Update on the slow delayed rectifier potassium current (I(Ks)): role in modulating cardiac function

The slow delayed rectifier current (I(Ks)) is the slow component of cardiac delayed rectifier current and is critical for the late phase repolarization of cardiac action potential. This current is also an important target for Sympathetic Nervous System (SNS) to regulate the cardiac electivity to accommodate to heart rate alterations in response to exercise or emotional stress and can be up-regulated by β- adrenergic or other signal molecules. I(Ks) channel is originated by the co-assembly of pore-forming KCNQ1 α-subunit and accessory KCNE1 β-subunit. Mutations in any subunit can bring about severe long QT syndrome (LQT-1, LQT-5) as characterized by deliquium, seizures and sudden death. This review summarizes the normal physiological functions and molecular basis of I(Ks) channels, as well as illustrates up-to-date development on its blockers and activators. Therefore, the current extensive survey should generate fundamental understanding of the role of I(Ks) channel in modulating cardiac function and donate some instructions to the progression of I(Ks) blockers and activators as potential antiarrhythmic agents or pharmacological tools to determine the physiological and pathological function of I(Ks).     

Increase in action potential duration and inhibition of the delayed rectifier outward current IK by berberine in cat ventricular myocytes.

1. In the present work, the effects of the antiarrhythmic drug, berberine, on action potential and ionic currents of cat ventricular myocytes were studied. 2. Berberine prolonged action potential duration in cat ventricular myocytes without altering other variables of the action potential. 3. The drug at concentrations of 0.3-30 microM blocked only the delayed rectifier (IK) current with an IC50 = 4.1 microM. Berberine produced a tonic block and a phasic block that was increased with the duration of the depolarizing pulse. The blocking effect on IK was use-dependent, but not frequency-dependent. 4. In cardiac preparations two delayed rectifier currents have been found: a rapid (IKr) current and a slow (IKs) current. In the present work it has been found that berberine at the concentrations used, selectively blocked IKr. 5. At concentrations higher than 10 microM it also decreased the transient outward (Ito1) current. The drug did not have effects on the inward rectifier (IK1) or the high threshold calcium current (Ica-L). 6. These results show that berberine is a specific potassium channel blocker. The increase in action potential duration induced by berberine can be explained mainly by its blocking effects on IK.     

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 μM) 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 μM, 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.     

Profile of I(Ks) during the action potential questions the therapeutic value of I(Ks) 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).     

Bepridil differentially inhibits two delayed rectifier K(+) currents, I(Kr) and I(Ks), in guinea-pig ventricular myocytes

1. We investigated the effects of bepridil on the two components of the delayed rectifier K(+) current, i.e., the rapidly activating (I(Kr)) and the slowly activating (I(Ks)) currents using tight-seal whole-cell patch-clamp techniques in guinea-pig ventricular myocytes, under blockade of L-type Ca(2+) current with nitrendipine (5 microM) or D600 (1 microM). 2. Bepridil decreased I(Ks) under blockade of I(Kr) with E4031 (5 microM), in a concentration-dependent manner. The concentration-dependent inhibition of I(Ks) by bepridil was fitted by a curve, assuming one-to-one interactions between the channel and the drug molecule. The concentration of half-maximal inhibition (IC(50)) was found to be 6.2 microM. 3. The effect of bepridil on I(Kr) was assessed using an envelope-of-tails test. In the control condition, a ratio of the tail current to the time-dependent current measured during depolarization was large (>1) at shorter pulses (<200 ms), and it decreased to a steady state value of approximately 0.4 with increases in the pulse duration. Bepridil at a concentration of 2 microM did not decrease this ratio at shorter pulses. 4. In a short-pulse (duration=50 ms) experiment that largely activates I(Kr), the drug was found to block I(Kr) in a cooperative manner (Hill coefficient=3.03) and the IC(50) was 13.2 microM. 5. These results suggest that bepridil at a clinical therapeutic concentration ( approximately 2 microM) selectively blocks I(Ks) but does not inhibit I(Kr). This may relate to the characteristic frequency-dependent effects of bepridil on the action potential duration (APD), e.g., the non-reverse use-dependent prolongation of APD.     

Stimulatory action of protein kinase C(epsilon) isoform on the slow component of delayed rectifier K+ current in guinea-pig atrial myocytes

BACKGROUND AND PURPOSE: Protein kinase C (PKC) comprises at least twelve isoforms and has an isoform-specific action on cardiac electrical activity. The slow component of delayed rectifier K(+) current (I (Ks)) is one of the major repolarizing currents in the hearts of many species and is also potentiated by PKC activation. Little is known, however, about PKC isoform(s) functionally involved in the potentiation of I (Ks) in native cardiac myocytes. EXPERIMENTAL APPROACH: I (Ks) was recorded from guinea-pig atrial myocytes, using the whole-cell configuration of patch-clamp method. KEY RESULTS: Bath application of phenylephrine enhanced I (Ks) concentration-dependently with EC(50) of 5.4 microM and the maximal response (97.1+/-11.9% increase, n=16) was obtained at 30 microM. Prazosin (1 microM) almost totally abolished the potentiation of I (Ks) by phenylephrine, supporting the involvement of alpha(1)-adrenoceptors. The stimulatory action of phenylephrine was significantly, if not entirely, inhibited by the general PKC inhibitor bisindolylmaleimide I but was little affected by G?-6976, G?-6983 and rottlerin. Furthermore, this stimulatory effect was significantly reduced by dialyzing atrial myocytes with PKCepsilon-selective inhibitory peptide epsilonV1-2 but was not significantly affected by conventional PKC isoform-selective inhibitory peptide betaC2-4. Phorbol 12-myristate 13-acetate (PMA) at 100 nM substantially increased I (Ks) by 64.2+/-1.3% (n=6), which was also significantly attenuated by an internal dialysis with epsilonV1-2 but not with betaC2-4. CONCLUSIONS AND IMPLICATIONS: The present study provides experimental evidence to suggest that, in native guinea-pig cardiac myocytes, activation of PKC contributes to alpha(1)-adrenoceptor-mediated potentiation of I (Ks) and that epsilon is the isoform predominantly involved in this PKC action.     

苄基四氢巴马汀对心室肌动作电位和延迟整流钾电流的频率依赖性作用

目的　研究BTHP对豚鼠乳头状肌动作电位及单个心室肌细胞延迟整流钾电流影响的频率依赖性。方法　用标准微电极方法在不同基础周长 (BCL)时测定动作电位 ;采用全细胞膜片钳技术测定延迟整流钾电流 (IK:IKr、IKs)。结果　 10 0 μmol·L-1BTHP在BCL为 :2 0 0 0、2 5 0ms时 ,使APD2 0 分别延长 11 35 %和 2 5 5 5 % ;使APD90 分别延长15 97%和 32 5 6 %。 30 μmol·L-1BTHP在刺激频率为 :0 2 5和 2 0Hz时分别使Ikr,tial从 (0 94± 0 .44 ) pA·pF-1和(0 92± 0 31) pA·pF-1降至 (0 6 0± 0 32 ) pA·pF-1和 (0 43± 0 18)pA·pF-1。在刺激频率为 :0 1和 2 0Hz时分别使Iks,tial从 (4 2 2± 0 5 6 ) pA·pF-1和 (5 14± 0 2 8)降至 (2 5 8±0 41)pA·pF-1和 (2 6 2± 0 37)pA·pF-1。结论　BTHP可频率依赖性地阻滞IKr、IKs,其延长动作电位也呈频率依赖性     

Blocking action of chromanol 293B on the slow component of delayed rectifier K(+) current in guinea-pig sino-atrial node cells

1. In guinea-pig sino-atrial (SA) node cells the delayed rectifier K(+) current (I(K)) is composed of rapidly and slowly activating components of I(K) (I(Kr) and I(Ks), respectively). The present study was undertaken to characterize the blocking action of the chromanol derivative 293B on I(Ks) in guinea-pig SA node cells using whole-cell patch-clamp technique. 2. Bath application of 293B blocked I(Ks), elicited by 4-s depolarizing voltage pulses from a holding potential of -50 mV, under conditions in which the L-type Ca(2+) current (I(Ca,L)) and I(Kr) were inhibited; the effect was concentration-dependent with an IC(50) of 5.3 microM, when evaluated by the decrease in the amplitude of I(Ks) tail current following 4-s depolarizing voltage steps to +50 mV. 3. The 293B block of I(Ks) progressed with time during depolarizing voltage steps with a more rapid block at higher concentrations. 4. The block of I(Ks) by 293B was fully reversed within a few minutes after washing off the drug, even when a maximal effect (a nearly full block) was achieved at high drug concentration (50 microM). 5. Bath application of 293B at 50 microM greatly and reversibly reduced the amplitude of I(Ks) which is maximally stimulated by beta-adrenergic agonist isoprenaline (1 microM), while the degree of 293B block of the isoprenaline-stimulated I(Ks) was slightly but significantly smaller than that of non-stimulated I(Ks) (94.0+/-0.98% block, n=6 vs 99.4+/-0.45% block, n=6; P<0.01). 6. We conclude that, in guinea-pig SA node cells (i) 293B is a potent and fully reversible blocker of I(Ks) in control and during beta-adrenergic stimulation and (ii) block with 293B occurs in a time-dependent manner during depolarizing voltage steps.     

Pharmacological antagonism of the slow-activating delayed rectifying potassium channel (I(Ks)) has no effect on cochlear structure and function in vivo

The experimental class III antiarrhythmic drug, L-768673, prolongs the refractory period of cardiac myocytes by selectively blocking the slow-activating delayed rectifying potassium (I(Ks)) channel. The I(Ks) channel has also been identified in vestibular dark cells and in the marginal cells of the stria vascularis. In the stria vascularis, the I(Ks) channel plays an important role in cochlear homeostasis. Genetic null deletion of the I(Ks) channel in mice and man results in profound hearing loss and cochlear pathology. Therefore, the purpose of the present study was to investigate the effect of L-768673 on the auditory function and cochlear morphology in rats using auditory brainstem-evoked response and light microscopy. Auditory testing was performed one week prior to dosing, following 14 days of administration and 28 days after the completion of dosing. L-768673 (50 or 250 mg/kg/day for 14 days), had no significant effects on auditory function or cochlear morphology. The results of this study suggest that high doses of L-768673 are not toxic to the inner ear of adult rats treated for 14 consecutive days, and that the ototoxic potential of orally administered L-768673 and similar I(Ks)-selective compounds is unlikely at doses within the therapeutic range.     

Propofol inhibited the delayed rectifier potassium current (I(k)) via activation of protein kinase C epsilon in rat parietal cortical neurons

Propofol has been shown to exert neuroprotective effects. Delayed rectifier potassium current (I(K)) was reported to be closely related to neuronal damage. This study was designed to test the effects of propofol on I(K) in rat parietal cortical neurons and the involvement of PKC in this activity. Whole-cell patch-clamp recordings were performed in rat parietal cortical neurons. The amplitudes of I(K) were recorded before and after the addition of different concentrations of propofol. Propofol concentration-dependently inhibited I(K) with an IC50 value of 36.3±2.7 μM. Moreover, propofol caused a downward shift of the I-V curve of I(K) in a concentration dependent manner. The kinetics of I(K) was altered by propofol, with decreased activation and delayed recovery of I(K). Pretreatment with calphostin-C (a non-selective inhibitor of PKC) or PKC epsilon translocation inhibitor peptide (PKC epsilon inhibitor) abrogated the inhibition of I(K) by propofol. In conclusion, propofol inhibited I(K) via the activation of PKC epsilon in rat cerebral parietal cortical neurons.     

Block of the delayed rectifier current (IK) by the 5-HT3 antagonists ondansetron and granisetron in feline ventricular myocytes.

1. We investigated the effects of two 5-HT3 antagonists, ondansetron and granisetron, on the action potential duration (APD) and the delayed rectifier current (IK) of feline isolated ventricular myocytes. Whole-cell current and action potential recordings were performed at 37 degrees C with the patch clamp technique. 2. Ondansetron and granisetron blocked IK with a KD of 1.7 +/- 1.0 and 4.3 +/- 1.7 microM, respectively. At a higher concentration (30 microM), both drugs blocked the inward rectifier (IKl). 3. The block of IK was dependent on channel activation. Both drugs slowed the decay of IK tail currents and produced a crossover with the pre-drug current trace. These results are consistent with block and unblock from the open state of the channel. 4. Granisetron showed an intrinsic voltage-dependence as the block increased with depolarization. The equivalent voltage-dependency of block (delta) was 0.10 +/- 0.04, suggesting that granisetron blocks from the intracellular side at a binding site located 10% across the transmembrane electrical field. 5. Ondansetron (1 microM) and granisetron (3 microM) prolonged APD by about 30% at 0.5 Hz. The prolongation of APD by ondansetron was abolished at faster frequencies (3 Hz) showing reverse rate dependence. 6. In conclusion, the 5-HT3 antagonists, ondansetron and granisetron, are open state blockers of the ventricular delayed rectifier and show a clear class III action.     

Effects on transmembrane action potential,slow inward current and force of contraction in ventricular cardiac muscle of BRL 31660, a new antiarrhythmic drug with class I and class IV activity

Summary The effects of 1–30 mol l^–1 BRL 31660 on transmembrane action potential and force of contraction were investigated in guinea-pig electrically driven papillary muscles. Lidocaine was studied for comparison.BRL 31660 depressedV _max of the action potential without changing the resting potential, decreased the force of contraction and decreased the action potential duration. Similar effects were obtained with lidocaine.BRL 31660 inhibited the recovery ofV _max from inactivation, the time constant of which was estimated to be about 1,100 ms in the presence of 10 mol l^–1 BRL 31660. The depressive effect onV _max was particularly pronounced at low (less negative) membrane potentials. BRL 31660 can thus be classified as a class I antiarrhythmic agent of the lidocaine type.Additional voltage-clamp experiments in cow ventricular trabeculae provided evidence that BRL 31660 also depressed the slow inward current at concentrations similar to those producing the effects on the transmembrane action potential. BRL 31660 thus exerted an additional class IV action. This effect was not shared by lidocaine.It is concluded that BRL 31660 is a new antiarrhythmic agent which depresses both the fast and slow inward current at similar concentrations. The dual effects of BRL 31660 conceivably contribute to its antiarrhythmic activity, but the clinical relevance of these results remains to be elucidated.     

Prolonged action potential duration and positive inotropy induced by the novel class III antiarrhythmic agent H 234/09 (Almokalant) in isolated human ventricular muscle.

The electromechanical properties of H 234/09 (Almokalant), a novel class III antiarrhythmic agent, was examined in isolated human ventricular muscle strips excised from patients undergoing mitral valve replacement. Using transmembrane microelectrode recording techniques, we demonstrated that H 234/09 markedly prolonged the action potential duration (APD) without affecting the maximal rate of depolarization or action potential amplitude. At 75 and 90% repolarization APD was prolonged to a similar extent, whereas the lengthening at 50% repolarization was somewhat less marked. In isometrically contracting muscle strips, H 234/09 increased peak developed force and its maximal rate of rise (dF/dt) and fall (-dF/dt) in a concentration-dependent manner, whereas time to peak developed force was unaltered. We conclude from these studies that H 234/09 is a class III agent in human ventricular muscle and that the class III effect is linked with a positive inotropic response.     

Gender differences in the slow delayed (IKs) but not in inward (IK1) rectifier K+ currents of canine Purkinje fibre cardiac action potential: key roles for IKs, beta-adrenoceptor stimulation, pacing rate and gender

As the beagle dog is a commonly used preclinical species to test the effects of new drugs on cardiac repolarisation and Purkinje fibres have become an established in vitro preparation to assess the effects of these new drugs on action potential duration (APD), the main aim of this study was therefore to evaluate the relative contribution of the inward (I(K1)) and slow delayed (I(Ks)) rectifier cardiac K(+) currents to action potential repolarisation in beagle Purkinje fibres under three different experimental conditions: (i) selective block of I(K1) with BaCl(2), (ii) selective block of I(Ks) with (-) chromanol 293B under basal conditions and (iii) selective block of I(Ks) during beta-adrenoceptor stimulation. Furthermore, the dependence of this contribution on gender and pacing rate was investigated. Microelectrode techniques were employed to measure APD in Purkinje fibres from adult female and male dogs. At stimulation rates of 3.33, 1.0 and 0.2 Hz, the degree of prolongation of APD evoked by BaCl(2) (10 microM) was comparable in fibres from female and male dogs. At the same stimulation rates, 10 microM (-) chromanol 293B did not change the APD in fibres from female and male dogs. During beta-adrenoceptor stimulation with 0.1 microM isoproterenol, an APD prolonging effect of (-) chromanol 293B was detected. In the presence of isoproterenol, action potentials in fibres from male dogs get shorter when changing the stimulation rate from 1.0 to 0.2 Hz, while the opposite is seen in fibres from female dogs. This alteration was completely reversed by (-) chromanol 293B. In conclusion, our findings confirm that beta-adrenoceptor stimulation is one condition where there may be an increased role of I(Ks) in action potential repolarisation. Gender differences in the autonomic modulation of I(Ks) could be a contributing factor to the reported increased susceptibility of female hearts to arrhythmias.     

Draft ICH guideline S7B: guideline on safety pharmacology studies for assessing the potential for delayed ventricular repolarization (QT interval prolongation) by human pharmaceuticals

Nonclinical assessment of potential of QT interval prolongation caused by non-antiarrhythmic drugs has been an issue for drug development because QT interval prolongation increases the risk of ventricular tachyarrhythmia, including torsade de pointes when combined with other risk factors. However, there is no scientific consensus on approaches and no international consensus on regulatory recommendations. This guideline is being developed to provide the general nonclinical testing strategy for evaluating the potential risk of QT prolongation and presents some major principles for in vitro and in vivo electrophysiology studies. The basis of this guideline is the integrated risk assessment that provides overall evaluations based on nonclinical study results and chemical/pharmacological class information to predict the potential of a test substance to prolong QT interval in humans (i.e., evidence of risk) and that contributes clinical study design and interpretation of clinical results. Safety margins are also components of integrated risk assessment. Since this guideline addresses a field of research that is in a state of rapid evolution, the proposed concept for evidence of risk and safety margins needs to be further refined based on the data being collected by international initiatives. In this article, the draft S7B guideline is outlined.     

Comparative analysis of the action of class I antiarrhythmic drugs (lidocaine, quinidine, and prajmaline) in rabbit atrial and ventricular myocardium.

Effects of three class I antiarrhythmic drugs (quinidine, lidocaine, and prajmaline) on transmembrane resting (RMP) and action potentials (AP) of isolated rabbit atrial and ventricular myocardium were studied at different stimulation rates. The frequency-dependent depression of the maximal upstroke velocity (Vmax) of the AP (sodium channel block) was analyzed according to the "guarded receptor" hypothesis. The resting block (Vmax depression after a resting period) induced by prajmaline (10(-6) M), quinidine (2.2 x 10(-5) M), and lidocaine (4.3 x 10(-5) M) was more expressed in the atrium (44, 28, and 19%, respectively) than in the ventricle (32, 9, and 0%, respectively). There were also significant (p less than 0.05) atrioventricular differences in the frequency-dependent extra block (Vmax reduction on stimulation at 3.3 Hz) for quinidine (39 vs. 26%) and lidocaine (4 vs. 25%). From the analysis, according to the guarded receptor hypothesis, it follows that the three compounds bind preferentially to inactivated sodium channels with about the same affinity to the atrium and ventricle, except for quinidine which shows a significantly smaller dissociation constant in the atrium (5 x 10(-6) M vs. 2.7 x 10(-5) M; p less than 0.001). We conclude that the atrioventricular differences in the resting block are mainly due to atrioventricular differences in the RMP, whereas the differences in the frequency-dependent extra block are based on the shorter atrial AP duration (lidocaine) or are due to higher affinity to atrial sodium channels (quinidine).