ACE Inhibitor Delapril Prevents Ca(2+)-Dependent Blunting of IK1 and Ventricular Arrhythmia in Ischemic Heart Disease

Angiotensin-converting enzyme inhibitors (ACE-I) improve clinical outcome in patients with myocardial infarction (MI) and chronic heart failure. We investigated potential anti-arrhythmic (AA) benefits in a mouse model of ischemic HF. We hypothesized that normalization of diastolic calcium (Ca(2+)) by ACE-I may prevent Ca(2+)-dependent reduction of inward rectifying K(+) current (IK1) and occurrence of arrhythmias after MI. Mice were randomly assigned to three groups: Sham, MI, and MI-D (6 weeks of treatment with ACE-I delapril started 24h after MI). Electrophysiological analyses showed that delapril attenuates MI-induced prolongations of electrocardiogram parameters (QRS complex, QT, QTc intervals) and conduction time from His bundle to ventricular activation. Delapril improved the sympatho-vagal balance (LF/HF) and reduced atrio-ventricular blocks and ventricular arrhythmia. Investigations in cardiomyocytes showed that delapril prevented the decrease of IK1 measured by patch-clamp technique. IK1 reduction was related to intracellular Ca(2+) overload. This reduction was not observed when intracellular free-Ca(2+) was maintained low. Conversely, increasing intracellular free-Ca(2+) in Sham following application of SERCA2a inhibitor thapsigargin reduced IK1. Thapsigargin had no effect in MI animals and abolished the benefits of delapril on IK1 in MI-D mice. Delapril prevented both the prolongation of action potential late repolarization and the depolarization of resting membrane potential, two phenomena known to trigger abnormal electrical activities, promoted by MI. In conclusion, early chronic therapy with delapril after MI prevented Ca(2+)-dependent reduction of IK1. This mechanism may significantly contribute to the antiarrhythmic benefits of ACE-I in patients at risk for sudden cardiac death.

ACE inhibition prevents diastolic Ca2+ overload and loss of myofilament Ca2+ sensitivity after myocardial infarction

Prevention of adverse cardiac remodeling after myocardial infarction (MI) remains a therapeutic challenge. Angiotensin-converting enzyme inhibitors (ACE-I) are a well-established first-line treatment. ACE-I delay fibrosis, but little is known about their molecular effects on cardiomyocytes. We investigated the effects of the ACE-I delapril on cardiomyocytes in a mouse model of heart failure (HF) after MI. Mice were randomly assigned to three groups: Sham, MI, and MI-D (6 weeks of treatment with a non-hypotensive dose of delapril started 24h after MI). Echocardiography and pressure-volume loops revealed that MI induced hypertrophy and dilation, and altered both contraction and relaxation of the left ventricle. At the cellular level, MI cardiomyocytes exhibited reduced contraction, slowed relaxation, increased diastolic Ca2+ levels, decreased Ca2+-transient amplitude, and diminished Ca2+ sensitivity of myofilaments. In MI-D mice, however, both mortality and cardiac remodeling were decreased when compared to non-treated MI mice. Delapril maintained cardiomyocyte contraction and relaxation, prevented diastolic Ca2+ overload and retained the normal Ca2+ sensitivity of contractile proteins. Delapril maintained SERCA2a activity through normalization of P-PLB/PLB (for both Ser16- PLB and Thr17-PLB) and PLB/SERCA2a ratios in cardiomyocytes, favoring normal reuptake of Ca2+ in the sarcoplasmic reticulum. In addition, delapril prevented defective cTnI function by normalizing the expression of PKC, enhanced in MI mice. In conclusion, early therapy with delapril after MI preserved the normal contraction/relaxation cycle of surviving cardiomyocytes with multiple direct effects on key intracellular mechanisms contributing to preserve cardiac function.

The p42/44mitogen-activated protein kinase inhibitor PD 98059, but not U 0126, increases a K+ current in cardiomyocytes

1. The effects of the mitogen-activated protein kinase (MAPK) inhibitors PD 98059 and U 0126, useful tools to investigate MAPK involvement in intracellular signal transduction pathways, were assessed on cardiomyocytes. 2. In rat freshly isolated ventricular myocytes, under current-clamp conditions, PD 98059 (40 micro mol/L) shortened the action potential. Under whole-cell patch-clamp, this compound slowly induced a fast activating sustained outward K+ current that was sensitive to 1 mmol/L Ba2+, 100 micro mol/L Gd3+, 3 mmol/L 4-aminopyridine and 100 micro mol/L tetracain. The PD 98059-induced current was prevented by 40 micro mol/L AACOCF3, a cytosolic phospholipase A2 inhibitor. 3. U 0126 (1 micro mol/L), a recently developed highly potent p42/44 MAPK inhibitor, did not alter K+ currents. 4. PD 98059, but not U 0126, increased arachidonic acid content, probably as a consequence of its reported cyclo-oxygenase inhibitory effect. 5. These observations indicate that PD 98059 activates a TREK-1 like current. Thus, this MAPK inhibitor has to be used with caution because alterations in cell metabolism can be secondary to changes in electrophysiological behaviour.

Simultaneous activation of p38 MAPK and p42/44 MAPK by ATP stimulates the K+ current ITREK in cardiomyocytes

Living cells exhibit multiple K(+) channel proteins; among these is the recently reported atypical two-pore domain K(+) channel protein TREK-1. Most K(+) currents are modulated by neurohormones and under various pathological conditions. Here, in rat ventricular cardiomyocytes using the whole-cell patch-clamp technique, we characterize for the first time a native TREK-1-like current (I(TREK)) that is activated by ATP, a purine agonist applied at a micromolar range. This current is sensitive to arachidonic acid, intracellular acidosis, and various K(+) current inhibitors. Reverse transcription-polymerase chain reaction reveals the presence of a TREK-1-like mRNA in rat cardiomyocytes that shows 93% identity with mouse TREK-1. ATP effects are greatly attenuated in the presence of arachidonic acid or HCO(-)(3)-induced intracellular acidosis. Using a series of inhibitors, we further demonstrate that the ATP-induced stimulation of I(TREK) implies the activation of cytosolic phospholipase A(2) and the release of arachidonic acid. These events require the simultaneous involvement of p38 MAPK and p42/44 MAPK, respectively, via a cAMP-dependent protein kinase and a tyrosine kinase pathway, whereas the two MAPKs conjugate to activate a mitogen- and stress-activated protein kinase (MSK-1). Our results thus demonstrate the occurrence of a TREK-1-like current in cardiac cells whose activation by purine agonists implies a dual-MAPK cytosolic pathway.

Late post-myocardial infarction induces a tetrodotoxin-resistant Na(+)Current in rat cardiomyocytes

Left ventricular remodeling after myocardial infarction is accompanied by electrical abnormalities that might predispose to rhythm disturbances. To get insight into the ionic mechanisms involved, we studied myocytes isolated from four different regions of the rat ventricles, 4-6 months after ligation of the left coronary artery. Using the whole-cell patch-clamp technique, we never observed T-type Ca(2+)current in both diseased and control hearts. In contrast, in 41 out of 78 cells isolated from 16 post-myocardial infarcted rats, analysed in the presence of 30 m m Na(+)ions, we found a tetrodotoxin (TTX)-resistant Na(+)current with quite variable amplitude in every investigated region. Albeit being resistant to 100 microM TTX, this Na(+)-dependent current was highly sensitive to lidocaine since 3 microM lidocaine induced about 65% tonic block. It was also inhibited by 5 microM nifedipine and 2 m m Co(2+), but was insensitive to 100 microM Ni(2+). The TTX-resistant Na(+)channel availability was shifted rightward by 25-30 mV with respect to TTX-sensitive Na(+)current; therefore, a large "window current" might flow in the voltage range from -70 to -20 mV. In conclusion, in late post-myocardial infarction, a Na(+)current with specific kinetics and pharmacology may provide inward charges in a critical range of membrane voltages that are able to alter action potential time course and trigger ventricular arrhythmia. These apparent new characteristics of the Na(+)channel might result in part from environmental changes during heart remodeling.

Cellular and in vivo electrophysiological effects of dronedarone in normal and postmyocardial infarcted rats

We studied the effects of dronedarone (SR 33589) on the action potentials, membrane ionic currents, and arrhythmic activity in control rats and in rats after myocardial infarction, a model known to develop anomalous electrical activity. Dronedarone increased action potential duration in normal hearts. It had little effect on the action potentials that were already prolonged in the postmyocardial infarcted (PMI) rats. Particularly, dronedarone reduced the late sustained K(+) current, I(K) (or Isus) by 69%. Dronedarone induced only a tonic block of I(K). Similar relative inhibitions of I(K) by dronedarone were obtained in young, sham, and PMI rats, even if I(K) was less in sham than in young and further reduced in PMI rats. The EC(50) values were 0.78 and 0.85 microM in sham and PMI rats. Dronedarone induced a weak increase in the fast transient outward current, I(to). Time-to-peak and inactivation time constant of I(to) were decreased by dronedarone that also induced a marked slowing of I(to) recovery from inactivation. Similar effects were observed on the reduced I(to) recorded in PMI rats. Holter monitoring study in control, unthetered animals showed that dronedarone had no proarrhythmic effect. On rats, which after myocardial infarction exhibited ventricular premature beats, dronedarone significantly decreased beat occurrence during the 7-day treatment; this effect was sustained for two more weeks. Thus, dronedarone exerts antiarrhythmic effects on PMI rat heart. Its effects are attributable for the most part to the inhibition of outward K(+) currents and the increase in effective refractory period.

Ionic basis of ventricular arrhythmias in remodeled rat heart during long-term myocardial infarction

OBJECTIVE

Deleterious electrical abnormalities evolve during myocardial infarction. The goal of this study was to analyse current changes during the late decompensated phase of heart disease induced by coronary ligation and to compare them in various heart regions.

METHODS

Young rats were submitted to left coronary ligature. After 4-6 months, cells were enzymatically dissociated and isolated from the upper part basal region of the left ventricle, as well as from the septum, apex and the right ventricle before being studied under whole-cell patch-clamp.

RESULTS

Basal L-type Ca2+ current, ICaL elicited at +10 mV did not exhibit regional dependence neither in control nor after post-myocardial infarction (PMI). ICaL showed both a significantly reduced peak amplitude (17.1 +/- 2.8 pA/pF versus 9.9 +/- 1.4 pA/pF in seven control and seven PMI hearts, n = 32 and 40, respectively) and a slower inactivation, such that the amount of inward charges during a 200 ms-depolarizing pulse was nearly unchanged. beta-Adrenergic stimulation was less effective in increasing ICaL in PMI cells but it slowed inactivation further. Significant differences in the K+ currents were observed. A regional distribution was seen for Ito only, with the largest amplitude in the right ventricle (in pA/pF: 23.1 +/- 2.4, 18.2 +/- 3.9, 14.8 +/- 2.4, 8.3 +/- 1.7 in the right ventricle, apex, septum and left ventricle, respectively n = 8, 7, 8 and 9). This was also true in failing heart cells despite Ito being halved in each of the four regions (in pA/pF: 12.2 +/- 2.5, 11.2 +/- 1.9, 5.1 +/- 1.0 and 4.8 +/- 1.0, respectively n = 12, 12, 11 and 13). IK1 was also significantly reduced by 20% in the PMI cells. Two-way analyses of variance demonstrated the absence of interaction between the topographical origin of the cells and the physiological state of the rats. The alpha 1-adrenergic agonist, methoxamine significantly reduced Ito and IK1 to the same extent in both sham and PMI cells, by about 35% and 20% respectively.

CONCLUSIONS

Long-term left coronary occlusion induces significant alterations in both Ca2+ and K+ currents that occur with similar amplitude in both ventricles. They include a marked reduction in Ito amplitude as well as a slowing of ICaL inactivation. Both factors could contribute to the disturbances in cellular electrical behaviour and the occurrence of arrhythmias in the post-myocardial infarcted heart.