Julius Gyula Papp

Interaction of different potassium channels in cardiac repolarization in dog ventricular preparations: role of repolarization reserve.

The aim of this study was to investigate the possible role of the interaction of different potassium channels in dog ventricular muscle, by applying the conventional microelectrode and whole cell patch‐clamp techniques at 37°C. Complete block of IKr by 1 μM dofetilide lengthened action potential duration (APD) by 45.6±3.6% at 0.2 Hz (n=13). Chromanol 293B applied alone at 10 μM (a concentration which selectively blocks IKs) did not markedly lengthen APD (<7%), but when repolarization had already been prolonged by complete IKr block with 1 μM dofetilide, inhibition of IKs with 10 μM chromanol 293B substantially delayed repolarization by 38.5±8.2% at 0.2 Hz (n=6). BaCl2, at a concentration of 10 μM which blocks IKl without affecting other currents, lengthened APD by 33.0±3.1% (n=11), but when IKr was blocked with 1 μM dofetilide, 10 μM BaCl2 produced a more excessive rate dependent lengthening in APD, frequently (in three out of seven preparations) initiating early afterdepolarizations. These findings indicate that if only one type of potassium channels is inhibited in dog ventricular muscle, excessive APD lengthening is not likely to occur. Dog ventricular myocytes seem to repolarize with a strong safety margin (‘repolarization reserve’). However, when this normal ‘repolarization reserve’ is attenuated, otherwise minimal or moderate potassium current inhibition can result in excessive and potentially proarrhythmic prolongation of the ventricular APD. Therefore, application of drugs which are able to block more than one type of potassium channel is probably more hazardous than the use of a specific inhibitor of one given sort of potassium channel, and when simultaneous blockade of several kinds of potassium channel may be presumed, a detailed study is needed to define the determinants of ‘repolarization reserve’.

Ionic currents and action potentials in rabbit, rat, and guinea pig ventricular myocytes

SummaryDistinct differences exist in action potentials and ionic currents between rabbit, rat, and guinea pig ventricular myocytes. Data obtained at room temperature indicate that about half of the rabbit myocytes show prominent phase 1 repolarization and transient outward current. Action potentials in guinea pig ventricular myocytes resemble those from rabbit myocytes not exhibiting phase 1 repolarization; and guinea pig myocytes do not develop transient outward current. Rat ventricular action potentials are significantly shorter than those from rabbit and guinea pig ventricular myocytes. Unlike rabbit and guinea pig myocytes, rat ventricular myocytes also exhibit a prominent phase 1 and lack a well defined plateau phase during repolarization. All rat ventricular myocytes exhibit a transient outward current which can be best fitted by a double exponential relation. There are no significant differences between the amplitude, voltage dependence and inactivation kinetics of the inward calcium currents observed in rabbit, rat and guinea pig. The steady-state current-voltage relations between −120 mV and −20 mV, which mostly represent the inward rectifier potassium current are similar in rabbit and guinea pig. The amplitude of this current is significantly less in rat ventricular myocytes. The outward currents activated upon depolarization to between −10 and +50 mV are different in the three species. Only a negligible, or absent, delayed rectifier outward current has been observed in rabbit and rat; however, a relatively large delayed rectifier current has been found in guinea pig. These large interspecies variations in outward membrane currents help explain the differences in action potential configurations observed in rabbit, rat, and guinea pig.

Electrophysiological effects of dronedarone (SR 33589), a noniodinated amiodarone derivative in the canine heart: comparison with amiodarone

The electrophysiological effects of dronedarone, a new nonionidated analogue of amiodarone were studied after chronic and acute administration in dog Purkinje fibres, papillary muscle and isolated ventricular myocytes, and compared with those of amiodarone by applying conventional microelectrode and patch‐clamp techniques. Chronic treatment with dronedarone (2×25 mg−1 kg−1 day p.o. for 4 weeks), unlike chronic administration of amiodarone (50 mg−1 kg−1 day p.o. for 4 weeks), did not lengthen significantly the QTc interval of the electrocardiogram or the action potential duration (APD) in papillary muscle. After chronic oral treatment with dronedarone a small, but significant use‐dependent Vmax block was noticed, while after chronic amiodarone administration a strong use‐dependent Vmax depression was observed. Acute superfusion of dronedarone (10 μM), similar to that of amiodarone (10 μM), moderately lengthened APD in papillary muscle (at 1 Hz from 239.6±5.3 to 248.6±5.3 ms, n=13, P<0.05), but shortened it in Purkinje fibres (at 1 Hz from 309.6±11.8 to 287.1±10.8 ms, n=7, P<0.05). Both dronedarone (10 μM) and amiodarone (10 μM) superfusion reduced the incidence of early and delayed afterdepolarizations evoked by 1 μM dofetilide and 0.2 μM strophantidine in Purkinje fibres. In patch‐clamp experiments 10 μM dronedarone markedly reduced the L‐type calcium current (76.5±0.7 %, n=6, P<0.05) and the rapid component of the delayed rectifier potassium current (97±1.2 %, n=5, P<0.05) in ventricular myocytes. It is concluded that after acute administration dronedarone exhibits effects on cardiac electrical activity similar to those of amiodarone, but it lacks the ‘amiodarone like’ chronic electrophysiological characteristics.

Pharmacological block of the slow component of the outward delayed rectifier current (IKs) fails to lengthen rabbit ventricular muscle QTc and action potential duration

The effects of IKs block by chromanol 293B and L‐735,821 on rabbit QT‐interval, action potential duration (APD), and membrane current were compared to those of E‐4031, a recognized IKr blocker. Measurements were made in rabbit Langendorff‐perfused whole hearts, isolated papillary muscle, and single isolated ventricular myocytes. Neither chromanol 293B (10 μM) nor L‐735,821 (100 nM) had a significant effect on QTc interval in Langendorff‐perfused hearts. E‐4031 (100 nM), on the other hand, significantly increased QTc interval (35.6±3.9%, n=8, P<0.05). Similarly both chromanol 293B (10 μM) and L‐735,821 (100 nM) produced little increase in papillary muscle APD (less than 7%) while pacing at cycle lengths between 300 and 5000 ms. In contrast, E‐4031 (100 nM) markedly increased (30 – 60%) APD in a reverse frequency‐dependent manner. In ventricular myocytes, the same concentrations of chromanol 293B (10 μM), L‐735,821 (100 nM) and E‐4031 (1 μM) markedly or totally blocked IKs and IKr, respectively. IKs tail currents activated slowly (at +30 mV, τ=888.1±48.2 ms, n=21) and deactivated rapidly (at −40 mV, τ=157.1±4.7 ms, n=22), while IKr tail currents activated rapidly (at +30 mV, τ=35.5±3.1 ms, n=26) and deactivated slowly (at −40 mV, τ1=641.5±29.0 ms, τ2=6531±343, n=35). IKr was estimated to contribute substantially more to total current density during normal ventricular muscle action potentials (i.e., after a 150 ms square pulse to +30 mV) than does IKs. These findings indicate that block of IKs is not likely to provide antiarrhythmic benefit by lengthening normal ventricular muscle QTc, APD, and refractoriness over a wide range of frequencies.

Functional Role of Potassium Channels in the Vasodilating Mechanism of Levosimendan in Porcine Isolated Coronary Artery

Levosimendan, a new type of inodilator drugs, is known to activate membrane adenosine 3′,5′-triphosphate-sensitive potassium (KATP) channels in some vascular smooth muscles and causes vasorelaxation. The involvement of potassium channels in the mechanism of the coronary artery relaxing effect of the drug has not been established. In the present study performed in the porcine epicardial coronary artery, the effect of levosimendan (0.009–3.2 μM) was compared to cromakalim (0.0125–5 μM), the known activator of ATP-sensitive potassium (KATP) channels, in the presence of glibenclamide (GLI), an inhibitor of KATP channels and tetraethylammonium (TEA), the non-selective inhibitor of potassium channels. The interaction of levosimendan with the specific calcium-activated potassium channel (KCa) blocker, iberiotoxin (IBTX), and the voltage-sensitive potassium channel (KV) blocker, 4-aminopyridine (4-AP), was also studied. All the experiments were performed in the isometric tension of endothelium denuded porcine isolated epicardial coronary arteries precontracted with 20 mM potassium chloride. 1 μM GLI decreased the maximum of cromakalim-induced relaxation by 60% but did not affect the action of levosimendan. In contrast, 2 mM TEA decreased only the coronary artery relaxing effect of levosimendan. 100 nM IBTX suppressed the maximum effect of levosimendan by only 15% while 0.5 mM 4-AP significantly shifted the concentration-response curve of the inodilator to the right. 5 mM 4-AP caused a maximum of 33% decrease of levosimendan-induced relaxation. These results indicate that, in porcine isolated epicardial coronary artery, the vasorelaxing mechanism of levosimendan involves the activation of voltage-sensitive and, at large concentrations, calcium-activated potassium channels.

Combined pharmacological block of IKr and IKs increases short-term QT interval variability and provokes torsades de pointes

Assessing the proarrhythmic potential of compounds during drug development is essential. However, reliable prediction of drug‐induced torsades de pointes arrhythmia (TdP) remains elusive. Along with QT interval prolongation, assessment of the short‐term variability of the QT interval (STV(QT)) may be a good predictor of TdP. We investigated the relative importance of IKs and IKr block in development of TdP together with correlations between QTc interval, QT interval variability and incidence of TdP.

Pharmacology of moxonidine, an I1-imidazoline receptor agonist

&NA; Moxonidine is a second‐generation, centrally acting antihypertensive drug with a distinctive mode of action. Moxonidine activates I1‐imidazoline receptors (I1‐receptors) in the rostroventrolateral medulla (RVLM), thereby reducing the activity of the sympathetic nervous system. Moxonidine leads to a pronounced and longlasting blood pressure reduction in different animal models of hypertension, e.g., spontaneously hypertensive rats, renal hypertensive rats, and renal hypertensive dogs. Blood pressure reduction with moxonidine is usually accompanied by a reduction in heart rate which, however, in most studies is of shorter duration and lesser magnitude than the fall in blood pressure. Chronic administration of moxonidine to SHRs with established hypertension causes normalization of myocardial fibrosis, capillarization, and regressive changes in myocytes, in parallel with the reduction of blood pressure. Left ventricular hypertrophy and renal glomerulosclerosis are also significantly reduced. After withdrawal of chronic moxonidine treatment, blood pressure gradually rises to pretreatment values. Direct injection of moxonidine into the vertebral artery of cats elicits a more pronounced fall in blood pressure compared with i.v. injection of an equivalent dose. This observation and others clearly indicate that moxonidines antihypertensive activity is centrally mediated. The RVLM is the site of action within the CNS that mediates pronounced blood pressure reduction after direct administration of moxonidine into the RVLM of anesthetized SHRs. Selective I1‐receptor antagonists introduced into this area abolish the action of systemic moxonidine. Receptor binding studies have shown high and selective affinity of moxonidine for I1‐receptors vs. &agr;2‐adrenergic receptors. In vivo studies using a variety of selective I1 or &agr;2‐adrenergic agonists and antagonists have confirmed the primary role of I1‐receptors in blood pressure regulation by moxonidine. In addition to lowering blood pressure, moxonidine possesses further properties that appear likely to be relevant in its therapeutic application in the hypertensive syndrome. Moxonidine increases urine flow rate and sodium excretion after central and direct intrarenal administration. It is active against ventricular arrhythmias in a variety of experimental settings. It lacks the respiratory depressant effects attributed to central &agr;2 activation. It exerts beneficial effects on glucose metabolism and blood lipids in genetically hypertensive obese rats. It exhibits anti‐ulcer activity. And, finally, moxonidine lowers intraocular pressure, suggesting a possible benefit in glaucoma. Therefore, moxonidine, by its novel mode of action, represents a new therapeutic principle in the treatment of hypertension. Because of its unique profile, moxonidine may prove to be effective in slowing progression of the disease by providing protective effects beyond merely blood pressure reduction. Further studies are needed to verify this potential.

Slow Delayed Rectifier Potassium Current (IKs) and the Repolarization Reserve

The aim of this review is to present the properties of the slow component of the delayed rectifier potassium current (IKs) in the human ventricle. The review gives a detailed description of the physiology, molecular biology and pharmacology of the IKs current, including kinetic properties, genetic structures, agonists and antagonists. The authors also present the role of the IKs current in the human cardiac repolarization focusing on several pathophysiological situations, such as the LQT syndrome and the Torsade de Pointes arrhythmia.

Theoretical Possibilities for the Development of Novel Antiarrhythmic Drugs

One possible mechanism of action of the available K-channel blocking agents used to treat arrhythmias is to selectively inhibit the HERG plus MIRP channels, which carry the rapid delayed rectifier outward potassium current (I(Kr)). These antiarrhythmics, like sotalol, dofetilide and ibutilide, have been classified as Class III antiarrhythmics. However, in addition to their beneficial effect, they substantially lengthen ventricular repolarization in a reverse-rate dependent manner. This latter effect, in certain situations, can result in life-threatening polymorphic ventricular tachycardia (torsades de pointes). Selective blockers (chromanol 293B, HMR-1556, L-735,821) of the KvLQT1 plus minK channel, which carriy the slow delayed rectifier potassium current (I(Ks)), were also considered to treat arrhythmias, including atrial fibrillation (AF). However, I(Ks) activates slowly and at a more positive voltage than the plateau of the action potential, therefore it remains uncertain how inhibition of this current would result in a therapeutically meaningful repolarization lengthening. The transient outward potassium current (I(to)), which flows through the Kv 4.3 and Kv 4.2 channels, is relatively large in the atrial cells, which suggests that inhibition of this current may cause substantial prolongation of repolarization predominantly in the atria. Although it was reported that some antiarrhythmic drugs (quinidine, disopyramide, flecainide, propafenone, tedisamil) inhibit I(to), no specific blockers for I(to) are currently available. Similarly, no specific inhibitors for the Kir 2.1, 2.2, 2.3 channels, which carry the inward rectifier potassium current (I(kl)), have been developed making difficult to judge the possible beneficial effects of such drugs in both ventricular arrhythmias and AF. Recently, a specific potassium channel (Kv 1.5 channel) has been described in human atrium, which carries the ultrarapid, delayed rectifier potassium current (I(Kur)). The presence of this current has not been observed in the ventricular muscle, which raises the possibility that by specific inhibition of this channel, atrial repolarization can be lengthened without similar effect in the ventricle. Therefore, AF could be terminated and torsades de pointes arrhythmia avoided. Several compounds were reported to inhibit I(Kur)(flecainide, tedisamil, perhexiline, quinidine, ambasilide, AVE 0118), but none of them can be considered as specific for Kv 1.5 channels. Similarly to Kv 1.5 channels, acetylcholine activated potassium channels carry repolarizing current (I(KAch)) in the atria and not in the ventricle during normal vagal tone and after parasympathetic activation. Specific blockers of I(KAch) can, therefore, also be a possible candidate to treat AF without imposing proarrhythmic risk on the ventricle. At present several compounds (amiodarone, dronedarone, aprindine, pirmenol, SD 3212) were shown to inhibit I(KAch) but none of them proved to be selective. Further research is needed to develop specific K-channel blockers, such as I(Kur)and I(KAch) inhibitors, and to establish their possible therapeutic value.

Relevance of anaesthesia for dofetilide-induced torsades de pointes in α1-adrenoceptor-stimulated rabbits

No information is available concerning the effects of anaesthetics in the most frequently used in vivo pro‐arrhythmia model. Accordingly, in this study we examined the effect of pentobarbital, propofol or α‐chloralose anaesthesia on the pro‐arrhythmic activity of the class III anti‐arrhythmic dofetilide in α1‐adrenoceptor‐stimulated rabbits.