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Cell calcium. 01/2006; 40:205-220.
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ABSTRACT: Defective calcium (Ca2+) signaling and impaired contractile function have been observed in skeletal muscle secondary to impaired myocardial function. However, the molecular basis for these muscle defects have not been identified. In this study, we evaluated the alterations of the ryanodine-sensitive Ca2+ release channels (RyR1) by analyzing global and local Ca2+ signaling in a rat postmyocardial infarction (PMI) model of myocardial overload. Ca2+ transients, measured with multiphoton imaging in individual fibers within a whole extensor digitorum longus (EDL) muscle, exhibited significantly reduced amplitude and a prolonged time course in PMI. Spatio-temporal properties of spontaneous Ca2+ sparks in fibers isolated from PMI EDL muscles were also significantly altered. In addition, RyR1 from PMI skeletal muscles were PKA-hyperphosphorylated and depleted of the FK506 binding protein (FKBP12). These data show that PMI skeletal muscles exhibit altered local Ca2+ signaling, associated with hyperphosphorylation of RyR1. The observed changes in Ca2+ signaling may contribute to defective excitation-contraction coupling in muscle that can contribute to the reduced exercise capacity in PMI, out of proportion to the degree of cardiac dysfunction.
The FASEB Journal 09/2003; 17(11):1517-9. · 5.71 Impact Factor
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ABSTRACT: 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.
Clinical and Experimental Pharmacology and Physiology 05/2003; 30(4):273-7. · 1.85 Impact Factor
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ABSTRACT: The L-type Ca 2+ current (I Ca-L) plays a key role in the cardiac excitation-contraction (E-C) coupling. Thus, it is a major target for many transmitters and hormones modulating cardiac function and, therefore, for pharmacological drugs to regulate inotropy. Ca 2+ (and other) ion currents are commonly studied in animal tissues for practical reasons. Investigations in human cardiomyocytes started extensively only ten years ago with the development of patch-clamp techniques, enzymatic cell dissociation procedures, and surgical techniques. These studies have already provided valuable information concerning the nature, biophysics, pharmacology and regulation of human cardiac ionic currents in normal and diseased tissues. Interesting advances have been made to understand the role of I Ca-L in the development of chronic atrial fibrillation (AF). Alterations of single channel activity and regulation of macroscopic I Ca-L have also been found in heart failure (HF), although some of the data are divergent and puzzling. The T-type Ca 2+ current (I Ca-T) has never been recorded in human cardiomyocytes. After a rapid overview of the basic properties of human cardiac Ca 2+ currents, we focus on selected aspects of pathophysiology that are still unsolved.
Archiv für Kreislaufforschung 04/2002; 97(7). · 7.35 Impact Factor
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ABSTRACT: The L-type Ca2+ current (I(Ca-L)) plays a key role in the cardiac excitation-contraction (E-C) coupling. Thus, it is a major target for many transmitters and hormones modulating cardiac function and, therefore, for pharmacological drugs to regulate inotropy. Ca2+ (and other) ion currents are commonly studied in animal tissues for practical reasons. Investigations in human cardiomyocytes started extensively only ten years ago with the development of patch-clamp techniques, enzymatic cell dissociation procedures, and surgical techniques. These studies have already provided valuable information concerning the nature, biophysics, pharmacology and regulation of human cardiac ionic currents in normal and diseased tissues. Interesting advances have been made to understand the role of I(Ca-L) in the development of chronic atrial fibrillation (AF). Alterations of single channel activity and regulation of macroscopic I(Ca-L) have also been found in heart failure (HF), ugh some of the data are divergent and puzzling. The T-type Ca2+ current (I(Ca-T)) has never been recorded in human cardiomyocytes. After a rapid overview of the basic properties of human cardiac Ca2+ currents, we focus on selected aspects of pathophysiology that are still unsolved.
Archiv für Kreislaufforschung 02/2002; 97 Suppl 1:I11-8. · 7.35 Impact Factor
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ABSTRACT: 1. Aldosterone, a major ionic homeostasis regulator, might also regulate cardiac ion currents. Using the whole-cell patch-clamp technique, we investigated whether aldosterone affects the 4-aminopyridine-sensitive transient outward K+ current (I(to1)). 2. Exposure to 100 nM aldosterone for 48 h at 37 degrees C produced a 1.6-fold decrease in the I(to1) density compared to control myocytes incubated without aldosterone. Neither the time- nor voltage-dependent properties of the current were significantly altered after aldosterone treatment. RU28318 (1 microM), a specific mineralocorticoid receptor antagonist, prevented the aldosterone-induced decrease in I(to1) density. 3. When myocytes were incubated for 24 h with aldosterone, concentrations up to 1 microM did not change I(to1) density, whereas L-type Ca(2+) current (I(Ca,L)) density increased. After 48 h, aldosterone caused a further increase in I(Ca,L). The delay in the I(to1) response to aldosterone might indicate that it occurs secondary to an increase in I(Ca,L). 4. After 24 h of aldosterone pretreatment, further co-incubation for 24 h either with an I(Ca,L) antagonist (100 nM nifedipine) or with a permeant Ca(2+) chelator (10 microM BAPTA-AM) prevented a decrease in I(to1) density. 5. After 48 h of aldosterone treatment, we observed a 2.5-fold increase in the occurrence of spontaneous Ca(2+) sparks, which was blunted by co-treatment with nifedipine. 6. We conclude that aldosterone decreases I(to1) density. We suggest that this decrease is secondary to the modulation of intracellular Ca(2+) signalling, which probably arises from the aldosterone-induced increase in I(Ca,L). These results provide new insights into how cardiac ionic currents are modulated by hormones.
The Journal of Physiology 12/2001; 537(Pt 1):151-60. · 4.72 Impact Factor
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ABSTRACT: BACKGROUND: Heart failure (HF) frequently follows the occurrence of myocardial infarction (MI). Questions about how HF develops and what cellular defects contribute to this dysfunction led to this study. Methods and Results-- MI was induced in rats by coronary artery ligation. Clinical examination of the post-MI (PMI) surviving animals indicated that they were in overt HF by all measures. Cellular examination of the cardiomyocytes by patch-clamp and confocal [Ca(2+)](i) imaging methods indicated that cellular function was significantly compromised. At the single-cell level, [Ca(2+)](i) transient amplitudes were reduced and contractions were decreased and slowed, although Ca(2+) current (I(Ca)) remained unchanged. The excitation-contraction coupling (ECC) gain function measured as Delta[Ca(2+)](i)/I(Ca) was significantly decreased. Ouabain, a cardiotonic steroid that blocks the Na(+),K(+)-ATPase and activates Ca(2+) entry via cardiac Na(+) channels, largely alleviated this defect. CONCLUSIONS: After MI, I(Ca) becomes less able to trigger release of Ca(2+) from the sarcoplasmic reticulum. This failure of ECC is a major factor contributing to the development of contractile dysfunction and HF in PMI animals. The improved ECC gain, enhanced Ca(2+) entry, and augmented Ca(2+) signaling due to cardiotonic steroids contribute to the beneficial effects of these agents.
Circulation 09/2001; 104(6):688-93. · 14.74 Impact Factor
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ABSTRACT: Using the whole-cell patch-clamp configuration in rat ventricular myocytes, we recently reported that microtubule disruption increases calcium current (I(Ca)) and [Ca(2+)](i) transient and accelerates their kinetics by adenylyl cyclase activation. In the present report, we further analyzed the effects of microtubule disruption by 1 micromol/L colchicine on Ca(2+) signaling in cardiac myocytes with intact sarcolemma. In quiescent intact cells, it is possible to investigate ryanodine receptor (RyR) activity by analyzing the characteristics of spontaneous Ca(2+) sparks. Colchicine treatment decreased Ca(2+) spark amplitude (F/F(0): 1.78+/-0.01, n=983, versus 1.64+/-0.01, n=1660, recorded in control versus colchicine-treated cells; P<0.0001) without modifying the sarcoplasmic reticulum Ca(2+) load and enhanced their time to peak (in ms: 6.85+/-0.09, n=1185, versus 7.33+/-0.13, n=1647; P<0.0001). Microtubule disruption also induced the appearance of Ca(2+) sparks in doublets. These alterations may reflect RyR phosphorylation. To further investigate Ca(2+) signaling in cardiac myocytes with intact sarcolemma, we analyzed [Ca(2+)](i) transient evoked by field stimulation. Cells were loaded with the fluorescence Ca(2+) indicator, Fluo-3 cell permeant, and stimulated at 1 HZ: [Ca(2+)](i) transient amplitude was greater and its decay was accelerated in colchicine-treated, field-stimulated myocytes. This effect is reversible. When colchicine-treated myocytes were placed in a colchicine-free solution for 30 minutes, tubulin was repolymerized into microtubules, as shown by immunofluorescence, and the increase in [Ca(2+)](i) transient was reversed. In summary, we demonstrate that microtubule disruption by colchicine reversibly modulates Ca(2+) signaling in cardiac cells with intact sarcolemma.
Circulation Research 04/2001; 88(7):E59-65. · 9.49 Impact Factor
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ABSTRACT: 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.
Journal of Biological Chemistry 01/2001; 275(50):39110-6. · 4.77 Impact Factor
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ABSTRACT: Sodium ions have been reported to alter the permeation properties of L- and N-type Ca2+ channels. Here in frog atrial cardiomyocytes under whole-cell patch-clamp conditions, we have examined the effects of lowering the external Na+ concentration on the amplitude of T-type Ca2+ current, ICaT, and on the relief of its steady-state inactivation by large depolarizing prepulses, ICaT facilitation. A partial reduction in Na+ ion concentration did not significantly alter ICaT amplitude elicited at -50 mV. However, after a large depolarization, low- Na+ solutions enhanced the relief of inactivation and induced ICaT facilitation. This facilitation occurred independently of the divalent charge carrier, high intracellular Ca2+ buffering or the intracellular Na+ content. Its effects were additional to the beta-adrenergic effects mediated by a decrease of Gi/o-protein inhibitory tone. In Ca2+-free solution the very large T-type current, then carried by Na+ ions, showed only a weak relief of inactivation. In conclusion, ICaT facilitation--which, as previously reported, is modulated by the transient voltage-dependent relief of Gi-protein inhibitory tone--is further enhanced in a low-Na+ solution. In Ca2+-free solution, relief of inactivation due to re-openings dependent on the divalent charge carrier is improbable. It thus appears that for a short while after a large depolarization, external Na+ compete with Ca2+ ions on permeation-controlling sites, so as to modulate channel re-openings and thus the amplitude of voltage-facilitated ICaT independently of the control exerted by the inhibitory G-protein.
Pflügers Archiv - European Journal of Physiology 12/2000; 441(1):39-48. · 4.46 Impact Factor
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ABSTRACT: gamma-Hydroxybutyrate (GHB), an anesthetic adjuvant analog of gamma-aminobutyrate (GABA), depresses cell excitability in hippocampal neurons by inducing hyperpolarization through the activation of a prominent inwardly rectifying K(+) (Kir3) conductance. These GABA type B (GABA(B))-like effects are clearly shown at high concentrations of GHB corresponding to blood levels usually reached during anesthesia and are mimicked by the GABA(B) agonist baclofen. Recent studies of native GABA(B) receptors (GABA(B)Rs) have favored the concept that GHB is also a selective agonist. Furthermore, cloning has demonstrated that GABA(B)Rs assemble heteromeric complexes from the GABA(B)R1 and GABA(B)R2 subtypes and that these assemblies are activated by GHB. The surprisingly high tissue content, together with anti-ischemic and protective effects of GHB in the heart, raises the question of a possible influence of GABA(B) agonists on excitable cardiac cells. In the present study, we provide electrophysiological evidence that GHB activates an inwardly rectifying K(+) current in rat ventricular myocytes. This effect is mimicked by baclofen, reversibly inhibited by GABA(B) antagonists, and prevented by pertussis toxin pretreatment. Both GABA(B)R1 and GABA(B)R2 are detected in cardiomyocytes by Western blotting and are shown to coimmunoprecipitate. Laser scanning confocal microscopy discloses an even distribution of the two receptors in the sarcolemma and along the transverse tubular system. Hence, we conclude that GABA(B)Rs are distributed not only in neuronal tissues but also in the heart, where they can be activated and induce electrophysiological alterations through G-protein-coupled inward rectifier potassium channels.
Proceedings of the National Academy of Sciences 08/2000; 97(15):8664-9. · 9.68 Impact Factor
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ABSTRACT: 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.
Journal of Molecular and Cellular Cardiology 08/2000; 32(7):1169-79. · 5.17 Impact Factor
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ABSTRACT: The signaling role of the Ca(2+) releaser inositol 1,4, 5-trisphosphate (IP(3)) has been associated with diverse cell functions. Yet, the physiological significance of IP(3) in tissues that feature a ryanodine-sensitive sarcoplasmic reticulum has remained elusive. IP(3) generated by photolysis of caged IP(3) or by purinergic activation of phospholipase Cgamma slowed down or abolished autonomic Ca(2+) spiking in neonatal rat cardiomyocytes. Microinjection of heparin, blocking dominant-negative fusion protein, or anti-phospholipase Cgamma antibody prevented the IP(3)-mediated purinergic effect. IP(3) triggered a ryanodine- and caffeine-insensitive Ca(2+) release restricted to the perinuclear region. In cells loaded with Rhod2 or expressing a mitochondria-targeted cameleon and TMRM to monitor mitochondrial Ca(2+) and potential, IP(3) induced transient Ca(2+) loading and depolarization of the organelles. These mitochondrial changes were associated with Ca(2+) depletion of the sarcoplasmic reticulum and preceded the arrest of cellular Ca(2+) spiking. Thus, IP(3) acting within a restricted cellular region regulates the dynamic of calcium flow between mitochondria and the endoplasmic/sarcoplasmic reticulum. We have thus uncovered a novel role for IP(3) in excitable cells, the regulation of cardiac autonomic activity.
Molecular Biology of the Cell 06/2000; 11(5):1845-58. · 4.94 Impact Factor
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ABSTRACT: 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.
Journal of Pharmacology and Experimental Therapeutics 02/2000; 292(1):415-24. · 3.83 Impact Factor
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ABSTRACT: "Ca(2+)-current facilitation" describes several features of increase in current amplitude often associated with a reduction in inactivation rate. The aim of this study was to investigate the mechanism of frequency-dependent increase in L-type Ca2+ current, I(Ca) taking advantage of recent knowledge on the control of Ca2+ current inactivation in cardiac cells. The frequency-dependent increase in I(Ca) was studied in adult rat ventricular myocytes using the whole-cell patch-clamp technique. I(Ca) was elicited by a train of 200-ms depolarizing pulses to +20 mV applied at various frequencies (0.2 up to 1.3 Hz). The increase in frequency induced a rate-dependent enhancement of I(Ca), or facilitation phenomena. In most cells, that showed two inactivation phases of I(Ca), facilitation was mainly related to slowing of the fast I(Ca) inactivation phase that occurred besides increase in peak I(Ca) amplitude. Both the decrease and slowing of the fast component of inactivation phase were attenuated on beta -adrenergic-stimulated current. Frequency-dependent I(Ca) facilitation paralleled a reduction in Ca2+ transient measured with fluo-3. After blocking sarcoplasmic reticulum-Ca2+ release by thapsigargin, the fast I(Ca) inactivation phase was reduced and facilitation was eliminated. Facilitation could not then be restored by 1 microM isoprenaline. Thus in rat ventricular myocytes, frequency-dependent facilitation of I(Ca)reflects a reduced Ca(2+)-dependent inactivation consecutive, in most part, to reduced Ca2+ load and Ca2+ release by the sarcoplasmic reticulum rather than being an intrinsic characteristic of the L-type Ca2+ channel.
Journal of Molecular and Cellular Cardiology 11/1999; 31(10):1783-93. · 5.17 Impact Factor
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ABSTRACT: The anion exchangers (AE) are encoded by a multigenic family that comprises at least three genes, AE1, AE2 and AE3, and numerous splicoforms. Besides regulating intracellular pH (pHi) via the Cl-/HCO3- exchange, the AEs exert various cellular functions including generation of a senescent antigen, anchorage of the cytoskeleton to the membrane and regulation of metabolism. Most cells express several AE isoforms. Despite the key role of this family of proteins, little is known about the function of specific AE isoforms in any tissue, including the heart. We therefore chose isolated cardiac cells, in which a tight control of pHi is mandatory for the excitation-contraction coupling process, to thoroughly investigate the expression of the AE genes at both the mRNA and protein levels. RT-PCR revealed the presence of AE1, AE2 and AE3 mRNAs in both neonatal and adult rat cardiomyocytes. AE1 is expressed both as the erythroid form (Band 3 or eAE1) and a novel alternate transcript (nAE1), which was more specifically characterized using a PCR mapping strategy. Two variants of AE2 (AE2a and AE2c) were found at the mRNA level. Cardiac as well as brain AE3 mRNAs were expressed in both neonatal and adult rat cardiomyocytes. Several AE protein isoforms were found, including a truncated form of AE1 and two AE3s, but there was no evidence of AE2 protein in adult rat cardiomyocytes. In cardiomyocytes transfected with an AE3 oligodeoxynucleotide antisense, AE3 immunoreactivity was dramatically decreased but the activity of the Cl-/HCO3- exchange was unchanged. In contrast, intracellular microinjection of blocking anti-AE1 antibodies inhibited the AE activity. Altogether, our findings suggest that a specific and novel AE1 splicoform (nAE1) mediates the cardiac Cl-/HCO3- exchange. The multiple gene and protein expression within the same cell type suggest numerous functions for this protein family.
Journal of Cell Science 06/1999; 112 ( Pt 10):1519-28. · 6.11 Impact Factor
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ABSTRACT: 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.
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.
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.
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.
Cardiovascular Research 06/1999; 42(2):402-15. · 6.06 Impact Factor
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ABSTRACT: In human ventricular cells, the inwardly rectifying K+ current (IK1) is very similar to that of other mammalian species, but detailed knowledge about the K+-dependent distribution of open and blocked states during rectification and about the K+-dependent modulation of inactivation on hyperpolarization is currently lacking.
We used the whole-cell patch-clamp technique to record IK1 in myocytes isolated from subendocardial layers of left ventricular septum from patients with nonfailing hearts with aortic stenosis and cardiac hypertrophy who were undergoing open-heart surgery. Outward currents were very small at voltages positive to the reversal potential but increased at high external [K+]. Chord conductance measurements and kinetic analyses allowed us to estimate the proportion of channels in the open state and of those showing either slow unblock or instantaneous unblock (the so-called slow or instantaneous "activation") on hyperpolarization: the distribution in the individual states was dependent on external [K+]. The proportion of channels unblocking slowly was greater than that of channels unblocking instantaneously on hyperpolarization from the plateau voltage range. Hence, because of the previously reported link between the presence of highly protonated blocking molecules and slow unblock kinetics, it is suggested that high cellular concentrations of spermine may account for the low outward current density recorded in these cells. The current decrease observed on extended hyperpolarization was significantly relieved by an increase in external [K+].
The pattern of IK1 current alterations observed in the present model of human ventricular hypertrophy might favor enhanced excitability and underlie ventricular arrhythmias, possibly via increased intracellular polyamine levels.
Circulation 01/1999; 98(24):2753-9. · 14.74 Impact Factor
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ABSTRACT: The two novel dihydropyridines, oxodipine and elgodipine greatly depressed the KCl-induced contraction of rabbit aorta and decreased the cardiac force of contraction of rat ventricular strips with lower potency. Both compounds markedly shortened cardiac action potentials. In rat cultured neonatal ventricular myocytes, oxodipine and elgodipine decreased the L-type Ca2+ current (I(CaL)) with IC50 of 0.24 and 0.33 microM respectively while oxodipine was slightly more potent on the T-type Ca2+ current (I(CaT)) than elgodipine (IC50 = 0.41 vs. 2.18 microM). Both compounds were less potent in inhibiting I(CaL) of adult cardiomyocytes. Oxodipine exhibited mostly a tonic block of both currents while elgodipine induced mainly a use-dependent block. Oxodipine and elgodipine increased by at least one order of magnitude their inhibitory potency on I(CaT) and I(CaL) when the cells were partially depolarized. We conclude that the mechanisms of inhibition of Ca2+ channels by these two dihydropyridines are different and suggest that the underlying mechanism of vascular selectivity is the voltage-dependent block of I(CaL), with the use-dependent inhibition of Ca2+ currents by elgodipine further contributing to this selectivity.
European Journal of Pharmacology 10/1998; 357(1):93-105. · 2.52 Impact Factor
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ABSTRACT: The present study was designed to investigate whether and how the purinergic stimulation of rat ventricular myocytes modulates the cAMP-dependent pathway. Stimulation of cardiomyocytes with ATPgammaS in the presence of the phosphodiesterase inhibitor IBMX increases by 3-fold intracellular cAMP content. In contrast to beta-adrenergic stimulation, the purinergic stimulation of adenylyl cyclase was not inhibited by activation or enhanced by inhibition of a Gi protein. Forskolin did not potentiate the effect of extracellular ATPgammaS on intracellular cAMP content but the effect of isoproterenol did. Like isoproterenol, the purinergic agonist decreased subsequent ADP-ribosylation of a 45 kDa G(alpha s) by cholera toxin. ATPgammaS also increased cAMP content in neonatal rat cardiomyocytes, a cell type that expresses a long form of Gs protein (alpha(s), 52 kDa) in contrast to adult rat cardiomyocytes that express mostly a short form of Gs protein (alpha(s), 45 kDa). Both purinergic and beta-adrenergic agonists increased cAMP in HEK 293 cells expressing type V adenylyl cyclase while cAMP was only increased by beta-adrenergic stimulation of HEK expressing type IV or VI adenylyl cyclases. Thus, we propose that the purinergic and beta-adrenergic stimulations differentially activate adenylyl cyclase isoforms in rat cardiomyocytes and that adenylyl cyclase V is the specific target of the purinergic stimulation.
FEBS Letters 08/1998; 431(2):189-94. · 3.54 Impact Factor
