Currents In Cardiac Cells Research Articles

New Aspects of HERG K+ Channel Function Depending upon Cardiac Spatial Heterogeneity

HERG K+ channel, the genetic counterpart of rapid delayed rectifier K+ current in cardiac cells, is responsible for many cases of inherited and drug-induced long QT syndromes. HERG has unusual biophysical properties distinct from those of other K+ channels. While the conventional pulse protocols in patch-clamp studies have helped us elucidate these properties, their limitations in assessing HERG function have also been progressively noticed. We employed AP-clamp techniques using physiological action potential waveforms recorded from various regions of canine heart to study HERG function in HEK293 cells and identified several novel aspects of HERG function. We showed that under AP-clamp IHERG increased gradually with membrane repolarization, peaked at potentials around 20–30 mV more negative than revealed by pulse protocols and at action potential duration (APD) to 60%-70% full repolarization, and fell rapidly at the terminal phase of repolarization. We found that the rising phase of IHERG was conferred by removal of inactivation and the decaying phase resulted from a fall in driving force, which were all determined by the rate of membrane repolarization. We identified regional heterogeneity and transmural gradient of IHERG when quantified with the area covered by IHERG trace. In addition, we observed regional and transmural differences of IHERG in response to dofetilide blockade. Finally, we characterized the influence of HERG function by selective inhibition of other ion currents. Based on our results, we conclude that the distinct biophysical properties of HERG reported by AP-clamp confer its unique function in cardiac repolarization thereby in antiarrhythmia and arrhythmogenesis.

Efficacy and Safety of Ranolazine in Patients With Chronic Stable Angina

Chronic stable angina (CSA) impairs patient quality of life, is associated with increased patient mortality, and is a prominent symptom of coronary artery disease (CAD), the latter being prevalent worldwide in patients. Currently, therapeutic options for patients with CSA include β-blockers, calcium channel blockers, nitrates, and ranolazine. Ranolazine is a first-in-class piperazine derivative that inhibits the late inward sodium current in cardiac cells and is considered an effective and safe option for treating patients with CSA. As with any first-in-class agent, it is important for the practitioner to be familiar with the safety profile of the drug. Therefore, the objective of our article is to review safety data on the use of ranolazine in patients with CSA. Clinical data show that ranolazine is well tolerated: major treatment-associated adverse events include dizziness, nausea, headache, and constipation. Ranolazine treatment is also associated with QTc-interval prolongation; however, QTc-interval prolongation with ranolazine does not appear to have clinical consequences–in fact, several studies suggest that ranolazine therapy may have an antiarrhythmic effect in patients. Notably, ranolazine is hemodynamically neutral in that it exerts its antianginal effect without significantly impacting patient heart rate or blood pressure. In addition, small decreases in glycosylated hemoglobin levels have been seen in patients with type 2 diabetes mellitus. Overall, ranolazine (in doses of 500 mg and 1000 mg, twice daily) is a safe and effective option for monotherapy or add-on therapy to reduce anginal symptoms in patients with CSA.

Individual Cell Electrophysiology (ICE) of Cardiac Myocytes _ Using Innovative ‘Onion-Peeling’ Technique to Study the ICE of Excitable Cells

The action potential (AP) of a cardiac cell is a finely choreographed dance involving many ion channels and transporters interacting with one another via membrane potential and intracellular Ca2+. The ionic mechanisms that cause arrhythmogenic APs (early or delayed afterdepolarizations, short or long AP duration) are still incompletely understood. Traditional voltage-clamp experiments measured one current from one cell and in most cases without Ca2+ transients. Data averaged from many cells were then used to build a canonical AP model. However, the canonical AP may be very different from the AP of any particular cell due to cell-to-cell variations. Recently, we developed an innovative ‘Onion-Peeling’ (O-P) technique to achieve (a) a direct recording of ionic current under in situ condition (during AP, with Ca2+ cycling, in a physiological milieu, undergoing contraction) and (b) recording of multiple currents from a single cell to study how different currents interact and integrate to shape AP. Here we report the results of using O-P to investigate: (1) recording of key ionic currents (IKs, IKr, IK1, ICa,L, INCX, INa-Late) during AP in the guinea pig ventricular myocyte; (2) effects of the Ca2+ transient on modulating these currents during AP; (3) changes of multiple currents under beta-adrenergic stimulation and how concerted changes in currents integrate to reshape AP. The ability to measure many currents in a single cell has also revealed two hitherto unknown characteristics of the currents in cardiac cells: coordination of currents within a cell and large variations of currents between cells. Hence, our O-P technique presents a significant step beyond the traditional way of measuring only one current in any one cell, and provides a powerful new tool for studying the individual cell electrophysiology.

Propofol and arrhythmias: two sides of the coin

The hypnotic agent propofol is effective for the induction and maintenance of anesthesia. However, recent studies have shown that propofol administration is related to arrhythmias. Propofol displays both pro- and anti-arrhythmic effects in a concentration-dependent manner. Data indicate that propofol can convert supraventricular tachycardia and ventricular tachycardia and may inhibit the conduction system of the heart. The mechanism of the cardiac effects remains poorly defined and may involve ion channels, the autonomic nervous system and cardiac gap junctions. Specifically, sodium, calcium and potassium currents in cardiac cells are suppressed by clinically relevant concentrations of propofol. Propofol shortens the action potential duration (APD) but lessens the ischemia-induced decrease in the APD. Furthermore, propofol suppresses both sympathetic and parasympathetic tone and preserves gap junctions during ischemia. All of these effects cumulatively contribute to the antiarrhythmic and proarrhythmic properties of propofol.

Sequential dissection of multiple ionic currents in single cardiac myocytes under action potential-clamp

The cardiac action potential (AP) is shaped by myriad ionic currents. In this study, we develop an innovative AP-clamp Sequential Dissection technique to enable the recording of multiple ionic currents in the single cell under AP-clamp. This new technique presents a significant step beyond the traditional way of recording only one current in any one cell. The ability to measure many currents in a single cell has revealed two hitherto unknown characteristics of the ionic currents in cardiac cells: coordination of currents within a cell and large variation of currents between cells. Hence, the AP-clamp Sequential Dissection method provides a unique and powerful tool for studying individual cell electrophysiology.

A computational investigation of cardiac caveolae as a source of persistent sodium current

Recent studies of cholesterol-rich membrane microdomains, called caveolae, reveal that caveolae are reservoirs of “recruitable” sodium ion channels. Caveolar channels constitute a substantial and previously unrecognized source of sodium current in cardiac cells. In this paper we model for the first time caveolar sodium currents and their contributions to cardiac action potential morphology. We show that the β-agonist-induced opening of caveolae may have substantial impacts on peak overshoot, maximum upstroke velocity, and ultimately conduction velocity. Additionally, we show that prolonged action potentials and the formation of potentially arrhythmogenic afterdepolarizations, can arise if caveolae open intermittently throughout the action potential. Our simulations suggest that caveolar sodium current may constitute a route, which is independent of channelopathies, to delayed repolarization and the arrhythmias associated with such delays.

Ranolazine delays Ca2+‐induced mitochondrial permeability transition pore opening and membrane potential depolarization in guinea pig heart mitochondria

Ranolazine (RAN) is an antianginal drug that may block a late Na+ current in cardiac cells during ischemia. We showed previously that RAN improved cardiac function and reduced ROS emission during ischemia and reperfusion (IR). Here we tested if these effects of RAN are associated with delay in opening of the mitochondrial (m) permeability transition pore (PTP) and membrane potential (ΔΨm) depolarization induced by incrementally adding buffer CaCl2 to mitochondrial suspension. Guinea pig hearts were perfused with RAN (5 μM) for 1 min followed by 30 min ischemia and 120 min reperfusion to assess m[Ca2] and infarct size (IS) in intact hearts, or reperfused for only 10 min to isolate mitochondria. ΔΨm and matrix Ca2+ uptake were measured using fluorescence spectrophotometry with rhodamine 123 and indo‐1. We found that m[Ca2+] increased less during early ischemia in RAN (296±25 nM) vs control hearts (436±34 nM), and m[Ca2+] was less in RAN treated (280±39 nM) vs control hearts (429±45 nM) during early reperfusion. RAN reduced IS (24±3%) vs control (37±3%). More buffer CaCl2 (20–25%) was required to open PTP and to induce full ΔΨm depolarization in RAN vs control hearts. Reduced ROS emission during ischemia may delay cytosolic Na+, Ca2+ loading, and consequently mCa2+ loading. The delay in PTP opening by Ca2+ in RAN treated hearts could result from improved mitochondrial bioenergetics with increased capacity to extrude Ca2+.This work was supported in part by the American Heart Association [0355608Z, 0855940G to D.F. Stowe]; the National Institutes of Health [K01 HL73246 to A.K.S. Camara, R01 HL089514 to D.F. Stowe]

Loss of Intracellular and Intercellular Synchrony of Calcium Release in Systolic Heart Failure

Normal contraction of cardiac muscle requires a coordinated cellular mechanical response to the electric pacemaker signal that begins in the sinoatrial node and spreads through the conduction system and ultimately across the ventricular myocardium via gap junctions. In ventricular myocytes, depolarization by the action potential causes clusters of L-type calcium channels (LCCs) to open, allowing a small amount of Ca to enter the diadic cleft space between the sarcolemma and the sarcoplasmic reticulum (SR). When the Ca concentration in this space becomes sufficiently high to gate ryanodine receptors (RyRs), they open and release SR Ca into the cytoplasm, so that it can bind to and activate myofilaments, resulting in force development. Each cluster of LCCs on the sarcolemma, its opposing cluster of RyRs across the 10- to 12-nm diameter diadic cleft space, and associated SR regulatory proteins is a couplon.1 According to the local control theory of cardiac excitation-contraction (EC) coupling, couplons are activated independently. Independent activation of couplons depends on the extent of LCC activation, providing a mechanism for regulation of contractile force by stochastic recruitment of couplons.2 In systolic heart failure, numerous cellular defects in EC coupling and Ca regulation have been identified that contribute to contractile dysfunction (and ventricular arrhythmias), although there is a host of other major abnormalities leading to loss of force development, including fibrosis,3 defective energy metabolism,4 pH changes,5 heterogeneous conductance across connexin hemichannels,6 and defects in sarcomeric proteins.7 Thus, it is important to keep in mind that no single defect can explain the contractile dysfunction of systolic failure. Article see p 223 Recent studies of defective EC coupling in heart failure have addressed not only reductions in the extent and kinetics of Ca released in response to the action potential (ie, reduced recruitment of couplons and …

L-type Ca2+ channels in the heart: Structure and regulation

This review analyzes the structure and regulation mechanisms of voltage-dependent L-type Ca(2+) channel in the heart. L-type Ca(2+) channels in the heart are composed of four different polypeptide subunits, and the pore-forming subunit alpha(1) is the most important part of the channel. In cardiac myocytes, Ca(2+) enter cell cytoplasm from extracellular space mainly through L-type Ca(2+) channels; these channels are very important system in heart Ca(2+) uptake regulation. L-type Ca(2+) channels are responsible for the activation of sarcoplasmic reticulum channels (RyR2) and force of muscle contraction generation in heart; hence, activity of the heart depends on L-type Ca(2+) channels. Phosphorylation of channel-forming subunits by different kinases is one of the most important ways to change the activity of L-type Ca(2+) channel. Additionally, the activity of L-type Ca(2+) channels depends on Ca(2+) concentration in cytoplasm. Ca(2+) current in cardiac cells can facilitate, and this process is regulated by phosphorylation of L-type Ca(2+) channels and intracellular Ca(2+) concentration. Disturbances in cellular Ca(2+) transport and regulation of L-type Ca(2+) channels are directly related to heart diseases, life quality, and life span.

Late sodium current in the pathophysiology of cardiovascular disease: consequences of sodium–calcium overload

Late sodium current in cardiac cells is very small compared with the fast component, but as it flows throughout the action potential it may make a substantial contribution to sodium...

Molecular Identification and Functional Roles of a Ca2+-activated K+ Channel in Human and Mouse Hearts

The repolarization phase of cardiac action potential is prone to aberrant excitation that is common in cardiac patients. Here, we demonstrate that this phase is markedly sensitive to Ca2+ because of the surprising existence of a Ca2+-activated K+ currents in cardiac cells. The current was revealed using recording conditions that preserved endogenous Ca2+ buffers. The Ca2+-activated K+ current is expressed differentially in atria compared with ventricles. Because of the significant contribution of the current toward membrane repolarization in cardiac myocytes, alterations of the current magnitude precipitate abnormal action potential profiles. We confirmed the presence of a small conductance Ca2+-activated K+ channel subtype (SK2) in human and mouse cardiac myocytes using Western blot analysis and reverse transcription-polymerase chain reaction and have cloned SK2 channels from human atria, mouse atria, and ventricles. Because of the marked differential expression of SK2 channels in the heart, specific ligands for Ca2+-activated K+ currents may offer a unique therapeutic opportunity to modify atrial cells without interfering with ventricular myocytes.

Alosetron and the rapid component of delayed rectifying potassium current in cardiac cells

Some drugs acting on 5-hydroxytryptamine receptors inhibit the rapid component of delayed rectifying potassium currents (I Kr) in cardiac muscle cells. This is associated with lengthening of the QT interval in the cardiac cycle and can lead to fatal arrhythmias. We investigated whether alosetron, a novel 5HT 3 antagonist proposed for treatment of irritable bowel syndrome (IBS), blocks I Kr in guinea pig cardiac myocytes. I Kr was isolated under whole-cell voltage clamp, and was identified by its sensitivity to the selective I Kr antagonist E4031. Cisapride (10 −6 M) inhibited the E4031-sensitive current while alosetron (10 −10 − 10 −6 M) had no effect on I Kr. We also found that alosetron did not inhibit I Ks. Therefore, use of alosetron for treatment of IBS should not be confounded by long QT syndrome.

Azimilide dihydrochloride: a new class III anti-arrhythmic agent

Azimilide dihydrochloride (Stedicor™) is a new class III anti-arrhythmic agent that is being developed by Proctor & Gamble to treat supraventricular and ventricular arrhythmias. Development of this agent is being undertaken due to the high prevalence of atrial fibrillation and the lack of satisfactory therapy for this arrhythmia, along with the desire to develop therapy to reduce the risk of life-threatening ventricular arrhythmias in patients following myocardial infarction. The mechanism of action of azimilide is to block both the slowly conducting (IKs) and rapidly conducting (IKr) rectifier potassium currents in cardiac cells. This differs from other class III agents that block IKr exclusively or in combination with sodium, calcium, or transient outward (Ito) potassium current channels. Azimilide is distinguished by a relative lack of reverse use-dependence, excellent oral absorption, no need for dose titration, an option for out-patient initiation, no need for adjustment associated with renal or liver failure and a lack of interaction with warfarin or digoxin. It carries some risk of torsade de pointes and rarely, neutropoenia. Azimilide has shown dose-related efficacy in prolonging the time to recurrence of atrial fibrillation. A large trial examining the impact of azimilide on mortality in high-risk patients following myocardial infarction has completed enrolment and should yield data in the next couple of years and further studies are planned. Even if this trial fails to show a survival benefit, a neutral effect on mortality will make the agent attractive for atrial arrhythmias.

Choline modulates cardiac membrane repolarization by activating an M3 muscarinic receptor and its coupled K+ channel.

Choline is a necessary substrate of the lipid membrane and for acetylcholine synthesis. Accumulating evidence indicates that besides being a structural component, choline is also a functional modulator of the membrane. It has been shown to be a muscarinic acetylcholine receptor (mAChR) agonist and can induce a novel K+ current in cardiac cells. However, the potential role of choline in modulating cardiac functions remained unstudied despite that mAChRs are known to be important in regulating heart functions. With microelectrode techniques, we found that choline produced concentration-dependent (0.1 approximately 10 mm) decreases in sinus rhythm and action potential duration in isolated guinea pig atria. The effects were reversed by 2 nm 4DAMP (an M3-selective antagonist). Whole-cell patch-clamp recordings in dispersed myocytes from guinea pig and canine atria revealed that choline is able to induce a K+ current with delayed rectifying properties. The choline-induced current was suppressed by low concentrations of 4DAMP (2 approximately 10 nm). Antagonists toward other subtypes (M1, M2 or M4) all failed to alter the current. The affinity of choline (Kd) at mAChRs derived from displacement binding of [3H]-NMS in the homogenates from dog atria was 0.9 mm, consistent with the concentration needed for the current induction and for the HR and APD modulation. Our data indicate that choline modulates the cellular electrical properties of the hearts, likely by activating a K+ current via stimulation of M3 receptors.

Effect of isoproterenol on the L-type Ca2+ current in cardiac cells from rats and hybernating ground squirrels.

The perforated patch clamp method was used to study the effect of the agonist of beta-adrenoreceptors isoproterenol on L-type Ca2+ current in cardiocytes of rats and ground squirrels in two states: active and hibernating. It is shown that isoproterenol exerts a dual effect on Ca2+ currents of rats and ground squirrels in the active state: at Vh = -50 mV, the current increases, whereas at Vh = -30 mV, it decreases. In hibernating ground squirrels, the dual effect of isoproterenol is not observed: isoproterenol increases Ca2+ current at any Vh values. The hypothesis is put forward that, during the entrance of ground squirrels into hibernation, the phosphorylation of one of the sites (not cAMP-dependent) of L-type Ca2+ channels is blocked.

Embryonic lethality and abnormal cardiac myocytes in mice lacking ryanodine receptor type 2.

The ryanodine receptor type 2 (RyR-2) functions as a Ca2+-induced Ca2+ release (CICR) channel on intracellular Ca2+ stores and is distributed in most excitable cells with the exception of skeletal muscle cells. RyR-2 is abundantly expressed in cardiac muscle cells and is thought to mediate Ca2+ release triggered by Ca2+ influx through the voltage-gated Ca2+ channel to constitute the cardiac type of excitation-contraction (E-C) coupling. Here we report on mutant mice lacking RyR-2. The mutant mice died at approximately embryonic day (E) 10 with morphological abnormalities in the heart tube. Prior to embryonic death, large vacuolate sarcoplasmic reticulum (SR) and structurally abnormal mitochondria began to develop in the mutant cardiac myocytes, and the vacuolate SR appeared to contain high concentrations of Ca2+. Fluorometric Ca2+ measurements showed that a Ca2+ transient evoked by caffeine, an activator of RyRs, was abolished in the mutant cardiac myocytes. However, both mutant and control hearts showed spontaneous rhythmic contractions at E9.5. Moreover, treatment with ryanodine, which locks RyR channels in their open state, did not exert a major effect on spontaneous Ca2+ transients in control cardiac myocytes at E9.5-11.5. These results suggest no essential contribution of the RyR-2 to E-C coupling in cardiac myocytes during early embryonic stages. Our results from the mutant mice indicate that the major role of RyR-2 is not in E-C coupling as the CICR channel in embryonic cardiac myocytes but it is absolutely required for cellular Ca2+ homeostasis most probably as a major Ca2+ leak channel to maintain the developing SR.

A Conservation Principle and its Effect on the Formulation of Na–Ca Exchanger Current in Cardiac Cells

In this paper we show the presence of a latent conservation principle in the formulation of ionic currents in cardiac cells and examine its effect on a formulation of the sodium–calcium exchange current appearing in the Noble model of the sinoatrial node cell in the mammalian heart [see Nobleet al. (1989) or Winslowet al. (1991)]. Our objective is to show that this formulation, if not corrected, will result in a serious instability in this cardiac cell model. In particular, under certain initial conditions, the solutions of the model equations will blow-up in finite time. We also propose a correction to the model equation of the sodium–calcium exchange current, and we show that the modified model agrees favorably with the original model. These phenomena also occur in the other cardiac cell models, such as those modeling the Purkinje fiber, the atrial cell and the ventricular cell. The changes proposed in this paper can be applied directly to these models as well.

Current-voltage relations and steady-state characteristics of Na+-Ca2+ exchange: characterization of the eight-state consecutive transport model

An analytical expression for Na+-Ca2+ exchange currents in cardiac cells has been obtained for an eight-state model. The equation obtained has been used to derive theoretical expressions for current-voltage relationships, maximum Na+-Ca2+ exchange currents, and half-saturating concentrations for Na+ and Ca2+. These equations were analyzed over a wide range of cytoplasmic and extracellular Na+ and Ca2+ concentrations, under forward and reverse "zero-trans" conditions. Correspondence of theoretical results with those obtained from giant excised patch experiments are presented. Rate constants from published reports were used to evaluate turnover rates for Na+-Ca2+ exchange in the forward and reverse directions. A factor, epsilon, is introduced that permits prediction of the extent to which the Na+-Ca2+ exchange cycle is under voltage or diffusion control. This factor can be conveniently used for data interpretation and comparison. The derived equations also provide a foundation for continuing experimental evaluation of the fidelity of this model.

Effects of class III antiarrhythmic drugs on the Na(+)-activated K+ channels in guinea-pig ventricular cells.

1. Class III antiarrhythmic drugs are known to block the outward currents through voltage-gated K+ channels. However, effects of class III antiarrhythmic drugs on the ligand-gated K+ channels have not been thoroughly examined. In this study effects of amiodarone and newer class III antiarrhythmic drugs, E-4031 and MS-551, on the Na+-activated K+ (KNa) current were examined in inside-out membrane patches and in whole cells isolated from guinea-pig ventricle. 2. The KNa channel current was activated by increasing [Na+]i from 0 mM to 30-100 mM with 150 mM [K+]o in inside-out membrane patches of ventricular myocytes. The channel current showed a larger slope conductance (210 pS), inward-going rectification and subconductance levels of various amplitudes. 3. E-4031 and MS-551 at high concentrations (300 microM) inhibited the K+ current by decreasing the open time (flickering block). On the other hand, amiodarone at relatively low concentrations (0.1-10 microM) inhibited the KNa channel current by decreasing the open probability rather than by decreasing the open time. The IC50 value of amiodarone for inhibiting the KNa channel current was 1.0 microM. 4. These drugs also inhibited the whole-cell outward current activated by intracellular loading of 50 mM [Na+]i and extracellular application of 10 microM ouabain. 5. These results indicate that class III antiarrhythmic drugs inhibit the KNa channel current in cardiac cells. However, there are sharp differences in the effective concentrations and the mode of inhibition between amiodarone and the newer class III antiarrhythmic drugs.

Local calcium transients triggered by single L-type calcium channel currents in cardiac cells

Excitation-contraction coupling was studied in mammalian cardiac cells in which the opening probability of L-type calcium (Ca2+) channels was reduced. Confocal microscopy during voltage-clamp depolarization revealed distinct local transients in the concentration of intracellular calcium ions ([Ca2+]i). When voltage was varied, the latency to occurrence and the relative probability of occurrence of local [Ca2+]i transients varied as predicted if Ca2+ release from the sarcoplasmic reticulum (SR) was linked tightly to Ca2+ flux through L-type Ca2+ channels but not to that through the Na-Ca exchanger or to average [Ca2+]i. Voltage had no effect on the amplitude of local [Ca2+]i transients. Thus, the most efficacious "Ca2+ signal" for activating Ca2+ release from the SR may be a transient microdomain of high [Ca2+]i beneath an individual, open L-type Ca2+ channel.