Delayed rectifier potassium current in undiseased human ventricular myocytes

Objective: The purpose of the study was to investigate the properties of the delayed rectifier potassium current (I-K) in myocytes isolated from undiseased human left ventricles. Methods: The whole-cell configuration of the patch-clamp technique was applied in 28 left ventricular myocytes from 13 hearts at 35 degrees C. Results: An E-4031 sensitive tail current identified the rapid component of I-K (I-Kr) in the myocytes, but there was no evidence for an E-4031 insensitive slow component of I-K (I-Kb). When nifedipine (5 mu M) was used to block the inward calcium current (I-Ca), I-Kr activation was fast (tau=31.0+/-7.4 ms, at +30 mV, n=5) and deactivation kinetics were biexponential and relatively slow (tau(1) =600.0+/-53.9 ms and tau(2)-6792.2+/-875.7 ms, at -40 mV, n=7). Application of CdCl2 (250 mu M) to block I-Ca altered the voltage dependence of the I-Kr considerably, slowing its activation (tau=657.1+/-109.1 ms, at +30 mv n=5) and accelerating its deactivation ( tau=104.0+/-18.5 ms, at -40 mV, n=8). Conclusions: In undiseased human ventricle at 35 degrees C I-Kr exists having fast activation and slow deactivation kinetics; however, there was no evidence found for an expressed I-Ks. I-Kr probably plays an important role in the frequency dependent modulation of repolarization in undiseased human ventricle, and is a target for many Class III antiarrhythmic drugs

The slow component of the delayed rectifier potassium current in undiseased human ventricular myocytes

Objective: The purpose of this study was to investigate the properties of the slow component of the delayed rectifier potassium current (I-Ks) in myocytes isolated from undiseased human left ventricles. Methods: The whole-cell configuration of the patch-clamp technique was applied in 58 left ventricular myocytes from 15 hearts at 37 degreesC. Nisoldipine (1 muM) was used to block inward calcium current (I-Ca) and E-4031 (1-5 muM) was applied to inhibit the rapid component of the delayed rectifier potassium current (I-Ks). Results: In 31 myocytes an E-4031 insensitive, but L-735,821 and chromanol 293B sensitive, tail current was identified which was attributed to the slow component of I-K (I-Ks). Activation of I-Ks was slow (tau = 903 +/- 101 ms at 50 mV, n = 24), but deactivation of the current was relatively rapid ( tau =122.4 +/- 11.7 ms at -40 mV, n = 19). The activation of I-Ks was voltage independent but its deactivation showed clear voltage dependence. The deactivation was faster at negative voltages (about 100 ms at -50 mV) and slower at depolarized potentials (about 300 ms at 0 mV). In six cells, the reversal potential was -81.6 +/- 2.8 mV on an average which is close to the K+ equilibrium potential suggesting K+ as the main charge carrier. Conclusion: In undiseased human ventricular myocytes, I-Ks exhibits slow activation and fast deactivation kinetics. Therefore, in humans I-Ks differs from that reported in guinea pig, and it best resembles I-Ks described in dog and rabbit ventricular myocytes

Diabetes mellitus attenuates the repolarization reserve in mammalian heart

Objective: In diabetes mellitus several cardiac electrophysiological parameters are known to be affected. In rodent experimental diabetes models changes in these parameters were reported, but no such data are available in other mammalian species including the dog. The present study was designed to analyse the effects of experimental type I diabetes on ventricular repolarization and its underlying transmembrane ionic currents and channel proteins in canine hearts. Methods and results: Diabetes was induced by a single injection of alloxan, a subgroup of dogs received insulin substitution. After the development of diabetes (8 weeks) electrophysiological studies were performed using conventional microelectrodes, whole cell voltage clamp, and ECG. Expression of ion channel proteins was evaluated by Western blotting. The QT(c) interval and the ventricular action potential duration in diabetic dogs Were moderately prolonged. This was accompanied by significant reduction in the density of the transient outward K+ current (I-to) and the slow delayed rectifier K+ current (I-Ks), to 54.6% and 69.3% of control, respectively. No differences were observed in the density of the inward rectifier K+ current (I-K1), rapid delayed rectifier K+ current (I-Kr), and L-type Ca2+ current (I-Ca). Western blot analysis revealed a reduced expression of Kv4.3 and MinK (to 25 +/- 21% and 48 +/- 15% of control, respectively) in diabetic dogs, while other channel proteins were unchanged (HERG, MiRP1, alpha(1c)) or increased (Kv1.4, KChIP2, KvLQT1). Insulin substitution fully prevented the diabetes-induced changes in I-Ks, KvLQT1 and MinK, however, the changes in I-to, Kv4.3, and Kv1.4 were only partially diminished by insulin. Conclusion: It is concluded that type I diabetes mellitus, although only moderately, lengthens ventricular repolarization, attenuates the repolarization reserve by decreasing I-to and I-Ks currents, and thereby may markedly enhance the risk of sudden cardiac death

Long-term endurance training-induced cardiac adaptation in new rabbit and dog animal models of the human athlete’s heart

Sudden cardiac death in athletes is rare and most often unexpectable. For a better understanding of cardiac re- modeling, this study presents the effects of chronic vigor- ous exercise on cardiac structure and electrophysiology in new rabbit and dog athlete’s heart models. Rabbits and dogs were randomized into sedentary (’Sed’), exer- cised (subjected to 16 weeks chronic treadmill exercise (’Ex’) groups, and a testosterone-treated (’Dop’) group in dogs. Echocardiography and electrocardiogram were performed. Proarrhythmic sensitivity and autonomic re- sponses were tested in conscious dogs. ‘Ex’ animals exhibited left ventricular enlargement with bradycardia (mean RR in ‘Ex’ vs. ‘Sed’ rabbits: 335 ± 15 vs. 288 ± 19 ms, p ≤ 0.05, and in ‘Dop’ vs. ‘Ex’ vs. ‘Sed’ dogs: 718 ± 6 vs. 638 ± 38 vs. 599 ± 49 ms) accompanied by an increase of heart rate variability in both species (e.g. SD RR in ‘Ex’ vs. ‘Sed’ rabbits: 3.4 ± 0.9 vs. 1.4 ± 0.1 ms, p ≤ 0.05, and in ‘Dop’ vs. ‘Ex’ vs. ‘Sed’ dogs: 156 ± 59 vs. 163 ± 44 vs. 111 ± 49 ms) indicating an increased vagal tone. A lower response to parasym- patholytic agent atropine and more pronounced QT c in- terval lengthening after dofetilide challenge were found in ’Ex’ and ’Dop’ dogs compared to the ‘Sed’ group. No morphological and functional changes were found after chronic steroid treatment in dogs. The structural-functional findings share more similarities with human athlete’s heart. Slight repolarization sensitivity in the exercised dogs may indicate an increased risk of arrhythmias in athletes under different circumstances. These animal models might be useful for the further investigations of the cardiovascular effects of competitive training

Levosimendan Efficacy and Safety: 20 years of SIMDAX in Clinical Use

Levosimendan was first approved for clinic use in 2000, when authorisation was granted by Swedish regulatory authorities for the haemodynamic stabilisation of patients with acutely decompensated chronic heart failure. In the ensuing 20 years, this distinctive inodilator, which enhances cardiac contractility through calcium sensitisation and promotes vasodilatation through the opening of adenosine triphosphate-dependent potassium channels on vascular smooth muscle cells, has been approved in more than 60 jurisdictions, including most of the countries of the European Union and Latin America. Areas of clinical application have expanded considerably and now include cardiogenic shock, takotsubo cardiomyopathy, advanced heart failure, right ventricular failure and pulmonary hypertension, cardiac surgery, critical care and emergency medicine. Levosimendan is currently in active clinical evaluation in the US. Levosimendan in IV formulation is being used as a research tool in the exploration of a wide range of cardiac and non-cardiac disease states. A levosimendan oral form is at present under evaluation in the management of amyotrophic lateral sclerosis. To mark the 20 years since the advent of levosimendan in clinical use, 51 experts from 23 European countries (Austria, Belgium, Croatia, Cyprus, Czech Republic, Estonia, Finland, France, Germany, Greece, Hungary, Italy, the Netherlands, Norway, Poland, Portugal, Russia, Slovenia, Spain, Sweden, Switzerland, UK and Ukraine) contributed to this essay, which evaluates one of the relatively few drugs to have been successfully introduced into the acute heart failure arena in recent times and charts a possible development trajectory for the next 20 years

Analysis of the electrophysiological effects of ambasilide, a new antiarrhythmic agent, in canine isolated ventricular muscle and Purkinje fibers

The aim of the study was to determine the in vitro rate-dependent cellular electrophysiological effects of ambasilide (10 and 20 mu M/l), a new investigational antiarrhythmic agent, in canine isolated ventricular muscle and Purkinje fibers by applying the standard microelectrode technique. At the cycle length (CL) of 1000 ms, ambasilide significantly prolonged the action potential duration measured at 90% repolarization (APD(90)) in both ventricular muscle and Purkinje fibers. Ambasilide (10 mu M/l) produced a more marked prolongation of APD(90) at lower stimulation frequencies in Purkinje fibers (at CL of 2000 ms = 56.0 +/- 16.1%, n = 6, versus CL of 400 ms = 15.1 +/- 3.7%, n = 6; p < 0.05), but, in 20 mu M/l, this effect was considerably diminished (15.2 +/- 3.6%, n = 6, versus 7.3 +/- 5.1%, n = 6, p < 0.05). In ventricular muscle, however, both concentrations of the drug induced an almost frequency-independent lengthening of APD(90) in response to a slowing of the stimulation rate (in 20 mu M/l at CL of 5000 ms = 19.0 +/- 1.5%, n = 91 versus CL of 400 ms = 16.9 +/- 1.4%, n = 9). Ambasilide induced a marked rate-dependent depression of the maximal rate of rise of the action potential upstroke (V-max) (in 20 mu M/l at CL of 300 ms = -45.1 +/- 3.9%, n = 6, versus CL of 5000 ms = -8.5 +/- 3.9%, n = 6, p < 0.05, in ventricular muscle) and the corresponding recovery of V-max time constant was tau = 1082.5 +/- 205.1 ms (n = 6). These data suggest that ambasilide, in addition to its Class III antiarrhythmic action, which is presumably due to its inhibitory effect on the delayed rectifier potassium current, possesses I/B type antiarrhythmic properties as a result of the inhibition of the fast sodium channels at high frequency rate with relatively fast kinetics. This latter effect may play an important role in its known less-pronounced proarrhythmic ("torsadogenic") potential

Multiple cellular electrophysiological effects of azimilide in canine cardiac preparations

The cellular electrophysiological effect of azimilide (0.1-30 muM) was analyzed in canine ventricular preparations by applying the standard microelectrode and patch-clamp techniques at 37 degreesC. In papillary muscle, the drug prolonged the action potential duration (APD) in a concentration-dependent manner at a cycle length (CL) of 1000 ms. In Purkinje fibers, at the same CL, the concentration-dependent lengthening of the APD was observed in the presence of up to 3 muM azimilide (at 3.0 muM: 24.1 +/- 4.2%, n = 9); at higher drug concentration, no further APD prolongation was observed. Azimilide lengthened APD in a reverse frequency-dependent manner in papillary muscle and Purkinje fibers alike. Azimilide (10 muM) caused a rate-dependent depression in the maximal upstroke velocity of the action potential (V-max) in papillary muscle. The time and rate constants of the offset and onset kinetics of this V-max block were 1754 +/- 267 ms (n = 6) and 5.1 +/- 0.4 beats (n = 6), respectively. Azimilide did not prevent the APD shortening effect of 10 muM pinacidil in papillary muscle, suggesting that the drug does not influence the ATP-sensitive K+ current. Azimilide inhibited the rapid (I-Kr) and slow component (I-Ks) of the delayed rectifier K+ current and the L-type Ca2+ current (I-Ca). The estimated EC50 value of the drug was 0.59 muM for I-Ks, 0.39 muM for I-Kr and 7.5 muM for I-Ca. The transient outward (I-to) and the inward rectifier (I-kl) K+ currents were not influenced by the drug. It is concluded that the site of action of azimilide is multiple, it inhibits not only K+ (I-Kr, I-Ks) currents but, in higher concentrations, it also exerts calcium- and use-dependent sodium channel block

Cellular electrophysiological effect of terikalant in the dog heart

The cellular mechanism of action of terikalant, an investigational antiarrhythmic agent known to block the inward rectifier and other potassium currents, has not yet been fully clarified. The aim of the present study was therefore to analyse the in vitro electrophysiological effects of terikalant in canine isolated ventricular muscle and Purkinje fibers by applying the standard microelectrode technique. The effects of terikalant on the duration of action potential at a stimulation cycle length of 1000 ms and on the maximum upstroke velocity of the action potential in right ventricular papillary muscle were examined at 1, 2.5, 10, and 20 mu M concentrations. Terikalant significantly prolonged the action potential duration measured both at 50% and 90% of repolarization in concentration-dependent manner. The maximum upstroke velocity of the action potential was unaffected at 1 and 2.5 mu M concentrations. However, this parameter was significantly reduced at 10 and 20 mu M concentrations of terikalant. In Purkinje fibers terikalant (2.5 mu M) also produced a marked action potential lengthening effect. Frequency dependence (cycle length of 300-5000 ms) of the action potential lengthening effect of terikalant was studied at a concentration of 2.5 mu M. Prolongation of the duration of action potential occurred in a reverse frequency-dependent manner both in papillary muscle and Purkinje fibers, with a more pronounced frequency-dependence observed in Purkinje fibers. The onset kinetics of the terikalant (10 mu M) induced block of the maximum upstroke velocity of the action potential was rapid (0.6 +/- 0.1 beat(-1) n=6) like that of Class I/B antiarrhythmics, and the offset (recovery) kinetics of the drug (2956 696 ms, n=6) best resembled that of Class I/A antiarrhythmic drugs. It was concluded that terikalant, unlike pure Class III antiarrhythmic drugs, has combined mode of action by lengthening repolarization and blocking the inward sodium current in a use-dependent manner

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 + 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 29313, HMR-1556, L-735,821) of the KvLQT1 + minK channel, which carriy the slow delayed rectifier potassium current GO, 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 0 118), 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