Contribution of I Ks to ventricular repolarization in canine myocytes

The role of the slow delayed rectifier K⁺ current (I _Ks) in cardiac repolarization seems to be largely influenced by the experimental conditions including the species and tissue studied. The aim of this study was to determine the contribution of I _Ks to repolarization in canine ventricular myocytes by measuring the frequency dependent action potential lengthening effect of 10 μM chromanol 293B using sharp microelectrodes. Pretreatment with isoproterenol (2 nM), E-4031 (1 μM), and injection of inward current pulses were applied to modify action potential configuration. Chromanol alone caused moderate but statistically significant lengthening of action potentials at cycle lengths longer than 500 ms. The lengthening effect of chromanol, which was strongly enhanced in the presence of either isoproterenol or E-4031, was proportional to the amplitude of plateau, whereas poor correlation was found with action potential duration. Similar results were obtained when action potential configuration was modified by injection of depolarizing current pulses. Computer simulations revealed that activation of I _Ks is a sharp function of the plateau amplitude within the physiological range, while elongation of repolarization may enhance I _Ks only when it is excessive. It was concluded that the effect of I _Ks on ventricular repolarization critically depends on the level of action potential plateau; however, other factors, like action potential duration, cycle length, or suppression of other K⁺ currents can also influence its contribution.     

Role of external Ca2+ and K+ in gating of cardiac delayed rectifier K+ currents

We sought to determine whether extracellular Ca²⁺ (Ca _e ²⁺ ) and K⁺ (K _e ⁺ ) play essential roles in the normal functioning of cardiac K⁺ channels. Reports by others have shown that removal of Ca _e ²⁺ and K _e ⁺ alters the gating properties of neural delayed rectifier (I _K) and A-type K⁺ currents, resulting in a loss of normal cation selectivity and voltage-dependent gating. We found that removal of Ca _e ²⁺ and K _e ⁺ from the solution bathing guinea pig ventricular myocytes often induced a leak conductance, but did not affect the ionic selectivity or time-dependent activation and deactivation properties of I _K. The effect of [K⁺]_e on the magnitude of the two components of cardiac I _K was also examined. I _K in guinea pig myocytes is comprised of two distinct types of currents: I _Kr (rapidly activating, rectifying) and I _Ks (slowly activating). The differential effect of Ca _e ²⁺ on the two components of I _K (previously shown to shift the voltage dependence of activation of the two currents in opposite directions) was exploited to determine the role of K _e ⁺ on the magnitude of I _Ks and I _Kr. Lowering [K⁺]_e from 4 to 0 mM increased I _Ks, as expected from the change in driving force for K⁺, but decreased I _Kr. The differential effect of [K⁺]_e on the two components of cardiac I _K may explain the reported discrepancies regarding modulation of cardiac I _K conductance by this cation.     

Involvement of tyrosine kinase in the hyposmotic stimulation of I Ks in guinea-pig ventricular myocytes

The objective of this study was to investigate the involvement of tyrosine phosphorylation in the hyposmotic stimulation of cardiac I _Ks, a slowly activating delayed-rectifier K⁺ current that promotes repolarization of the action potential. The current was recorded from whole-cell-configured guinea-pig ventricular myocytes before, during, and after their exposure to solution whose osmolarity was 0.75 times normal. Exposure to hyposmotic solution caused a near-doubling of the amplitude of I _Ks, with little change in the voltage dependence of current activation. Stable, hyposmotically stimulated I _Ks (I _Ks,Hypo) was decreased by broadspectrum tyrosine kinase (TK) inhibitors tyrphostin A23 (IC₅₀ ≈ 5 μM) and tyrphostin A25 (IC₅₀ 15.8 ± 1.6 μM) but not by TK-inactive tyrphostin analogs, suggesting that tyrosine phosphorylation is important for maintenance of the current. In agreement with that view, we found that the TK-inhibitor action on I _Ks,Hypo was strongly antagonized by vanadate compounds known to inhibit phosphotyrosyl phosphatase. When myocytes were pretreated with TK inhibitors, the stimulation of I _Ks was attenuated in a concentration-dependent manner. The attenuation was not due to concomitant attenuation of a stimulation of tyrosine phosphorylation because neither the stimulation of I _Ks nor its rate of decay following removal of hyposmotic solution was affected by pretreatment with vanadates. We suggest that the stimulation of I _Ks by hyposmotic solution is dependent on a basal tyrosine phosphorylation that modulates a swelling-induced I _Ks-stimulatory signal and/or the receptivity of Ks channels to that signal.     

Beta-adrenergic stimulation reverses the I Kr–I Ks dominant pattern during cardiac action potential

β-Adrenergic stimulation differentially modulates different K⁺ channels and thus fine-tunes cardiac action potential (AP) repolarization. However, it remains unclear how the proportion of I _Ks, I _Kr, and I _K1 currents in the same cell would be altered by β-adrenergic stimulation, which would change the relative contribution of individual K⁺ current to the total repolarization reserve. In this study, we used an innovative AP-clamp sequential dissection technique to directly record the dynamic I _Ks, I _Kr, and I _K1 currents during the AP in guinea pig ventricular myocytes under physiologically relevant conditions. Our data provide quantitative measures of the magnitude and time course of I _Ks, I _Kr, and I _K1 currents in the same cell under its own steady-state AP, in a physiological milieu, and with preserved Ca²⁺ homeostasis. We found that isoproterenol treatment significantly enhanced I _Ks, moderately increased I _K1, but slightly decreased I _Kr in a dose-dependent manner. The dominance pattern of the K⁺ currents was I _Kr?>?I _K1?>?I _Ks at the control condition, but reversed to I _Kr?I _K1?I _Ks following β-adrenergic stimulation. We systematically determined the changes in the relative contribution of I _Ks, I _Kr, and I _K1 to cardiac repolarization during AP at different adrenergic states. In conclusion, the β-adrenergic stimulation fine-tunes the cardiac AP morphology by shifting the power of different K⁺ currents in a dose-dependent manner. This knowledge is important for designing antiarrhythmic drug strategies to treat hearts exposed to various sympathetic tones.     

Inhibition of IKs in guinea pig cardiac myocytes and guinea pig IsK channels by the chromanol 293B

The chromanol derivative 293B was previously shown to inhibit a cAMP regulated K⁺ conductance in rat colon crypts. Subsequent studies on cloned K⁺ channels from the rat demonstrated that 293B blocks specifically I_sK channels expressed in Xenopus oocytes, but does not affect the delayed and inward rectifier Kv1.1 and Kir2.1, respectively. In the present study, the specificity of 293B for the cardiac K⁺ conductances I_Ks and I_Kr, and for the cloned guinea pig I_sK channel and the human HERG channel, which underly I_Ks and I_Kr, respectively, was analyzed. 293B inhibited both the slowly activating K⁺ conductance I_Ks in cardiac myocytes and guinea pig I_sK channels expressed in Xenopus oocytes with a similar IC₅₀ (2-6 μmol/1). In contrast, high concentrations of 293B had only a negligible effect on the more rapid activating I_Kr. Similarly, 293B exerted no effect on HERG channels expressed in Xenopus oocytes. In summary, 293B appears to be a rather specific inhibitor of I_Ks and the underlying I_sK channels.     

Angiotensin II type 1 receptor mediates partially hyposmotic-induced increase of I Ks current in guinea pig atrium

A repolarizing conduction in the heart augmented by hyposmotic or mechanically induced membrane stretch is the slow component of delayed rectifier K⁺ current (I _Ks). I _Ks upregulation is recognized as a factor promoting appearance of atrial fibrillation (AF) since gain-of-function mutations of the channel genes have been detected in congenital AF. Mechanical stretch activates angiotensin II type 1 (AT₁) receptor in the absence of its physiological ligand angiotensin II. We investigated the functional role of AT₁ receptor in I _Ks enhancement in hyposmotically challenged guinea pig atrial myocytes using the whole-cell patch-clamp method. In atrial myocytes exposed to hyposmotic solution with osmolality decreased to 70% of the physiological level, I _Ks was enhanced by 84.1%, the duration of action potential at 90% repolarization (APD₉₀) was decreased by 16.8%, and resting membrane potential was depolarized (+4.9 mV). The hyposmotic-induced effects on I _Ks and APD₉₀ were significantly attenuated by specific AT₁ receptor antagonist candesartan (1 and 5 μM). Pretreatment of atrial myocytes with protein tyrosine kinase inhibitors tyrphostin A23 and A25 suppressed but the presence of tyrosine phosphatase inhibitor orthovanadate augmented hyposmotic stimulation of I _Ks. The above results implicate AT₁ receptor and tyrosine kinases in the hyposmotic modulation of atrial I _Ks and suggest acute antiarrhythmic properties of AT₁ antagonists in the settings of stretch-related atrial tachyarrhythmias.     

Angiotensin II modulates the delayed rectifier potassium current of guinea pig ventricular myocytes

Amplitude of the delayed rectifier (I_K) tail current measured following long (5000msec) depolarizing pulses (–10 to +50mV) was decreased 19±3% (P<0.05) in a voltage-independent manner by angiotensin II (AII) 100nM. In contrast, amplitude of tail current measured following short (250msec) depolarizing pulses to potentials > + 10mV was increased 13±3 % (P< 0.05) by AII 30nM. Deactivation kinetics of I_K measured at –30mV were altered by AII 30nM and 100nM; time constant of the faster deactivating phase ( ₁) was decreased 1.4-fold. In summary, data obtained demonstrated that physiological concentrations of AII modulates major outward potassium currents involved in cardiac repolarization. Results suggest that AII increases amplitude of the rapid component of I_k (I_Kr) but decreases its slow component I_Ks. Thus, we postulate that modulators of AII effects may exhibit direct cardiac electrophysiological properties.     

Cellular basis of sex disparities in human cardiac electrophysiology

Aim: Sex disparities in electrocardiogram variables and dysrhythmia susceptibility exist, notably in long QT syndrome (LQTS) and Brugada syndrome, but the underlying mechanisms in man are unknown. We studied the cellular basis of sex distinctions in human cardiac electrophysiology and dysrhythmia susceptibility using mathematical models of human ventricular myocytes. Methods: We implemented sex differences in the Priebe–Beuckelmann and ten Tusscher–Noble–Noble–Panfilov human ventricular cell models by modifying densities of the L‐type Ca²⁺ current (I_Ca,L), transient outward K⁺ current (I_to), and rapid delayed rectifier K⁺ current (I_Kr), according to experimental data from male and female hearts of various species. Sex disparities in transmural repolarization were studied in transmural strands of cells with ion current densities based on canine experimental data. Results: Female cells have longer action potential duration (APD), steeper APD‐heart rate relationship, larger transmural APD heterogeneity, and a greater susceptibility to pro‐dysrhythmogenic early afterdepolarizations (EADs) than male cells. Conversely, male cells have more prominent phase‐1 repolarization and are more susceptible to all‐or‐none repolarization. Conclusion: Sex differences in I_Ca,L, I_to and I_Kr densities may explain sex disparities in human cardiac electrophysiology. Female cells exhibit a limited ‘repolarization reserve’ as demonstrated by their larger susceptibility to EADs, which, combined with their larger transmural electrical heterogeneity, renders them more vulnerable to tachydysrhythmias in LQTS. Conversely, male cells have a limited ‘depolarization reserve’, as shown by their larger susceptibility to all‐or‐none repolarization, which facilitates tachydysrhythmias in Brugada syndrome. These general principles may also apply to dysrhythmia susceptibility in common disease.     

Insulin suppresses IKs (KCNQ1/KCNE1) currents,which require β-subunit KCNE1

Abnormal QT prolongation in diabetic patients has become a clinical problem because it increases the risk of lethal ventricular arrhythmia. In an animal model of type 1 diabetes mellitus, several ion currents, including the slowly activating delayed rectifier potassium current (I_Ks), are altered. The I_Ks channel is composed of KCNQ1 and KCNE1 subunits, whose genetic mutations are well known to cause long QT syndrome. Although insulin is known to affect many physiological and pathophysiological events in the heart, acute effects of insulin on cardiac ion channels are poorly understood at present. This study was designed to investigate direct electrophysiological effects of insulin on I_Ks (KCNQ1/KCNE1) currents. KCNQ1 and KCNE1 were co-expressed in Xenopus oocytes, and whole cell currents were measured by a two-microelectrode voltage-clamp method. Acute application of insulin suppressed the KCNQ1/KCNE1 currents and phosphorylated Akt and extracellular signal-regulated kinase (ERK), the two major downstream effectors, in a concentration-dependent manner. Wortmannin (10^?6 M), a phosphoinositide 3-kinase (PI3K) inhibitor, attenuated the suppression of the currents and phosphorylation of Akt by insulin, whereas U0126 (10^?5 M), a mitogen-activated protein kinase kinase (MEK) inhibitor, had no effect on insulin-induced suppression of the currents. In addition, insulin had little effect on KCNQ1 currents without KCNE1, which indicated an essential role of KCNE1 in the acute suppressive effects of insulin. Mutagenesis studies revealed amino acid residues 111–118 within the distal third C-terminus of KCNE1 as an important region. Insulin has direct electrophysiological effects on I_Ks currents, which may affect cardiac excitability.     

Multiple effects of 4-aminopyridine on feline and rabbit sinoatrial node myocytes and multicellular preparations

4-aminopyridine (4-AP) is commonly used to block the transient outward potassium current, I_to, in cardiac and noncardiac tissues. In the present work, we found that 4-AP inhibited the rapid component of the delayed rectifier potassium current, I_Kr, in rabbit-isolated sinoatrial node myocytes by 25% (1 mM) and 51% (5 mM) and inhibited the slow component of the delayed rectifier potassium current, I_Ks, in cat- isolated sinoatrial node myocytes by 39% (1 mM) and 62% (5 mM). In cat- and rabbit-isolated sinoatrial node myocytes, 4-AP activated muscarinic receptors in a voltage-dependent manner to increase the acetylcholine-activated potassium current, I_KACh. In multicellular preparations of the central region of the sinoatrial node from nonreserpinized rabbits, 4-AP produced an increase in action potential overshoot, frequency, and rate of diastolic depolarization. In the presence of the β-adrenergic antagonist propranolol, 4-AP produced a marked increase in duration and a marked decrease in maximum diastolic potential and eventually, cessation of the spontaneous activity in preparations from the sinoatrial central region. In multicellular preparations from reserpinized rabbits, 4-AP produced similar effects to those observed in the presence of propranolol. We conclude that 4-AP inhibits multiple cardiac K⁺ currents, including I_to, I_Kr, and I_Ks, and that these activities mask I_KACh activation. In addition, in multicellular preparations, 4-AP produces neurotransmitter release from the autonomic nerve terminals. These multiple effects need to be considered when using 4-AP as a “specific” I_to blocker.     

[Ca2+]i-induced augmentation of the inward rectifier potassium current (IK1) in canine and human ventricular myocardium

The inward rectifier K⁺ current (I_K1) plays an important role in terminal repolarization and stabilization of the resting potential in cardiac cells. Although I_K1 was shown to be sensitive to changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i), the nature of this Ca²⁺ sensitivity—in spite of its deep influence on action potential morphology—is controversial. Therefore, we aimed to investigate the effects of a nonadrenergic rise in [Ca²⁺]_i on the amplitude of I_K1 in canine and human ventricular myocardium and its consequences on cardiac repolarization. I_K1, defined as the current inhibited by 10 μM Ba²⁺, was significantly increased in isolated canine myocytes following a steady rise in [Ca²⁺]_i. Enhanced I_K1 was also observed when [Ca²⁺]_i was not buffered by ethylene glycol tetraacetic acid, and [Ca²⁺]_I transients were generated. This [Ca²⁺]_i-dependent augmentation of I_K1 was largely attenuated after inhibition of CaMKII by 1 μM KN-93. Elevation of [Ca²⁺]_o in multicellular canine and human ventricular preparations resulted in shortening of action potentials and acceleration of terminal repolarization. High [Ca²⁺]_o enhanced the action potential lengthening effect of the Ba²⁺-induced I_K1 blockade and attenuated the prolongation of action potentials following a 0.3-μM dofetilide-induced I_Kr blockade. Blockade of I_Ks by 0.5 μM HMR-1556 had no significant effect on APD₉₀ in either 2 mM or 4 mM [Ca²⁺]_o. It is concluded that high [Ca²⁺]_i leads to augmentation of the Ba²⁺-sensitive current in dogs and humans, regardless of the mechanism of the increase. This effect seems to be at least partially mediated by a CaMKII-dependent pathway and may provide an effective endogenous defense against cardiac arrhythmias induced by Ca²⁺ overload.     

Potassium currents in isolated human atrial and ventricular cardiocytes

The whole-cell configuration of the patch-clamp technique was applied to study and compare ion currents in single ventricular and atrial cardiocytes isolated from human myocardium. In ventricular cardiocytes the K⁺ inward rectifier current (I_K1) was three times larger than in atrial cardiocytes, while its inactivation kinetics were twice as slow when measured at -140 mV. The magnitude of these variables depended on the test potential but was independent of changes in holding potential. A transient outward current (I_to) was observed in both ventricular and atrial cardiocytes. The amplitude of the inactivating component of I_to was not significantly different in atrial and ventricular cells, but the time course of inactivation was significantly longer in atrial than in ventricular cardiocytes. Steady-state inactivation of I_to in atrial cells was well described by a two-state Boltzmann function having a midpoint potential of - 41.4mV and a slope factor of 6.9 mV^-1. No discernible K⁺ delayed rectifier current (I_K) was observed in either cell type. In four of the 12 atrial cells studied, a time dependent inward current was observed at negative test potentials having a 240 ± 21 ms time constant for activation and an amplitude of 101±28pA. This current, which resembled the pacemaker current (I_t), was not observed in any of the ventricular cells examined.     

Kir 2.1 channelopathies: the Andersen–Tawil syndrome

As a multisystem disorder, Andersen–Tawil syndrome (ATS) is rather unique in the family of channelopathies. The full spectrum of the disease is characterized by ventricular arrhythmias, dysmorphic features, and periodic paralysis. Most ATS patients have a mutation in the ion channel gene, KCNJ2, which encodes the inward rectifier K⁺ channel Kir2.1, a component of the inward rectifier I _K1. I _K1 provides repolarizing current during the most terminal phase of repolarization and is the primary conductance controlling the diastolic membrane potential. Thus, ATS is a disorder of cardiac repolarization. The chapter will discuss the most recent data concerning the genetic, cellular, and clinical data underlying this unique disorder.     

An analysis of the delayed outward current in single ventricular cells of the guinea-pig

Properties of the delayed outward current (I _K) in ventricular myocytes of the guinea-pig were studied using the whole cell clamp method. The experiments were performed under conditions in whichI _K was enhanced by application of isoproterenol while the Ca²⁺ current was eliminated by Ca²⁺-removal and by the addition of Cd²⁺. The reversal potential (E _rev) ofI _K, determined from the current tails, was about 10 mV less negative than the K⁺ equilibrium potential. This was estimated by examining the reversal potential of the inward rectifier K⁺ current in Ba²⁺-containing solution, or from the Nernst equation. TheE _rev-log[K⁺]₀ relationship had a slope of 49 mV per tenfold change in [K⁺]₀. In Na⁺-free solution,E _rev became more negative. Thus, although the major charge carriers inI _K are K⁺ ions, Na⁺ ions may also contribute in part to this current. TheP _Na/P _K ratio inI _K, calculated by applying a Goldman-Hodgkin-Katz relation to the reversal potential, was 0.016. The activation ofI _K during depolarization showed a sigmoidal time course at the onset, while the time course of the current tails was monoexponential at voltages more negative than –50 mV, but biexponential at more positive voltages. These observations can be explained by the conductance equation of the Hodgkin-Huxley type in which the kinetic variable is raised to the second power. These and other features ofI _K observed in the ventricular cells are discussed in comparison to the properties of similar current systems reported in other cardiac preparations.     

Sex differences in repolarization and slow delayed rectifier potassium current and their regulation by sympathetic stimulation in rabbits

Slow delayed rectifier potassium current (I_Ks) is important in action potential (AP) repolarization and repolarization reserve. We tested the hypothesis that there are sex-specific differences in I_Ks, AP, and their regulation by β-adrenergic receptors (β-AR’s) using whole-cell patch-clamp. AP duration (APD₉₀) was significantly longer in control female (F) than in control male (M) myocytes. Isoproterenol (ISO, 500 nM) shortened APD₉₀ comparably in M and F, and was largely reversed by β₁-AR blocker CGP 20712A (CGP, 300 nM). Inhibition of I_Ks with chromanol 293B (10 μM) resulted in less APD prolongation in F at baseline (3.0 vs 8.9 %, p?<?0.05 vs M) and even in the presence of ISO (5.4 vs 20.9 %, p?<?0.05). This suggests that much of the ISO-induced APD abbreviation in F is independent of I_Ks. In F, baseline I_Ks was 42 % less and was more weakly activated by ISO (19 vs 68 % in M, p?<?0.01). ISO enhancement of I_Ks was comparably attenuated by CGP in M and F. After ovariectomy, I_Ks in F had greater enhancement by ISO (72 %), now comparable to control M. After orchiectomy, I_Ks in M was only slightly enhanced by ISO (23 %), comparable to control F. Pretreatment with thapsigargin (to block SR Ca release) had bigger impact on ISO-induced APD shortening in F than that in M (p?<?0.01). In conclusion, we found that there are sex differences in I_Ks, AP, and their regulation by β-AR’s that are modulated by sex hormones, suggesting the potential for sex-specific antiarrhythmic therapy.     

An early outward transient K+ current that depends on a preceding Na+ current and is enhanced by insulin

A whole-cell early transient outward current occurs in rat myoballs if and only if there is an immediatly preceding current of large amplitude through the voltage-gated, tetrodotoxin-inhibitable Na⁺ channel. This early outward transient is a K⁺ current, designated I _K(Na⁺). Under the conditions in which I _K(Na⁺) appears, simultaneous measurement of voltage and current, under voltage clamp, demonstrates that there is transient voltage escape to depolarized levels, peaking at about the time of peak inward Na⁺ current arid resembling an action potential. I _K(Na⁺) was never seen in the absence of this breach of the voltage clamp, suggesting that I _K(Na⁺) might be an artefact due to transient depolarization from the clamp. However, when the voltage escape was mimicked by voltage commands under conditions in which the Na⁺ channel was not activated, there was no I _K(Na). Insulin increased or produced I _K(Na⁺) even though insulin had no effect on I _Na or on the delayed rectifier K⁺ current or on the escape from voltage clamp. It is concluded that there is a population of rat myoballs in which there is an early outward K⁺ current that requires an immediately preceding current through the voltagegated tetrodotoxin-inhibitable Na⁺ channel and is enhanced by insulin.     

Human Kir2.1 channel carries a transient outward potassium current with inward rectification

We have previously reported a depolarization-activated 4-aminopyridine-resistant transient outward K⁺ current with inward rectification (I _to.ir) in canine and guinea pig cardiac myocytes. However, molecular identity of this current is not clear. The present study was designed to investigate whether Kir2.1 channel carries this current in stably transfected human embryonic kidney (HEK) 293 cells using whole-cell patch-clamp technique. It was found that HEK 293 cells stably expressing human Kir2.1 gene had a transient outward current elicited by voltage steps positive to the membrane potential (around −70 mV). The current exhibited a current–voltage relationship with intermediate inward rectification and showed time-dependent inactivation and rapid recovery from inactivation. The half potential (V _0.5) of availability of the current was −49.4 ± 2.1 mV at 5 mM K⁺ in bath solution. Action potential waveform clamp revealed two components of outward currents; one was immediately elicited and then rapidly inactivated during depolarization, and another was slowly activated during repolarization of action potential. These properties were similar to those of I _to.ir observed previously in native cardiac myocytes. Interestingly, inactivation of the I _to.ir was strongly slowed by increasing intracellular free Mg²⁺ (Mg²⁺ _i , from 0.03 to 1.0, 4.0, and 8.0 mM). The component elicited by action potential depolarization increased with the elevation of Mg²⁺ _i . Inclusion of spermine (100 μM) in the pipette solution remarkably inhibited both the I _to.ir and steady-state current. These results demonstrate that the Mg²⁺ _i -dependent current carried by Kir2.1 likely is the molecular identity of I _to.ir observed previously in cardiac myocytes.     

Novel transient outward K+ current of mature murine hippocampal neurones

 Hippocampal neurones were freshly isolated from the brain of adult mice and voltage-dependent K⁺ currents were recorded with the whole-cell patch-clamp technique. Three components of transient K⁺ current (IA) were isolated when analyzing data with exponential functions or treating neurones with a variety of voltage protocols and pharmacologic agents. Subtraction of the delayed rectifier current (I _K) from the K⁺ currents elicited after prepulses to –120 mV of varying duration revealed fast (IAf) and slow (IAs) components with decay time constants of 45 ± 8 and 612 ± 140 ms, respectively; the corresponding time constants for the removal of inactivation were 12.3 and 189.6 ms. Both tetraethylammonium and dendrotoxin selectively inhibited IAs. 4-Aminopyridine (4-AP) specifically blocked IAf and 40% of IAs with different affinities. Therefore, the properties of a 4-AP-resistant (IAsR) and a 4-AP-sensitive (IAsS) component of IAs were compared. These data suggest that three distinct subtypes of K⁺ currents contribute to the IA of mature murine hippocampal neurones. Received: 10 April 1996 / Accepted: 11 February 1997     

Phosphatidylinositol 4,5-bisphosphate interactions with the HERG K+ channel

Regulation of ion channel activity plays a central role in controlling heart rate, rhythm, and contractility responses to cardiovascular demands. Dynamic beat-to-beat regulation of ion channels is precisely adjusted by autonomic stimulation of cardiac G protein-coupled receptors. The rapidly activating delayed rectifier K⁺ current (I _Kr) is produced by the channel that is encoded by human ether-a-gogo-related gene (HERG) and is essential for the proper repolarization of the cardiac myocyte at the end of each action potential. Reduction of I _Kr via HERG mutations or drug block can lead to lethal cardiac tachyarrhythmias. Autonomic regulation of HERG channels is an area of active investigation with the emerging picture of a complex interplay of signal transduction events, including kinases, second messengers, and protein–protein interactions. A recently described pathway for regulation of HERG is through channel interaction with the phospholipid phosphatidylinositol 4,5-bisphosphate (PIP2). Changes in cellular PIP2 concentrations may occur with Gq-coupled receptor activation. Here, we review the evidence for PIP2–HERG interactions, its potential biological significance, and unfilled gaps in our understanding of this regulatory mechanism.