Dynamics of the background outward current of single guinea pig ventricular myocytes. Ionic mechanisms of hysteresis in cardiac cells

The Effectiveness in Activating M-Type K+ Current Produced by Solifenacin ([(3R)-1-azabicyclo[2.2.2]octan-3-yl] (1S)-1-phenyl-3,4-dihydro-1H-isoquinoline-2-carboxylate): Independent of Its Antimuscarinic Action

Solifenacin (Vesicare®, SOL), known to be a member of isoquinolines, is a muscarinic antagonist that has anticholinergic effect, and it has been beneficial in treating urinary incontinence and neurogenic detrusor overactivity. However, the information regarding the effects of SOL on membrane ionic currents is largely uncertain, despite its clinically wide use in patients with those disorders. In this study, the whole-cell current recordings revealed that upon membrane depolarization in pituitary GH3 cells, the exposure to SOL concentration-dependently increased the amplitude of M-type K+ current (IK(M)) with effective EC50 value of 0.34 μM. The activation time constant of IK(M) was concurrently shortened in the SOL presence, hence yielding the KD value of 0.55 μM based on minimal reaction scheme. As cells were exposed to SOL, the steady-state activation curve of IK(M) was shifted along the voltage axis to the left with no change in the gating charge of the current. Upon an isosceles-triangular ramp pulse, the hysteretic area of IK(M) was increased by adding SOL. As cells were continually exposed to SOL, further application of acetylcholine (1 μM) failed to modify SOL-stimulated IK(M); however, subsequent addition of thyrotropin releasing hormone (TRH, 1 μM) was able to counteract SOL-induced increase in IK(M) amplitude. In cell-attached single-channel current recordings, bath addition of SOL led to an increase in the activity of M-type K+ (KM) channels with no change in the single channel conductance; the mean open time of the channel became lengthened. In whole-cell current-clamp recordings, the SOL application reduced the firing of action potentials (APs) in GH3 cells; however, either subsequent addition of TRH or linopirdine was able to reverse SOL-mediated decrease in AP firing. In hippocampal mHippoE-14 neurons, the IK(M) was also stimulated by adding SOL. Altogether, findings from this study disclosed for the first time the effectiveness of SOL in interacting with KM channels and hence in stimulating IK(M) in electrically excitable cells, and this noticeable action appears to be independent of its antagonistic activity on the canonical binding to muscarinic receptors expressed in GH3 or mHippoE-14 cells.

Effective Perturbations of the Amplitude, Gating, and Hysteresis of IK(DR) Caused by PT-2385, an HIF-2α Inhibitor

PT-2385 is currently regarded as a potent and selective inhibitor of hypoxia-inducible factor-2α (HIF-2α), with potential antineoplastic activity. However, the membrane ion channels changed by this compound are obscure, although it is reasonable to assume that the compound might act on surface membrane before entering the cell´s interior. In this study, we intended to explore whether it and related compounds make any adjustments to the plasmalemmal ionic currents of pituitary tumor (GH₃) cells and human 13-06-MG glioma cells. Cell exposure to PT-2385 suppressed the peak or late amplitude of delayed-rectifier K⁺ current (I_K(DR)) in a time- and concentration-dependent manner, with IC₅₀ values of 8.1 or 2.2 µM, respectively, while the K_D value in PT-2385-induced shortening in the slow component of I_K(DR) inactivation was estimated to be 2.9 µM. The PT-2385-mediated block of I_K(DR) in GH₃ cells was little-affected by the further application of diazoxide, cilostazol, or sorafenib. Increasing PT-2385 concentrations shifted the steady-state inactivation curve of I_K(DR) towards a more hyperpolarized potential, with no change in the gating charge of the current, and also prolonged the time-dependent recovery of the I_K(DR) block. The hysteretic strength of I_K(DR) elicited by upright or inverted isosceles-triangular ramp voltage was decreased during exposure to PT-2385; meanwhile, the activation energy involved in the gating of I_K(DR) elicitation was noticeably raised in its presence. Alternatively, the presence of PT-2385 in human 13-06-MG glioma cells effectively decreased the amplitude of I_K(DR). Considering all of the experimental results together, the effects of PT-2385 on ionic currents demonstrated herein could be non-canonical and tend to be upstream of the inhibition of HIF-2α. This action therefore probably contributes to down-streaming mechanisms through the changes that it or other structurally resemblant compounds lead to in the perturbations of the functional activities of pituitary cells or neoplastic astrocytes, in the case that in vivo observations occur.

Beat-to-beat ECG restitution: A review and proposal for a new biomarker to assess cardiac stress and ventricular tachyarrhythmia vulnerability

BACKGROUND

Cardiac restitution is the ability of the heart to recover from one beat to the next. Ventricular arrhythmia vulnerability can occur when the heart does not properly adjust to sudden changes in rate or in hemodynamics leading to excessive temporal and/or spatial heterogeneity in conduction or repolarization. Restitution has historically been used to study, by invasive means, the dynamics of the relationship between action potential duration (APD) and diastolic interval (DI) in sedated subjects using various pacing protocols. Even though the analogous measures of APD and DI can be obtained using the surface ECG to acquire the respective QT and TQ intervals for ECG restitution, this methodology has not been widely adopted for a number of reasons.

METHODS

Recent development of more advanced software algorithms enables ECG intervals to be measured accurately, on a continuous beat-to-beat basis, in an automated manner, and under highly dynamic conditions (i.e., ambulatory or exercise) providing information beyond that available in the typical resting state.

RESULTS

Current breakthroughs in ECG technology will allow ECG restitution measures to become a practical approach for providing quantitative measures of the risks for ventricular arrhythmias as well as cardiac stress in general.

CONCLUSIONS

In addition to a review of the underlying principles and caveats of ECG restitution, a new approach toward an advancement of more integrated restitution biomarkers is proposed.

The Effects of Puerarin on Rat Ventricular Myocytes and the Potential Mechanism

Puerarin, a known isoflavone, is commonly found as a Chinese herb medicine. It is widely used in China to treat cardiac diseases such as angina, cardiac infarction and arrhythmia. However, its cardioprotective mechanism remains unclear. In this study, puerarin significantly prolonged ventricular action potential duration (APD) with a dosage dependent manner in the micromolar range on isolated rat ventricular myocytes. However, submicromolar puerarin had no effect on resting membrane potential (RMP), action potential amplitude (APA) and maximal velocity of depolarization (Vmax) of action potential. Only above the concentration of 10 mM, puerarin exhibited more aggressive effect on action potential, and shifted RMP to the positive direction. Millimolar concentrations of puerarin significantly inhibited inward rectified K⁺ channels in a dosage dependent manner, and exhibited bigger effects upon Kir2.1 vs Kir2.3 in transfected HEK293 cells. As low as micromolar range concentrations of puerarin significantly inhibited Kv7.1 and IKs. These inhibitory effects may due to the direct inhibition of puerarin upon channels not via the PKA-dependent pathway. These results provided direct preclinical evidence that puerarin prolonged APD via its inhibitory effect upon Kv7.1 and IKs, contributing to a better understanding the mechanism of puerarin cardioprotection in the treatment of cardiovascular diseases.

Driving force-dependent block by internal Ba(2+) on the Kir2.1 channel: Mechanistic insight into inward rectification

The Kir2.1 channel is characterized by strong inward rectification; however, the mechanism of the steep voltage dependence near the equilibrium potential remains to be investigated. Here, we studied the internal Ba(2+) block of the Kir2.1 channel expressed in Xenopus oocytes. We showed that the driving force and thus the K(+) ion flux significantly influenced the apparent affinity of the block by internal Ba(2+). Kinetic analysis revealed that the binding rate shifted with the driving force and changed steeply near the equilibrium point, either in the presence or absence of the transmembrane electrical field. The unbinding rate was determined by the intrinsic affinity of the site. Mutagenesis studies revealed that the high-affinity binding site for Ba(2+) was located near T141 at the internal entrance of the selectivity filter. The steep change of the blocking affinity near the equilibrium potential may result from the flux-coupling effect in the single-file, multi-ion cytoplasmic pore.

Effects of pacing magnitudes and forms on bistability width in a modeled ventricular tissue

Bistability in periodically paced cardiac tissue is relevant to cardiac arrhythmias and its control. In the present paper, one-dimensional tissue of the phase I Luo-Rudy model is numerically investigated. The effects of various parameters of pacing signals on bistability width are studied. The following conclusions are obtained: (i) Pacing can be classified into two types: pulsatile and sinusoidal types. Pulsatile pacing reduces bistability width as its magnitude is increased. Sinusoidal pacing increases the width as its amplitude is increased. (ii) In a pacing period the hyperpolarizing part plays a more important role than the depolarizing part. Variations of the hyperpolarizing ratio in a period evidently change the width of bistability and its variation tendency. (iii) A dynamical mechanism is proposed to qualitatively explain the phenomena, which reveals the reason for the different effects of pulsatile and sinusoidal pacing on bistability. The methods for changing bistability width by external pacing may help control arrhythmias in cardiology.

Ionic mechanism of shock-induced arrhythmias: role of intracellular calcium

BACKGROUND

Strong electrical shocks can cause focal arrhythmias, the mechanism of which is not well known. Strong shocks have been shown to produce diastolic Ca(i)(2+) increase, which may initiate focal arrhythmias via spontaneous Ca(i)(2+) rise (SCR), activation of inward Na(+)/Ca(2+) exchange current (I(NCX)), and rise in membrane potential (V(m)). It can be hypothesized that this mechanism is responsible for generation of shock-induced arrhythmias.

OBJECTIVE

The purpose of this study was to examine the roles of SCRs and I(NCX) in shock-induced arrhythmias.

METHODS

The occurrence of SCRs during shock-induced arrhythmias was assessed in neonatal rat myocyte cultures.

RESULTS

Simultaneous V(m)-Ca(i)(2+) optical mapping at arrhythmia source demonstrated that V(m) upstrokes always preceded Ca(i)(2+) transients, and V(m)-Ca(i)(2+) delays were not different between arrhythmic and paced beats (5.5 ± 0.9 and 5.7 ± 0.4 ms, respectively, P = .5). Shocks caused gradual rise of diastolic Ca(i)(2+) consistent with membrane electroporation but no significant Ca(i)(2+) rises immediately before V(m) upstrokes. Application of the Ca(i)(2+) chelator BAPTA-AM (10 μmol/L) decreased the duration of shock-induced arrhythmias whereas application of the I(NCX) inhibitor KB-R7943 (2 μmol/L) increased it, indicating that, despite the absence of SCRs, changes in Ca(i)(2+) affected arrhythmias. It is hypothesized that this effect is mediated by Ca(i)(2+) inhibition of outward I(K1) current and destabilization of resting V(m). The possible role of I(K1) was supported by application of the I(K1) inhibitor BaCl(2) (0.2 mmol/L), which increased the arrhythmia duration.

CONCLUSION

Shock-induced arrhythmias in neonatal rat myocyte monolayers are not caused by SCRs and inward I(NCX). However, these arrhythmias depend on Ca(i)(2+) changes, possibly via Ca(i)(2+)-dependent modulation of outward I(K1) current.

Ventricular fibrillation: dynamics and ion channel determinants

Ventricular fibrillation (VF) is the leading cause of sudden cardiac death. This brief review addresses issues relevant to the dynamics of the rotors responsible for functional reentry and VF. It also makes an attempt to summarize present-day knowledge of the manner in which the dynamic interplay between inward and outward transmembrane currents and the heterogeneous cardiac structure establish a substrate for the initiation and maintenance of rotors and VF. The fragmentary nature of our current understanding of ionic VF mechanisms does not even allow an approach toward a "Theory of VF". Yet some hope is provided by recently obtained insight into the roles played in VF by some of the sarcolemmal ion channels that control the excitation-recovery process. For example, strong evidence supports the idea that the interplay between the rapid-inward sodium current and the inward-rectifier potassium current controls rotor formation, as well as rotor stability and frequency. Solid evidence also exists for an involvement of L-type calcium current in the control of rotor frequency and in determining VF-to-ventricular tachycardia conversion. Less clear, however, is whether or not time dependent outward currents through voltage-gated potassium channels affect the fibrillatory process. Hopefully, taking advantage of currently available approaches of structural, molecular and cellular biology, together with computational and imaging techniques, will afford us the opportunity to further advance knowledge on VF mechanisms.

Hysteresis and bistability in periodically paced cardiac tissue

Hysteresis in periodically paced cardiac tissue is an important issue due to its relevance to cardiac arrhythmias. In the present paper, the mechanism of hysteresis formation and the related properties are interpreted by numerically investigating the phase I Luo-Rudy model. A formula calculating the width of hysteresis is proposed and well confirmed by numerical simulations. We also find that hysteresis in cardiac tissue shows several characteristics due to couplings among cardiac cells which are absent in a single cell. The influences of the physiological parameters are studied in detail. The model dependence of hysteresis is elucidated by considering a number of well-known models of excitable media. Moreover, the influence of bistability on controlling arrhythmias is revealed.

The short QT syndrome as a paradigm to understand the role of potassium channels in ventricular fibrillation

The recently discovered hereditary channelopathy, the Short QT Syndrome (SQTS), is an important advance in clinical and molecular cardiology that has opened new doors for investigating the manner in which alterations in excitability and action potential morphology may facilitate the occurrence of ventricular fibrillation. In this brief review we address the molecular and genetic features of SQTS in which specific mutations in one of three different potassium channels involved in cardiac repolarization substantially increase the risk of life-threatening tachyarrhythmias. We then summarize new knowledge on the mechanism of wavebreak, which is the hallmark of reentry initiation, and on the role of potassium channels in the ionic mechanisms underlying cardiac excitation and its frequency dependence. The article argues for a detailed understanding of the ionic mechanisms that promote wavebreaks and stable rotors as an essential tool for successful anti-arrhythmic therapy in SQTS and other diseases leading to sudden cardiac death.

The inward rectifier current (IK1) controls cardiac excitability and is involved in arrhythmogenesis

The cardiac inwardly rectifying potassium current (I(K1)) stabilizes the resting membrane potential and is responsible for shaping the initial depolarization and final repolarization of the action potential. The inwardly rectifying potassium channel (Kir2.x) subfamily members primarily mediate cardiac I(K1), but other inward rectifiers, including the acetylcholine-sensitive (Kir3.x) and ATP-sensitive (Kir6.x) inward rectifiers, also may modulate cardiac excitability. Studies suggest I(K1) plays a role in ventricular arrhythmias, highlighted by the recently described Andersen's syndrome and studies in the guinea pig heart model of ventricular fibrillation. This article describes the salient properties of cardiac I(K1) and discusses the role of this current in the cardiac action potential and in underlying regional differences in cardiac excitability. The mechanism of channel block, assembly, and structure are reviewed. The article discusses the role of I(K1) in ventricular fibrillation and speculates on modulation of I(K1) as a preventative antiarrhythmic mechanism.

The Electrical Restitution Curve Revisited:

The electrical restitution curve (ERC) traditionally describes the recovery of action potential duration (APD) as a function of the interbeat interval or, more correctly, the diastolic interval (DI). Often overlooked in modeling studies, the normal ventricular ERC is triphasic, starting with a steep initial recovery at the shortest DIs, a transient decline, and a final asymptotic rise to a plateau phase reached at long DIs. Recent studies have proposed that it would be advantageous to lower the slope of the ERC by drug intervention, as this might reduce the potential for electrical alternans and ventricular fibrillation. This review discusses the pros and cons of a flat versus steep slope of the ERC and draws attention to mechanisms thatjustify the (physiologically) steep slope, rather than a flat slope, as a better design against arrhythmias. Five potential mechanisms are discussed, which allows for a different interpretation of the effect of the slope on arrhythmogenicity. The most important appears to be the physiologic rate adaptive shortening of APD that, by reciprocal lengthening of the DI, allows the subsequent APD to move more quickly from the steep initial ERC phase onto the flat phase. A less steep initial ERC phase would protract the transition toward more fully recovered APD and, in fact, may perpetuate electrical alternans. The triphasic ERC time course in normal myocardium cannot be explained by or fitted to single exponentials or single ion channel recovery kinetics. A simple tri-ionic model is suggested that may help explain the shape of the ERC at various repolarization levels and place APD recovery into perspective with intracellular calcium recycling and recovery of contractile force.

Signature currents: a patch-clamp method for determining the selectivity of ion-channel blockers in isolated cardiac myocytes

BACKGROUND

We describe a simple method using membrane potential ramps for rapidly determining the ion-channel selectivity of drugs that affect action-potential duration in isolated cardiac myocytes. The method allows the simultaneous assay of compounds on a number of ionic currents in a single cardiac cell.

METHODS

Trains of membrane potential ramps were applied from -90 to +70 mV at 0.33 Hz to obtain a consistent "signature current," in which the major individual currents involved in the cardiac action potential could be easily identified. Confirmatory experiments were performed using known inhibitors of these currents.

RESULTS

The identities of the currents in the signature were established by varying the concentrations of extracellular cations and by adding known ion channel blockers to superfusion solutions. Inhibition of each current had a characteristic and reproducible effect on the overall signature current.

CONCLUSIONS

The consistent current signature in the presence and absence of blockers suggests that this method could be used for tertiary electrophysiological evaluation of compounds, eg, in a drug discovery program focusing on antiarrhythmic agents. The ability to assay for secondary effects of novel compounds against multiple currents in the target cell type is convenient and avoids the artefacts associated with using artificial expression systems.

Kinetics of rate-dependent shortening of action potential duration in guinea-pig ventricle; effects of IK1 and IKr blockade

1. The kinetics of shortening of action potential duration (APD) following an increase in pacing rate, from 2 to 3.3 Hz, was characterized in guinea-pig ventricular preparations. Terikalant (RP62719), an inhibitor of the inwardly rectifying K+ current (IK1), and dofetilide, a specific inhibitor of the rapidly activating delayed-rectifier current (IKr), were applied to determine the effect of inhibition of these ion currents on slow APD shortening. 2. Action potentials were recorded from isolated guinea-pig ventricular myocytes using the perforated-patch patch-clamp technique, and monophasic action potentials were recorded from Langendorff-perfused guinea-pig ventricles using a contact epicardial probe. 3. Under control conditions, after an increase in pacing rate, APD immediately decreased, and then shortened slowly with an exponential time course. In ventricular myocytes, the time constant of this exponential shortening was 28+/-4 s and the amount of slow shortening was 21.9+/-0.9 ms (n=8) for an increase in rate from 2 to 3.3 Hz. Similar values were observed in Langendorff-perfused ventricles. 4. Terikalant dose-dependently increased APD and the increase was enhanced by rapid pacing ('positive' rate-dependence). The drug dose-dependently decreased the time constant of shortening and amount of slow APD shortening. In contrast, dofetilide, an inhibitor of IKr, which shows 'reverse' rate-dependent APD widening, had no significant effect on the time constant or amount of slow shortening. 5. These observations suggest that IK1 plays a role in rate-dependent shortening of APD. The results appear to support the hypothesis that 'reverse' rate-dependent effects of IKr blockers are due to these drugs not affecting the ion current(s) mediating intrinsic rate-dependent slow shortening of APD.

Transmembrane ICa contributes to rate-dependent changes of action potentials in human ventricular myocytes

The mechanism of action potential abbreviation caused by increasing rate in human ventricular myocytes is unknown. The present study was designed to determine the potential role of Ca2+ current (ICa) in the rate-dependent changes in action potential duration (APD) in human ventricular cells. Myocytes isolated from the right ventricle of explanted human hearts were studied at 36 degreesC with whole cell voltage and current-clamp techniques. APD at 90% repolarization decreased by 36 +/- 4% when frequency increased from 0.5 to 2 Hz. Equimolar substitution of Mg2+ for Ca2+ significantly decreased rate-dependent changes in APD (to 6 +/- 3%, P < 0.01). Peak ICa was decreased by 34 +/- 3% from 0.5 to 2 Hz (P < 0.01), and ICa had recovery time constants of 65 +/- 12 and 683 +/- 39 ms at -80 mV. Action potential clamp demonstrated a decreasing contribution of ICa during the action potential as rate increased. The rate-dependent slow component of the delayed rectifier K+ current (IKs) was not observed in four cells with an increase in frequency from 0.5 to 3.3 Hz, perhaps because the IKs is so small that the increase at a high rate could not be seen. These results suggest that reduction of Ca2+ influx during the action potential accounts for most of the rate-dependent abbreviation of human ventricular APD.

Spiral waves in two-dimensional models of ventricular muscle: formation of a stationary core

Previous experimental studies have clearly demonstrated the existence of drifting and stationary electrical spiral waves in cardiac muscle and their involvement in cardiac arrhythmias. Here we present results of a study of reentrant excitation in computer simulations based on a membrane model of the ventricular cell. We have explored in detail the parameter space of the model, using tools derived from previous numerical studies in excitation-dynamics models. We have found appropriate parametric conditions for sustained stable spiral wave dynamics (1 s of activity or approximately 10 rotations) in simulations of an anisotropic (ratio in velocity 4:1) cardiac sheet of 2 cm x 2 cm. Initially, we used a model that reproduced well the characteristics of planar electrical waves exhibited by thin sheets of sheep ventricular epicardial muscle during rapid pacing at a cycle length of 300 ms. Under these conditions, the refractory period was 147 ms; the action potential duration (APD) was 120 ms; the propagation velocity along fibers was 33 cm/s; and the wavelength along fibers was 4.85 cm. Using cross-field stimulation in this model, we obtained a stable self-sustaining spiral wave rotating around an unexcited core of 1.75 mm x 7 mm at a period of 115 ms, which reproduced well the experimental results. Thus the data demonstrate that stable spiral wave activity can occur in small cardiac sheets whose wavelength during planar wave excitation in the longitudinal direction is larger than the size of the sheet. Analysis of the mechanism of this observation demonstrates that, during rotating activity, the core exerts a strong electrotonic influence that effectively abbreviates APD (and thus wavelength) in its immediate surroundings and is responsible for the stabilization and perpetuation of the activity. We conclude that appropriate adjustments in the kinetics of the activation front (i.e., threshold for activation and upstroke velocity of the initiating beat) of currently available models of the cardiac cell allow accurate reproduction of experimentally observed self-sustaining spiral wave activity. As such, the results set the stage for an understanding of functional reentry in terms of ionic mechanisms.

Flecainide and the electrophysiologic matrix: the effects of flecainide acetate on the determinants of cardiac excitability in sheep Purkinje fibers

Flecainide was associated with excess mortality distributed virtually equally throughout the period of the Cardiac Arrhythmia Suppression Trial, suggesting the intersection of two events, drug effect and perhaps ischemia. Flecainide's effect on active properties has been studied extensively, but nothing is known of its effects on passive properties or on the balance among active and passive cellular properties that determines cardiac excitability. The multiple microelectrode method of intracellular current application and transmembrane voltage recording was used in sheep Purkinje fibers to determines strength- and charge-duration as well as constant current-voltage relationships and to estimate active properties, liminal length, and cable properties at a normal [K+]o and in a setting of hyperkalemia analogous to that of ischemia. A computer tracked in time the alterations in the active and passive properties relevant to excitability. Flecainide slightly decreased excitability at a normal [K+]o, primarily by depressing the sodium system with some contributory effect of passive properties. At high [K+]o, flecainide caused a frequency-dependent decrease in excitability and conduction, the latter best interpreted as a failure of the fiber to attain the liminal length requirements to produce a local action potential due primarily to an effect on sodium conductance. Together, the observations suggest that the action potential is the local phenomenon and that the propagated event is the sequential fulfillment of liminal length requirements. The data were interpreted in terms of the electrophysiologic matrix first proposed in detail in this Journal, which indicated that the electrophysiologic universe moved as a system in response to the drug and a change in [K+]o, the presumed antiarrhythmic and proarrhythmic electrophysiologic matrices for flecainide were quite similar, and the matrical configuration shared characteristics with the matrices of other drugs with known proarrhythmic potential.

beta-Adrenergic modulation of the inwardly rectifying potassium channel in isolated human ventricular myocytes. Alteration in channel response to beta-adrenergic stimulation in failing human hearts

The beta-adrenergic modulation of the inwardly-rectifying K+ channel (IK1) was examined in isolated human ventricular myocytes using patch-clamp techniques. Isoproterenol (ISO) reversibly depolarized the resting membrane potential and prolonged the action potential duration. Under the whole-cell C1- -free condition, ISO applied via the bath solution reversibly inhibited macroscopic IdK1. The reversal potential of the ISO-sensitive current was shifted by approximately 60 mV per 10-fold change in the external K+ concentration and was sensitive to Ba2+. The ISO-induced inhibition of IK1 was mimicked by forskolin and dibutyrl cAMP, and was prevented by including a cAMP-dependent protein kinase (PKA) inhibitor (PKI) in the pipette solution. In single-channel recordings from cell-attached patches, bath applied ISO could suppress IK1 channels by decreasing open state probability. Bath application of the purified catalytic sub-unit of PKA to inside-out patches also inhibited IK1 and the inhibition could be antagonized by alkaline phosphatase. When beta-adrenergic modulation of IK1 was compared between ventricular myocytes isolated from the failing and the nonfailing heart, channel response to ISO and PKA was significantly reduced in myocytes from the failing heart. Although ISO inhibited IK1 in a concentration-dependent fashion in both groups, a half-maximal concentration was greater in failing (0.12 microM) than in nonfailing hearts (0.023 microM). These results suggest that IK1 in human ventricular myocytes can be inhibited by a PKA-mediated phosphorylation and the modulation is significantly reduced in ventricular myocytes from the failing heart compared to the nonfailing heart.

Characterization of an E4031-sensitive potassium current in quiescent AT-1 cells

INTRODUCTION

A cardiac culture cell line (AT-1) recently has been generated from transgenic mice. Initial studies have yielded opposing results as to the nature of the major repolarizing current(s) in these cells. The present study describes the ion selectivity, voltage dependence, and E4031 sensitivity of the major time-dependent outward current present in AT-1 cells. In addition, we have determined whether an outward current with the characteristics we observed could be capable of modulating action potential duration in a frequency-dependent manner (for stimulation cycle lengths between 250 and 1000 msec).

METHODS AND RESULTS

Action potentials and membrane currents were recorded from nonconfluent cultures of quiescent AT-1 cells using the "perforated patch" technique. AT-1 cells showed a round appearance 1 or 2 days after plating. An E4031-insensitive transient outward current seemed to be absent in these cells. The main time-dependent outward current was a rapidly activating and rectifying potassium current with properties similar to those of IKr. Most of the potassium current was sensitive to the benzenesulfonamide E4031 (5 microM). The same concentration of E4031 led to a 38% increase in action potential duration. Action potential parameters were independent of the stimulation cycle length within the range of 250 to 1000 msec, thus suggesting that the membrane currents involved in the action potential of AT-1 cells are completely reset within a diastolic interval of approximately 150 msec.

CONCLUSION

AT-1 cells present a unique electrophysiologic phenotype, which is clearly different from that reported for freshly dissociated adult atrial or ventricular myocytes from other species. AT-1 cells may be a good model to study IKr, since there seems to be minimal contamination by other outward conductances (such as IKs). In addition, the feasibility of culturing AT-1 cells provides us with a system where electrophysiologic experiments on IKr currents could be combined with biochemical or molecular biological studies requiring significant periods of incubation in a cell culture system.

Electrotonic inhibition and active facilitation of excitability in ventricular muscle

INTRODUCTION

The effects of subthreshold electrical pulses on the response to subsequent stimulation have been described previously in experimental animal studies as well as in the human heart. In addition, previous studies in cardiac Purkinje fibers have shown that diastolic excitability may decrease after activity (active inhibition) and, to a lesser extent, following subthreshold responses (electrotonic inhibition). However, such dynamic changes in excitability have not been explored in isolated ventricular muscle, and it is uncertain whether similar phenomena may play any role in the activation patterns associated with propagation abnormalities in the myocardium.

METHODS AND RESULTS

Experiments were performed in isolated sheep Purkinje fibers and papillary muscles, and in enzymatically dissociated guinea pig ventricular myocytes. In all types of preparations introduction of a conditioning subthreshold pulse between two suprathreshold pulses was followed by a transient decay in excitability (electrotonic inhibition). The degree of inhibition was directly related to the amplitude and duration of the conditioning pulse and inversely related to the postconditioning interval. Yet, inhibition could be demonstrated long after (> 1 sec) the end of the conditioning pulse. Electronic inhibition was found at all diastolic intervals and did not depend on the presence of a previous action potential. In Purkinje fibers, conditioning action potentials led to active inhibition of subsequent responses. In contrast, in muscle cells, such action potentials had a facilitating effect (active facilitation). Electrotonic inhibition and active facilitation were observed in both sheep ventricular muscle and guinea pig ventricular myocytes. Accordingly, during repetitive stimulation with pulses of barely threshold intensity, we observed: (1) bistability (i.e., with the same stimulating parameters, stimulus:response patterns were either 1:1 or 1:0, depending on previous history), and (2) abrupt transitions between 1:1 and 1:0 (absence of intermediate Wenckebach-like patterns). Simulations utilizing an ionic model of cardiac myocytes support the hypothesis that electrotonic inhibition in well-polarized ventricular muscle is the result of partial activation of IK following subthreshold pulses. On the other hand, active facilitation may be the result of an activity-induced decrease in the conductance of IK1.

CONCLUSION

Diastolic excitability of well-polarized ventricular myocardium may be transiently depressed following local responses and transiently enhanced following action potentials. On the other hand, diastolic excitability decreases during quiescence. Active facilitation and electrotonic inhibition may have an important role in determining the dynamics of excitation of the myocardium in the presence of propagation abnormalities.