Fendiline inhibits L-type calcium channels in guinea-pig ventricular myocytes: a whole-cell patch-clamp study.

1. Fendiline, a diphenylalkylamine type of antianginal drug, was examined for its effects on L-type calcium channels in guinea-pig ventricular myocytes by the whole-cell patch-clamp technique. 2. Fendiline (0.3-100 microM) applied extracellularly inhibited the calcium channel current (ICa) in a concentration- and time-dependent manner. The IC50 of fendiline was 17.0 +/- 2.43 microM and the Hill slope was 1.39 +/- 0.23. 3. Inhibition of ICa by fendiline appeared with an onset of less than 3 s. 4. Fendiline inhibited ICa at all the membrane potentials tested and shifted the current-voltage curve upwards. The overall calcium channel conductance (gCa) of the cell was reduced and conductance-voltage curve was shifted to the left in the presence of fendiline. 5. Isoprenaline (0.5-1 microM), a beta-adrenoceptor agonist, partially reversed the inhibitory effect of fendiline on ICa. 6. It is suggested that fendiline applied extracellularly blocks L-type calcium channels and reduces calcium channel conductance of the cell. The calcium channels thus inhibited are, nevertheless, still available for beta-adrenoceptor stimulation.     

N-salicyloyltryptamine, a new anticonvulsant drug, acts on voltage-dependent Na+, Ca2+, and K+ ion channels

1. The aim of this work was to study the effects of N-salicyloyltryptamine (STP), a novel anticonvulsant agent, on voltage-gated ion channels in GH3 cells. 2. In this study, we show that STP at 17 microM inhibited up to 59.2+/-10.4% of the Ito and 73.1+/-8.56% of the IKD K+ currents in GH3 cells. Moreover, the inhibitory activity of the drug STP on K+ currents was dose-dependent (IC50=34.6+/-8.14 microM for Ito) and partially reversible after washing off. 3. Repeated stimulation at 1 Hz (STP at 17 microM) led to the total disappearance of Ito current, and an enhancement of IKD. 4. In the cell-attached configuration, application of STP to the bath increased the open probability of large-conductance Ca2+-activated K+ channels. 5. STP at 17 microM inhibited the L-type Ca2+ current by 54.9+/-7.50% without any significant changes in the voltage dependence. 6. STP at 170 microM inhibited the TTX-sensitive Na+ current by 22.1+/-2.41%. At a lower concentration (17 microM), no effect on INa was observed. 7. The pharmacological profile described here might contribute to the neuroprotective effect exerted by this compound in experimental 'in vivo' models.     

Frequency- and time-dependent effects of fendiline on action potentials of guinea pig papillary muscle

The action of fendiline on cardiac electrical activity was investigated in guinea pig papillary muscle by monitoring frequency- and time-dependent changes in membrane potential, action potential (AP) configuration and conduction velocity. Isolated guinea pig papillary muscles driven at 0.1 to 3 Hz showed a concentration-dependent reduction of +Vmax, overshoot, and AP duration at -20mV (APD20) in the presence of fendiline (1-320 microM), reflecting inhibition of Na+ and L-type Ca2+ channels, respectively. No significant change in resting potential and AP duration at 90% repolarization (APD90) were observed. Inhibition of +Vmax and APD20 was more prominent at higher frequency of stimulation (2 Hz) than at lower ones (0.2 Hz), demonstrating frequency-dependent block of Na+ and Ca2+ channels including an open channel block. A good relationship between changes in +Vmax and APD20 suggested some commonality in the mechanism of inhibition of Na+ and Ca2+ channels by fendiline. Time-dependence of effects of fendiline, observed in presence of bolus dose (200 microM), showed an earlier onset of inhibition of +Vmax and APD20, particularly at higher frequencies. Missed beats and conduction block also appeared earlier in preparations driven at higher frequency. These findings suggest a frequency-dependent (and open channel) block of Na+ and Ca2+ channels by fendiline, leading to inhibition of fast and slow conduction in addition to its reported inactivated Ca2+ channel block.     

Electrophysiological mechanisms for antiarrhythmic efficacy and positive inotropy of liriodenine, a natural aporphine alkaloid from Fissistigma glaucescens.

1. The antiarrhythmic potential and electromechanical effects of liriodenine, an aporphine alkaloid isolated from the plant, Fissistigma glaucescens, were examined. 2. In the Langendorff perfused (with constant pressure) rat heart, at a concentration of 0.3 to 3 microM, liriodenine was able to convert a polymorphic ventricular tachyrhythmia induced by the ischaemia-reperfusion (EC50 = 0.3 microM). 3. In isolated atrial and ventricular muscle, liriodenine increased the contractile force and slowed the spontaneous beating of the right atrium. 4. The liriodenine-induced positive inotropy was markedly attenuated by a transient outward K+ channel blocker, 4-aminopyridine (4-AP) but was not significantly affected by prazosin, propranolol, verapamil or carbachol. 5. In rat isolated ventricular myocytes, liriodenine prolonged action potential duration and decreased the maximal upstroke velocity of phase 0 depolarization (Vmax) and resting membrane potential in a concentration-dependent manner. The action potential amplitude was not significantly changed. 6. Whole-cell voltage clamp study revealed that liriodenine blocked the Na+ channel (INa) concentration-dependently (IC50 = 0.7 microM) and caused a leftward shift of its steady-state inactivation curve. However, its recovery rate from the inactivated state was not affected. The L-type Ca2+ currents (Ica) were also decreased, but to a lesser degree (IC50 = 2.5 microM, maximal inhibition = 35%). 7. Liriodenine inhibited the 4-AP-sensitive transient outward current (Ito) (IC50 = 2.8 microM) and moderately accelerated its rate of decay. The block of Ito was not associated with changes in the voltage-dependence of the steady-state inactivation curve or in the process of recovery from inactivation of the current. Liriodenine also reduced the amplitude of a slowly inactivating, steady-state outward current (Iss) (IC50 = 1.9 microM). These effects were consistent with its prolonging effect on action potential duration. The inwardly rectifying background K+ current (IK1), was also decreased but to a less degree. 8. Compared to quinidine, liriodenine exerted a stronger degree of block on INa, comparable degree of block on IK1, and lesser extent of block on ICa and Ito. 9. It is concluded that, through inhibition of Na+ and the Ito channel, liriodenine can suppress ventricular arrhythmias induced by myocardial ischaemia reperfusion. The positive inotropic effect can be explained by inhibition of the Ito channel and the subsequent prolongation of action potential duration. These results provide a satisfactory therapeutic potential for the treatment of cardiac arrhythmias.     

Tonic and phasic Vmax block induced by 5-hydroxypropafenone in guinea pig ventricular muscles

The effects of 5-hydroxypropafenone (5-OH-P) on transmembrane action potentials were studied in isolated guinea pig papillary muscles. 5-Hydroxypropafenone, 10(-7) M to 5 X 10(-5) M, produced a dose-dependent decrease in the Vmax and amplitude of the action potential and a depolarization of the resting membrane potential, whereas the action potential duration was not affected by the drug. In the presence of 5-OH-P, trains of stimuli at rates between 0.2 and 2 Hz led to an exponential decline in Vmax (onset rate at 1 Hz 0.07 +/- 0.01 per action potential) to a new steady-state level. This phasic, use-dependent Vmax block was augmented at higher stimulation frequencies. The time constant for the recovery of Vmax from the phasic block (offset) was 200.0 +/- 9.3 s. In papillary muscles depolarized with 10 mM K the inhibitory effect of 5-OH-P on Vmax of the first action potential after a long quiescent period (tonic block) was markedly increased, but the rates of onset and offset of the phasic block were similar to those found in normally polarized fibers under 5.4 mM K. 5-OH-P also shifted the curves relating membrane potential and Vmax in the hyperpolarizing direction. These findings suggest that 5-OH-P exhibited similar phasic inhibitory action of the fast sodium channels than other class Ic antiarrhythmic drugs. This phasic Vmax block, and its greater inhibition of Vmax in depolarized muscles through the increase of tonic block, would play a major role for the antiarrhythmic effect of the drug.     

Blockade of HERG cardiac K+ current by antifungal drug miconazole

1. Miconazole, an imidazole antifungal agent, is associated with acquired long QT syndrome and ventricular arrhythmias. Miconazole increases the plasma concentration of QT-prolonging drugs by inhibiting the hepatic cytochrome P450 metabolic pathway, but whether it has direct effects on cardiac ion channels has not been elucidated. 2. To determine the mechanism underlying these clinical findings, we investigated the effect of miconazole on human ether-a-go-go-related gene (HERG) K+ channels. 3. HERG channels were heterologously expressed in human embryonic kidney 293 (HEK293) cells and whole-cell currents were recorded using a patch-clamp technique (23 degrees C). 4. Miconazole inhibited HERG peak tail current in a concentration-dependent manner (0.4-40 microM) with an IC50 of 2.1 microM (n=3-5 cells at each concentration, Hill coefficient 1.2). HERG block was not frequency-dependent. It required channel activation, occurred rapidly, and had very slow dissociation properties. 5. The activation curve was shifted in a negative direction (V(1/2): -9.5+/-2.3 mV in controls and -15.3+/-2.4 mV after 4 microM miconazole, P<0.05, n=6). Miconazole did not change other channel kinetics (activation, deactivation, onset of inactivation, recovery from inactivation, steady-state inactivation). 6. The S6 domain mutation, F656C, abolished the inhibitory action of miconazole on HERG current indicating that miconazole preferentially binds to an aromatic amino-acid residue within the pore-S6 region. 7. Our findings indicate that miconazole causes HERG channel block by binding to a common drug receptor, and this involves preferential binding to activated channels. Thus, miconazole prolongs the QT interval by direct inhibition of HERG channels.     

Open channel block of HERG K(+) channels by vesnarinone.

Vesnarinone, a cardiotonic agent, blocks I(Kr) and, unlike other I(Kr) blockers, produces a frequency-dependent prolongation of action potential duration (APD). To elucidate the mechanisms, we studied the effects of vesnarinone on HERG, the cloned human I(Kr) channel, heterologously expressed in Xenopus laevis oocytes. Vesnarinone caused a concentration-dependent inhibition of HERG currents with an IC(50) value of 17.7 +/- 2.5 microM at 0 mV (n = 6). When HERG was coexpressed with the beta-subunit MiRP1, a similar potency for block was measured (IC(50): 15.0 +/- 3.0 microM at 0 mV, n = 5). Tonic block of the HERG channel current was minimal (<5% at 30 microM, n = 5). The rate of onset of block and the steady-state value for block of current were not significantly different for test potentials ranging from -40 to +40 mV [time constant (tau) = 372 +/- 76 ms at +40 mV, n = 4]. Recovery from block at -60, -90, and -120 mV was not significantly different (tau = 8.5 +/- 1.5 s at -90 mV, n = 4). Vesnarinone produced similar effects on inactivation-removed mutant (G628C/S631C) HERG channels. The IC(50) value was 10.7 +/- 3.7 microM at 0 mV (n = 5), and the onset and recovery from block of current findings were similar to those of wild-type HERG. Amino acids important for the binding of vesnarinone were identified using alanine-scanning mutagenesis of residues believed to line the inner cavity of the HERG channel. Six important residues were identified, including G648, F656, and V659 located in the S6 domain and T623, S624, and V625 located at the base of the pore helix. These residues are similar but not identical to those determined previously for MK-499, an antiarrhythmic drug. In conclusion, vesnarinone preferentially blocks open HERG channels, with little effect on channels in the rested or inactivated state. These actions may contribute to the favorable frequency-dependent prolongation in APD.     

Raloxifene inhibits transient outward and ultra-rapid delayed rectifier potassium currents in human atrial myocytes

The selective estrogen receptor modulator raloxifene is widely used in the treatment of postmenopausal osteoporosis, and has cardioprotective properties. However, effects of raloxifene on cardiac ion channels are unclear. The present study was designed to investigate the effects of raloxifene and beta-estradiol on transient outward and ultra-rapid delayed rectifier potassium currents (Ito1 and IKur) in human atrial myocytes with a whole cell patch-clamp technique. Ito1 was inhibited by raloxifene in a concentration-dependent manner with an IC50 of 0.9 microM. Raloxifene at 1 microM decreased Ito1 by 40.2+/-1.9% (at +50 mV, n=14, P<0.01 vs control). Time-dependent recovery from inactivation was slowed, and time to peak and time-dependent inactivation of Ito1 were significantly accelerated, while steady-state voltage dependent activation and inactivation of Ito1 were not affected by raloxifene. In addition, raloxifene remarkably suppressed IKur (IC50=0.7 microM). Raloxifene at 1 microM decreased IKur by 57.3+/-3.3% (at +50 mV, n=10, P<0.01 vs control). However, beta-estradiol inhibited Ito1 (IC50=10.3 microM) without affecting IKur. The inhibitory effects of raloxifene and beta-estradiol on Ito1 and/or IKur were unaffected by the estrogen receptor antagonist ICI 182,780. Our results indicate that raloxifene directly inhibits the human atrial repolarization potassium currents Ito1 and IKur. Whether raloxifene is beneficial for supraventricular arrhythmias remains to be studied.     

Effects of propafenone on K currents in human atrial myocytes

1. The class Ic anti-arrhythmic agent, flecainide is known to inhibit the transient outward K current (Ito) selectively in human atrium. We studied the effects of propafenone, another class Ic antiarrhythmic agent, on K currents in human atrial myocytes using a whole-cell voltage-clamp method. 2. Propafenone inhibited both Ito and the sustained or ultra-rapid delayed rectifier K current (Isus or Ikur) evoked by depolarization pulses. The concentration for half-maximal inhibition (IC50) was 4.9 microM for Ito and 8.6 microM for Isus. Propafenone blocked Ito and Isus in a voltage- and use-independent fashion and accelerated the inactivation time constant of Ito [from 28.3 to 6.7 ms at 10 microM propafenone]. 3. The steady-state inactivation curve for Ito was unaffected by propafenone. Propafenone did not affect the initial current at depolarizing potentials, but it did produce a block that increased as a function of time after depolarization (time constant of 3.4 ms). This suggests that propafenone preferentially blocked Ito in the open state. 4. Propafenone had no significant effect on the rate at which Ito recovered from inactivation at -80 mV suggesting that propafenone dissociates rapidly from the channel. 5. The steady-state activation curve for Isus was not affected by propafenone. Propafenone slowed the time course of the onset of the Isus tail current. This suggests that propafenone blocked Isus in the open state. 6. The present results suggest that, unlike flecainide, propafenone blocks both Ito and Isus in human atrial myocytes in the open state at clinically relevant concentrations.     

Blockade of HERG human K+ channel and IKr of guinea pig cardiomyocytes by prochlorperazine

Prochlorperazine, a drug for the symptomatic control of nausea, vomiting and psychiatric disorders, can induce prolonged QT, torsades de pointes and sudden death. We studied the effects of prochlorperazine on human ether-a-go-go-related gene (HERG) channels expressed in Xenopus oocytes and also in the delayed rectifier K+ current of guinea pig cardiomyocytes. Prochlorperazine induced a concentration-dependent decrease in current amplitudes at the end of the voltage steps and tail currents of HERG. The IC50 for a prochlorperazine block of HERG current in Xenopus oocytes progressively decreased relative to the degree of depolarization, from 42.1 microM at -40 mV to 37.4 microM at 0 mV to 22.6 microM at +40 mV. The block of HERG by prochlorperazine was use-dependent, exhibiting a more rapid onset and a greater steady-state block at higher frequencies of activation, while there was partial relief of the block with reduced frequencies. In guinea pig ventricular myocytes, bath applications of 0.5 and 1 muM prochlorperazine at 36 degrees C blocked rapidly activating delayed rectifier K+ current by 38.9% and 76.5%, respectively, but did not significantly block slowly activating delayed rectifier K+ current. Our findings suggest that the arrhythmogenic side effects of prochlorperazine are caused by a blockade of HERG and the rapid component of the delayed rectifier K+ current rather than by a blockade of the slow component.     

Barnidipine block of L-type Ca(2+) channel currents in rat ventricular cardiomyocytes

The effects of barnidipine and nifedipine on L-type Ca(2+) current (I(Ca(L))) were investigated in ventricular cardiomyocytes from rats. Both barnidipine and nifedipine reduced I(Ca(L)) in a concentration and voltage dependent manner; the EC(50) were 80 and 130 nM at a holding potential of -80 mV, respectively, and 18 and 6 nM at -40 mV, respectively. Both drugs induced a leftward shift of the steady-state inactivation curve of I(Ca(L)). Using a twin pulse protocol, the relationships between the amount of block of I(Ca(L)) by either drug, seen during the second pulse, and the length of the first pulse were described by monoexponential functions reflecting onset of block, dependent on drug concentration. The onset of block by barnidipine was three times faster than that by nifedipine. With both drugs, recovery of I(Ca(L)) was 50 times slower than under control conditions and described by monoexponential functions reflecting offset of block (independent of drug concentration). The offset of block with barnidipine was three times slower than that with nifedipine. The time constants of block and unblock of I(Ca(L)) by both drugs were used to calculate binding and unbinding and to predict their effects at two frequencies. It is suggested that barnidipine exhibits a higher affinity to the inactivated Ca(2+) channel state as compared to nifedipine.     

States and sites of actions of flecainide on guinea-pig cardiac sodium channels.

The states and sites of actions of flecainide on sodium channels were investigated in guinea-pig single cardiac cells, using the whole-cell voltage-clamp technique at 22 degrees C. External application of flecainide caused tonic and use-dependent block of the sodium current (INa). The tonic block and the steady state use-dependent block increased with increasing drug concentrations. The dose-response curve for the use-dependent block was fitted by the equation for 1:1 drug-receptor binding and yielded a KD of 7.0 microM flecainide. At 5 microM flecainide, the use-dependent block of INa with 10 and 200 ms depolarizing pulses at an interpulse interval of 400 ms was 31.1 +/- 2.7 (mean +/- S.E.) and 36.8 +/- 2.7%, respectively. The two values were not significantly different. The block developed as a single exponential function with onset rate of 0.041 +/- 0.005/pulse. Recovery from flecainide block consisted of two components as reported previously. The mean time constant of the initial fast component was 48 +/- 17 ms, which was comparable but significantly longer than that in the absence of the drug. The late slow component was only seen after drug application and the time constant was 26 +/- 7 s at -100 mV. Internal application of 5 and 50 microM flecainide for 30 min after rupture of the cell membrane produced a non-significant block and values of 1.7 +/- 0.8 and 6.9 +/- 2.4%, respectively, for the use-dependent block of INa.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of TYB-3823, a new antiarrhythmic drug, on sodium current in isolated cardiac cells.

1. Sodium current (INa) blockade by TYB-3823, a newly synthesized antiarrhythmic agent, was investigated in isolated single ventricular myocytes by use of the whole cell patch-clamp technique. 2. TYB-3823 blocked INa under steady-state conditions (Kd,rest = 500 microM, Kd,i = 4.9 microM), findings consistent with a shift in the steady state INa availability curve to more negative potentials. 3. TYB-3823 produced use-dependent block at 2 Hz in conjunction with increase in pulse duration (5-300 ms), that was markedly enhanced at less negative holding potentials. 4. The time course of the onset of block was accelerated and the degree of use-dependent block was decreased at more negative holding potential. The time course of the onset of block was accentuated with enhancing block at more positive holding potentials. 5. The time course of recovery from use-dependent block was accelerated at more negative holding potentials but was accentuated at more positive holding potentials. 6. These results suggest that both tonic block and use-dependent block of sodium channels in cardiac tissue might result from an interaction of TYB-3832 with sodium channels mainly in the inactivated channel states and the kinetics of the interaction between drug and receptor may be modulated by the inactivation gate.     

The voltage- and time-dependent blocking effect of trifluoperazine on T lymphocyte Kv1.3 channels

Phenothiazines are well-known calmodulin inhibitors that interact with many receptors and channels including a variety of potassium channels. In this study, we report a blocking effect of trifluoperazine (TFP) on voltage-gated Kv1.3 channels expressed in human T lymphocytes. Application of TFP in the concentration range from 1 to 20 microM reduced the current amplitude to about a half of the control value. The currents were blocked to less than 0.05 of the control value at 50 microM TFP concentration. The blocking effect was accompanied by a substantial increase in the current inactivation rate, whereas the activation rate and the steady-state activation and inactivation were not changed significantly. The blocking effect of TFP was voltage dependent being most potent at +60mV and least potent at -20mV. The blocking effect of TFP on the currents and the recovery from block was time dependent. Other calmodulin antagonists: tamoxifen (TMX) and thioridazine also inhibited the channels at micromolar concentrations. The effects exerted by TMX and thioridazine resembled the inhibitory effect of TFP. The blocking effect of thioridazine was time dependent and appeared to be more potent that the inhibition by TFP and TMX. TFP, TMX and thioridazine inhibited the activity of Kv1.3 channels only when applied extracellularly. The inhibitory effect of all the compounds was reversible. The possible physiological significance of the current inhibition is discussed.     

Effects of U50,488H on transient outward and ultra-rapid delayed rectifier K+ currents in young human atrial myocytes

The effects of trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate salt (U50,488H), a selective kappa-opioid receptor agonist, on transient outward K+ current (Ito1) and ultra-rapid delayed rectifier K+ current (IKur) in young human atrial myocytes were evaluated with a whole-cell patch-clamp technique. At +10 mV, U50,488H decreased Ito1 in a concentration-dependent manner (IC50=12.4+/-3.5 microM), while at +50 mV, U50,488H produced biphasic effects on Ito1-increasing and decreasing the current at 1-3 and 10-30 microM, respectively. U50,488H at 10 microM shifted the midpoint (V0.5) of Ito1 activation in a depolarizing direction by approximately 5 mV, accelerated the inactivation, and slowed the recovery from inactivation of Ito1. In addition, U50,488H inhibited IKur in a concentration-dependent manner (IC50=3.3+/-0.6 microM). The effects of U50,488H on the two types of K+ currents were not antagonized by either 5 microM nor-binaltorphimine or 300 nM naloxone. These results indicate that U50,488H affects both Ito1 and IKur in young human atrial myocytes in an opioid receptor-independent manner.     

Effects of imipramine on the transient outward current in rabbit atrial single cells.

1. The effects of imipramine on action potential characteristics and transient outward potassium current (It) of rabbit isolated atrial myocytes were studied using the whole-cell configuration of the patch-clamp technique. 2. Imipramine, 3 microM, decreased action potential amplitude and lengthened the action potential duration measured at 50% of repolarization, whereas it did not modify the final phase of repolarization or the resting membrane potential. These results are similar to those reported in multicellular rabbit atrial preparations. 3. Imipramine, 0.1-100 microM, induced a concentration-dependent inhibition of the peak amplitude of It, a shortening of the time to peak current and an increase in the inactivation rate. The acceleration of the current inactivation is to a major extent responsible for the decrease in the integral of the outward current measured at 50 ms after the start of the pulse. 4. The drug-induced block of It was not associated with changes in the voltage-dependence of the steady-state inactivation curve or in the process of recovery from inactivation of the current. Extrapolation to zero block shows that imipramine did not block It before its activation at the onset of the depolarization. These results suggested that imipramine does not affect the inactivated or the resting state of It channels. 5. It is concluded that in rabbit isolated atrial cells, imipramine inhibits It and that this effect is responsible for the lengthening of the action potential duration produced by this drug.     

Depression of maximum rate of depolarization of guinea-pig ventricular action potentials by metabolites of encainide.

1. Standard microelectrode methods have been used to record action potentials from guinea-pig ventricular myocardium and dog Purkinje fibres, and to study the effects of the two major metabolites of encainide, O-desmethyl encainide (ODE) and 3-methoxy-O-desmethyl encainide (MODE). 2. In concentrations similar to those found in patients during chronic encainide therapy, neither ODE nor MODE produced significant depression of maximum rate of depolarization (Vmax) of action potentials in unstimulated tissue. Repetitive stimulation, however, was associated with depression of Vmax which increased with increasing driving rates (rate-dependent block, RDB). At the fastest rate studied (interstimulus interval = 300 ms) ODE 1 microM depressed Vmax by 47.5 +/- 5.7% and MODE 1 microM, reduced Vmax by 52.2 +/- 12%. 3. The onset and offset kinetics of this rate-dependent block were very slow. Full development of RDB during a train required over 100 action potentials and the time constants of recovery of Vmax from RDB were 86.4 +/- 37 s for ODE and 100.4 +/- 18 s for MODE. The amount of RDB and its rate of onset increased with drug concentration. The recovery time constants were independent of inter-stimulus interval or drug concentration. Both metabolites also produced rate-dependent depression of conduction velocity in canine Purkinje fibres, but no evidence of selective depression of conduction of interpolated premature potentials was seen. 4. Early afterdepolarizations occurred spontaneously in three preparations in the presence of MODE, 1 microM and one preparation in ODE, 1 microM. 5. It is concluded that these metabolites of encainide may play a role in producing both its antiarrhythmic and its proarrhythmic effects.     

Transient outward current inhibition by propafenone and 5-hydroxypropafenone in cultured neonatal rat ventricular myocytes

The antiarrhythmic agent propafenone and its primary electropharmacologically active metabolite, 5-hydroxypropafenone, are known inhibitors of cardiac myocyte repolarizing currents. We recently documented potent propafenone inhibition of the transient outward potassium current (Ito) in human atrial myocytes from patients in the newborn and infant age range. In the current study we characterized ventricular Ito inhibition by propafenone and 5-hydroxypropafenone in neonatal myocytes enzymatically isolated from 2-day-old Sprague-Dawley rat pups. Using the whole-cell patch-clamp technique in ventricular myocytes kept in primary culture for 1-4 days, we observed comparably potent Ito inhibition by both agents, yielding 50% maximal inhibitory concentration (IC50) values of 2.1 +/- 0.5 and 1.5 +/- 0.2 microM for propafenone and 5-hydroxypropafenone, respectively. Ito blockade by both of these agents was time, concentration, and voltage dependent, but use independent. There was no drug effect on steady-state voltage dependence of Ito inactivation, or on the time course of Ito recovery from inactivation. These findings are consistent with an open channel-blocking mechanism as suggested by other models. We conclude that both propafenone and 5-hydroxypropafenone are potent Ito inhibitors in neonatal rat ventricular myocytes, with potencies exceeding those demonstrated for propafenone in adult rat ventricular myocytes or in human atrial myocytes from patients of all ages.     

Effects of the antifungal antibiotic clotrimazole on human cardiac repolarization potassium currents

The antifungal antibiotic clotrimazole (CLT) shows therapeutic effects on cancer, sickle cell disease, malaria, etc. by inhibiting membrane intermediate-conductance Ca2+ -activated K+ channels (IKCa). However, it is unclear whether this drug would affect human cardiac K+ currents. The present study was therefore designed to investigate the effects of CLT on transient outward K+ current (Ito1), and ultra-rapid delayed rectifier K+ current (IKur) in isolated human atrial myocytes, and cloned hERG channel current (IhERG) and recombinant human cardiac KCNQ1/KCNE1 channel current (IKs) expressed in HEK 293 cells. It was found that CLT inhibited Ito1 with an IC50 of 29.5 microM, accelerated Ito1 inactivation, and decreased recovery of Ito1 from inactivation. In addition, CLT inhibited human atrial I(Kur) in a concentration-dependent manner (IC50 = 7.6 microM). CLT substantially suppressed IhERG (IC50 = 3.6 microM), and negatively shifted the activation conductance of IhERG. Moreover, CLT inhibited IKs (IC50 = 15.1 microM), and positively shifted the activation conductance of the current. These results indicate that the antifungal antibiotic CLT substantially inhibits human cardiac repolarization K+ currents including Ito1, IKur, IhERG, and IKs. However, caution is recommended when correlating the observed in vitro effects on cardiac ion currents to the clinical relevance.     

Block and modified gating of cardiac calcium channel currents by terodiline.

1. Terodiline, an anticholinergic/antispasmodic drug effective in the treatment of urinary incontinence, is presently restricted due to adverse side effects on cardiac function. To characterize its effects on cardiac L-type Ca2+-channel current carried by Ca2+ (ICa, L) and Ba2+ (IBa,L), concentrations ranging from 0.1 to 100 microM were applied to whole-cell-configured guinea-pig ventricular myocytes. 2. Although sub-micromolar concentrations of terodiline had no effect on ICa,L at 0 mV, 100 microM drug reduced its amplitude to ca. 10% of pre-drug control. The estimated IC50 (15.2 microM in K+-dialysed cells, 12.2 microM in Cs+-dialysed cells; 0.1 Hz pulsing rate) is eight times higher than reported for ICa,L in bladder smooth muscle myocytes. 3. Terodiline affected ICa,L in a use-dependent manner; block increased when the pulsing rate was increased from 0.1 to 2 - 3 Hz, and when holding potential was lowered from -43 mV. The drug accelerated the decay of ICa,L at 0 mV in a concentration-dependent manner, and slowed the recovery of channels from inactivation. 4. Terodiline reduced peak IBa,L more effectively than peak ICa,L, and markedly accelerated the rate of inactivation of the current. 5. The results are discussed in terms of mechanisms of Ca2+ channel block and relation to the therapeutic and cardiotoxic effects of the drug.