Mechanisms of improved ischemic regional dysfunction by bradycardia. Studies on UL-FS 49 in swine

A pathophysiological compass to personalize antianginal drug treatment

Myocardial ischaemia results from coronary macrovascular or microvascular dysfunction compromising the supply of oxygen and nutrients to the myocardium. The underlying pathophysiological processes are manifold and encompass atherosclerosis of epicardial coronary arteries, vasospasm of large or small vessels and microvascular dysfunction - the clinical relevance of which is increasingly being appreciated. Myocardial ischaemia can have a broad spectrum of clinical manifestations, together denoted as chronic coronary syndromes. The most common antianginal medications relieve symptoms by eliciting coronary vasodilatation and modulating the determinants of myocardial oxygen consumption, that is, heart rate, myocardial wall stress and ventricular contractility. In addition, cardiac substrate metabolism can be altered to alleviate ischaemia by modulating the efficiency of myocardial oxygen use. Although a universal agreement exists on the prognostic importance of lifestyle interventions and event prevention with aspirin and statin therapy, the optimal antianginal treatment for patients with chronic coronary syndromes is less well defined. The 2019 guidelines of the ESC recommend a personalized approach, in which antianginal medications are tailored towards an individual patient's comorbidities and haemodynamic profile. Although no antianginal medication improves survival, their efficacy for reducing symptoms profoundly depends on the underlying mechanism of the angina. In this Review, we provide clinicians with a rationale for when to use which compound or combination of drugs on the basis of the pathophysiology of the angina and the mode of action of antianginal medications.

Prediction of Significant Coronary Artery Disease Through Advanced Echocardiography: Role of Non-invasive Myocardial Work

Background: Non-invasive prediction of critical coronary artery stenosis (CAST) in patients with coronary artery disease (CAD) is challenging. Strain parameters can often capture an impairment of regional longitudinal function; however, they are load dependent. A novel non-invasive method to estimate Myocardial Work (MW) has been recently proposed, showing a strong correlation with invasive work measurements. Our aim was to investigate the ability of non-invasive MW to predict the ischaemic risk area underlying a CAST. Methods and Results: The study population comprises 80 individuals: 50 patients with CAST and 30 controls (CTRL). Echocardiography recordings were obtained before coronary angiography to measure global longitudinal strain (GLS), Myocardial Work Index (MWI), Myocardial Constructive Work (MCW), Myocardial Wasted work (MWW), Myocardial Work Efficiency (MWE). Global MWI (p = 0.048), MWE (p < 0.001), and MCW (p = 0.048) at baseline were significantly reduced in patients with CAST compared to controls (p < 0.05). Regional MWE within the myocardial segments underlying the CAST, but not LS, was significantly reduced compared to non-target segments (p < 0.001). At ROC analysis, the diagnostic performance to predict CAST for regional MWE (AUC = 0.920, p < 0.001) was higher compared to both regional post-systolic shortening index (PSI) (AUC = 0.600, p = 0.129) and regional LS (AUC = 0.546, p = 0.469). Conclusions: Non-invasive estimation of MW work indices is able to predict a CAST before invasive angiography.

Non-invasive myocardial work is reduced during transient acute coronary occlusion

Background

During ischemia a close relationship exists between sub-endocardial blood flow and myocardial function. Strain parameters can capture an impairment of regional longitudinal function but are load dependent. Recently, a novel non-invasive method to calculate Myocardial Work (MW) showed a strong correlation with invasive work measurements.

Our aim was to investigate the ability of non-invasive MW indices to identify the ischaemic risk area during transient acute coronary occlusion (TACO).

Methods and results

The study population comprises 50 individuals with critical coronary stenosis (CCS). Echocardiography recordings were obtained before coronary angiography, during TACO and after revascularization to measure global longitudinal strain (GLS), Myocardial Work Index (MWI), Myocardial Constructive Work (MCW), Myocardial Wasted work (MWW), Myocardial work efficiency (MWE).

Compared to baseline, we found a significant reduction of GLS (p = 0.005), MWI, MCW and MWE (p<0.001) during TACO.

Conclusions

The non-invasive measurement of MW parameters is a sensitive and early marker of myocardial ischemia during TACO.

Old unsolved problems: when and how to treat silent ischaemia

Silent myocardial ischaemia (SMI) is defined as objective evidence of ischaemia without angina (or equivalent symptoms) in the presence of coronary artery disease, differing from silent coronary artery disease. Silent myocardial ischaemia represents the majority of episodes of myocardial ischaemia at Holter monitoring. During transient myocardial ischaemia, the symptoms appear after the contraction anomalies of the left ventricle and after the ECG changes. The cause of silent myocardial ischaemia is still not well established. The severity and duration of ischaemia have been theorized as important elements in the SMI mechanism. Another possible mechanism responsible for SMI is represented by changes in the perception of painful stimuli with an increased pain threshold. Finally, a neuronal dysfunction of the diabetic, in post-infarction or a cardiac neuronal ‘stunning’ could play a role in SMI. In the pre-stent era, the SMI was associated with a worse prognosis. In patients with diabetes mellitus, SMI seems to be more represented because autonomic dysfunction is present in this category of patients. In conclusion, SMI is more frequent than symptomatic ischaemia. However, despite the presence of countless studies on the subject, it is not clear today whether medical therapy has equalized the risk and what the real prognosis of SMI is.

A murine model of increased coronary sinus pressure induces myocardial edema with cardiac lymphatic dilation and fibrosis

Myocardial edema is a consequence of many cardiovascular stressors, including myocardial infarction, cardiac bypass surgery, and hypertension. The aim of this study was to establish a murine model of myocardial edema and elucidate the response of cardiac lymphatics and the myocardium. Myocardial edema without infarction was induced in mice by cauterizing the coronary sinus, increasing pressure in the coronary venous system, and inducing myocardial edema. In male mice, there was rapid development of edema 3 h following coronary sinus cauterization (CSC), with associated dilation of cardiac lymphatics. By 24 h, males displayed significant cardiovascular contractile dysfunction. In contrast, female mice exhibited a temporal delay in the formation of myocardial edema, with onset of cardiovascular dysfunction by 24 h. Furthermore, myocardial edema induced a ring of fibrosis around the epicardial surface of the left ventricle in both sexes that included fibroblasts, immune cells, and increased lymphatics. Interestingly, the pattern of fibrosis and the cells that make up the fibrotic epicardial ring differ between sexes. We conclude that a novel surgical model of myocardial edema without infarct was established in mice. Cardiac lymphatics compensated by exhibiting both an acute dilatory and chronic growth response. Transient myocardial edema was sufficient to induce a robust epicardial fibrotic and inflammatory response, with distinct sex differences, which underscores the sex-dependent differences that exist in cardiac vascular physiology.NEW & NOTEWORTHY Myocardial edema is a consequence of many cardiovascular stressors, including myocardial infarction, cardiac bypass surgery, and high blood pressure. Cardiac lymphatics regulate interstitial fluid balance and, in a myocardial infarction model, have been shown to be therapeutically targetable by increasing heart function. Cardiac lymphatics have only rarely been studied in a noninfarct setting in the heart, and so we characterized the first murine model of increased coronary sinus pressure to induce myocardial edema, demonstrating distinct sex differences in the response to myocardial edema. The temporal pattern of myocardial edema induction and resolution is different between males and females, underscoring sex-dependent differences in the response to myocardial edema. This model provides an important platform for future research in cardiovascular and lymphatic fields with the potential to develop therapeutic interventions for many common cardiovascular diseases.

Investigation of Metabolite Profile of YM758, a Novel If Channel Inhibitor

Background

YM758 monophosphate is a novel If channel inhibitor that has an inhibitory action for If current and shows a strong and specific activity, selectively lowering the heart rate and decreasing oxygen consumption by heart muscle.

Objectives

The objectives of the current study were to investigate the in vivo metabolic profiles of YM758 in mice, rats, rabbits, dogs, and monkeys and to elucidate the structures of YM758 metabolites.

Methods

Biological samples were analyzed by liquid chromatography hyphenated with a radiometric detection system and liquid chromatography coupled with a mass spectrometer to clarify their metabolic patterns. To elucidate their structures, metabolites were isolated and analyzed by mass spectrometry and nuclear magnetic resonance spectroscopy.

Results

Our results from in vivo metabolic profiling in humans and animals indicated there is no significant species difference in the metabolism of YM758, and the metabolic pathways of YM758 are considered to be oxidation, hydration, and demethylation followed by sulfate or glucuronide conjugation.

The instantaneous wave-free ratio (iFR) for evaluation of non-culprit lesions in patients with acute coronary syndrome and multivessel disease

BACKGROUND

Adenosine administration is currently required for evaluation of stenosis severity with fractional flow reserve (FFR). The instantaneous wave-free ratio (iFR) was recently introduced as an adenosine-free alternative in patients with stable CAD. The aim of the present study was to replicate the findings of previous iFR studies using an independent calculation algorithm and to evaluate the iFR for the assessment of non-culprit vessels in patients with acute coronary syndrome (ACS).

METHODS AND RESULTS

53 patients with ACS (65%) and at least one non-culprit intermediate lesion and 29 (35%) with stable CAD were included. A total of 123 stenoses were evaluated with both FFR and iFR. Classification match of iFR in ACS was not inferior to stable CAD (79.5% in ACS and 84.4% in CAD; p=0.497). Accordingly, no difference was observed in iFR/FFR correlation between ACS and stable CAD (r=0.66 in ACS vs. r=0.69 in CAD). Overall, a significant correlation was found between iFR and FFR (r=0.68; p<0.001) with a good diagnostic efficiency at ROC analysis (area under the curve: 0.87). In addition, neither the localization of the stenosis within the coronary tree (p=0.147) nor the time interval from the acute event (p=0.550) significantly influenced the concordance of iFR with FFR.

CONCLUSIONS

The iFR is a promising method for the assessment of non-culprit lesion severity in patients with acute coronary syndrome.

Investigation of long-term retention of unchanged (-)-N-{2-[(R)-3-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline-2-carbonyl)piperidino]ethyl}-4-fluorobenzamide, a novel "funny" If current channel inhibitor, and its metabolites in the eyeball and thoracic aorta of rats

(-)-N-{2-[(R)-3-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline-2-carbonyl)piperidino]ethyl}-4-fluorobenzamide (YM758), a novel "funny" If current channel inhibitor, was being developed as a treatment for stable angina and atrial fibrillation. After a single oral administration of (14)C-YM758, extensive accumulation and long-term retention of radioactivity were observed in the eyeballs of nonalbino rats and in the thoracic aorta of albino/nonalbino rats. Radioluminograms of the eyeballs of nonalbino rats indicated that the radioactivity was localized to the uveal tract, which suggests that the radioactivity may be positively charged and bound mainly to the melanins. Treatment with a mixture of 2 mol/l hydrochloric acid and methanol (5:95, v/v) allowed for the recovery of the major portion of radioactivity from the eyeball, which suggests reversible binding. The radioactive constituents in eyeballs consisted of the unchanged drug (YM758) and three metabolites [mainly 6,7-dimethoxy-2-[(3R)-piperidin-3-ylcarbonyl]-1,2,3,4-tetrahydroisoquinoline (YM-252124)]. Using the organic solvent mixture described above, almost all of the radioactivity was not collected from the thoracic aorta, and approximately 90% was recovered by treatment with elastase, which suggests that some metabolites covalently bind to the elastin fiber localized in the tunica media.

Relationship between exposure of (-)-N-{2-[(R)-3-(6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline-2-carbonyl)piperidino]ethyl}-4-fluorobenzamide (YM758), a "funny" if current channel inhibitor, and heart rate reduction in tachycardia-induced beagle dogs

(-)-N-{2-[(R)-3-(6,7-Dimethoxy-1,2,3,4-tetrahydroisoquinoline-2-carbonyl)piperidino]ethyl}-4-fluorobenzamide (YM758), a novel "funny" If current channel (If channel) inhibitor, is developed as a treatment for stable angina and atrial fibrillation. In this study, the pharmacokinetic/pharmacodynamic (PK/PD) relationship after intravenous administration of YM758 to tachycardia-induced dogs was investigated and described based on the simplified compartment model. The PK of YM758 in dogs did not differ between the nontreated and tachycardia-induced groups. A drug-induced reduction in heart rate (HR) was clearly observed, and the half-life of the duration of the effect (approximately 4.0 h) was longer than that of the plasma concentration of the unchanged drug. The fitting and simulation procedure from the PK/PD relationship between the time profiles for YM758 plasma concentration and HR reduction had an ECe(50) value (YM758 concentration in the effective compartment resulting in a 50% decrease of the maximum effect) of 6.0 ng/ml, which did not agree with the results of the in vitro experiment using right atria isolated from guinea pigs (EC(30), 70.4 ng/ml). In addition, in the in vitro experiments, YM758 metabolites had a weak inhibitory effect, if any, on the spontaneous beat rate of the right atria from guinea pigs. These data, along with the previous finding that YM758 and its metabolites are eliminated rapidly from rat hearts, indicate that the duration of the pharmacological effect of YM758 (compared with the rapid elimination of the plasma drug concentration) may be the result of strong binding and/or slower dissociation of YM758 in the If channel. Such PK/PD analyses allow the pharmacological profiles of many drugs, especially cardiovascular drugs, to be more readily understood and better predicted during the clinical stages.

Evaluation of the inhibitory and induction potential of YM758, a novel If channel inhibitor, for human P450-mediated metabolism

This study was designed to examine the in vitro metabolism of YM758, a novel cardiovascular agent, and to evaluate its potential to cause drug interactions and induction of CYP isozymes. After incubation with pooled human liver microsomes, YM758 was converted to two major metabolites (AS2036313-00, and YM-394111 or YM-394112). The formation of AS2036313-00, and YM-394111 or YM-394112 were mediated by CYP2D6 and CYP3A4, respectively, which was elucidated by using a bank of human liver microsomes and recombinant CYP enzymes in combination with the utilization of typical substrates and inhibitors. The Ki values of YM758 for midazolam, nifedipine, and metoprolol metabolism ranged from 59 to 340 microM, being much higher than the YM758 concentration in human plasma. The formation of AS2036313-00, and YM-394111 or YM-394112 was inhibited by quinidine and ketoconazole with Ki values of 140 and 0.24 microM, respectively, which indicates that YM758 metabolism may be affected by coadministration of strong CYP2D6 and 3A4 inhibitors in vivo, given the clinical plasma concentrations of quinidine and ketoconazole. After human hepatocytes were exposed to 10 microM YM758, microsomal activity and mRNA level for CYP1A2 were not induced while those for CYP3A4 were slightly induced. The tested concentration was much higher than that in human plasma, which suggests that the induction potential of YM758 is also negligible.

Tissue distribution of YM758, a novel If channel inhibitor, in pregnant and lactating rats

In this study the tissue distribution of radioactivity in pregnant and lactating rats was investigated by quantitatively determining radioactivity concentrations and by whole-body autoradioluminograms after a single oral administration of 14C-YM758. In addition, the transfer of radioactivity into the reproductive tissues, foetus, and milk is discussed in terms of the localization of transporters in syncytiotrophoblast and mammary gland. The radioactivity concentrations in the liver were the highest of all the tissues and organs tested at all the sampling times. The radioactivity in main tissues (liver and kidney), including reproductive tissues (amniotic fluid, placenta, ovary, and uterus), was not retained for a long time, as in the plasma. The tissue/plasma (T/P) ratio of radioactivity in the foetus was below 1.0, which might be due to Mdr1-mediated export of YM758 into blood via the blood-placenta barrier since YM758 is a substrate for hMDR1, not for hBCRP/rBcrp. The T/P ratio of radioactivity in the maternal milk 1 and 4 h after oral administration of 14C-YM758 was 7.2 and 11.0, respectively. To understand better the distribution of new drugs into the reproductive tissues/milk, and to interpret further the results of reproductive safety studies for drug development, the contribution of transporters expressed in the blood-placenta barrier and mammary gland to the drug-transfer into placenta and milk should be considered.

Heart rate in the pathophysiology of coronary blood flow and myocardial ischaemia: benefit from selective bradycardic agents

Starting out from a brief description of the determinants of coronary blood flow (perfusion, pressure, extravascular compression, autoregulation, metabolic regulation, endothelium-mediated regulation and neurohumoral regulation) the present review highlights the overwhelming importance of metabolic regulation such that coronary blood flow is increased at increased heart rate under physiological circumstances and the overwhelming importance of extravascular compression such that coronary blood flow is decreased at increased heart rate through reduction of diastolic duration in the presence of severe coronary stenoses. The review goes on to characterize the role of heart rate in the redistribution of regional myocardial blood flow between a normal coronary vascular tree with preserved autoregulation and a poststenotic vasculature with exhausted coronary reserve. When flow is normalized by heart rate, there is a consistent close relationship of regional myocardial blood flow and contractile function for each single cardiac cycle no matter whether or not there is a coronary stenosis and what the actual blood flow is. beta-Blockade improves both flow and function along this relationship. When the heart rate reduction associated with beta-blockade is prevented by pacing, alpha-adrenergic coronary vasoconstriction is unmasked and both flow and function are deteriorated. Selective heart rate reduction, however, improves both flow and function without any residual negative effect such as unmasked alpha-adrenergic coronary vasoconstriction or negative inotropic action.

[Selection and pharmacological characterisation of Procoralan, a selective inhibitor of the pacemaker If current]

The screening of a series of benzocycloalkane derivatives led to the selection of Procoralan (ivabradine), the first selective inhibitor of the depolarizing If (funny) current of the sinus node, for the treament of myocardial ischaemia. In vitro, this compound reduces the spontaneous beating rate of isolated right rat atria and the firing rate of the action potential of rabbit sinus node preparations. This effect is explained by a reduction in the diastolic depolarisation slope of the action potential and underlies a selective inhibition of the pacemaker If current. In vivo, it induces a selective reduction in heart rate both at rest and during exercise. It preserves myocardial contractility, atrioventricular conduction and ventricular repolarisation duration. Ivabradine exerts a similar anti-ischaemic activity in exercise-induced myocardial ischaemia in pigs to that of a beta-blocker and, furthermore, it limits to a greater extent ischaemic myocardial contractile dysfunction.

Myocardial hibernation: a delicate balance

The pathophysiology of myocardial hibernation is characterized as a situation of reduced regional contractile function distal to a coronary artery stenosis that recovers after removal of the coronary stenosis. A subacute "downregulation" of contractile function in response to reduced regional myocardial blood flow exists, which normalizes regional energy and substrate metabolism but does not persist for more than 12-24 h. Chronic hibernation develops in response to one or more episodes of myocardial ischemia-reperfusion, possibly progressing from repetitive stunning with normal blood flow to hibernation with reduced blood flow. An upregulation of a protective gene program is seen in hibernating myocardium, putting it into the context of preconditioning. The morphology of hibernating myocardium is characterized by both adaptive and degenerative features.

Fréquence cardiaque et ischémie myocardique expérimentale

Every increase in heart rate represents a poor prognostic factor in cardiology, and multiple arguments have now led to the belief that reducing heart rate is a major therapeutic challenge. A comparison of the pharmacological effects of If current inhibitors such as zatebradine, and more recently ivabradine (Procoralan) and beta-blockers, have demonstrated experimentally that reductions in heart rate and myocardial contractile force contribute equally to the reduction in myocardial oxygen consumption in the normal heart. Conversely, at a similar level of reduction in heart rate, the lack of a concomitant negative inotropic effect with ivabradine affords longer diastolic perfusion times than beta-blockers. In other words, a negative inotropic effect is deleterious when an increase in coronary blood flow is required. Hence, if the anti-ischaemic effects afforded by an If current inhibitor and a beta-blocker are roughly comparable, the former are clearly of higher benefit than beta-blockers in the treatment of myocardial dysfunction accompanying cardiac ischaemia-reperfusion, especially myocardial stunning.

Effects of tachycardia on regional wall motion in acute ischemic canine heart

Tachycardia accompanies the preload reduction. Our aim is to assess the effect of the heart rate change on wall motion in ischemic heart. In 8 dogs with occlusion of left anterior descending artery, we changed the heart rate (heart rate 90, 120, and 150 beats/minute) after using UL-FS49, a selective bradycardic agent, with atrial pacing. Preload was changed by inferior vena caval occlusion at a heart rate of 90 beats/minute. With either an increase in heart rate or an inferior vena caval occlusion, the end-diastolic length was decreased, but the end-diastolic length relationships between the non-ischemic and the ischemic region made different lines from those of the heart rate change and inferior vena caval occlusion. When increasing the heart rate, isovolumetric shortening was unchanged in the non-ischemic region with more expansion in the ischemic region. While inferior vena caval occlusion at a heart rate of 90 beats/minute, isovolumetric shortening was increased in the non-ischemic region, with more expansion in the ischemic region. Both in tachycardia and by the inferior vena caval occlusion, ejectional shortenings decreased in the non-ischemic and ischemic regions. Our results suggest that, in ischemic heart, tachycardia changes both in the end-diastolic length relationship between the non-ischemic and the ischemic region and at the isovolumetric contraction phase. The changes seem to be not only due to the inferior vena caval occlusion, but also due to tachycardia itself.

Anti-Ischemic Effects of Ivabradine, a Selective Heart Rate-Reducing Agent, in Exercise-Induced Myocardial Ischemia in Pigs

The effects of ivabradine, a novel heart rate-reducing agent that inhibits the cardiac pacemaker current If, were compared with those of the beta-adrenergic blocker propranolol, in a model of exercise-induced regional myocardial ischemia in pigs. Five Yucatan micropigs were chronically instrumented to measure hemodynamics, regional myocardial contractility, and local electrograms, and a fixed stenosis of the left anterior descending coronary artery was induced using a clip. Each animal underwent three experiments on different days, each consisting of two treadmill exercise sessions, 4 hours apart. Ivabradine 5 mg/kg, propranolol 5 mg/kg, or vehicle was administered orally 3 hours before the second exercise session. Exercises before treatment and after vehicle produced reproducible hemodynamic changes and regional myocardial ischemia in the area perfused by the stenosed coronary artery, indicated by ST segment shift and regional contractile dysfunction. Ivabradine and propranolol were equipotent in reducing heart rate at rest and limiting tachycardia during exercise. Ivabradine, unlike propranolol, did not reduce left ventricular contractility at rest or during exercise, and did not increase atrio-ventricular conduction time. Both compounds reduced the exercise-induced ST segment shift in the ischemic region by approximately 80%, but ivabradine preserved systolic shortening to a significantly greater degree than propranolol (P < 0.05).

Synthesis and pharmacological evaluation of 2-(3-piperidyl)-1,2,3,4-tetrahydroisoquinoline derivatives as specific bradycardic agents

Novel 1,2,3,4-tetrahydroisoquinoline derivatives bearing directly a cyclic amine at the 2-position were prepared and examined for their bradycardic activities in isolated right atria and in anesthetized rats. The structure-activity relationships (SAR) study revealed that the 2-(3-piperidyl)-1,2,3,4-tetrahydroisoquinoline skeleton is essential for the appearance of potent in vitro activity, and that the presence of at least one methoxy group at the 6- or 7-position of the 1,2,3,4-tetrahydroisoquinoline ring is important to exert potent in vitro activity. In vivo tests of selected compounds demonstrated that 2-(1-benzyl-3-piperidyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline (6c) exhibited potent bradycardic activity with negligible influence on mean blood pressure in rats, although its potency is a half of that of Zatebradine.

Contributions of heart rate and contractility to myocardial oxygen balance during exercise

The respective contributions of heart rate (HR) reduction and left ventricular (LV) negative inotropy to the effects of antianginal drugs are debated. Accordingly, eight instrumented dogs were investigated during exercise at spontaneous and paced HR (250 beats/min) after administration of either saline, atenolol, or ivabradine (selective pacemaker current channel blocker). During exercise, atenolol and ivabradine (both 1 mg/kg iv) similarly reduced HR (-30% from 222 +/- 5 beats/min), and LV mean ejection wall stress was not altered. LV dP/dt(max) was reduced by atenolol but not ivabradine. Diastolic time (DT) was increased by atenolol versus saline (195 +/- 6 vs. 123 +/- 4 ms, respectively) and to a greater extent by ivabradine (233 +/- 11 ms). Myocardial oxygen consumption (MVo(2)) was lower under ivabradine and atenolol versus saline (6.7 +/- 0.6 and 4.7 +/- 0.4 vs. 8.1 +/- 0.6 ml/min, respectively, P < 0.05). Under pacing, DT and MVo(2) were similar between ivabradine and saline but significantly reduced with atenolol. Thus HR reduction and negative inotropy equally contribute to the reduction in MVo(2) during exercise in the normal heart. The negative inotropy limits the increase in DT afforded by HR reduction.

Acute effects of zatebradine on cardiac conduction and repolarization

Zatebradine, a potent bradycardic agent, is believed to act selectively at the sinoatrial node. The selectivity of such a property relative to various electrophysiologic classes of action is not well defined. To characterize the electrophysiologic properties of zatebradine, the corrected sinus node recovery time, sinoatrial conduction time, conduction intervals, atrial effective refractory period and monophasic action potential duration in the ventricle were measured before and after incremental doses of zatebradine (0.1-1.5 mg/kg) in 15 anesthetized dogs. The electrophysiologic effects of zatebradine developed immediately after a single i.v. bolus dose, reaching steady-steady-state at 15 minutes with the maximum effect evident at 0.75 mg/kg. The IC(50) was 0.23 mg/kg. There was no significant effect on the sinus node recovery time. The PR interval on the electrocardiogram was significantly increased when the dose was higher than 0.25 mg/kg. The duration of the P wave and the PA interval were not changed. Zatebridine greatly increased the AH (from 135 to 178 milliseconds) without changing the HH and HV intervals in His bundle recordings. The EC(50) of this effect was 0.58 mg/kg. The QRS interval was not changed. The QTc was significantly increased from 0.43 to 0.56 s(1/2) (P < 0.05). The action potential duration was significantly increased by high dose zatebradine (> 0.5 mg/kg), the EC(50) for this effect was 0.76 mg/kg. The atrium effective refractory period was significantly increased (31%) with an EC(50) 0.69 mg/kg. These results indicate that zatebradine selectively inhibits sinus node automaticity at low doses. The inhibition of the AV nodal conduction and the lengthening of the refractory period and repolarization in the atria and the ventricles occur at higher dose.

Exercise training improves diastolic perfusion time in patients with coronary artery disease

Exercise training elicits an improvement in work capacity and in left-ventricular function in patients with coronary artery disease. An improvement in myocardial oxygen supply accounts for these effects. The aim of this study was to test the hypothesis that exercise training could favorably influence diastolic perfusion time, a major determinant of subendocardial perfusion. Twenty-two male patients with coronary artery disease were randomized to a training or control group. At the study entry and after one year, all patients underwent an exercise stress test. After one year, rest heart rate was lower and diastolic perfusion time was higher in the training group but not in the control group. At peak of exercise, diastolic perfusion time increased and ST-segment depression decreased significantly in the training group but not in the control group. A significant relation was found between the R-R interval and the diastolic perfusion time either before or after training, with a difference in the intercepts of two regressions. Training shifted updown-line regression, effecting a higher value of diastolic perfusion time for a given value of heart rate. Thus, training increases diastolic perfusion time, independently from the effect on heart rate. This mechanism may contribute to the improvement of myocardial perfusion.

Beneficial effects of heart rate reduction on cardiac mechanics and energetics in patients with left ventricular dysfunction

It has been shown recently that the force-frequency relationship is blunted in experimental heart failure models. Furthermore, tachycardia is thought to have adverse effects on the diseased heart for several reasons, one of which is an increase in myocardial oxygen consumption. Inversely, the oxygen-saving effects of bradycardia may be beneficial for the treatment of heart failure. The aim of this study was to elucidate how heart rate (HR) modulates cardiac mechanics and energetics in patients with left ventricular (LV) dysfunction. LV pressure-volume data and myocardial oxygen consumption (MVO2) was assessed using conductance and coronary sinus thermodilution catheters in 14 patients with moderate LV dysfunction (mean ejection fraction 34%) under 3 conditions: (a) basal, (b) HR increased by 20% using atrial pacing, and (c) HR decreased by 16% using a specific bradycardic agent, zatebradine (7.5 mg p.o.). Atrial pacing decreased external work (EW) (from 0.39 to 0.31 J beat(-1) m(-2), p<0.05) at a comparable MVO2 per beat with a marginal increase in LV contractility index (Ees) (from 2.34 to 2.76 mm Hg ml(-1) m(-2), p = 0.08), resulting in a decrease in mechanical efficiency (EW/MVO2) (from 25.9 to 22.1%, p<0.05). In contrast, zatebradine did not decrease Ees (from 2.34 to 2.24 mm Hg ml(-1) m(-2), NS), but increased EW (from 0.39 to 0.42 J beat(-1) m(-2), p<0.05 vs. basal level) without a change in MVO2 per beat, resulting in improved mechanical efficiency (from 25.9 to 29.7%, p<0.05 vs. basal level). These results suggest that mild bradycardia is energetically advantageous and does not decrease myocardial contractility and performance, whereas pacing-induced tachycardia worsens cardiac mechanics and energetics in patients with LV dysfunction. Thus, the oxygen-saving effect of bradycardia may be beneficial for the treatment of heart failure.

The relation of contractile function to myocardial perfusion. Perfusion-contraction match and mismatch

During normoperfusion, both myocardial blood flow and contractile function are heterogeneously distributed throughout the left ventricle. Midwall segment shortening is greater at the apex than at the base of the left ventricle, and greater in the anterior than in the posterior wall. Also, transmural heterogeneity of myocardial deformation exists, with greater segment shortening and wall thickening in inner than in outer myocardial layers. Myocardial blood flow is greater in inner than in outer myocardial layers. Apart from transmural heterogeneities, there are adjacent regions with largely different resting flow in the same heart. While an increase in myocardial contractile function will lead to a metabolically mediated increase in myocardial blood, an increase in regional coronary perfusion within or above the autoregulatory range does not increase regional myocardial contractile function. During hypoperfusion induced by a proximal coronary stenosis, the reduction in subendocardial blood flow is more pronounced than that in subepicardial blood flow, and contractile function in the inner myocardial layers ceases more rapidly than in the outer myocardial layers. The reduced regional myocardial contractile function is closely matched to the reduced regional myocardial blood flow; however, such a coupling between reduced flow and function is lost when ischemia is prolonged for several hours in that function for a given flow is further reduced. Also, acute embolization of the coronary microcirculation induces a progressive loss of regional myocardial function at an unchanged regional myocardial blood flow, i.e. perfusion-contraction mismatch. During reperfusion, regional myocardial contractile function remains depressed for a prolonged period of time, depending on the severity, duration and location of the preceding ischemic episode, while regional myocardial blood flow is restored to almost normal. Recovery of contractile function in the outer myocardial layers is faster than in the inner myocardial layers.

Hibernating myocardium

Decreased myocardial contraction occurs as a consequence of a reduction in blood flow. The concept of hibernation implies a downregulation of contractile function as an adaptation to a reduction in myocardial blood flow that serves to maintain myocardial integrity and viability during persistent ischemia. Unequivocal evidence for this concept exists in scenarios of myocardial ischemia that lasts for several hours, and sustained perfusion-contraction matching, recovery of energy and substrate metabolism, the potential for recruitment of inotropic reserve at the expense of metabolic recovery, and lack of necrosis are established criteria of short-term hibernation. The mechanisms of short-term hibernation, apart from reduced calcium responsiveness, are not clear at present. Experimental studies with chronic coronary stenosis lasting more than several hours have failed to continuously monitor flow and function. Nevertheless, a number of studies in chronic animal models and patients have demonstrated regional myocardial dysfunction at reduced resting blood flow that recovered upon reperfusion, consistent with chronic hibernation. Further studies are required to distinguish chronic hibernation from cumulative stunning. With a better understanding of the mechanisms underlying short-term hibernation, it is hoped that these adaptive responses can be recruited and reinforced to minimize the consequences of acute myocardial ischemia and delay impending infarction. Patients with chronic hibernation must be identified and undergo adequate reperfusion therapy.

Stereospecific in vitro and in vivo effects of the new sinus node inhibitor (+)-S 16257

The effects of the two isomers, (+)-S 16257 and (-)-S 16260, of a new bradycardic agent, (+/-)-S 15544 (7,8-dimethoxy 3-[3-[[(4.5-dimethoxybenzocyclobutan-1-yl)methyl] methylamino]propyl]1,3,4,5-tetrahydro-2H-3-benzazepin-2-one), were compared in vitro and in vivo on cardiac spontaneous rate and repolarization time. In the isolated rabbit sino-atrial node, the three compounds (3 microM) were equi-effective to reduce the action potential firing rate. In anesthetized pigs, both isomers (0.03, 0.1, 0.3 and 1 mg kg(-1) i.v.) were equipotent to reduce heart rate. For all compounds, the negative chronotropic effect resulted from a reduction in the slope of diastolic depolarization of pacemaker cells. In sino-atrial node cells, (-)-S 16260 (3 microM) increased action potential duration while (+)-S 16257 had a smaller effect. In driven guinea-pig papillary muscles exposed to increasing concentrations of compounds (0.1 to 10 microM) a small prolongation of action potential duration was observed. This prolongation was more marked in rabbit Purkinje fibers stimulated at a low rate. In all cardiac preparations the highest prolongation was observed with (-)-S 16260. In vivo, (-)-S 16260 prolonged QTc at the two highest doses tested while (+)-S 16257 had no effect. In conclusion, resolution of (+/-)-S 15544 into its two enantiomers yielded compounds with the same bradycardic effects. Of the isomers, (+)-S 16257 has an increased specificity with minimal direct effect on action potential repolarization.

Zatebradine slows ectopic ventricular rhythms in canine heart 24 hours after coronary artery ligation

Arrhythmias occur 24 h after occlusion of the left anterior descending (LAD) coronary artery in the canine heart and have been attributed to the abnormal spontaneous activity in subendocardial Purkinje fibers, which are markedly depolarized. The major current underlying normal automaticity in these fibers is i(f). Although the i(f) activation range is generally considered to be more negative than the diastolic membrane potential in these depolarized fibers in infarcts, this activation range has been shown to shift in a positive direction in response to hormonal influences. Thus i(f) could still mediate automaticity in these fibers in infarcts. Furthermore, recent reports indicate that a depolarizing diastolic current, probably i(f), also can be measured in ventricular muscle during abnormal experimental conditions, which may occur during ischemia. To test whether there is a role of i(f) currents in sustaining ventricular ectopy, we administered the selective i(f) channel blocker, zatebradine, 24 h after LAD ligation in canine hearts. We report that intravenous injections of zatebradine (0.25 or 1.0 mg/kg) significantly slow ventricular rhythms (with average reductions of 19 or 26%, respectively). Moreover, because zatebradine also slows sinus nodal rate, it can lead to an increased incidence of ectopic beats. However, during right atrial pacing, when sinus slowing has no effect on ventricular rhythms, capture of ventricular rhythms occurs at lower rates in the presence of zatebradine. The reduction of capture threshold is comparable to the reduction in the rate of the ectopic rhythm. Thus zatebradine eliminated the arrhythmia when the right atrium was paced at the original sinus rate.

Force-frequency relations in the failing rabbit heart and responses to adrenergic stimulation

BACKGROUND

Recent experiments have documented the importance of beta-adrenergic regulation of the force-frequency relation (FFR) in the normal and failing heart. As in isolated human cardiac muscle, a descending limb of the FFR occurs at high frequencies in the intact rabbit heart, and therefore a new model of atrial pacing-induced heart failure was developed in the rabbit. Responses of the FFR to beta-adrenergic stimulation were then assessed in the conscious state before and after the induction of heart failure.

METHODS AND RESULTS

Rapid atrial pacing for an average of 19.5 days in instrumented rabbits produced severe left ventricular dilation with reduced cardiac output (echocardiography) and depressed myocardial contractility and relaxation rate (left ventricular dP/dt, catheter-tip micromanometer), associated with reductions in beta-adrenergic receptor density and adenylyl cyclase activity. Before heart failure, heart rate was slowed in the conscious animal from 280 +/- 30 (SD) to about 225 beats/min using a sinus node inhibitor (zatebradine), and heart rate was then increased in steps by atrial pacing from 250 to 450 beats/min; the heart rate-versus-left ventricular dP/dtmax (FFR) response showed an ascending response (increasing contractility), with a descending limb at rates greater than 375 beats/min, and dobutamine infusion amplified the ascending limb of the FFR (increased slope) and attenuated the descending limb. In heart failure the basal FFR was severely depressed with a descending limb over 350 beats/min; dobutamine shifted the FFR upward somewhat without change in the slope of the ascending limb, whereas dobutamine prevented the descending limb of the FFR. Similar responses were observed in the relations between heart rate and cardiac output.

CONCLUSIONS

A new model of heart failure in the conscious rabbit was developed using rapid atrial pacing and applied to study force-frequency effects. In heart failure, normal beta-adrenergic amplification of the ascending limb of the FFR by dobutamine was absent, but a marked descending limb of the FFR at higher heart rates was prevented by dobutamine. Observed reductions in components of the beta-adrenergic receptor system likely were responsible for impaired beta-adrenergic FFR amplification, but the mechanism(s) for the descending limb and its correction by dobutamine are not yet established. These responses of the FFR may influence importantly the ability of the failing heart to respond to exercise and stress.

Inotropic stimulation by dobutamine increases left ventricular regional function at the expense of metabolism in hibernating myocardium

The mechanism by which dobutamine increases the contraction of chronically dysfunctional myocardium and its effects on metabolism are still unknown. The aim of this study was to assess regional myocardial metabolism at rest and during an intracoronary dobutamine infusion in patients with hibernating myocardium. Eleven asymptomatic patients with single proximal stenosis of the left anterior descending coronary artery and persistent left ventricular dysfunction at rest (undergoing percutaneous transluminal coronary angioplasty [PTCA]) were studied prospectively. Regional left ventricular function was assessed by two-dimensional (2D) echocardiography and regional perfusion by thallium-201 single-proton-emission computed tomography. Great cardiac vein and aortic blood samples were obtained for measurements of lactate and plasma free fatty acid (FFA) concentrations. Inotropic challenge, obtained by using intracoronary dobutamine infusion, increases regional left ventricular function. However, the arteriovenous AV lactate difference was 0.206 = 0.070 mmol/L at rest, and it decreased to 0.018 = 0.069 mmol/L (p < 0.05 vs baseline) and 0.066 = 0.068 mmol/L (p < 0.05 vs baseline) at 4 and 10 minutes of dobutamine infusion, respectively. Thus the hibernating myocardium does not produce lactate at rest. However, when regional contraction is stimulated, dobutamine-induced inotropic challenge may cause a perfusion-contraction mismatch with an activation of anaerobic glycolysis.

Effects of a sinus node inhibitor on the normal and failing rabbit heart

The effects on cardiac function of slowed frequency produced by a sinus node inhibitor (zatebradine, or UL-FS 49) were studied in the conscious rabbit under control conditions (n = 16) and after heart failure was produced by rapid atrial pacing for an average of 18.5 days (n = 8). Echocardiography was used to verify severe left ventricular (LV) dysfunction, and high-fidelity micromanometry and cardiac output measurements (Doppler echo) were performed. Echocardiographic fractional shortening was 40.3 +/- 4.1 % (SD) in controls; in heart failure it was 18.0 +/- 1.6 %, and the LV was enlarged. In controls, as heart rate (HR) was decreased from 279 beats per minute (bpm) by incremental doses of zatebradine (up to 0.75 mg/kg), maximal changes occurred when the heart reached 218 bpm with a maximum decrease of the first derivative of LV pressure (LV dP/dtmax) of 15.9 %; LV enddiastolic pressure (EDP) increased from 4.3 to 8.4 mmHg along with a significant decrease in cardiac index (CI) of 15.2 %, while LV systolic pressure (SP) was stable. In heart failure, LV dP/dtmax and CI were markedly reduced compared to controls and with reduction of HR from 257 to 221 bpm LV dP/dtmax was unchanged, LVEDP increased slightly (NS), LVSP was unchanged and CI fell by 13.5 % at the highest dose. In subgroups (control n = 9, failure n = 6), in order to eliminate the hemodynamic effects of cardiac slowing by zatebradine the sinus rate present before zatebradine was matched by atrial pacing; this procedure eliminated all hemodynamic abnormalities accompanying cardiac slowing in both groups. In conclusion, slowed HR due to a sinus node inhibitor was well tolerated in severe heart failure, and all negative hemodynamic responses in both controls and in heart failure were due entirely to a negative forcefrequency effect, without a direct depressant action of zatebradine on the myocardium.

Altered inotropic and lusitropic responses to heart rate in conscious dogs with tachycardia-induced heart failure

OBJECTIVES

The effects of increasing heart rate on left ventricular contraction and relaxation were examined in conscious dogs with tachycardia-induced heart failure under autonomically blocked conditions.

BACKGROUND

Previous studies using isolated myocardium have shown attenuated positive inotropic responses to stimulation frequency in heart failure. However, these responses have not been well examined in intact preparations in the presence of heart failure with autonomic system blockade, where the intrinsic ventricular responses to increasing heart rate could be revealed.

METHODS

Seven dogs were instrumented with a micromanometer and a conductance volume catheter. After autonomic blockade to eliminate neural reflexes, left ventricular contractile properties were quantified by the slope of the end-systolic pressure-volume relation (ventricular elastance), and left ventricular relaxation was assessed by the time constant of isovolumetric ventricular pressure decay.

RESULTS

Increasing the heart rate by 60 beats/min enhanced ventricular elastance by 71 +/- 18% (mean +/- SD) and decreased end-systolic volume by 6 +/- 5% in normal hearts. In failing hearts, ventricular elastance increased by only 21 +/- 20%, and end-systolic volume did not change appreciably. Although the reduction in left ventricular end-diastolic and minimal pressures by tachycardia was smaller in the failing heart, ventricular relaxation rate remained unaltered both in the normal heart and in the failing heart.

CONCLUSIONS

Under conscious but autonomically blocked conditions, effects of increasing heart rate on the failing left ventricle are characterized by a predominant attenuation of the inotropic response rather than of the lusitropic response.

Intracoronary amiloride prevents contractile dysfunction of postischemic "stunned" myocardium: role of hemodynamic alterations and inhibition of Na+/H+ exchange and L-type Ca2+ channels

OBJECTIVES

This study sought to establish the effect of amiloride on stunned myocardium and to determine the role of hemodynamic alterations and inhibition of sodium/proton (Na+/H+) exchange and L-type cytosolic calcium (Ca2+) channels.

BACKGROUND

Amiloride is a nonspecific agent that may reduce reperfusion injury, but its effect on reversible dysfunction or stunned myocardium is unclear.

METHODS

Ninety-seven open chest dogs undergoing 15 min of left anterior descending coronary artery occlusion and 3 h of reperfusion with monitoring of hemodynamic variables, systolic shortening and myocardial blood flow were randomized to seven intracoronary infusions: control dogs (5% dextrose, n = 16); low dose amiloride (1 mg/min, n = 14); high dose amiloride (5 mg/min) with (n = 12) and without (n = 16) atrial pacing; sodium nitroprusside (20 micrograms/min, n = 16); hexamethylene amiloride (a specific inhibitor of Na+/H+ exchange, 60 micrograms/min, n = 14); and nifedipine (a specific inhibitor of L-type Ca2+ channels, 5 micrograms/min, n = 9). Drug infusions were started 40 min before occlusion and stopped at 30 min after reperfusion.

RESULTS

Forty-three dogs were excluded because of ventricular fibrillation or high collateral flow. The incidence of ventricular fibrillation was similar in all groups to that in control dogs. Systolic shortening completely recovered (p = NS vs. baseline; p < 0.01 vs. control group) by 2 h after reperfusion in the low dose amiloride group and 30 min in the high dose group (p < 0.01 vs. low dose). High dose amiloride increased myocardial blood flow and had positive inotropic and negative chronotropic effects (p < 0.05 vs. control group). Atrial pacing did not attenuate recovery. The only effect of low dose amiloride was increased myocardial blood flow after reperfusion. Systolic shortening did not deteriorate after washout of drug effects. Sodium nitroprusside and nifedipine similarly increased myocardial blood flow, but systolic shortening never recovered. Hexamethylene amiloride had no hemodynamic effects, and systolic shortening never recovered.

CONCLUSIONS

Amiloride prevented the contractile dysfunction of myocardial stunning but did not prevent arrhythmias. Hemodynamic alterations, increased myocardial blood flow and inhibition of Na+/H+ exchange or L-type Ca2+ channels alone did not account for the improved function. Inhibition of Na+/Ca2+ exchange may be the mechanism of improved postischemic function.

Insulin-like growth factor-1 enhances ventricular hypertrophy and function during the onset of experimental cardiac failure

To determine whether additional hypertrophy would be beneficial or maladaptive in cardiac failure, the effects of insulin-like growth factor (IGF-1) were investigated in rats with left ventricular (LV) dysfunction. In normal rats, 3 mg/kg per d of recombinant human IGF-1 for 14 d augmented LV wt (32%) and increased LV/body wt ratio (P < 0.01). 2 d after coronary occlusion, rats were randomized to IGF-1 (3 mg/kg per d) or placebo. After 2 wk, IGF-1-treated rats showed significant increases in LV wt (13%) and LV wt/tibial length ratio, but LV/body wt ratio was unchanged. By microangiography, compared with controls (n = 12) IGF-1-treated rats (n = 16) showed increased LV end-diastolic volume (19%) and stroke volume (31%) (both significant normalized to tibial length, but not to body wt). Average infarct size did not differ between groups. The LV ejection fraction (EF) was not significantly different between groups, but estimated cardiac output was higher in treated rats; there was a significant interaction for the EF between infarct size and treatment (P = 0.029) and a trend for EF to be higher in treated rats with large infarctions (EF 33.4 vs 25.1% in controls). Myocyte cross-sectional areas in noninfarcted LV zones tended to be larger in treated rats (232.1 vs 205.4 microns 2; P = 0.10), but there was no difference in capillary density and collagen content did not differ between groups. In conclusion, IGF-1 administration caused hypertrophy of the normal heart in vivo. When stimulated by IGF-1, the severely dysfunctional heart in evolving myocardial infarction is capable of undergoing additional hypertrophy with evidence of improved function, suggesting a beneficial effect. Further investigation of the potential role of growth factor therapy in heart failure appears warranted.

Electrophysiological effects of S 16257, a novel sino-atrial node modulator, on rabbit and guinea-pig cardiac preparations: comparison with UL-FS 49

1. S 16257 is a new bradycardic agent. Its electropharmacological profile has been compared to that of the known bradycardic compound UL-FS 49 (Zatebradine). Intracellular recordings of action potentials (APs) were performed with conventional glass microelectrodes. 2. In the rabbit isolated sino-atrial node (SAN) tissue, S 16257 and UL-FS 49 (1 microM, 3 microM and 10 microM) were equipotent in slowing spontaneous APs firing predominantly by decreasing the rate of diastolic depolarization (at 3 microM, -23.8 +/- 3.9% and -27.9 +/- 2.6%, respectively). For the two compounds a maximal effect was obtained at 3 microM. In these preparations, action potential duration at 50% of total repolarization (APD50) was more affected by UL-FS 49 than S 16257 at any concentration tested (at 3 microM, +8.9 +/- 2.9% and +29.1 +/- 3.7% for S 16257 and UL-FS 49, respectively; P < or = 0.01). 3. To estimate the direct effects on AP duration, driven cardiac preparations were exposed to these agents. In guinea-pig papillary muscles, paced at a frequency of 1 Hz, increasing concentrations of S 16257 or UL-FS 49 (0.1 to 10 microM, 30 min exposure for each concentration) slightly prolonged AP repolarization. This prolongation was more marked for UL-FS 49 (at 1 microM, +6.1 +/- 0.6% and +11.2 +/- 1.3% elevation of APD50, for S 16257 and UL-FS 49, respectively). 4. Application of UL-FS 49 (3 microM) to rabbit Purkinje fibres, triggered at a frequency of 0.25 Hz, induced a marked prolongation of APD50 and APD90 (+149.4 +/- 51.2% and +86.0 +/- 15.4%, respectively). S 16257 (3 MicroM) induced only a weak prolongation of AP (+ 14.1 +/- 5.0% and + 14.8 +/- 3.3% for APD50 and APD90, respectively) significantly smaller than in the case of UL-FS 49.5. These results show that S 16257 slows the rate of spontaneous AP firing in isolated SAN mainly by a reduction of the diastolic depolarization of the cells, which suggests an inhibition of the pace-maker current (If). S 16257 and UL-FS 49 are equipotent in their bradycardic effect but S 16257 is more specific as it induces less increase in myocardial repolarization time.

Anti-arrhythmic efficacy of beta-adrenergic blockade during acute ischemia in myocardium with scar

Ventricular arrhythmia production in the ischemic heart is considered to be influenced by prior infarction. Although beta-adrenergic blockade is known to have beneficial effects during acute ischemia, its anti-arrhythmic efficacy during post-infarction ischemia is not known. To explore this question, we have used a model with a relatively high incidence of ischemic arrhythmias. Mongrel dogs 2 to 3 years of age were studied intact under anesthesia. An irreversible injury of the infero-posterior myocardium was produced with an electrode catheter 1 week earlier. The arrhythmic response to acute ischemia was assessed using serial, transient 15-minute occlusions of the left-anterior descending coronary artery with a balloon catheter. During ischemia alone, the incidence of ventricular fibrillation in animals who underwent all phases of the study was 6 of 9; with atenolol (0.2 mg/kg intravenously) and ischemia, 1 of 9 (p < 0.05). To assess the role of the bradycardic response, the latter was repeated 1 week subsequently during atrial pacing at the heart rate that existed before ischemia. Fibrillation occurred in 8 of 9, a significant reversal of the therapeutic effect. To exclude the potential artifact of a fixed intervention protocol, a study was undertaken with the short-acting esmolol, in which three ischemic periods were alternated at 1-hour intervals: (A) ischemia without treatment, (B) ischemia with continuous infusion of 150 micrograms/kg/min esmolol, and (C) same as B except that heart rate was maintained by atrial pacing.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The role of heart rate in myocardial ischemia and infarction: implications of myocardial perfusion-contraction matching

The pathophysiology of myocardial ischemia traditionally has been attributed to disturbances of oxygen demand, as observed in classic effort-induced angina pectoris, or to a primary disruption of coronary blood supply, as in unstable angina or acute myocardial infarction. Laboratory research eliciting various types of perfusion-contraction matching has challenged such a historical distinction between supply and demand-induced determinants of myocardial ischemia. A growing number of clinical studies analyzing the role of heart rate in the course of coronary heart disease suggest the possibility that a common perfusion-contraction scheme may underlie these diverse clinical manifestations. During experimental myocardial ischemia, produced by a low coronary blood flow, regional perfusion-contraction matching exists in which the energy demands and regional contraction are reduced to match the diminished myocardial substrate supply. Heart rate is a major factor influencing transmural blood flow distribution and regional function, because when coronary vasodilation is maximal there is an inverse relation between the level of heart rate and subendocardial perfusion. Thus, in experimental regional ischemia, increasing heart rate reduces subendocardial flow and contraction, whereas slowing of heart rate causes improvement of contraction associated with increased subendocardial blood flow, accompanied by a decrease in outer wall blood flow. Also, "interventricular steal" of blood from the left ventricle by the right ventricle during ischemia can be reversed by slowing the heart rate in the presence of regional ischemia. Improvement of contraction by heart rate slowing is more than would be expected on the basis of the increase in subendocardial perfusion alone, reflecting a combination of decreased oxygen demand and increased oxygen supply, and separate curves relating blood flow per minute to contractile function are observed at different heart rates. However, when perfusion is normalized for heart rate by expressing subendocardial blood flow in units per beat, a single relation is observed at different heart rates. This observation supports the concept of a close coupling between subendocardial blood flow per beat and regional performance, or perfusion-contraction matching, during various levels of ischemia. Based on these principles, it can be predicted that exercise-induced regional ischemia in the presence of coronary stenosis will be attenuated by several mechanisms when heart rate is slowed using either a beta-blocking agent, or a specific bradycardic drug.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

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

The effects of UL-FS 49, a specific bradycardic agent, on systemic hemodynamics, regional myocardial function (sonomicrometry, percentage of segment shortening), and regional coronary blood flow (radioactive microspheres) were studied in open-chest, anesthetized dogs with severe left circumflex coronary artery (LCX) stenosis. UL-FS 49 was administered as two sequential bolus injections of 0.25 mg/kg. Heart rate decreased from 149 +/- 13 beats/min to 102 +/- 6 and 77 +/- 4 beats/min after 0.25 and 0.5 mg/kg cumulative doses of UL-FS 49, respectively. The reduction in heart rate was not associated with any significant change in left ventricular pressure or mean arterial pressure, left ventricular dp/dt, or coronary vascular resistance. Similarly no hemodynamic changes occurred with atrial pacing to the initial heart rate. Application of an LCX stenosis of sufficient severity to produce a 50% reduction in mean LCX blood flow (44 +/- 4 to 22 +/- 2 ml/min) resulted in a significant reduction in the percentage of segment shortening in the ischemic zone (9.8 +/- 1.6% to 6.5 +/- 1.1%). The percentage of segment shortening in the ischemic zone progressively improved to 8.4 +/- 1.2% and 9.4 +/- 0.5% after 0.25 and 0.5 mg/kg UL-FS 49, respectively. Subepicardial perfusion in the ischemic zone was decreased and subendocardial perfusion was increased after administration of UL-FS 49. Consequently the ischemic zone endocardial/epicardial ratio increased from 0.43 +/- 0.08 to 1.12 +/- 0.22 and 1.48 +/- 0.32 with low and high doses of UL-FS 49.(ABSTRACT TRUNCATED AT 250 WORDS)

The selective bradycardic effects of zatebradine (UL-FS 49) do not adversely affect left ventricular function in conscious pigs with chronic coronary artery occlusion

This study was designed to test whether the selective bradycardic effects of zatebradine (UL-FS 49) were altered in the setting of chronic mild left ventricular dysfunction secondary to a myocardial infarction. We therefore administered four doses of UL-FS 49 at 15-min intervals (cumulative doses of 10, 30, 100, and 300 micrograms/kg) to eight normal conscious pigs and to seven pigs in which the left circumflex coronary artery was occluded 3 weeks previously. Left ventricular dysfunction in this second group of animals was manifested by an increase in left ventricular end-diastolic pressure (LVEDP, 11 +/- 2 mmHg vs. 7 +/- 1 mmHg, respectively; p less than 0.05) and a decrease in LVdP/dtmax (3020 +/- 210 mmHg vs. 3720 +/- 210 mmHg, respectively; p less than 0.05). The results showed that UL-FS 49 was equally effective in reducing heart rate in both groups of animals [from 126 +/- 4 to 95 +/- 2 beats/min and from 140 +/- 5 to 98 +/- 6 beats/min for the normal animals and for the animals with a chronic myocardial infarction (MI), respectively]. The duration of left ventricular systole was not affected, but the duration of diastole was prolonged from 290 +/- 10 msec to 420 +/- 20 msec in the normal animals and from 250 +/- 10 msec to 430 +/- 30 msec in the animals with MI (both p less than 0.05). Up to 100 micrograms/kg UL-FS 49 did not affect arterial blood pressure, whereas LVdP/dtmax and cardiac output decreased by less than 10% in either group.(ABSTRACT TRUNCATED AT 250 WORDS)