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Sylvain Richard

Publications and source records attributed to Sylvain Richard.

At least 19 recordsLinked to original sources
PubMed2006

Nitric oxide pathway counteracts enhanced contraction to membrane depolarization in aortic rings of rats on high-sodium diet.

Vascular smooth muscle cell contraction and endothelium-dependent relaxation was evaluated in aortic rings isolated from weaned, 5-mo-old Sprague-Dawley rats fed a normal (NS; 0.8% NaCl) or high (HS; 8% NaCl) sodium diet. Arterial pressure was 140 +/- 6 (NS) and 145 +/- 6 mmHg (HS). In endothelium-denuded rings, the response to phenylephrine (PE) was not modified by the sodium diet, while that of depolarizing agent KCl and intracellular calcium releasing agent caffeine increased in the HS group. When endothelium was preserved, PE-evoked contraction was reduced in both NS and HS groups, the contraction being yet lower in the HS group. This effect was partially obliterated by addition of N(G)-nitro-L-arginine methyl ester (L-NAME), independently of the sodium diet. Relaxation to ACh in intact rings and to sodium nitroprusside (SNP) and 8-bromoadenosine 3'5' cyclic guanosine monophosphate (8-BrcGMP) in the absence of endothelium was enhanced in rings isolated from HS rats. In addition, the dose-response curve to 8-BrcGMP was shifted to the right in the presence of iberiotoxin, an inhibitor of large conductance, voltage-dependent, and calcium-sensitive potassium channel (BK(Ca)). However, shift was more marked in rings from HS rats. Present results provide evidence that response of vascular smooth muscle cell to nitric oxide/cGMP-related compounds is increased in HS rings and is associated with a greater activation of the repolarizing BK(Ca) channels. Such changes might counterbalance enhanced contractile response to membrane depolarization and thus participate in maintenance of arterial pressure in the present model of early and long-term HS feeding in rats.

Acetylcholine↗
PubMed2006

Heparin binding EGF is necessary for vasospastic response to endothelin.

Endothelin-1 (ET-1), a powerful vasoconstrictor, is involved in vasospastic diseases such as coronary artery disease and subarachnoidal hemorrhage, as well as in renal and cardiovascular fibrotic remodeling. Transactivation of the epidermal growth factor receptor (EGFR) mediates ET-1 signaling in vascular smooth muscle cells (VSMCs) and isolated arteries. Moreover, EGFR is required for a full constrictive response to ET-1. However, the relevant mechanisms mediating EGFR transactivation in response to ET-1 have not been identified. The present study used isolated arteries and VSMCs to investigate the role of the EGFR ligand heparin binding-epidermal growth factor (HB-EGF) in ET-1-induced transactivation of EGFR, intracellular calcium mobilization, and VSMCs contraction. While baseline blood pressures were similar in HB-EGF-deficient and in wild-type littermate mice, the vasoconstrictor actions of ET-1 were attenuated in HB-EGF-/- animals. In isolated mouse carotid artery segments mounted in an arteriograph, ET-1 caused only a weak increase in isovolumetric tone in HB-EGF-deficient vessels, and this effect was mimicked by inhibition of EGFR tyrosine kinase or phosphoinositide 3-kinase (PI3K) in wild-type arteries with or without endothelium, indicating a specific role in VSMCs. EGFR or PI3K inhibitors had no effect on KCl-induced contraction, which was normal in HB-EGF-deficient mice. To confirm that the abnormal responses in HB-EGF-deficient mice were due to impaired EGFR signaling, we studied VSMCs from waved-2 (wa2) mice; these animals have a mutation causing a partial loss of function of EGFR tyrosine kinase activity. The ET-1-induced calcium peak was reduced by 30% in VSMCs from wa2 mice and from HB-EGF-/- mice. This effect was reproduced by preincubation of wild-type VSMCs with EGFR inhibitor AG1478 and PI3K inhibitors LY294002 and wortmannin. ProHB-EGF is bound to the cell membrane and released after cleavage by metalloproteinases; its action may contribute to effects of GPCR agonists on cell growth. Pretreatment of mouse VSMCs with batimastat, a metalloproteinase inhibitor, significantly attenuated ET-1-induced [Ca(2+)](i) response in wild-type cells. Human proHB-EGF has been shown to be the endogenous receptor for Corynebacterium diphteriae toxin (DT). Mutated DT toxin (CRM197) is devoid of toxicity but it neutralizes HB-EGF binding to EGFR. Pretreatment of human VSMCs from internal mammary arteries with CRM197 significantly blunted ET-1-stimulated calcium transients. In conclusion, these findings suggest that the mechanism of ET-1-induced vasoconstriction involves HB-EGF-mediated transactivation of the EGFR. This functional cascade requires modulation of agonist-induced calcium transient by EGFR and PI3K with extremely fast kinetics, suggesting a novel paradigm for GPCR-mediated calcium signaling, which may offer future therapeutic targets.

Animals↗
PubMed2006

Mechanisms of [Ca2+]i transient decrease in cardiomyopathy of db/db type 2 diabetic mice.

Cardiovascular disease is the leading cause of death in the diabetic population. However, molecular mechanisms underlying diabetic cardiomyopathy remain unclear. We analyzed Ca2+-induced Ca2+ release and excitation-contraction coupling in db/db obese type 2 diabetic mice and their control littermates. Echocardiography showed a systolic dysfunction in db/db mice. Two-photon microscopy identified intracellular calcium concentration ([Ca2+]i) transient decrease in cardiomyocytes within the whole heart, which was also found in isolated myocytes by confocal microscopy. Global [Ca2+]i transients are constituted of individual Ca2+ sparks. Ca2+ sparks in db/db cardiomyocytes were less frequent than in +/+ myocytes, partly because of a depression in sarcoplasmic reticulum Ca2+ load but also because of a reduced expression of ryanodine receptor Ca2+ channels (RyRs), revealed by [3H]ryanodine binding assay. Ca2+ efflux through Na+/Ca2+ exchanger was increased in db/db myocytes. Calcium current, I(Ca), triggers sarcoplasmic reticulum Ca2+ release and is also involved in sarcoplasmic reticulum Ca2+ refilling. Macroscopic I(Ca) was reduced in db/db cells, but single Ca2+ channel activity was similar, suggesting that diabetic myocytes express fewer functional Ca2+ channels, which was confirmed by Western blots. These results demonstrate that db/db mice show depressed cardiac function, at least in part, because of a general reduction in the membrane permeability to Ca2+. As less Ca2+ enters the cell through I(Ca), less Ca2+ is released through RyRs.

Animals↗
PubMed2005

A direct relationship between plasma aldosterone and cardiac L-type Ca2+ current in mice.

Aldosterone is involved in a variety of pathophysiological processes that ultimately cause cardiovascular diseases. Despite this, the physiological role of aldosterone in heart function remains elusive. We took advantage of transgenic mouse models characterized by a renal salt-losing (SL) or salt-retaining (SR) phenotype, thus exhibiting chronically high or low plasma aldosterone levels, respectively, to investigate the chronic effects of aldosterone in cardiomyocytes devoid of pathology. On a diet containing normal levels of salt, these animals do not develop any evidence of cardiovascular disease. Using the whole cell patch-clamp technique on freshly isolated adult ventricular cardiomyocytes, we observed that the amplitude of L-type Ca(2)(+) currents (I(Ca)) correlates with plasma aldosterone levels. Larger values of I(Ca) are associated with high aldosterone concentrations in SL models, whereas smaller values of I(Ca) were observed in the SR model. Neither the time- nor the voltage-dependent properties of I(Ca) varied measurably. In parallel, we determined whether modulation of I(Ca) by blood concentration of aldosterone has a major physiological impact on the excitation-contraction coupling of the cardiomyocytes. Action potential duration, [Ca(2)(+)](i) transient amplitude and contraction are increased in the SL model and decreased in the SR model. In conclusion, we demonstrate that the blood concentration of aldosterone exerts chronic regulation of I(Ca) in mouse cardiomyocytes. This regulation has important consequences for excitation-contraction coupling and, potentially, for other Ca(2)(+)-regulated functions in cardiomyocytes.

Aldosterone↗
PubMed2005

Ca2+-dependent reduction of IK1 in rat ventricular cells: a novel paradigm for arrhythmia in heart failure?

OBJECTIVES: We investigated the inward rectifier potassium current (I(K1)), which can be blocked by intracellular Ca(2+), in heart failure (HF). METHODS: We used the whole-cell patch-clamp technique to record I(K1) from single rat ventricular myocytes in voltage-clamp conditions. Fluorescence measurements of diastolic Ca(2+) were performed with Indo-1 AM. HF was examined 8 weeks after myocardial infarction (coronary artery ligation). RESULTS: I(K1) was reduced and diastolic Ca(2+) was increased in HF cells. The reduction of I(K1) was attenuated when EGTA was elevated from 0.5 to 10 mM in the patch pipette and prevented with high BAPTA (20 mM). Ryanodine (100 nM) and FK506 (10 microM), both of which promote spontaneous SR Ca(2+) release from ryanodine receptor (RyR2) during diastole, reproduced the effect of HF on I(K1) in normal cells but had no effect in HF cells. The effects of ryanodine and FK506 were not additive and were prevented by BAPTA. Rapamycin (10 microM), which removes FKBP binding proteins from RyR2 with no effect on calcineurin, mimicked the effect of FK506 on I(K1). Cyclosporine A (10 microM), which inhibits calcineurin via cyclophilins, had no effect. In both HF cells and normal cells treated by FK506, the protein kinase C (PKC) inhibitor staurosporine totally restored the inward component of I(K1), but only partially restored its outward component at potentials corresponding to the late repolarizing phase of the action potential (-80 to -40 mV). CONCLUSIONS: I(K1) is reduced by elevated diastolic Ca(2+)in HF, which involves in parallel PKC-dependent and PKC-independent mechanisms. This regulation provides a novel paradigm for Ca(2+)-dependent modulation of membrane potential in HF. Since enhanced RyR2-mediated Ca(2+)release also reduces I(K1), this paradigm might be relevant for arrhythmias related to acquired or inherited RyR2 dysfunction.

Action Potentials↗
PubMed2005

'Ca(2+)-induced Ca(2+) entry' or how the L-type Ca(2+) channel remodels its own signalling pathway in cardiac cells.

The adjustment of Ca(2+) entry in cardiac cells is critical to the generation of the force necessary for the myocardium to meet the physiological needs of the body. In this review, we present the concept that Ca(2+) can promote its own entry through Ca(2+) channels by different mechanisms. We refer to it under the general term of 'Ca(2+)-induced Ca(2+) entry' (CICE). We review short-term mechanisms (usually termed facilitation) that involve a stimulating effect of Ca(2+) on the L-type Ca(2+) current (I(Ca-L)) amplitude (positive staircase) or a lessening of Ca(2+)-dependent inactivation of I(Ca-L). This latter effect is related to the amount of Ca(2+) released by ryanodine receptors (RyR2) of the sarcoplasmic reticulum (SR). Both effects are involved in the control of action potential (AP) duration. We also describe a long-term mechanism based on Ca(2+)-dependent down-regulation of the Kv4.2 gene controlling functional expression of the repolarizing transient outward K(+) current (I(to)) and, thereby, AP duration. This mechanism, which might occur very early during the onset of hypertrophy, enhances Ca(2+) entry by maintaining Ca(2+) channel activation during prolonged AP. Both Ca(2+)-dependent facilitation and Ca(2+)-dependent down-regulation of I(to) expression favour AP prolongation and, thereby, promote sustained voltage-gated Ca(2+) entry used to enhance excitation-contraction (EC) coupling (with no change in the density of Ca(2+) channels per se). These self-maintaining mechanisms of Ca(2+) entry have significant functions in remodelling Ca(2+) signalling during the cardiac AP. They might support a prominent role of Ca(2+) channels in the establishment and progression of abnormal Ca(2+) signalling during cardiac hypertrophy and congestive heart failure.

Animals↗
PubMed2005

Vascular effects of calcium channel antagonists: new evidence.

Calcium channel antagonists have a well-established role in the management of cardiovascular diseases. L-type calcium channels in vascular cells are a key therapeutic target in hypertension and are the preferred molecular target of the initial calcium channel antagonists. However, third-generation dihydropyridine (DHP) calcium channel antagonists, including manidipine, nilvadipine, benidipine and efonidipine, appear to have effects in addition to blockade of the L-type calcium channel. Voltage-gated calcium channels are widely expressed throughout the cardiovascular system. They constitute the main route for calcium entry, essential for the maintenance of contraction. Cardiac and vascular cells predominantly express L-type calcium channels. More recently, T-type channels have been discovered, and there is emerging evidence of their significance in the regulation of arterial resistance. A lack of functional expression of L-type channels in renal efferent arterioles may be consistent with an important role of T-type channels in the regulation of efferent arteriolar tone. Although the exact role of T-type calcium channels in vascular beds remains to be determined, they could be associated with gene-activated cell replication and growth during pathology. The three major classes of calcium channel antagonists are chemically distinct, and exhibit different functional effects depending on their biophysical, conformation-dependent interactions with the L-type calcium channel. The DHPs are more potent vasodilators, and generally have less cardiodepressant activity than representatives of other classes of calcium channel antagonist such as diltiazem (a phenylalkylamine) and verapamil (a benzothiazepine). In contrast to older calcium channel antagonists, the newer DHPs, manidipine, nilvadipine, benidipine and efonidipine, dilate not only afferent but also efferent renal arterioles, a potentially beneficial effect that may improve glomerular hypertension and provide renoprotection. The underlying mechanisms for the heterogenous effects of calcium channel antagonists in the renal microvasculature are unclear. A credible hypothesis suggests a contribution of T-type calcium channels to efferent arteriolar tone, and that manidipine, nilvadipine and efonidipine inhibit both L and T-type channels. However, other mechanisms, including an effect on neuronal P/Q-type calcium channels (recently detected in arterioles), the microheterogeneity of vascular beds, and other types of calcium influx may also play a role. This article presents recent data about the expression and physiological role of calcium channels in arteries and the molecular targets of the calcium channel antagonists, particularly those exhibiting distinct renovascular effects.

Animals↗
PubMed2004

Neuropeptide Y rapidly enhances [Ca2+]i transients and Ca2+ sparks in adult rat ventricular myocytes through Y1 receptor and PLC activation.

Neuropeptide Y (NPY) is the most abundant peptide in the mammalian heart, but its cardiac actions are not fully understood. Here we investigate the effect of NPY in intracellular Ca2+ release, using isolated rat cardiac myocytes and confocal microscopy. Cardiac myocytes were field-stimulated at 1 Hz. The evoked [Ca2+]i transient was of higher amplitude and of faster decay in the presence of 100 nM NPY. Cell contraction was also increased by NPY. We analyzed the occurrence of Ca2+ sparks and their characteristics after NPY application. NPY significantly increased Ca2+ sparks frequency in quiescent cells. The Ca2+ spark amplitude was enhanced by NPY but the other characteristics of Ca2+ sparks were not significantly altered. Because cardiac myocytes express both Y1 and Y2 NPY receptors, we repeated the experiments in the presence of the receptor blockers, BIBP3226 and BIIE0246. We found that Y1 NPY receptor blockade completely inhibited NPY effects on [Ca2+]i transient. PTX-sensitive G-proteins and/or phospholypase C (PLC) have been invoked to mediate NPY effects in other cell types. We tested these two hypotheses. In PTX-treated myocytes NPY was still effective, which suggests that the observed NPY actions are not mediated by PTX-sensitive G-proteins. In contrast, the increase in [Ca2+]i transient by NPY was completely inhibited by the PLC inhibitor U73122. In conclusion, we find that NPY has a positive inotropic effect in isolated rat cardiac myocytes, which involves increase in Ca2+ release after activation of Y1 NPY receptor and subsequent stimulation of PLC.

Aging↗
PubMed2004

Frequency-dependent and proarrhythmogenic effects of FK-506 in rat ventricular cells.

FK-506, a widely used immunosuppressant, has caused a few clinical cases with QT prolongation and torsades de pointe at high blood concentration. The proarrhytmogenic potential of FK-506 was investigated in single rat ventricular cells using the whole cell clamp method to record action potentials (APs) and ionic currents. Fluorescence measurements of Ca2+ transients were performed with indo-1 AM using a multiphotonic microscope. FK-506 (25 micromol/l) hyperpolarized the resting membrane potential (RMP; -3 mV) and prolonged APs (AP duration at 90% repolarization increased by 21%) at 0.1 Hz. Prolongation was enhanced by threefold at 3.3 Hz, and early afterdepolarizations (EADs) occurred in 59% of cells. EADs were prevented by stronger intracellular Ca2+ buffering (EGTA: 10 vs. 0.5 mmol/l in the patch pipette) or replacement of extracellular Na+ by Li+, which abolishes Na+/Ca2+ exchange [Na+/Ca2+ exchanger current (INaCa)]. In indo-1-loaded cells, FK-506 generated doublets of Ca(2+) transients associated with increased diastolic Ca2+ in one-half of the cells. FK-506 reversibly decreased the L-type Ca2+ current (ICaL) by 25%, although high-frequency-dependent facilitation of ICaL persisted, and decreased three distinct K+ currents: delayed rectifier K+ current (IK; >80%), transient outward K+ current (<20%), and inward rectifier K+ current (IK1; >40%). A shift in the reversal potential of IK1 (-5 mV) accounted for RMP hyperpolarization. Numerical simulations, reproducing all experimental effects of FK-506, and the use of nifedipine showed that frequency-dependent facilitation of ICaL plays a role in the occurrence of EADs. In conclusion, the effects of FK-506 on the cardiac AP are more complex than previously reported and include inhibitions of IK1 and ICaL. Alterations in Ca2+ release and INaCa may contribute to FK-506-induced AP prolongation and EADs in addition to the permissive role of ICaL facilitation at high rates of stimulation.

Action Potentials↗
PubMed2004

Morphine mimics the antiapoptotic effect of preconditioning via an Ins(1,4,5)P3 signaling pathway in rat ventricular myocytes.

Morphine has cardioprotective effects against ischemic-reperfusion injuries. This study investigates whether morphine could mimic the antiapoptotic effect of preconditioning using a model of cultured neonatal rat cardiomyocytes subjected to metabolic inhibition (MI). To quantify MI-induced apoptosis, DNA fragmentation and mitochondrial cytochrome c release levels were measured by ELISA. MI-dependent DNA fragmentation was prevented by both Z-VAD-fmk (20 microM), a pan-caspase inhibitor, and cyclosporine A (CsA; 5 microM), a mitochondrial pore transition blocker, added during MI (36% and 54% decrease, respectively). MI-dependent cytochrome c release was not blocked by Z-VAD-fmk but was decreased (38%) by CsA during MI. Metabolic preconditioning (MIP) and preconditioning with morphine (1 microM) were also assessed. MI-dependent DNA fragmentation and cytochrome c release were prevented by MIP (40% and 45% decrease, respectively) and morphine (34% and 45%, respectively). The antiapoptotic effect of morphine was abolished by naloxone (10 nM), a nonselective opioid receptor antagonist, or xestospongin C (XeC, 400 nM), an inhibitor of inositol (1,4,5)-trisphosphate [Ins(1,4,5)P(3)]-mediated Ca(2+) release. Ca(2+) preconditioning, induced by increasing extracellular Ca(2+) from 1.8 to 3.3 mM, mimicked the antiapoptotic effect of morphine on DNA fragmentation (24% decrease) and cytochrome c release (57% decrease). This effect mediated by extracellular Ca(2+) was also abolished by XeC. Measurements of intracellular Ca(2+) concentration using fura-2 microspectrofluorimetry showed that morphine induces Ins(1,4,5)P(3)-dependent Ca(2+) transients abolished by 2-aminoethoxydiphenyl borate (2-APB), a cell-permeable Ins(1,4,5)P(3) antagonist. These results suggest that morphine preconditioning prevents simulated ischemia-reperfusion-induced apoptosis via an Ins(1,4,5)P(3) signaling pathway in rat ventricular myocytes.

Animals↗
PubMed2004

Mineralocorticoid receptor antagonism prevents the electrical remodeling that precedes cellular hypertrophy after myocardial infarction.

BACKGROUND: Cardiac hypertrophy underlies arrhythmias and sudden death, for which mineralocorticoid receptor (MR) activity has recently been implicated. We sought to establish the sequence of ionic events that link the initiating insult and MR to hypertrophy development. METHODS AND RESULTS: Using whole-cell, patch-clamp and quantitative reverse transcription-polymerase chain reaction techniques on right ventricular myocytes of a myocardial infarction (MI) rat model, we examined the cellular response over time. One week after MI, no sign of cellular hypertrophy was found, but action potential duration (APD) was lengthened. Both an increase in Ca2+ current (I(Ca)) and a decrease in K+ transient outward current (I(to)) underlay this effect. Consistently, the relative expression of mRNA coding for the Ca2+ channel alpha1C subunit (Ca(v)1.2) increased, and that of the K+ channel K(v)4.2 subunit decreased. Three weeks after MI, AP prolongation endured, whereas cellular hypertrophy developed. I(Ca) density, Ca(v)1.2, and K(v)4.2 mRNA levels regained control levels, but I(to) density remained reduced. Long-term treatment with RU28318, an MR antagonist, prevented this electrical remodeling. In a different etiologic model of abdominal aortic constriction, we confirmed that APD prolongation and modifications of ionic currents precede cellular hypertrophy. CONCLUSIONS: Electrical remodeling, which is triggered at least in part by MR activation, is an initial, early cellular response to hypertrophic insults.

Action Potentials↗
PubMed2004

Ca2+ controls functional expression of the cardiac K+ transient outward current via the calcineurin pathway.

The transient outward K+ current (Ito) modulates transmembrane Ca2+ influx into cardiomyocytes, which, in turn, might act on Ito. Here, we investigated whether Ca2+ modifies functional expression of Ito. Whole-cell Ito were recorded using the patch clamp technique in single right ventricular myocytes isolated from adult rats and incubated for 24 h at 37 degrees C in a serum-free medium containing various Ca2+ concentrations ([Ca2+]o). Increasing the [Ca2+]o from 0.5 to 1.0 and 2.5 mM produced a gradual decrease in Ito density without change in current kinetics. Quantitativereverse transcriptase-PCR showed that a decrease of the Kv4.2 mRNA could account for this decrease. In the acetoxymethyl ester form of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM)-loaded myocytes (a permeant Ca2+ chelator), Ito density increased significantly when cells were exposed for 24 h to either 1 or 2.5 mM [Ca2+]o. Moreover, 24-h exposure to the Ca2+ channel agonist, Bay K8644, in 1 mM [Ca2+]o induced a decrease in Ito density, whereas the Ca2+ channel antagonist, nifedipine, blunted Ito decrease in 2.5 mM [Ca2+]o. The decrease of Ito in 2.5 mM [Ca2+]o was also prevented by co-incubation with either the calmodulin inhibitor W7 or the calcineurin inhibitors FK506 or cyclosporin A. Furthermore, in myocytes incubated for 24 h with 2.5 mM [Ca2+]o, calcineurin activity was significantly increased compared with 1 mM [Ca2+]o. Our data suggest that modulation of [Ca2+]i via L-type Ca2+ channels, which appears to involve the Ca2+/calmodulin-regulated protein phosphatase calcineurin, down-regulates the functional expression of Ito. This effect might be involved in many physiological and pathological modulations of Ito channel expression in cardiac cells, as well other cell types.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed2004

FKBP12.6 overexpression decreases Ca2+ spark amplitude but enhances [Ca2+]i transient in rat cardiac myocytes.

Ryanodine receptors/Ca2+-release channels (RyR2) from the sarcoplasmic reticulum (SR) provide the Ca2+ required for contraction at each cardiac twitch. RyR2 are regulated by a variety of proteins, including the immunophilin FK506 binding protein (FKBP12.6). FKBP12.6 seems to be important for coupled gating of RyR2 and its deficit and alteration may be involved in heart failure. The role of FKBP12.6 on Ca2+ release has not been analyzed directly, but rather it was inferred from the effects of immunophilins, such us FK506 and rapamycin, which, among other effects, dissociates FKBP12.6 from the RyR2. Here, we investigated directly the effects of FKBP12.6 on local (Ca2+ sparks) and global [intracellular Ca2+ concentration ([Ca2+]i) transients] Ca2+ release in single rat cardiac myocytes. The FKBP12.6 gene was transfected in single myocytes using the adenovirus technique with a reporter gene strategy based on green fluorescent protein (GFP) to check out the success of transfections. Control myocytes were transfected with only GFP (Ad-GFP). Rhod-2 was used as the Ca2+ indicator, and cells were viewed with a confocal microscope. We found that overexpression of FKBP12.6 decreases the occurrence, amplitude, duration, and width of spontaneous Ca2+ sparks. FK506 had diametrically opposed effects. However, overexpression of FKBP12.6 increased the [Ca2+]i transient amplitude and accelerated its decay in field-stimulated cells. The associated cell shortening was increased. SR Ca2+ load, estimated by rapid caffeine application, was increased. In conclusion, FKBP12.6 overexpression decreases spontaneous Ca2+ sparks but increases [Ca2+]i transients, in relation with enhanced SR Ca2+ load, therefore improving excitation-contraction coupling.

Animals↗
PubMed2003

Ca2+ current-mediated regulation of action potential by pacing rate in rat ventricular myocytes.

OBJECTIVE: Pacing rate regulates the duration of the cardiac action potential (AP). It also regulates the decay kinetics of the L-type Ca(2+) current (I(Ca-L)) which occurs via modulation of Ca(2+)-dependent inactivation. We investigated whether and how this latter process contributes to frequency-dependent (FD) changes in the AP waveform in rat ventricular cells. METHODS: We recorded APs using a microelectrode technique in rat papillary muscles, and using the whole-cell current patch-clamp technique in single rat ventricular cells. RESULTS: The AP duration (APD) was increased by high rates encompassing the physiological range (0.1-5.7 Hz) in both papillary muscles and single cells. This prolongation was accompanied by concomitant depolarisation (approximately 7 mV at 5.7 Hz) of the membrane potential (MP) in papillary muscles. Equivalent artificial depolarisation of the MP enhanced the FD prolongation in single cells. The FD prolongation was enhanced in presence of the K(+) current blocker 4-aminopyridine (5 mmol/l), and decreased in absence of extracellular Ca(2+). It was antagonised by Ca(2+) channel blockers (Co(2+), nifedipine, nitrendipine) and decreased by use of high EGTA (10 vs. 0.5 mmol/l EGTA) or BAPTA (20 mmol/l) in the patch-pipette. It was prevented by ryanodine or thapsigargin, two drugs that reduce or abolish SR-Ca(2+) function. CONCLUSION: I(Ca-L) contributes to the FD modulation of the AP, which occurs following a sudden change in cardiac frequency in rat ventricular cells. This highly dynamic physiological process is related to SR-Ca(2+) release and occurs through beat-to-beat adaptation of Ca(2+)-dependent inactivation of I(Ca-L).

4-Aminopyridine↗
PubMed2003

Effects of amiodarone and dronedarone on voltage-dependent sodium current in human cardiomyocytes.

UNLABELLED: Effect of Dronedarone on Cardiac Na Current. INTRODUCTION: Amiodarone (AM) is a highly effective antiarrhythmic agent used in the management of both atrial and ventricular arrhythmias. Its noniodinated analogue dronedarone (SR) may have fewer side effects than AM. In this study, we compared the effects of AM and SR on the sodium current I(Na) in human atrial myocytes. METHODS AND RESULTS: INa was studied with the whole-cell, patch clamp technique. Both AM and SR induced a dose-dependent inhibition of I(Na) recorded at -40 mV from a holding potential of -100 mV. AM inhibited I(Na) by 41%+/- 11% (n = 4) at 3 microM, and by 80%+/- 7% (n = 5) at 30 microM. SR produced more potent block, inhibiting INa significantly at only 0.3 microM (23%+/- 10%, n = 4) and completely (97%+/- 4%, n = 4) at 3 microM. Both AM and SR had only moderate effects on voltage-dependent properties of I(Na) (current-voltage relationship, availability for activation) and had no effect on the current decay kinetics. CONCLUSION: Both AM and SR inhibit I(Na) significantly in single human atrial cells, showing that the two drugs have Class I antiarrhythmic properties. The acute effects of SR are more potent than those of AM. The study supports the idea that the iodinated form of the molecule has no part in the acute effect of AM on Na+ channels.

Adult↗