Contribution of I Ks to ventricular repolarization in canine myocytes

The role of the slow delayed rectifier K(+) current (I (Ks)) in cardiac repolarization seems to be largely influenced by the experimental conditions including the species and tissue studied. The aim of this study was to determine the contribution of I (Ks) to repolarization in canine ventricular myocytes by measuring the frequency dependent action potential lengthening effect of 10 microM chromanol 293B using sharp microelectrodes. Pretreatment with isoproterenol (2 nM), E-4031 (1 microM), and injection of inward current pulses were applied to modify action potential configuration. Chromanol alone caused moderate but statistically significant lengthening of action potentials at cycle lengths longer than 500 ms. The lengthening effect of chromanol, which was strongly enhanced in the presence of either isoproterenol or E-4031, was proportional to the amplitude of plateau, whereas poor correlation was found with action potential duration. Similar results were obtained when action potential configuration was modified by injection of depolarizing current pulses. Computer simulations revealed that activation of I (Ks) is a sharp function of the plateau amplitude within the physiological range, while elongation of repolarization may enhance I (Ks) only when it is excessive. It was concluded that the effect of I (Ks) on ventricular repolarization critically depends on the level of action potential plateau; however, other factors, like action potential duration, cycle length, or suppression of other K(+) currents can also influence its contribution.

L-364,373 fails to activate the slow delayed rectifier K+ current in canine ventricular cardiomyocytes

Activators of the slow delayed rectifier K+ current (I(Ks)) are promising tools to suppress ventricular arrhythmias originating from prolongation of action potentials. A recently synthesized compound, L-364,373, was shown to activate I(Ks) in ventricular cells isolated from guinea pigs and rabbits. Due to the interspecies differences known to exist in the properties of the delayed rectifier K+ currents, the effect of L-364,373 on I(Ks) was studied and compared with that of another I(Ks) activator mefenamic acid in canine ventricular myocytes. Mefenamic acid (100 microM) significantly increased the amplitude of the fully activated I(Ks) current, as well as the I(Ks) current tails, by shifting the voltage dependence of its activation towards negative voltages and increased the time constant for deactivation. In contrast, L-364,373, up to concentrations of 3 microM, failed to augment I(Ks) at any membrane potential studied, but slightly increased the time constant of deactivation. It is concluded that human studies are required to evaluate the therapeutically beneficial effects of I(Ks) activators. Rodent cardiac tissues are not suitable for this purpose.

Norfluoxetine and fluoxetine have similar anticonvulsant and Ca2+ channel blocking potencies

Norfluoxetine is the most important active metabolite of the widely used antidepressant fluoxetine but little is known about its pharmacological actions. In this study the anticonvulsant actions of norfluoxetine and fluoxetine were studied and compared to those of phenytoin and clonazepam in pentylenetetrazol-induced mouse epilepsy models. Pretreatment with fluoxetine or norfluoxetine (20mg/kg s.c.), as well as phenytoin (30 mg/kg s.c.) and clonazepam (0.1mg/kg s.c.) significantly increased both the rate and duration of survival, demonstrating a significant protective effect against pentylenetetrazol-induced epilepsy. These effects of norfluoxetine were similar to those of fluoxetine. According to the calculated combined protection scores, both norfluoxetine and fluoxetine were effective from the concentration of 10mg/kg, while the highest protective action was observed with clonazepam. Effects of norfluoxetine and fluoxetine on voltage-gated Ca2+ channels were evaluated by measuring peak Ba2+ current flowing through the Ca2+ channels upon depolarization using whole cell voltage clamp in enzymatically isolated rat cochlear neurons. The current was reduced equally in a concentration-dependent manner by norfluoxetine (EC50=20.4+/-2.7 microM, Hill coefficient=0.86+/-0.1) and fluoxetine (EC50=22.3+/-3.6 microM, Hill coefficient=0.87+/-0.1). It was concluded that the efficacy of the two compounds in neuronal tissues was equal, either in preventing seizure activity or in blocking the neuronal Ca2+ channels.

Effects of SEA0400 and KB-R7943 on Na+/Ca2+ exchange current and L-type Ca2+ current in canine ventricular cardiomyocytes

SEA0400 and KB-R7943 are compounds synthesised to block transsarcolemmal Na+/Ca2+ exchange current (I(Na/Ca)); however, they have also been shown to inhibit L-type Ca2+ current (I(Ca)). The potential value of these compounds depends critically on their relative selectivity for I(Na/Ca) over I(Ca). In the present work, therefore, the concentration-dependent effects of SEA0400 and KB-R7943 on I(Na/Ca) and I(Ca) were studied and compared in canine ventricular cardiomyocytes using the whole-cell configuration of the patch clamp technique. SEA0400 and KB-R7943 decreased I(Na/Ca) in a concentration-dependent manner, having EC50 values of 111+/-43 nM and 3.35+/-0.82 microM, when suppressing inward currents, while the respective EC50 values were estimated at 108+/-18 nM and 4.74+/-0.69 microM in the case of outward current block. SEA0400 and KB-R7943 also blocked I(Ca), having comparable EC50 values (3.6 microM and 3.2 microM, respectively). At higher concentrations (10 microM) both drugs accelerated inactivation of I(Ca), retarded recovery from inactivation and shifted the voltage dependence of inactivation towards more negative voltages. The voltage dependence of activation was slightly modified by SEA0400, but not by KB-R7943. Based on the relatively good selectivity of submicromolar concentrations of SEA0400--but not KB-R7943--for I(Na/Ca) over I(Ca), SEA0400 appears to be a suitable tool to study the role of I(Na/Ca) in Ca2+ handling in canine cardiac cells. At concentrations higher than 1 microM, however, I(Ca) is progressively suppressed by the compound.

Asymmetrical distribution of ion channels in canine and human left-ventricular wall: epicardium versus midmyocardium

The aim of the present study was to compare the distribution of ion currents and the major underlying ion channel proteins in canine and human subepicardial (EPI) and midmyocardial (MID) left-ventricular muscle. Ion currents and action potentials were recorded from canine cardiomyocytes derived from the very superficial EPI and central MID regions of the left ventricle. Amplitude, duration and the maximum velocity of depolarization of the action potential were significantly greater in MID than EPI myocytes, whereas phase-1 repolarization was more pronounced in the EPI cells. Amplitudes of the transient outwards K+ current (29.5+/-1.5 vs. 19.0+/-2.3 pA/pF at +50 mV) and the slow component of the delayed rectifier K+ current (10.3+/-2.3 vs. 6.5+/-1.0 pA/pF at +50 mV) were significantly larger in EPI than in MID myocytes under whole-cell voltage-clamp conditions. The densities of the inwards rectifier K+ current, rapid delayed rectifier K+ current and L-type Ca2+ current were similar in both cell types. Expression of channel proteins in both canine and human ventricular myocardium was determined by Western blotting. In the canine heart, the expression of Kv4.3, Kv1.4, KChIP2 and KvLQT1 was significantly higher, and that of Nav1.5 and MinK much lower, in EPI than in MID. No significant EPI-MID differences were observed in the expression of the other channel proteins studied (Kir2.1, alpha1C, HERG and MiRP1). Similar results were obtained in human hearts, although the HERG was more abundant in the EPI than in the MID layer. In the canine heart, the EPI-MID differences in ion current densities were proportional to differences in channel protein expression. Except for the density of HERG, the pattern of EPI-MID distribution of ion-channel proteins was identical in canine and human ventricles.

Effects of terpenoid phenol derivatives on calcium current in canine and human ventricular cardiomyocytes

Concentration-dependent (10-1000 microM) effects of terpenoid phenol derivatives were studied on L-type Ca(2+) current in isolated canine and human ventricular cardiomyocytes using the whole-cell configuration of patch clamp technique. Carvacrol, thymol and eugenol suppressed peak Ca(2+) current at +5 mV, having EC(50) values and Hill coefficients of 98+/-11, 158+/-7 and 187+/-15 microM and 1.42+/-0.05, 2.96+/-0.43 and 1.6+/-0.1, respectively, in canine myocytes. Zingerone displayed a weak effect (estimated EC(50): 2+/-0.37 mM, Hill coefficient: 0.73+/-0.07), while vanillin and guaiacol failed to substantially modify Ca(2+) current up to the concentration of 1 mM. In addition to tonic block, thymol and carvacrol, but not eugenol, evoked marked rate-dependent block at 2 Hz. Carvacrol and eugenol accelerated inactivation of Ca(2+) current and caused leftward shift in the voltage dependence of steady-state inactivation without altering activation kinetics. Carvacrol, but not eugenol, increased the time constant of recovery from inactivation. These effects of carvacrol and eugenol developed rapidly and were largely reversible. In myocytes isolated from undiseased human hearts, the effect of carvacrol was similar to that observed in canine cells. It is concluded that suppression of cardiac Ca(2+) currents by phenol derivatives is influenced by the substituent in the benzene ring, and the blocking effect of these drugs may involve interactions with the inactivation machinery of the channel.

Effects of norfluoxetine on the action potential and transmembrane ion currents in canine ventricular cardiomyocytes

Norfluoxetine is the most important active metabolite of the widely used antidepressant compound fluoxetine. Although the cellular electrophysiological actions of fluoxetine are well characterized in cardiac cells, little is known about the effects of its metabolite. In this study, therefore, the effects of norfluoxetine on action potential (AP) configuration and transmembrane ion currents were studied in isolated canine cardiomyocytes using the whole cell configuration of patch clamp techniques. Micromolar concentrations of norfluoxetine (1-10 microM) modified AP configuration: amplitude and duration of the AP and maximum velocity of depolarization were decreased in addition to depression of the plateau and elimination of the incisura of AP. Voltage clamp experiments revealed a concentration-dependent suppression of both L-type Ca(2+) current, I(Ca) (EC(50)=1.13+/-0.08 microM) and transient outward K(+) current, I(to) (EC(50)=1.19+/-0.17 microM) having Hill coefficients close to unity. The midpoint potential of the steady-state inactivation of I(Ca) was shifted from -20.9+/-0.75 mV to -27.7+/-1.35 mV by 3 microM norfluoxetine ( P<0.05, n=7). No such shift in the steady-state inactivation curve was observed in the case of I(to). Similarly, norfluoxetine caused no change in the steady-state current-voltage relationship of the membrane or in the density of the inward rectifier K(+) current, I(K1). All these effects of norfluoxetine developed rapidly and were fully reversible. Comparing present results with those obtained previously with fluoxetine, it can be concluded that norfluoxetine displays stronger suppression of cardiac ion channels than fluoxetine. Consequently, the majority of the cardiac side effects observed during fluoxetine treatment are likely to be attributed to its metabolite norfluoxetine.

Reopening of L-type calcium channels in human ventricular myocytes during applied epicardial action potentials

AIMS

Present study was performed to compare the dynamics of human L-type calcium current (ICa,L) flowing during rectangular voltage pulses, voltage ramps, and action potentials (APs) recorded from epicardiac and endocardiac canine ventricular cells.

METHODS

ICa,L was recorded in single myocytes isolated from undiseased human hearts using the whole cell voltage clamp technique.

RESULTS

The decay of ICa,L was monotonic when using rectangular pulses or endocardial APs as voltage commands, whereas the current became double-peaked (displaying a second rise and fall) during epicardial (EPI) APs or voltage ramps used to mimic EPI APs. These ICa,L profiles were associated with single-hooked and double-hooked phase-plane trajectories, respectively. No sustained current was observed during the AP commands. Kinetics of deactivation and recovery from inactivation of human ICa,L were determined using twin-pulse voltage protocols and voltage ramps, and the results were similar to those obtained previously in canine cells under identical experimental conditions.

CONCLUSIONS

ICa,L can inactivate partially before and deactivate during the phase-1 repolarization of the epicardiac AP, and reopening of these channels seems to be associated with formation of the dome.

Effects of Endothelins on Cardiac and Vascular Cells: New Therapeutic Target for the Future?

The predominant isoform of the endothelin peptide family. endothelin-1 (ET-1) exerts various biological effects. These include effects on arterial smooth muscle cells causing intense vasoconstriction and stimulation of cardiac cells. ET-1 promotes changes in cardiomyocytes that are consistent with electrical remodelling such as changes in ionic current density and inhomogeneous prolongation of action potential duration resulting in increased dispersion. As for the underlying mechanisms, ET-1 was shown to suppress several cAMP-dependent ionic currents, such as ICa, IK and ICl in various mammalian cardiac preparations including human myocytes; however, the degree of suppression of these currents is different and highly dependent on experimental conditions. The proposed arrhythmogenic effects of ET-1 may also involve enhancement of Ca2+ release from intracellular stores, generation of IP3, and acidosis due to stimulation of the Na+/H+ exchange. Furthermore, ET-1 acts as the natural counterpart to endothelium-derived nitric oxide, which exerts vasodilator, antithrombotic and antiproliferative effects, and inhibits leukocyte adhesion to the vascular wall. Effects of ET-1 are mediated through interaction with two major types of cell surface receptors. ETA receptors have been associated with electrical remodelling, vasoconstriction and cell growth, while ETB receptors are involved in the clearance of ET-1, inhibition of endothelial apoptosis, release of NO and prostacyclins, and inhibition of the expression of ET-1 converting enzyme. The derangement of endothelial function in various cardiovascular diseases, such as cardiomyopathies, hypertension or arteriosclerosis, is a crucial element of the pathomechanism, thus ET receptors are considered as important therapeutic targets. Indeed, ET receptor antagonists may be able to preserve or restore endothelial integrity and may have antiarrhythmic properties; therefore, they are promising tools in cardiovascular medicine.

Effect of thymol on calcium handling in mammalian ventricular myocardium

Concentration-dependent effects of thymol on calcium handling were studied in canine and guinea pig cardiac preparations (Langendorff-perfused guinea pig hearts, canine ventricular trabeculae, canine sarcoplasmic reticular vesicles and single ryanodine receptors). Thymol induced a concentration-dependent negative inotropic action in both canine and guinea pig preparations (EC(50) = 297 +/- 12 microM in dog). However, low concentrations of thymol reduced intracellular calcium transients in guinea pig hearts without decreasing contractility. At higher concentrations both calcium transients and contractions were suppressed. In canine sarcoplasmic reticular vesicles thymol induced rapid release of calcium (V(max) = 0.47 +/- 0.04 nmol s(-1), EC(50) = 258 +/- 21 microM, Hill coefficient = 3.0 +/- 0.54), and decreased the activity of the calcium pump (EC(50) = 253 +/- 4.7 microM, Hill coefficient = 1.62 +/- 0.05). Due to the less sharp concentration-dependence of the ATPase inhibition, this effect was significant from 50 microM, whereas the thymol-induced calcium release only from 100 microM. In single ryanodine receptors incorporated into artificial lipid bilayer thymol induced long lasting openings, having mean open times increased with 3 orders of magnitude, however, the specific conductance of the channel remained unaltered. This effect of thymol was not voltage-dependent and failed to prevent the binding of ryanodine. In conclusion, the negative inotropic action of thymol can be explained by reduction in calcium content of the sarcoplasmic reticulum due to the combination of the thymol-induced calcium release and inhibition of the calcium pump. The calcium-sensitizer effect, observed at lower thymol concentrations, indicates that thymol is likely to interact with the contractile machinery also.

Profile of IKs During the Action Potential Questions the Therapeutic Value of IKs Blockade

The goal of this paper is two fold. First, we attempt to review the reports available on the role of I(Ks) in myocardial repolarization. Based on theoretical considerations and experimental results, it seems reasonable to assume that I(Ks)blockade will lengthen the action potential. However, results obtained with I(Ks) blockers, like chromanol 293B or L-735,821, are conflicting, since from slight lengthening to marked prolongation of action potentials were equally obtained. Although these contradictory results were explained by interspecies or regional differences, the role of I(Ks) in repolarization is a matter of growing dispute. In the second part of this study, we simulated the performance of I(Ks) during cardiac action potentials. We compared the profile of the predicted current in three mathematical models in order to determine the relative role of the current in repolarization. We studied the effect of the cycle length, action potential duration and height of the plateau on the profile of I(Ks) in epicardiac, endocardiac and midmyocardiac ventricular action potentials. The results indicate that the height of the plateau is the most important parameter to control activation of I(Ks)in cardiac tissues, and accordingly, the interspecies and regional differences observed in the efficacy of I(Ks) blockers are likely due to the known differences in action potential morphology. We conclude also that I(Ks)blockade may have unpredictable effects on the length of the action potential in a diseased heart, questioning the possible therapeutic value of drugs blocking I(Ks).

The role of transmembrane chloride current in afterdepolarisations in canine ventricular cardiomyocytes

The physiological role of chloride currents (Icl) in cardiac cells is poorly understood. The aim of the present study was to reveal the role of Icl in the genesis of early and delayed afterdepolarisations (EADs and DADs, respectively). First we identified Icl under action potential voltage clamp conditions as the anthracene-9-carboxylic acid (ANTRA) (0.5 mmol/l)-sensitive current. The ANTRA-sensitive current was large and outwardly directed at the beginning, while it was moderate and inwardly directed at the end of the action potential. Application of ANTRA under current clamp conditions decreased the depth of the incisura, shifted the plateau upwards and lengthened the duration of action potentials. The effect of ANTRA was studied in three models of afterdepolarisations: the ouabain-induced DAD model, the caesium-induced EAD model, and in the presence of subthreshold concentration of isoproterenol. Preincubation of the cells with 0.5 mmol/l ANTRA failed to induce afterdepolarisations. Ouabain (200 nmol/l) alone caused DADs in 62.5% of the cells within 15 min. When ouabain was applied in the presence of ANTRA, 60% of the myocytes transiently displayed EADs before the development of DADs, and all cells developed DADs within 7 min. Isoproterenol (5 nmol/l) alone failed to induce afterdepolarisations. However, 75% of the cells produced DADs within 6 min when superfused with isoproterenol in the presence of ANTRA. Incubation of the myocytes with 3.6 mmol/l CsCl caused EADs in 71.4% of the cells within 30 min. Application of CsCl in the presence of ANTRA resulted in immediate depolarisation of the membrane from -79.6 +/- 0.4 to -54.2 +/- 3.5 mV. Summarizing our results we conclude that the ANTRA-sensitive current is an important mechanism of defence against afterdepolarisations. Suppression of Icl may thus increase the incidence and accelerate the rate of development of both EADs and DADs.

Beta-adrenoceptor activation plays a role in the reverse rate-dependency of effective refractory period lengthening by dofetilide in the guinea-pig atrium, in vitro

1. Blockers of the rapid component of the delayed rectifier potassium current (I(Kr)) prolong cardiac action potential duration (APD) and effective refractory period (ERP) in a reverse rate-dependent manner. Since activation of beta-adrenoceptors attenuates prolongation of APD evoked by I(Kr) blockers, rate-dependent neuronal noradrenaline liberation in the myocardium may contribute to the reverse rate-dependent nature of the effects of I(Kr) blockers. In order to test this hypothesis, we studied the effects of dofetilide, a pure I(Kr) blocker, on ERP after activation or blockade of beta-adrenoceptors and after catecholamine depletion in guinea-pig left atrial myocardium paced at 3, 2 and 1 Hz, in vitro. 2. Dofetilide (100 nM) lengthened ERP in a reverse rate-dependent manner in the left atrial myocardium of guinea-pigs. Strong activation of beta-adrenoceptors using 10 nM isoproterenol abolished the dofetilide-induced lengthening of ERP at all pacing rates. 3. Blockade of the beta-adrenoceptors with metoprolol (1 micro M), atenolol (3 micro M) or propranolol (300 nM) increased the dofetilide-evoked prolongation of ERP at 3 and 2 Hz, but not at 1 Hz. As a consequence, metoprolol attenuated while propranolol and atenolol fully eliminated the reverse rate-dependent nature of the dofetilide-induced ERP lengthening. In catecholamine-depleted atrial preparations of the guinea-pig (24 h pretreatment with 5 mg kg(-1) reserpine i.p.), the effect of dofetilide on ERP was not frequency dependent, and propranolol did not alter the effects of dofetilide. 4. In contrast to results obtained in guinea-pig atrial preparations, propranolol failed to change the reverse rate-dependent effect of dofetilide on ERP in the right ventricular papillary muscles of rabbits and guinea-pigs. 5. As an indication of the functional consequences of rate-dependent noradrenaline liberation, propranolol decreased twitch tension at 3 and 2 Hz but not at 1 Hz in the atrial myocardium of control guinea-pigs, whereas no such effect was detected in catecholamine-depleted atrial preparations. Propranolol failed to change contractility of ventricular myocardium in guinea-pigs and rabbits. 6. It is concluded that rate-dependent noradrenaline release and the ensuing beta-adrenoceptor activation contributed to the reverse rate-dependent nature of ERP prolongation caused by I(Kr) blockers in isolated guinea-pig atrial myocardium.

Effect of thymol on kinetic properties of Ca and K currents in rat skeletal muscle

BACKGROUND

Thymol is widely used as a general antiseptic and antioxidant compound in the medical practice and industry, and also as a stabilizer to several therapeutic agents, including halothane. Thus intoxication with thymol may occur in case of ingestion or improper anesthesia. In the present study, therefore, concentration-dependent effects of thymol (30-600 micro-grams) were studied on calcium and potassium currents in enzymatically isolated rat skeletal muscle fibers using the double vaseline gap voltage clamp technique.

RESULTS

Thymol suppressed both Ca and K currents in a concentration-dependent manner, the EC50 values were 193 +/- 26 and 93 +/- 11 microM, with Hill coefficients of 2.52 +/- 0.29 and 1.51 +/- 0.18, respectively. Thymol had a biphasic effect on Ca current kinetics: time to peak current and the time constant for inactivation increased at lower (100-200 microM) but decreased below their control values at higher (600 microM) concentrations. Inactivation of K current was also significantly accelerated by thymol (200-300 microM). These effects of thymol developed rapidly and were partially reversible. In spite of the marked effects on the time-dependent properties, thymol caused no change in the current-voltage relationship of Ca and K peak currents.

CONCLUSIONS

Present results revealed marked suppression of Ca and K currents in skeletal muscle, similar to results obtained previously in cardiac cells. Furthermore, it is possible that part of the suppressive effects of halothane on Ca and K currents, observed experimentally, may be attributed to the concomitant presence of thymol in the superfusate.

Differences in electrophysiological and contractile properties of mammalian cardiac tissues bathed in bicarbonate - and HEPES-buffered solutions

AIM

The aim of this study was to compare the action potential configuration, contractility, intracellular Ca2+ and H+ concentrations in mammalian cardiac tissues bathed with Krebs and Tyrode solutions at 37 degrees C.

RESULTS

In Langendorff-perfused guinea-pig hearts, loaded with the fluorescent Ca2+-indicator Fura-2, or H+-sensitive dye carboxy-SNARF, shifts from Krebs to Tyrode solution caused intra-cellular acidification, increased diastolic pressure and [Ca2+]i, decreased systolic pressure and [Ca2+]i, leading to a reduction in the amplitude of [Ca2+]i transients and pulse pressure. Contractility was also depressed in canine ventricular trabeculae when transferred from Krebs to Tyrode solution. Shifts from Krebs to Tyrode solution increased the duration of action potentials in multicellular cardiac preparations excised from canine and rabbit hearts but not in isolated cardiomyocytes. All these changes in action potential morphology, contractility, [Ca2+]i and [H+]i were readily reversible by addition of 26 mmol L(-1) bicarbonate to Tyrode solution. Effects of dofetilide and CsCl, both blockers of the delayed rectifier K current, on action potential duration were compared in Krebs and Tyrode solutions. Dofetilide lengthened rabbit ventricular action potentials in a significantly greater extent in Tyrode than in Krebs solution. Exposure of canine Purkinje fibres to CsCl evoked early after depolarizations within 40 min in all preparations incubated with Tyrode solution, but not in those bathed with Krebs solution.

CONCLUSION

It is concluded that the marked differences in action potential morphology, [Ca2+]i, [H+]i and contractility observed between preparations bathed with Krebs and Tyrode solutions are more likely attributable to differences in the intracellular buffering capacities of the two media.

Differential effects of fluoxetine enantiomers in mammalian neural and cardiac tissues

Racemic fluoxetine is a widely used SSRI antidepressant compound having also anticonvulsant effect. In addition, it was shown that it blocked several types of voltage gated ion channels including neural and cardiac calcium channels. In the present study the effects of enantiomers of fluoxetine (R(-)-fluoxetine and S(+)-fluoxetine) were compared on neuronal and cardiac voltage-gated Ca2+ channels using the whole cell configuration of patch clamp techniques, and the anticonvulsant action of these enantiomers was also evaluated in a mouse epilepsy model. In isolated pyramidal neurons of the dorsal cochlear nucleus of the rat the effect of fluoxetine (S(+), R(-) and racemic) was studied on the Ca2+ channels by measuring peak Ba2+ current during ramp depolarizations. All forms of fluoxetine reduced the Ba2+ current of the pyramidal cells in a concentration-dependent manner, with a Kd value of 22.3+/-3.6 microM for racemic fluoxetine. This value of Kd was higher by one order of magnitude than found in cardiac myocytes with fluoxetine enantiomers (2.4+/-0.1 and 2.8+/-0.2 microM). Difference between the effects of the two enantiomers on neuronal Ba2+ current was observed only at 5 microM concentration: R(-)-fluoxetine inhibited 28+/-3% of the peak current, while S(+)-fluoxetine reduced the current by 18+/-2% (n=13, P<0.05). In voltage clamped canine ventricular cardiomyocytes both enantiomers of fluoxetine caused a reversible concentration-dependent block of the peak Ca2+ current measured at 0 mV. Significant differences between the two enantiomers in this blocking effect was observed at low concentrations only: S(+)-fluoxetine caused a higher degree of block than R(-)-fluoxetine (56.3+/-2.2% versus 49.1+/-2.2% and 95.5+/-0.9% versus 84.5+/-3.1% block with 3 and 10 microM S(+) and R(-)-fluoxetine, respectively, P<0.05, n=5). Studied in current clamp mode, micromolar concentrations of fluoxetine shortened action potential duration of isolated ventricular cells, while higher concentrations also suppressed maximum velocity of depolarization and action potential amplitude. This shortening effect was significantly greater in the case of S(+) than R(-)-fluoxetine at 1 and 3 microM concentrations, whereas no differences in their effects on depolarization were observed. In pentylenetetrazole-induced mouse epilepsy model fluoxetine pretreatment significantly increased the 60 min survival rate, survival duration and seizure latency. These effects were more pronounced with the R(-) than the S(+) enantiomer. The results indicate that fluoxetine exerts much stronger suppressive effect on cardiac than neuronal calcium channels. At micromolar concentrations (between 1 and 10 microM) R(-)-fluoxetine is more effective than the S(+) enantiomer on neuronal, while less effective on cardiac calcium channels. The stronger anticonvulsant effect of the R(-) enantiomer may, at least partially, be explained by these differences. Used as an antidepressant or anticonvulsant drug, less severe cardiac side-effects are anticipated with the R(-) enantiomer.

Endocardial versus epicardial differences in L-type calcium current in canine ventricular myocytes studied by action potential voltage clamp

OBJECTIVES

The aim of the present study was to assess and compare the dynamics of L-type Ca(2+) current (I(Ca,L)) during physiologic action potential (AP) in canine ventricular cardiomyocytes of epicardial (EPI) and endocardial (ENDO) origin.

METHODS

I(Ca,L) was recorded on cells derived from the two regions of the heart using both AP voltage clamp and conventional whole cell voltage clamp techniques.

RESULTS

AP voltage clamp experiments revealed that the decay of I(Ca,L) is monotonic during endocardial AP, whereas the current is double-peaked (displaying a second rise) during epicardial AP. The amplitude of the first peak was significantly greater in ENDO (-4.6+/-0.8 pA/pF) than in EPI cells (-2.8+/-0.3 pA/pF). Application of epicardial APs as command pulses to endocardial cells yielded double-peaked I(Ca,L) profiles, and increased the net charge entry carried by I(Ca,L) during the AP from 0.187+/-0.059 to 0.262+/-0.056 pC/pF (n=5, P<0.05). No differences were observed in current densities and inactivation kinetics of I(Ca,L) between EPI and ENDO cells when studied under conventional voltage clamp conditions. Nisoldipine shortened action potentials and eliminated the dome of the epicardial AP.

CONCLUSION

I(Ca,L) was shown to partially inactivate before and deactivate during phase-1 repolarization and reopening of these channels is responsible for the formation of the dome in canine EPI cells. The transmural differences in the profile of I(Ca,L) could be well explained with differences in AP configuration.

Electrophysiological effects of risperidone in mammalian cardiac cells

In this study, the effects of risperidone, the widely used antipsychotic drug, on isolated canine ventricular myocytes and guinea-pig papillary muscles were analyzed using conventional microelectrode and whole cell voltage-clamp techniques. Risperidone concentration-dependently lengthened action potential duration in guinea-pig papillary muscles (EC(50)=0.29+/-0.02 micro M) and single canine ventricular myocytes (EC(50)=0.48+/-0.14 micro M). This effect was reversible, showed reverse rate dependence, and it was most prominent on the terminal portion of repolarization. No significant effect of risperidone on the resting membrane potential, action potential amplitude or maximum rate of depolarization was observed. In voltage-clamped canine ventricular myocytes risperidone caused concentration-dependent block of the rapid component of the delayed rectifier K(+) current ( I(Kr)), measured as outward current tails at -40 mV, with an IC(50) of 0.92+/-0.26 micro M. Suppression of I(Kr) was not associated with changes in activation or deactivation kinetics. High concentration of risperidone (10 micro M) suppressed also the slow component of the delayed rectifier K(+) current ( I(Ks)) by 9.6+/-1.5% at +50 mV. These effects of risperidone developed rapidly and were readily reversible. Risperidone had no significant effect on the amplitude of other K(+) currents ( I(K1) and I(to)). The inhibition of cardiac I(Kr) current by risperidone may explain the cardiac side-effects observed occasionally with the drug. Our results suggest that risperidone displays class III antiarrhythmic properties, and as such, may produce QTc prolongation, especially in patients with long QT syndrome. Therefore, in psychotic patients having also cardiac disorders, ECG control may be suggested during risperidone therapy.

Effects of thymol on calcium and potassium currents in canine and human ventricular cardiomyocytes

1. Concentration-dependent effects of thymol (1 - 1000 microM) was studied on action potential configuration and ionic currents in isolated canine ventricular cardiomyocytes using conventional microelectrode and patch clamp techniques. 2. Low concentration of thymol (10 microM) removed the notch of the action potential, whereas high concentrations (100 microM or higher) caused an additional shortening of action potential duration accompanied by progressive depression of plateau and reduction of V(max). 3. In the canine cells L-type Ca current (I(Ca)) was decreased by thymol in a concentration-dependent manner (EC(50): 158+/-7 microM, Hill coeff.: 2.96+/-0.43). In addition, thymol (50 - 250 microM) accelerated the inactivation of I(Ca), increased the time constant of recovery from inactivation, shifted the steady-state inactivation curve of I(Ca) leftwards, but voltage dependence of activation remained unaltered. Qualitatively similar results were obtained with thymol in ventricular myocytes isolated from healthy human hearts. 4. Thymol displayed concentration-dependent suppressive effects on potassium currents: the transient outward current, I(to) (EC(50): 60.6+/-11.4 microM, Hill coeff.: 1.03+/-0.11), the rapid component of the delayed rectifier, I(Kr) (EC(50): 63.4+/-6.1 microM, Hill coeff.: 1.29+/-0.15), and the slow component of the delayed rectifier, I(Ks) (EC(50): 202+/-11 microM, Hill coeff.: 0.72+/-0.14), however, K channel kinetics were not much altered by thymol. These effects on Ca and K currents developed rapidly (within 0.5 min) and were readily reversible. 5. In conclusion, thymol suppressed cardiac ionic channels in a concentration-dependent manner, however, both drug-sensitivities as well as the mechanism of action seems to be different when blocking calcium and potassium channels.

Multilateral in vivo and in vitro protective effects of the novel heat shock protein coinducer, bimoclomol: results of preclinical studies

Bimoclomol, the recently developed non-toxic heat shock protein (HSP) coinducer, was shown to display multilateral protective activities against various forms of stress or injuries at the level of the cell, tissue or organism. The compound enhanced the transcription, translation and expression of the 70 kD heat shock protein (HSP-70) in myogenic and HeLa cell lines exposed to heat stress, and increased cell survival on exposure to otherwise lethal thermal injury. Bimoclomol increased contractility of the working mammalian heart, this effect was associated with the increased intracellular calcium transients due to increased probability of opening of ryanodine receptors in the sarcoplasmic reticulum (SR). In healthy tissues these cardiac effects were evident only at relatively high concentrations of the drug, while in the ischemic myocardium bimoclomol exerted significant cardioprotective and antiarrhythmic effects at submicromolar concentrations. It decreased ischemia-induced reduction of contractility and of cardiac output, and dramatically decreased the elevation of the ST-segment during ischemia as well as the occurrence of ventricular fibrillation upon reperfusion. Bimoclomol was also active in various pathological animal models subjected to acute or chronic stress. In the spontaneously hypertensive rats chronic pretreatment with bimoclomol restored sensitivity of aortic rings to acetylcholine; this effect was accompanied by accumulation of HSP-70 in the tissues. Bimoclomol pretreatment significantly diminished the consequences of vascular disorders associated with diabetes mellitus. Diabetic neuropathy, retinopathy, and nephropathy were prevented or diminished, while wound healing was enhanced by bimoclomol. Enhancement of wound healing by bimoclomol was observed after thermal injury as well as following ultraviolet (UV) irradiation. In addition to the beneficial effects on peripheral angiopathies, bimoclomol antagonized the increase in permeability of blood-brain barrier induced by subarachnoid hemorrhager or arachidonic acid. A general and very important feature of the above effects of bimoclomol was that the drug failed to cause alterations under physiological conditions (except the enhanced calcium release from cardiac sarcoplasmic reticulum). Bimoclomol was effective only under conditions of stress. Consistent with its HSP-coinducer property, bimoclomol alone had very little effect on HSP production. Its protective activity became apparent only in the presence of cell damage. Currently, bimoclomol reached the end of the Phase II clinical trial in a group of 410 patients with diabetic complications. Results of this trial will answer the question, whether a compound with promising in vitro and in vivo preclinical findings will produce the anticipated beneficial effects in humans. In the event of a positive outcome of this trial, the indications for bimoclomol will be substantially extended.

Effects of the antiarrhythmic agent EGIS-7229 (S 21407) on calcium and potassium currents in canine ventricular cardiomyocytes

Based on earlier pharmacological studies performed using conventional microelectrodes EGIS-7229 (S 21407), the novel antiarrhythmic candidate, was suggested to have a combined mode of action in cardiac tissues isolated from various mammalian species. In order to characterize the electrophysiological effects of the compound, its effects on calcium and potassium currents of isolated canine ventricular cardiomyocytes were studied in the present work using the whole cell configuration of the patch clamp technique. L-type Ca current (ICa) was significantly depressed by EGIS-7229 at concentrations of 3 microM or higher with no concomitant changes in the voltage-dependence of activation and time course of inactivation of ICa. The drug reversibly suppressed the rapid component of the delayed rectifier K current (IKr) in a concentration-dependent manner, having a K0.5 value of 1.1+/-0.1 microM and a slope factor of close to unity (1.23+/-0.16), indicating that probably one single binding site of high affinity may be involved in binding of EGIS-7229 to the IKr channel. In contrast, no changes in the slow component of the delayed rectifier K current (IKs) was observed with the compound up to the concentration of 100 microM, even if the current was fully activated by 8-bromo-cAMP. At a concentration of 10 microM or higher, EGIS-7229 caused also a moderate but significant reduction in the inward rectifier K current (IK1) and the transient outward K current (Ito) with no change in the voltage-dependence of activation and steady-state inactivation of Ito. Present results indicate that EGIS-7229 can be considered as a selective IKr blocker at low (1 microM) concentration; however, its combined (class III + IV) mechanism of action is evident at concentrations of 3 microM or higher. Suppression of ICa may explain the lack of development of early afterdepolarizations in the presence of EGIS-7229, predicting a relatively safe clinical application in contrast to pure class III compounds.

Different effects of endothelin-1 on calcium and potassium currents in canine ventricular cells

Effects of endothelin-1 (ET-1) on the L-type calcium current (ICa) and delayed rectifier potassium current (IK) were studied in isolated canine ventricular cardiomyocytes using the whole-cell configuration of the patch-clamp technique. ET-1 (8 nM) was applied in three experimental arrangements: untreated cells, in the presence of 50 nM isoproterenol, and in the presence of 250 microM 8-bromo-cAMP. In untreated cells, ET-1 significantly decreased the peak amplitude of ICa by 32.3+/-4.8% at +5 mV (P<0.05) without changing activation or inactivation characteristics of ICa. ET-1 had no effect on the amplitude of IK, Ito (transient outward current) or IK1 (inward rectifier K current) in untreated cells; however, the time course of recovery from inactivation of Ito was significantly increased by ET-1 (from 26.5+/-4.6 ms to 59.5+/- 1.8 ms, P < 0.05). Amplitude and time course of intracellular calcium transients, recorded in voltage-clamped cells previously loaded with the fluorescent calcium indicator dye Fura-2, were not affected by ET-1. ET-1 had no effect on force of contraction in canine ventricular trabeculae. Isoproterenol increased the amplitude of ICa to 263+/-29% of control. ET-1 reduced ICa also in isoproterenol-treated cells by 17.8+/-2% (P<0.05); this inhibition was significantly less than obtained in untreated cells. IK was increased by isoproterenol to 213+/-18% of control. This effect of isoproterenol on IK was reduced by 31.8+/-4.8% if the cells were pretreated with ET-1. Similarly, in isoproterenol-treated cells ET-1 decreased IK by 16.2+/-1.5% (P<0.05). Maximal activation of protein kinase A (PKA) was achieved by application of 8-bromo-cAMP in the pipette solution. In the presence of 8-bromo-cAMP ET-1 failed to alter ICa or IK It was concluded that differences in effects of ET-1 on ICa and IK may be related to differences in cAMP sensitivity of the currents.

Effects of EGIS-7229 (S 21407), a novel class III antiarrhythmic drug, on myocardial refractoriness to electrical stimulation in vivo and in vitro

The I(Kr) blocker EGIS-7229 (S-21407), displays class Ib and class IV effects that may alter its pharmacologic profile compared with those of pure I(Kr) blockers. Therefore, the concentration- and frequency-dependent effects of EGIS-7229, and of the I(Kr) blockers d,l-sotalol and dofetilide, on the effective refractory period (ERP) were measured in isolated right ventricular papillary muscle of the rabbit in vitro. The effects of these drugs on right ventricular fibrillation threshold (RVFT) at increasing intravenous doses were also determined in anesthetized cats. Dofetilide and d,l-sotalol increased ERP in a concentration-dependent manner (dofetilide: 3-100 nM; d,l-sotalol: 3-100 microM) with strong reverse frequency dependence at high concentrations. EGIS-7229 concentration dependently lengthened ERP at 1-30 microM. Its effect on ERP was clearly reverse frequency dependent at 3 microM, but this feature of the drug diminished at 10 microM and was not apparent at 30 microM. The effect of EGIS-7229 (30 microM) on ERP was devoid of reverse frequency dependence as it was more effective (31%) than dofetilide (16 %) at high-pacing rate (3 Hz), whereas it was less effective (50%) than dofetilide (70%) at slow-pacing rate (1 Hz). Reverse frequency-dependent ERP effect of dofetilide (100 nM) was similarly abolished by the addition of lidocaine (30 microM). EGIS-7229 (1-8 mg/kg iv), d,l-sotalol (1-8 mg/kg iv), and dofetilide (10-80 microg/kg iv) caused a dose-dependent increase in RVFT. The minimum effective dose of d,l-sotalol and EGIS-7229 was 1 and 2 mg/kg, respectively, whereas that of dofetilide was 10 microg/kg. EGIS-7229 induced a smaller peak effect in RVFT than sotalol or dofetilide. In conclusion, EGIS-7229 markedly increased refractoriness to electrical stimulation in vitro and in vivo. Compared with pure I(Kr) blockers, the benefits of EGIS-7229 seem to be a greater lengthening of effective refractory period at rapid stimulation rates, suggesting a strong antiarrhythmic action, and a smaller effect at slow stimulation rates, suggesting low potential to induce early afterdepolarizations.

Effects of endothelin-1 on calcium and potassium currents in undiseased human ventricular myocytes

Endothelins have been reported to exert a wide range of electrophysiological effects in mammalian cardiac cells. These results are controversial and human data are not available. Our aim was to study the effects of endothelin-1 (ET-1, 8 nmol/l) on the L-type calcium current (ICa-L) and various potassium currents (rapid component of the delayed rectifier, IKr; transient outward current, Ito; and the inward rectifier K current, IK1) in isolated human ventricular cardiomyocytes. Cells were obtained from undiseased donor hearts using collagenase digestion via the segment perfusion technique. The whole-cell configuration of the patch-clamp technique was applied to measure ionic currents at 37 degrees C. ET-1 significantly decreased peak ICa-L from 10.2+/-0.6 to 6.8+/-0.8 pA/pF at +5 mV (66.7% of control, P<0.05, n=5). This reduction of peak current was accompanied by a lengthening of inactivation. The voltage dependence of steady-state activation and inactivation was not altered by ET- 1. IKr, measured as tail current amplitudes at 40 mV, decreased from 0.31+/-0.02 to 0.06+/-0.02 pA/pF (20.3% of control, P<0.05, n=4) after exposure to ET-1. ET-1 failed to change the peak amplitude of Ito, measured at +50 mV (9.3+/-4.6 and 9.0+/-4.4 pA/pF before and after ET-1, respectively), or steady-state IK1 amplitude, measured at the end of a 400-ms hyperpolarization to -100 mV (3.6+/-1.4 and 3.7+/-1.4 pA/pF, n=4). The present results indicate that in undiseased human ventricular myocytes ET-1 inhibits both ICa-L and IKr; however, the degree of suppression of the two currents is different.