Bernard Gout

Matrix metalloproteinase expression in cardiac myocytes following myocardial infarction in the rabbit

Myocardial infarction (MI), leads to cardiac remodeling, thinning of the ventricle wall, ventricular dilation, and heart failure, and is a leading cause of death. Interactions between the contractile elements of the cardiac myocytes and the extracellular matrix (ECM) help maintain myocyte alignment required for the structural and functional integrity of the heart. Following MI, reorganization of the ECM and the myocytes occurs, contributing to loss of heart function. In certain pathological circumstances, the ECM is modulated such that the structure of the tissue becomes damaged. The matrix metalloproteinases (MMPs) are a family of enzymes that degrade molecules of the ECM. The present experiments were performed to define the time-course, isozyme subtypes, and cellular source of increased MMP expression that occurs following MI in an experimental rabbit model. Heart tissue samples from infarcted and sham animals were analyzed over a time-course of 1-14 days. By zymography, it was demonstrated that, unlike the sham controls, MMP-9 expression was induced within 24 hours following MI. MMP-3 expression, also absent in sham controls, was induced 2 days after MI. MMP-2 expression was detected in both the sham and infarcted samples and was modestly up-regulated following MI. Tissue inhibitor of metalloproteinase-1 (TIMP-1) expression was evaluated and shown to be down-regulated following MI, inverse of MMP-9 and MMP-3 expression. Further, MMP-9 and MMP-3 expression was detected by immunohistochemistry in myocytes within the infarct. Additional studies were conducted in which cultured rat cardiac myocytes were exposed to a hypoxic environment (2% O2) for 24 hours and the media analyzed for MMP expression. MMP-9 and MMP-3 were induced following exposure to hypoxia. It is speculated that the net increase in proteolytic activity by myocytes is a contributing factor leading to myocyte misalignment and slippage. Additional studies with a MMP inhibitor would elucidate this hypothesis.

Cell Type-specific Localization of Human Cardiac S1P Receptors

Sphingosine 1-phosphate (S1P), which derives from the metabolism of sphingomyelin, is mainly synthesized, stored, and released from platelets after activation by physiological and pathophysiological events. S1P acts in cardiovascular tissues through cell surface G-protein-coupled receptors of the endothelial differentiation gene (EDG) family, i.e., EDG1, EDG3 and EDG5. The aim of the present study was to assess the precise distribution of EDG1, EDG3, and EDG5 receptors expressed in human cardiovascular tissues to investigate their respective physiological implication. When assessed by Northern blots, EDG1, EDG3, and EDG5 displayed wide expression levels in decreasing order, respectively. In particular, EDG3 was mainly detected in the aorta. Detailed analysis by in situ hybridization (ISH) and immunohistochemistry (IHC) revealed strong EDG1 expression in cardiomyocytes and in endothelial cells of cardiac vessels. In cardiomyocytes, the EDG1 receptor is likely to be co-expressed with EDG3 and EDG5, although EDG1 exhibits the most prominent expression pattern. Unlike EDG3 and EDG5, which are expressed in the smooth muscle cell layer of the human aorta, no signal corresponding to EDG1 expression could be detected in the aorta. Moreover, only EDG3 expression was also found in smooth muscle cells of cardiac vessels. The present results provide new insight into the expression pattern of S1P receptors in human cardiovascular tissues, indicating a differential pattern of expression for these receptors in human vessels.

Actions and interactions of E-4031 and tedisamil on reperfusion-induced arrhythmias and QT interval in rat in vivo.

SummaryThe effects of the Ito blocker, tedisamil (0.1, 1.0, and 3.0 mg/kg, IV), and the IK blocker, E-4031 (0.1, 1.0, and 3.0 mg/kg, IV), on the incidence and duration of reperfusion-induced arrhythmias were compared in the anesthetized rat (n=12 per group). Reperfusion arrhythmias were evaluated after a 5 minute occlusion period of the left main coronary artery. In the absence of any pronounced effect on blood pressure, tedisamil and E-4031 reduced heart rate in a dose-dependent manner. During the preischemic period, QTc interval was increased by tedisamil but was not changed by E-4031. Both compounds increased the QTc interval during the ischemic period and also during the reperfusion. E-4031 was unable to reduce the incidence and duration of reperfusion-induced ventricular arrhythmias after 5 minutes of coronary artery occlusion. Tedisamil dose-dependently reduced the duration of reperfusion arrhythmias and their incidence. In a second set of experiments, the combination of tedisamil (1.0 mg/kg) with E-4031 (1.0 mg/kg) was administered. The electrocardiographic action of this combination was similar to that observed with tedisamil given alone. However, with the combination the incidence of fibrillation was reduced from 83% in the control group to 8% in the treated group (p<0.001), and the mortality was reduced from 67% to 0% (p<0.001), that is, to a greater extent than with tedisamil (1.0 mg/kg) alone. The results show that the blockade of Ito by tedisamil allows a reduction of reperfusion-induced mortality and that a sepecific IK blocker (E-4031) is devoid of antifibrillatory action in the anesthetized rat. However, as E-4031 might potentiate the antifibrillatory effect of tedisamil, an action on other currents might be envisaged both for tedisamil and E-4031.

Vasodilatory Effect of Adrenomedullin in Mouse Aorta

Human adrenomedullin (AM) and human calcitonin gene-related peptide (CGRP) produced a concentration-dependent relaxation in mouse aorta, precontracted with noradrenaline. EC50 values for AM and CGRP were 9.8 ± 2.4 and 4.2 ± 0.1 nmol/l, respectively. AM-mediated vasorelaxation was partially (3-fold) shifted by AM(22–52), the C-terminal AM fragment, but not by CGRP(8–37), a selective CGRP1 antagonist. Both AM(22–52) and CGRP(8–37) failed to inhibit CGRP-mediated vasorelaxation of mouse aorta rings. Binding of rat [125I]AM to these membranes was specific. Both human AM and AM(22–52) displaced rat [125I]AM binding in a concentration-dependent manner with IC50 values of 12.0 ± 4 and 19.4 ± 8 nmol/l, respectively. In contrast, both human CGRP and CGRP(8–37) were weak in displacing [125I]AM binding. Very little specific binding was observed with [125I]CGRP. In conclusion, the data presented here demonstrate that the mouse aorta displays AM receptors that mediate vasorelaxation.

Synthesis, electrophysiological properties and analysis of structural requirements of a novel class of antiarrhythmic agents with potassium and calcium channel blocking properties.

Class III antiarrhythmic agents have been shown to prevent reentrant arrhythmias but also to be responsible for initiating arrhythmias characterised by afterdepolarizations and triggered activities. By combining potassium and calcium channel antagonistic actions, as with BRL-32872 (1), it might be possible to reduce the incidence of proarrhythmias albeit retaining antiarrhythmic efficacy. In the present study we synthesised and tested for their electrophysiological activity in guinea pig papillary muscle a wide panel of analogues of BRL-32872. Some qualitative relationships between compound structure and the inhibitory effect on the rapidly activating component of the delayed rectifier potassium current and/or the L-type calcium current will be presented. New derivatives depicting bell-shaped dose-response curves on action potential duration may therefore represent novel agents for improved antiarrhythmic therapy.

Antifibrillatory effects of BRL-32872 in anesthetized Yucatan minipigs with regional myocardial ischemia.

The antifibrillatory potential of BRL-32872, a novel antiarrhythmic compound with K+ and Ca2+ channel blocking activities, was examined in a minipig model of ischemia-induced arrhythmia. The effects of intravenous (i.v.) BRL-32872 (0.3 and 1.0 mg/kg, n = 8), dofetilide (0.3 mg/kg, n = 8), and flecainide (2.0 mg/kg, n = 8), were investigated on the incidence of ventricular fibrillation (VF) during a 20-min occlusion of the left anterior descending coronary artery (LAD). Ischemia-induced VF occurred in 6 of 9 vehicle-treated pigs. BRL-32872 reduced the incidence of ischemic VF to 13% at 0.3 mg/kg (p < 0.05) and to 0% at 1.0 mg/kg (p < 0.01). Dofetilide also prevented the occurrence of VF (0%, p < 0.01) In contrast, flecainide did not reduce the incidence of VF (63%). Indeed, flecainide shortened the time to onset of VF from 17 +/- 1 min in the vehicle group to 10 +/- 1 min (p < 0.001). The antifibrillatory effects of BRL-32872 and dofetilide were associated with a prolongation of QT interval on ECG. Flecainide did not prolong repolarization, but slowed the ventricular conduction velocity, as shown by significant increases in PR and QRS intervals. During early reperfusion, 1 of 8 surviving pigs in each group treated with BRL-32872 and 4 of 8 in the dofetilide group developed VF. This study demonstrated an antifibrillatory effect of BRL-32872 associated with prolonged ventricular repolarization and showed enhanced efficacy over dofetilide on reperfusion arrhythmias which is most likely a consequence of its Ca2+ blocking activity.

Electrophysiological characterization of BRL-32872 in canine Purkinje fiber and ventricular muscle. Effect on early after-depolarizations and repolarization dispersion.

Amongst the different pharmacological approaches to the treatment of cardiac arrhythmias, compounds with multiple electrophysiological activities appear to exhibit a reduced adverse effect profile. BRL-32872 (N-(3,4-dimethoxyphenyl)-N-[3[[2-(3,4-dimethoxyphenyl) ethyl] propyl]-4-nitrobenzamide hydrochloride) is a typical example of an antiarrhythmic agent with combined K(+) and Ca(2+) blocking actions. In this study, we investigated the effects of BRL-32872 on early after-depolarizations and on dispersion of repolarization. Action potentials were recorded either in canine cardiac Purkinje fibers alone or in preparations containing both ventricular muscle and the attached Purkinje fibers. In Purkinje fibers, BRL-32872 (0. 3-10 microM) induced a bell-shaped concentration-dependent increase in action potential duration. At 90% of repolarization, the action potential was prolonged at concentrations up to 1 microM and was shortened when the concentration of BRL-32872 was further increased. In all 17 experiments, BRL-32872 did not cause early after-depolarizations in Purkinje fibers. On the contrary, BRL-32872 (3 microM) systematically suppressed early after-depolarizations induced by clofilium (4-chloro-N, N-diethyl-N-heptylbenzenebutanaminium tosylate, 1 microM), a selective inhibitor of the delayed rectifier K(+) current. A similar effect was observed once with 1 microM BRL-32872, a concentration able to prolong Purkinje fiber action potentials. Simultaneous recording of action potentials in ventricular and Purkinje preparations showed that increasing concentrations of BRL-32872 (0. 3-10 microM) induced a limited increase in the difference of repolarization time between the two tissues. The selective K(+) channel inhibitor E-4031 (N-(4-(1-[2-(6-methyl-2-pyridyl) ethyl]-4-piperidyl)-carbonyl] phenyl) methanesulfonamide dihydrochloride dihydrate) exhibited a significant concentration-dependent increase in dispersion of repolarization. We conclude from the present results that the Ca(2+) blocking activity of BRL-32872 (i) prevents the occurrence of early after-depolarizations associated with action potential prolongation and (ii) limits an excessive increase in action potential duration heterogeneity. These electrophysiological features might represent the basis for antiarrhythmic compounds with reduced proarrhythmic profile.

Effect of Zatebradine, a Specific Bradycardic Agent, on Ischemia-Induced Arrhythmias in Anesthetized Rabbits

The effects of the specific bradycardic agent, zatebradine (UL-FS 49), on ventricular arrhythmias occurring during an acute ischemia were compared to those of verapamil. Anesthetized rabbits were submitted to a ligation of the left circumflex coronary artery for 20 min. Zatebradine (150 and 750 micrograms/kg, i.v.) dose-dependently reduced heart rate, but changed neither the left ventricular pressure nor the (+)dp/dtmax. In comparison, verapamil (150 and 750 micrograms/kg, i.v.) reduced heart rate, systemic blood pressure, left ventricular pressure and (+)dp/dtmax. The incidence of ventricular premature beats occurring during acute ischemia was changed neither by zatebradine nor by verapamil. Ventricular fibrillation, occurring in 36% of the saline-treated rabbits, was reduced to 18% in the presence of 750 micrograms/kg of zatebradine and 0% with verapamil (750 micrograms/kg, p < 0.05). The action of zatebradine on ischemia-induced ventricular fibrillation, albeit limited, was completely reversed by atrial pacing to the predrug heart rate. To further investigate the mechanisms involved in the antiarrhythmic potential of both zatebradine and verapamil, their electrophysiological actions were compared in canine Purkinje fibers. Both zatebradine and verapamil induced a dose-dependent increase in action potential duration (APD) measured at 90% repolarization. The APDs measured during the plateau level (APD30) and at 50% of the repolarization (APD50) were shortened by verapamil and increased by zatebradine showing that at the concentrations used, zatebradine did not exhibit any calcium antagonistic activity when compared to verapamil. The results of the present study suggest that the specific bradycardic agent zatebradine showed a beneficial action mainly because of its anti-ischemic properties. However, the present studies performed in anesthetized rabbits suggest that in this species pure reduction in heart rate is not sufficient to entirely prevent ischemic arrhythmias.

Influence of Sympathetic Heart Rate Modulation on RT Interval Rate Adaptation in Conscious Dogs

PLADYS, P., et al.: Influence of Sympathetic Heart Rate Modulation on R Interval Rate Adaptation In Conscious Dogs. The objective was to test if changes in autonomic tone still influenced the RT‐RR relationship when full RT adaptation is completed, when heart rate is controlled, and when beat‐to‐beat variability is abolished by atrial pacing. Eight dogs (8–11 kg) were chronically instrumented with atrial pacing electrodes. Digital ECG (1,000 Hz, 12 bits) were recorded from healthy conscious dogs during spontaneous sinus rhythm and during atrial pacing. The protocol was repeated before and after atenolol (2 mg/kg), prazosin (0.5 mg/kg), or atenolol + prazosin. A vocal incitation was used as sympathetic stimulation. Beat‐to‐beat quantitative analysis of the RT interval (from QRS apex to end of T wave) was correlated with the preceding RR by linear regression. In spontaneous rhythm, atenolol increased RR (P < 0.001), RT (P < 0.001), and short‐term heart rate variability (P < 0.01) and decreased RT‐RR slopes (P < 0.001). Prazosin did not significantly modify any parameter. Sympathetic stimulation decreased RR (P < 0.001), RT (P < 0.05), and short‐term heart rate variability (P < 0.01) and increased RT‐RR slopes (P < 0.001). In atrial pacing, the RTRR slopes were steeper during pacing than during spontaneous rhythm but were not modified by pharmacological manipulation of the autonomic nervous system. During sinus rhythm the RT‐RR relationship is increased by sympathetic stimulation and decreased by β–blockade. When heart rate modulation and the effects of the time delay in RT rate adaptation are abolished by atrial pacing, the influence of autonomic tone on RT rate adaptation disappears.

Influence of atenolol on the kinetics of RT interval rate adaptation in conscious dogs.

&NA; The objective was to test an effect of atenolol independent of heart rate on electrocardiographic RT rate adaptation by investigating RT adaptation during spontaneous rate and after an abrupt change of atrial rate (study of RT delay). Digital electrocardiograms were recorded from eight conscious dogs. Analysis of RT interval (measured from QRS apex to end of T) was performed on a beat‐to‐beat basis. The protocol was repeated in the control state and after atenolol administration (2 mg/kg). Regarding spontaneous heart rate, an increased or decreased RR duration did not modify the beat‐to‐beat relative adaptation of RT to a change of RR (2.15 ± 1% during control). Atenolol increased mean RR (p < 0.001) and decreased relative adaptation of RT (0.22 ± 0.18%, p < 0.001). The inverse correlation between mean RR and the relative RT adaptation (r = ‐0.76, p < 0.05) disappeared after atenolol administration. Regarding RT delay, complete adaptation of RT required 3 min; 48 ± 16% of this adaptation was observed after the first beat and 60 ± 11% was observed after the 20th. Atenolol attenuated this adaptation during the first six beats following the abrupt cycle length change (p < 0.05). We concluded that the attenuation of RT rate adaptation after atenolol is related to heart rate modulation and to the time delay in RT rate adaptation.