Edith Deroubaix

Angiotensin II Signaling Pathways Mediate Expression of Cardiac T-Type Calcium Channels

Abstract— Recent studies indicate that cardiac T-type Ca2+ current (ICaT) reappears in hypertrophied ventricular cells. The aim of this study was to investigate the role of angiotensin II (Ang II), a major inducer of cardiac hypertrophy, in the reexpression of T-type channel in left ventricular hypertrophied myocytes. We induced cardiac hypertrophy in rats by abdominal aorta stenosis for 12 weeks and thereafter animals were treated for 2 weeks with losartan (12 mg/kg per day), an antagonist of type 1 Ang II receptors (AT1). In hypertrophied myocytes, we showed that the reexpressed ICaT is generated by the CaV3.1 and CaV3.2 subunits. After losartan treatment, ICaT density decreased from 0.40±0.05 pA/pF (n=26) to 0.20±0.03 pA/pF (n=27, P <0.01), affecting CaV3.1- and CaV3.2-related currents. The amount of CaV3.1 mRNA increased during hypertrophy and retrieved its nonhypertrophic level after losartan treatment, whereas the amount of CaV3.2 mRNA was unaffected by stenosis. In cultured newborn ventricular cells, chronic Ang II application (0.1 &mgr;mol/L) also increased ICaT density and CaV3.1 mRNA amount. UO126, a mitogen-activated protein kinase kinase-1/2 (MEK1/2) inhibitor, reduced Ang II–increased ICaT density and CaV3.1 mRNA amount. Bosentan, an endothelin (ET) receptor antagonist, reduced Ang II–increased ICaT density without affecting the amount of CaV3.1 mRNA. Finally, cotreatment with bosentan and UO126 abolished the Ang II–increased ICaT density. Our results show that AT1-activated MEK pathway and autocrine ET-activated independent MEK pathway upregulate T-type channel expression. Ang II–increased of ICaT density observed in hypertrophied myocytes may play a role in the pathogenesis of Ca2+ overload and arrhythmias seen in cardiac pathology.

Ionic basis of the action potential prolongation in ventricular myocytes from Syrian hamsters with dilated cardiomyopathy

OBJECTIVEnThe aim of our study was to determine the main electrophysiological alterations associated with cardiac dilation in MS200 strain Syrian hamsters, a model of genetically determined cardiomyopathy.nnnMETHODSnVentricular action potentials (APs) were recorded with standard microelectrodes in isolated hearts from 120-day-old cardiomyopathic (strain MS200) and age-matched control (strain CHF148) Syrian hamsters. Ionic currents were recorded from single ventricular myocytes using the whole-cell patch-clamp technique.nnnRESULTSnIn MS200, AP was prolonged and the plateau phase was markedly increased as compared to CHF148. Differences in both AP duration and 4-aminopyridine-induced AP lengthening between epicardial and endocardial tissues were less marked in MS200 than in CHF148 ventricles. Cell size and membrane capacitance were not higher in MS200 than in CHF148 myocytes, indicating the absence of cell hypertrophy in myopathic ventricles. The L-type calcium current (ICa,L) density was significantly reduced in MS200 and the voltage-dependence of both steady-state activation and inactivation was altered. The voltage-dependent outward current was composed of both transient (Ito1) and sustained (Iss) components, respectively sensitive and insensitive to 4-aminopyridine. Ito1 density was strongly depressed in MS200 compared to CHF148, whereas Iss density was only slightly reduced. The conductance-voltage and steady-state inactivation relationships for Ito1 were shifted to more positive potentials in MS200. The Ito1 recovery process was markedly slower in MS200 than in CHF148. The steady-state current-voltage relationships, in the physiological voltage range, were superimposable in MS200 and CHF148.nnnCONCLUSIONSnIn ventricular myocytes from dilated heart of MS200 Syrian hamsters, Ito1 is more drastically depressed than ICa,L. Such an observation might partially explain dilation-induced AP lengthening.

Calcium current depression in isolated human atrial myocytes after cessation of chronic treatment with calcium antagonists.

The present study investigates the possibility that the slow calcium current of human atrial cardiac cells is modified by chronic treatment (1-24 months) with calcium antagonists (nifedipine, nicardipine, or diltiazem) in a manner different from a simple drug-induced blocking effect. Data from treated patients were recorded approximately 30 hours after cessation of the treatment and were compared with those of nontreated patients. In the treated group, the action potential plateau of atrial fibers was always markedly and irreversibly depressed, and action potential duration measured at 50% repolarization was markedly shortened (81 +/- 12 msec, n = 13) compared with normal values (155 +/- 9 msec, n = 28). In isolated atrial cells, peak calcium current density at +10 mV in treated cells was more than three times as small as that in nontreated cells. Steady-state inactivation relations of calcium current as a function of membrane potential were not significantly different in treated and nontreated cells. In contrast, in treated and nontreated cells superfused with 10(-6) M nifedipine, the curves were markedly shifted toward negative potentials. Cell superfusion with 10(-6) M Bay K 8644 increased calcium current to a larger extent in nontreated cells (sixfold increase) than in treated cells (threefold increase), whereas a 23-fold increase was observed in nontreated cells in which the current had been previously depressed by superfusion with 10(-6) M nifedipine. In contrast to Bay K 8644, 10(-7) M isoproterenol and 10(-8) M angiotensin II increased the calcium current to the same extent in both treated and nontreated groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of toxin II isolated from scorpion venom on action potential and contraction of mammalian heart

The effects of chemically pure toxin II (TII, 0.2 to 5.0 μg) obtained from the venom of scorpion Androctonus australis were studied in the isolated perfused heart of adult rat, guinea-pig and young rabbit and in isolated superfused new-born rat heart. Contraction and cellular action potentials were recorded at 24 to 25°C. Injections of TII produced a marked and persistent increase in amplitude and duration of the ventricular and atrial action potential plateau and a positive inotropic effect in normal as well as in reserpinized rat. Tetrodotoxin (TTX, 1 to 2 μg), calcium-rich media (10 mm) and procaine (15 μg) suppressed the TII-induced lengthening of the response; this lengthening was not suppressed by manganese. In calcium-free Tyrode solution containing manganese (1 to 4 mm) TII induced a small contraction suppressed by TTX. It is concluded that in rat heart, TII increases the plateau by slowing down the inactivation of the sodium conductance or by inducing an incomplete sodium inactivation. This effect favors (i) indirectly, the penetration of calcium through the slow channel and (ii) the development of the tonic component of cardiac contraction; both effects are responsible for the positive inotropic effect. In guinea-pig and rabbit heart, action potential was not lengthened by TII; this observation suggests the existence of species differences regarding the effects of the scorpion toxin on cardiac membranes.

Effect of 3-5-3′triiodothyronine treatment on cardiac action potential of streptozotocin-induced diabetic rat

Electrical activity of rat atrium of streptozotocin-diabetic and control rats was compared. (i) As occurs in the ventricle, diabetes lengthens the cardiac atrial action potential. (ii) Treatment by T3 of diabetic animals decreases action potential duration to normal values and causes partial recovery in plateau decay during the late phase of repolarization. (iii) T3 treatment however, does not completely normalized the action potential of the diabetic rat atrium, which remains abnormal during the early phase of repolarization. These results demonstrate that some defects in membrane mechanisms involved in the early phase of action potential repolarization are attributable solely to diabetes. The possible nature of these mechanisms is discussed.

Effect of maitotoxin on calcium current and background inward current in isolated ventricular rat myocytes

Maitotoxin (MTX) irreversibly suppressed the voltage-dependent calcium current after a variable delay, an effect which was preceded, in 61% of the cells, by a transient increase in calcium current partly attributable to a shift (4-7 mV) of the activation curve towards negative potentials. MTX also induced the development of a voltage-independent background inward current which did not occur in the absence of external calcium and was reduced by removal of external sodium, by calcium channel blockers and by high concentrations of quinidine. MTX-induced single channel activity consisted of long lasting bursts of inward current. Channel activity was voltage-independent, with a unitary conductance of 14 pS and an extrapolated reversal potential of +16 mV. Single-channel current amplitude was not detectably reduced in the absence of external calcium but strongly reduced in the absence of external sodium, in the presence of 2 mM nickel or when external sodium was replaced by 96 mM calcium or 50 mM barium. The channel activity was also inhibited by quinidine. It is concluded that MTX alters, then suppresses the voltage-activated calcium current and induces the development of a voltage-independent inward current, part of which results from the opening of nickel-sensitive cation channels, mostly permeable to sodium ions.

Differential effects of quinidine and flecainide on plateau duration of human atrial action potential.

SummaryQuinidine and flecainide, two class-I antiarrhythmics increase action potential duration (APD) at 90% repolarization and cellular refractory period in human atrial fibers without significant change in resting potential. On the other hand, quinidine decreases APD at 50%, whereas flecainide slightly increases, which suggests different effects on Ca2+ current. Using isolation cell procedure and whole cell recording, we found that 10 μM quinidine (34.77±6.5%, n=5) and 0.5 μM flecainide (50.46±6.2%, n=4) decrease calcium current in human atrium. It is concluded that, at therapeutical concentrations, quinidine and flecainide modify the action potential plateau phase in a different manner, which is not only related to the calcium current decrease.

Effect of diacetyl monoxime on transient outward current in rat ventricular myocytes.

Introduction Diacetyl monoxime, or 2,3-butanedione-2-monoxime (BDM) is a nucleophilic agent which is endowed with a phosphatase-like activity. BDM could therefore provide a tool for investigating the effects of phosphorylation of a channel constituent on channel function. In cat ventricular muscle, BDM has been shown to reversibly depress contraction and eliminate the plateau phase of the action potential (Wiggins et al. 1980), probably by removing phosphate groups from the slow channel protein, thus inducing slow inward current inhibition. However, we observed that in isolated perfused rat heart, 16 mM BDM enhanced the plateau amplitude and the action potential duration. Although these effects were accompanied by a negative inotropic effect, they could not be explained by inhibition of the calcium current. In view of these results, we studied the effect of BDM on the transient outward current, which is also largely involved in the plateau phase of the cardiac action potential.

Electrophysiological and inotropic effects of natural amino-deoxyglyco-cardenolides

The electrophysiological, inotropic and chronotropic effects of the two first natural amino-dexoyglyco-cardenolides, mitiphylline and holarosine B, were studied. Mitiphylline has qualitatively the same properties as those of common cardiac glycosides but is more potent; holarosine B has a similar action on cardiac electrophysiological parameters of the guinea pig heart but is less potent. The difference in activity between these two compounds could be attributed to the configuration in 5-and 17-positions of the genins. Holarisone B has the particularity of increasing the action potential plateau amplitude and its duration in rat heart; this could be part of the role of the aminosugar moiety.