Edouard Coraboeuf

Sensitivity to H, Li and Mg ions of the slow inward sodium current in frog atrial fibres.

Abstract Membrane currents have beenmeasured in frog atrial fibres under voltage clamp conditions (double sucrose gap method); the effects on these currents of (i) an increase in [H]o, (ii) the complete substitution of external Na by Li, (iii) the addition of Mg have been studied. The TTX-sensitive rapid inward current is decreased in acid Ringer (pH 5.6) as compared to values in normal Ringer (pH 7.8); it is unchanged in Li-Ringer and increased in Mg-Ringer. The slow inward calcium current (measured in TTX, Na-free solution) is decreased in acid Ringer and in Mg-Ringer and is unchanged in Li-Ringer. Increases or decreases affecting the rapid sodium current and the calcium current never exceed 40% and are generally smaller. On the other hand the slow inward sodium current (measured in TTX, Ca-free solution) is always suppressed in the three conditions studied. The participation of Na ions to the slow inward current in so called normal Ringer, that is in Mg-free Ringer is discussed. Indirect evidences are presented in favour of such a participation. After addition of Mg 1.8 m m to Ringer the slow inward current is carried only by Ca ions. Since the slow inward sodium current increases markedly in Mg-free Ca-free Ringer, it is concluded that the slow Na conductance is controlled by both Ca and Mg ions in addition to being strongly pH-dependent. It is also suggested that Na and Ca ions participating to the slow inward current do not penetrate necessarily through the same channel.

Barium- and calcium-permeable channels open at negative membrane potentials in rat ventricular myocytes

SummaryCa2+- and Ba2+-permeable channel activity from adult rat ventricular myocytes, spontaneously appeared in the three single-channel recording configurations: cell-attached, and excised inside-out or outside-out membrane patches. Single-channel activity was recorded at steady-state applied membrane potentials including the entire range of physiologic values, and displayed no “rundown” in excised patches. This activity occurred in irregular bursts separated by quiescent periods of 5 to 20 min in cell-attached membrane patches, whereas in excised patch experiments, this period was reduced to 2 to 10 min. During activity, a variety of kinetic behaviors could be observed with more or less complex gating patterns. Three conductance levels: 22, 45 and 78 pS were routinely observed in the same excised membrane patch, sometimes combining to give a larger level. These channels were significantly permeable to divalent cations and showed little or no permeability to potassium or sodium ions. The inorganic blockers of voltage-gated Ca channels, cobalt (2mm), cadmium (0.5mm) or nickel (3mm), had no apparent effect on these spontaneous unitary currents carried by barium ions. Under 10−5m bay K 8644 or nitrendipine, the activity was clearly increased in about half of the tested excised inside-out membrane patches. Both dihydropyridines enhanced openings of the larger conductance level, which was only very occasionally seen under control conditions. When the single-channel activity became sustained under 5×10−6m Bay K 8644, it was possible to calculate the mean unitary current at different membrane potentials and show that the mean current value increased with membrane potential.

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

OBJECTIVE The 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. METHODS Ventricular 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. RESULTS In 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. CONCLUSIONS In 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.

Abnormal pacemaking is modulated by sarcoplasmic reticulum in partially-depolarized myocardium from dilated right atria in humans

Fifty human atrial specimens removed at time of cardiac surgery were studied in vitro. Thirty-four samples were selected as presenting partial cell depolarization and exhibiting slow response action potentials. Twenty of these preparations were automatic whereas 14 were not. Neither mean maximum diastolic potential (-52.8 +/- 1.3 mV and -49.3 +/- 2.2 mV respectively) nor maximum rate of depolarization (Vmax) (1.1 +/- 0.1 V/s and 1.3 +/- 0.8 V/s) significantly differed between these two groups. Abnormal automaticity due to phase 4 depolarization occurred in 12/13 preparations dissected from markedly dilated atria whereas it occurred in only 2/10 preparations sampled from non-dilated atria. A statistically significant relationship between in vitro abnormal pacemaking and atrial dilatations was found. We investigated the effects on abnormal pacemaker depolarization and automaticity of a reduction in the extracellular Na and Ca and of sarcoplasmic reticulum (SR) inhibitors. Abnormal pacemaker depolarization appeared to be much more sensitive to a reduction in the extracellular Na than in the extracellular Ca. Both Sr and Mg slowed the automatic rate. Ryanodine 3 X 10(-6) M, a specific SR inhibitor, irreversibly lengthened the spontaneous basic cycle duration to about 300% of control. Epinephrine up to 10(-4) M was ineffective in accelerating the residual spontaneous rhythm that persists after ryanodine action, although epinephrine markedly enhanced the overshoot and Vmax of the slow responses. It is concluded that, in the human atrial myocardium, abnormal pacemaking that develops at low level of membrane potential: is promoted by chamber dilatation; is strongly modulated by SR-dependent processes.

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.

Release of acetylcholine by chick embryo heart before innervation

1. In chick embryo hearts, 3‐day‐old and not yet innervated, repetitive direct stimulation causes a transitory inhibition of the spontaneous rhythm.

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.

The antiarrhythmic agent bertosamil induces inactivation of the sustained outward K+ current in human atrial myocytes

1 In whole‐cell patch‐clamped human atrial myocytes, the antiarrhythmic agent bertosamil (10 μM) inhibited the sustained component, Isus (38.6±3.1%), and enhanced the inactivating component, It (9.1±6.1%), of the outward K+ current elicited by 750 ms test pulses from −60 mV to +50 mV. Higher concentrations of bertosamil (>10 μM) inhibited both It and Isus. 2 Suppression of Isus and stimulation of It by 10 μM bertosamil was observed on renewed stimulation following a 2 min rest period during which the drug was applied and persisted after washout, indicating a rest‐dependent effect of bertosamil on the outward K+ current. 3 Cell dialysis with an internal solution containing 10 μM bertosamil increased both It (78.0±14.7%) and Itotal (26.7±8.4%) and inhibited Isus (28.9±6.3%, n=6). In the presence of intracellular bertosamil, external application of the drug inhibited It and Isus in a concentration‐dependent and use‐dependent manner. 4 Following the suppression of Isus by 200 μM 4‐aminopyridine (4‐AP), bertosamil (10 μM) inhibited It. Washout of 4‐AP was associated with a larger It amplitude than that observed in control conditions. In myocytes characterized by a prominent Isus and lack of It, bertosamil (10 μM) induced a rapid and partial inactivation of the current, together with inward rectification of the current measured at the end of the test pulse. 5 In the presence of bertosamil the activation/voltage relationships, steady‐state inactivation and recovery from inactivation of It were markedly modified, pointing to changes in the conductance underlying It. 6 We conclude that bertosamil induces rapid inactivation of sustained outward current which leads to an apparent increase in It and decrease in Isus. This effect, which was distinct from the use‐dependent inhibition of the outward K+ current, could represent a new antiarrhythmic mechanism.

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.

Simulation of potassium accumulation in clefts of Purkinje fibers: Effect on membrane electrical activity☆

Purkinje fiber action potentials and concomitant intercellular cleft [K] variations were reconstructed by using modified McAllister, Noble & Tsien (1975) equations including the pump current, ip, and the pacemaker current, if. Three different mean cleft widths were chosen: 40, 200 and 1000 nm. Assuming a cylindrical arrangement of the cells in the bundle, the cleft [K] gradient across the bundle was calculated by using the radial cylindrical diffusion equation. The effects of varying several parameters (cleft width, tortuosity, ip and if) were studied in conditions corresponding to two different values of [K] in the bulk solution, namely 2.7 and 5.4 mM. The shortening influence on the action potential of the systolic increase in cleft [K] was detectable only in the case of the smallest cleft width. Reduction in electrogenic pump activity led to alterations of the electrical activity which depended on the cleft width. The evolution of the intercellular [K] during each action potential and the following diastolic period was normally biphasic; a small reaccumulation during the late part of the diastole was induced by the K component of the if current. Experimentally determined intercellular [K] variations described in the literature exhibit a monophasic evolution. Such a monophasic evolution could be reproduced after reduction of both if and the transient outward K current and suppression of the negative slope of the ik1-Em relationship. In this case the amplitude of the cyclic change in intercellular [K] was approximately equal to 0.2 mM (for a 200 nm cleft width), a value much lower than that experimentally recorded. Possible reasons for this discrepancy are discussed. A simplified three compartment model for K diffusion was also used. Results obtained with the two models demonstrated that the simplified model can be used as a reasonable approximation of the more complex radial diffusion model, with a reduction in computation time reaching 80% or more.