48 +/- 0.05 to 0.78 +/- 0.09) and allowed an increase in ischemic zone segment shortening. When normalized for changes in heart rate, ischemic zone subendocardial flow increased by 123 +/- 41% (0.39 +/- 0.09 to 0.71 +/- 0.12 ml/100 g per beat). Atrial pacing abolished the effects of zatebradine.

CONCLUSIONS

The present data suggest that the perfusion-contraction matching that accompanies a decrease in heart rate results in enhancement of inotropic stimulation of an ischemic zone. The actions of zatebradine are related to an increase in subendocardial blood flow per beat that allows improvement of regional contractile function.

Selective inhibition by E4080, a novel bradycardic agent, of positive chronotropic responses to norepinephrine in isolated dog hearts

E4080, a novel bradycardic agent acts on various ionic currents including the hyperpolarization-activated inward current (I(f)), L-type Ca2+ current (ICa) and ATP-sensitive K+ (K+ATP) current in mammalian heart and vascular tissues. We thus investigated the chronotropic and inotropic effects of E4080 and its interaction with the positive cardiac responses to norepinephrine, 3-isobutyl-1-methyl-xanthine (IBMX) and Bay k 8644 in the isolated, blood-perfused dog right atria and left ventricles. E4080 (0.01-1 mumol) decreased the sinus rate and atrial and ventricular contractile forces in a dose-related manner. Glibenclamide (3 mumol) partly blocked the decrease in atrial force but not the decreases in sinus rate and ventricular force induced by E4080. Atropine (10 nmol) did not affect the negative cardiac responses to E4080. E4080 (0.01-1 mumol) inhibited the positive chronotropic responses to norepinephrine and IBMX dose dependently, but did not inhibit the positive inotropic ones in isolated atria. E4080 affected neither positive chronotropic nor inotropic responses to Bay k 8644. These results suggest that (1) the activation of K+ATP channels by E4080 is partly related to the decrease in atrial force but not the decreases in sinus rate and ventricular force, and (2) the selective inhibition of E4080 of the cyclic AMP-dependent positive chronotropic responses but not inotropic ones is probably due to the inhibition of I(f) rather than other properties, e.g., activation of K+ATP channels and inhibition of ICa in the dog heart.

Tedisamil (KC 8857) is a new specific bradycardic drug: does it also influence myocardial contractility? Analysis by the conductance (volume) technique in coronary artery disease

To determine whether inotropism influences the bradycardic action of tedisamil, hemodynamic assessment was performed in 13 patients with ischemic coronary artery disease including analysis of end-systolic pressure-volume relationships after an infusion of tedisamil, 0.3 mg/kg, at rest, and during paced tachycardia stress. Slope Emax fell by 14% at rest (13 patients) and by 10% during tachycardia (6/13 patients), whereas loops of end-systolic pressure-volume relationships moved rightward; all parameter changes indicated a lack of significant inotropism loss with tedisamil (p > 0.05). Although the mean heart rate decreased from 77.5 to 64.7 beats/min and QTc duration increased by 14% (p < 0.05), filling pressure and dp/dtmin remained unchanged and vascular resistance increased by 30%. Parameters of left ventricular pump function (ejection fraction, stroke volume, left ventricular efficiency) decreased slightly (between 3% and 13%), whereas left ventricular volumes increased (end-diastolic volume by 6%, end-systolic volume by 23%). The respective parameter changes during tachycardia were comparable in tendency, and angina could no longer be induced during postdrug pacing stress. We concluded that the bradycardic effects of tedisamil are selectively generated without impairing either ventricular pump function or contractility in a clinically relevant fashion, whereas the postdrug anginal threshold appears elevated. Thus tedisamil can be used safely in ischemic coronary artery disease.