Milena Šimurdová

Slow inward current and action potentials of papillary muscles under non-steady state conditions.

Summary1.The relationship of the contractile response of cat papillary muscles and of the slow inward current, recorded under voltage clamp conditions (single sucrose gap), has been studied. The preparations were driven at a rate of 30 per min at 31° C. Both variables were recorded during a train of 7 identical clamp depolarizations (for 1 s from resting potential to −15 to +40 mV). The contractility increased severalfold and reached the steady state within 5–6 consecutive depolarizations.2.The voltage-dependence of slow inward current was confirmed: maximum was found at depolarizations near 0 mV. On repetition of clamp pulses the slow current gradually diminished in amplitude and was more slowly activated and inactivated. The shift of the current-voltage curve indicated a decrease of the reversal potential.3.Under non-steady state conditions the amplitude of the slow current was found to correlate closely with the magnitude of the contractile response at any given level of depolarization. The relation was linear with negative slope. The largest contractile response was not found at voltages which elicited maximum slow current.4.The progressive decrease of the slow current during repetition of voltage clamp depolarizations is not significantly affected by inadequate time for recovery of slowly changing conductances, since it occurs also at stimulation frequency 15 per min and the slow current remains virtually unaltered after 20 s period of quiescence.5.The course of total ionic current during phase 1 and 2 of action potential was reconstructed from a family of current curves obtained as a response to clamp depolarizations to various voltages, respecting the contractility-dependence of the current. The resulting course was correlated with the first derivative of action potential. A general conformity was ascertained.6.The correlation of slow inward current with action potential configuration indicates that the rate of its activation determines the depth of the notch separating spike and plateau, its magnitude determines the voltage of the plateau phase and its rate of inactivation affects repolarization.7.It is concluded that the described simultaneous changes of mechanical and electrical phenomena might be due to increased [Ca]i, which is responsible for more intense activation of the contractile proteins on the one hand, and decreased driving force of the slow inward current, carried by Ca ions, on the other.

Effect of ethanol on action potential and ionic membrane currents in rat ventricular myocytes

Aim:  Even though alcohol intoxication is often linked to arrhythmias, data describing ethanol effect on cardiac ionic channels are rare. In addition, ethanol is used as a solvent of hydrophobic compounds in experimental studies. We investigated changes of the action potential (AP) configuration and main ionic membrane currents in rat cardiomyocytes under 20–1500 mm ethanol.

Use-dependent effects of 4-aminopyridine on transient outward current in dog ventricular muscle

The use-dependent features of 4-aminopyridine-induced block of the transient outward current were investigated under voltage clamp conditions in dog ventricular myocardial bundles. The block depended strongly on the pattern of voltage clamp pulses. Its level (settled in rested membranes) became deeper at early time during depolarization. In contrast, prolonged depolarization (above 50 to 100 ms) produced relief of block. Our results suggest that the block strongly depends on the state of the channel gating system.

4-Aminopyridine sensitive transient outward current in dog ventricular fibres

Abstract(1) The depression of slow inward calcium current (ICa) induced by organic or inorganic inhibitors in voltage clamped dog ventricular preparations unmasks an early transient outward current (Ito). (2)Ito is depressed by 4-aminopyridine (1 mM) in a voltage dependent manner. (3)Ito appears in reponse to voltage steps above 40 mV (from holding voltage=resting voltage) and increases with raising the amplitude of clamp steps. (4) Within physiological range of membrane voltageIto is smaller and decays several times faster thanICa. Time course of the decline is approximately exponential (τ=25±6 ms at 80 mV above resting voltage). (5) Shifts of the holding voltage by 20 mV from the level of resting voltage alters the peak amplitude ofIto. It is increased by hyperpolarization and reduced by depolarization. (6) The recovery ofIto from inactivation at resting voltage was approximated by a single exponential. Time constant (390 ms) is about 15 times longer than the time constant of inactivation at 80 mV positive to the resting voltage.

Propafenone blocks ATP-sensitive K+ channels in rabbit atrial and ventricular cardiomyocytes

Propafenone, a class I antiarrhythmic agent, inhibits several membrane currents (I(Na), I(Ca), I(K), Ito), however, its effects on ATP-sensitive potassium current (I(K)ATP) of cardiac cells have not been tested. We evaluated the blocking effects of 0.1 to 100 microM propafenone applications at 35 degrees C on the whole-cell I(K)ATP as triggered by dinitrophenol (75 microM) in adult rabbit dissociated atrial and ventricular cardiomyocytes in comparison. The block of I(K)ATP by propafenone was dose-dependent, fully reversible and voltage-independent. The dose-response relation, as evaluated at 0 mV for atrial myocytes (ED50 = 1.26+/-0.17 microM, Hill number = 1.25+/-0.22) was significantly shifted to the left vs. that in ventricular myocytes (ED50 = 4.94+/-0.59 microM, Hill number = 1.22+/-0.14). It is concluded that propafenone blocks cardiac I(K)ATP at a single site with 4 times higher affinity for the drug in atrial myocytes. This block of cardiac I(K)ATP might play a role in the beneficial and adverse effects of the drug.

Activity-dependent changes of slow inward current in ventricular heart muscle

Abstract1.The relationships between membrane voltage, contractile force and slow inward current were studied in cat and dog papillary muscles or trabeculae employing the double sucrose gap voltage clamp technique. The experiments were performed at 30°C and the preparations were stimulated at a frequency of 0.5 Hz.2.The known relationships between steady state contractile force, slow inward current and membrane voltage were confirmed.3.Under non-steady state conditions the slow inward current decreases during ascending and increases during descending contraction staircases when the clamp steps of the test train exceed about 60 mV from resting level. Depolarization clamp steps below 60 mV produce parallel changes of the slow inward current and contractile force. Those clamp conditions which increase the contractile force shift the threshold of Isi and of contraction towards more negative values.4.During ascending staircases an increasing background outward current was regularly observed together with diminshing slow inward current.5.The reported current transients agree with the changes of action potential configuration during mechanical transients: the prolongation of plateau during descending staircases corresponds to an increase, and the shortening of action potential during late repolarization corresponds to a decrease of slow inward current in the respective voltage ranges.6.The slow inward current was tentatively separated into two components. The main component is inversely proportional to contractile force and it exhibits the well known current-voltage relationship for this current. The other one is directly proportional to contractile force and may be related to a regenerative response of reticular membranes.

Effect of epinephrine on the duration of action potential of papillary muscles

Im Gegensatz zur Kontrollbedingung (Tyrodelösung 1.8 mM Ca, 31°C, Reizfrequenz 30/min) ruft Adrenalin 6×10−6 M im Ca-freien Milieu eine significante Verlängerung der Aktionspotenziale hervor. Die Zugabe von Ca2+ kehrt die Aktionspotenzialdauer mit voller Entwicklung der positiv inotropen Wirkung zu Ausgangswerten zurück.

A contraction-related component of slow inward current in dog ventricular muscle and its relation to Na(+)-Ca2+ exchange.

1. The slow inward current component related to contraction (Isic) was studied in voltage clamp experiments on canine ventricular trabeculae at 30 degrees C with the aims of (a) estimating its relation to electrogenic Na(+)‐Ca2+ exchange and (b) comparing it with similar currents as reported in cardiac myocytes. 2. Isic may be recorded under conditions of augmented contractility in response to depolarizing pulses below the threshold of the classic slow inward current (presumably mediated by L‐type Ca2+ channels). In responses to identical depolarizing clamp pulses the peak value of Isic is directly related to the amplitude of contraction (Fmax). Isic peaks about 60 ms after the onset of depolarization and declines with a half‐time of about 110 ms. 3. The voltage threshold of Isic activation is the same as the threshold of contraction. The positive inotropic clamp preconditions shift both thresholds to more negative values of membrane voltage, i.e. below the threshold of the classic slow inward current. 4. Isic may also be recorded as a slowly decaying inwardly directed current ‘tail’ after depolarizing pulses. In this representation the peak value of Isic changes with duration of the depolarizing pulses, again in parallel with Fmax. In response to pulses shorter than 100 ms both variables increase with depolarization time. If initial conditions remain constant, further prolongation of the pulse does not significantly influence either one (tail currents follow a common envelope). 5. Isic differs from classic slow inward current by: (a) its direct relation to contraction, (b) the slower decay of the current tail on repolarization, (c) slower restitution corresponding to the mechanical restitution, (d) its relative insensitivity to Ca(2+)‐blocking agents (the decrease of Isic is secondary to the negative inotropic of Ca(2+)‐blocking agents (the decrease of Isic is secondary to the negative inotropic effect) and (e) its disappearance after Sr2+ substitution for Ca2+. 6. The manifestations of Isic in multicellular preparations do not differ significantly from those reported in isolated myocytes (in contrast to calcium current). 7. The analysis of the correlation between Isic and Fmax transients during trains of identical test depolarizing pulses at variable extra‐ and intracellular ionic concentrations (changes of [Ca2+]o, 50% Li+ substitution for Na+, strophanthidin) indicate that the observed effects conform to the predictions based on a quantitative model of Na(+)‐Ca2+ exchange. 8. It is concluded that Isic is activated by a transient increase of [Ca2+]i, in consequence of the release from the reticular stores.(ABSTRACT TRUNCATED AT 400 WORDS)

Voltage dependence of force- and slow inward current restitution in ventricular muscle

SummaryThis study was aimed to assess the relationship among the voltage-dependent processes underlying the excitation-contraction coupling, viz. force restitution (FR), transmembrane Ca fluxes and Ca release. The experiments (n=22) were performed on voltage-clamped dog trabeculae in which force and slow inward current were measured. Standard steady-state was achieved by clamp driving at 0.5 Hz, 300 ms, 70 mV depolarizing pulses from holding=resting potential at 30°C. Voltage and duration of single pulses and intervals in between were varied according to five protocols.The voltage dependence of Ca release was tested by varying single pulses at equal steady-state, i.e., at equal Ca availability. Contractions could be elicited in absence of ICa (20–30 mV step) and in the presence of disproportionately small ICa (above 80 mV).The voltage dependence of Ca availability for the release was tested by constant test pulses following either a variable conditioning clamp pulse or a period of rest at a variable voltage. After a low voltage pulse and, hence, depressed or absent ICa, the test contraction is diminished in presence of normal or even augmented Isi at any test interval (i.e., FR is depressed). Diminished Ca influx thus reduces the Ca availability of the subsequent beat. During prolonged depolarization (by 60 mV and more) a tonic response appears, but a phasic response cannot be elicited (FR is inhibited). Upon subsequent repolarization FR starts from zero and is significantly enhanced.It is concluded that, during depolarization, Ca release channels are in an open state, thus allowing free recirculation of Ca, but no build-up of a sufficient Ca gradient at the release site.

Inward rectifying potassium currents resolved into components: modeling of complex drug actions

Inward rectifier potassium currents (IKir,x) belong to prominent ionic currents affecting both resting membrane voltage and action potential repolarization in cardiomyocytes. In existing integrative models of electrical activity of cardiac cells, they have been described as single current components. The proposed quantitative model complies with findings indicating that these channels are formed by various homomeric or heteromeric assemblies of channel subunits with specific functional properties. Each IKir,x may be expressed as a total of independent currents via individual populations of identical channels, i.e., channels formed by the same combination of their subunits. Solution of the model equations simulated well recently observed unique manifestations of dual ethanol effect in rat ventricular and atrial cells. The model reflects reported occurrence of at least two binding sites for ethanol within IKir,x channels related to slow allosteric conformation changes governing channel conductance and inducing current activation or inhibition. Our new model may considerably improve the existing models of cardiac cells by including the model equations proposed here in the particular case of the voltage-independent drug-channel interaction. Such improved integrative models may provide more precise and, thus, more physiologically relevant results.