Single channel Ca2+ currents inHelix pomatia neurons

Unitary Ca²⁺ currents of TEA injected Helix neurons were recorded in the Giga seal situation (6, 7) from microscopic membrane patches exposed to 50 mM [Ca²⁺]_o, O [Na⁺]_o, 20 mM [TEA⁺]_o and 2.5 M [TTX]_o. Constant field assumptions yield a channel permeability of 2.9±1.0×10^–14 cm³s^–1 corresponding to slope conductances of 5 to 15 pS between 0 and –30 mV. Frequency of occurrence of the units strongly increased with depolarization. Mean open time of the Ca²⁺ channels was about 3 ms without obvious dependence on voltage. A similar open time was seen with [Ba²⁺]_o, yielding about double the current strength when compared with [Ca²⁺]_o.     

Activation of a slow outward current by the calcium released during contraction of cultured rat skeletal muscle cells

A slow outward current, activated during depolarization, which induced contraction in whole-cell patch-clamped rat skeletal muscle cells in primary culture [10], was extensively characterized in the present study. This current, I _O, was simultaneously recorded with the contraction as a slow outward current during the test pulse, and a slow outward bell-shaped tail after repolarization. I _O never appeared below the threshold potential for contraction, and the tail amplitude displayed a similar evolution with peak contraction amplitude as a function of membrane potential. This feature is consistent with the fact that I _O was suppressed when contraction was blocked by 5 M nifedipine [10], and it suggests that I _O was dependent on calcium released during contraction. This was confirmed by the fact that the presence of 10 mM EGTA in the patch pipette prevented the development of both contraction and I _O, and that I _O could be activated during caffeine-induced contractures without applying depolarizations. I _O could be carried by K⁺ or Cs⁺ ions, but not by Na⁺. The pharmacology of I _O was different from that of Ca²⁺-dependent BK and SK channels, since it was resistant to tetraethylammonium (135 mM), charybdotoxin (25 nM) and apamin (50 nM). I _o was also insensitive to 4-aminopyridine (1 mM) but blocked by 5 mM Ba²⁺ without change to contraction. It was concluded that rat cultured myoballs exhibit a Cs⁺ permeation through an atypical K⁺ channel type, which is activated by the calcium released during contraction.     

Effects of K-201 on the calcium pump and calcium release channel of rat skeletal muscle

The benzothiazepine derivative K-201 has been suggested as a potential therapeutic agent due to its antiarrhythmogenic action. To understand how the drug alters calcium release from the sarcoplasmic reticulum (SR), we investigated its effects on the SR calcium channel and calcium pump by single channel electrophysiology, whole-cell confocal microscopy, and ATPase activity measurements on control and post-myocardial infarcted (PMI) rat skeletal muscle. In bilayers, K-201 induced two subconductance states corresponding to ∼24% (S₁) and ∼13% (S₂) of the maximum conductance. Dependence of event frequency and of time spent in S₁ and S₂ on the drug concentration was biphasic both in control and in PMI rats, with a maximum at 50 μM. At this concentration, the channel spends 26 ± 4% and 24 ± 4%, respectively, of the total time in these subconductance states at positive potentials, while no subconductances are observed at negative potentials. K-201 altered the frequency of elementary calcium release events: spark frequency decreased from 0.039 ± 0.001 to 0.023 ± 0.001 s⁻¹ sarcomere⁻¹, while the frequency of embers increased from 0.011 ± 0.001 to 0.023 ± 0.001 s⁻¹ sarcomere⁻¹. Embers with different amplitude levels were observed after the addition of the drug. K-201 inhibited the Ca²⁺ ATPase characterized by IC_50,contr = 119 ± 21 μM and n _Hill,contr = 1.84 ± 0.48 for control and IC_50,PMI = 122 ± 18 μM and n _Hill,PMI = 1.97 ± 0.24 for PMI animals. These results suggest that although K-201 would increase the appearance of subconductance states, the overall calcium release is reduced by the drug. In addition, the effect of K-201 is identical on calcium release channels from control and PMI rats.     

Calcium-dependent slow outward current in visceral primary afferent neurones of the rabbit

Slow outward currents were recorded from voltage-clamped neurones in nodose ganglia excised from rabbits. In the majority of Type C neurones, a short depolarizing command pulse evoked a slow outward tail current (I _SAH) with a decay time constant ranging from 0.5 to 2 s. TheI _SAH was due to an increase in membrane conductance to K⁺ because its reversal potential was approximately equal to the Nernst potential for K⁺. TheI _SAH was reversibly blocked by removal of external Ca²⁺ or by Ca²⁺ antagonists. A Ca²⁺ ionophore, A23187, produced an outward current which was similar to theI _SAH. TheI _SAH was resistant to tetraethylammonium and depressed by Ba²⁺, whereas it was not affected by Cs⁺ and 4-aminopyridine. TheI _SAH was initially augmented and subsequently depressed by apamin (1–10 nM) and (+)-tubocurarine (100–600 M). It is concluded that theI _SAH in visceral primary neurones may be due to a long-lasting increase in K⁺ conductance caused by an increase in the concentration of intracellular Ca²⁺, resulting from Ca²⁺ entry during the depolarizing command pulse.     

Acetylcholine reduces the slow calcium current in embryonic skeletal muscle cells in culture

Xenopus skeletal muscle cells when grown in culture develop a slow inward calcium current that is sensitive to dihydropyridines. Acetylcholine (ACh, 10 M) applied through a puffer pipette caused a large inward current in these cells (at the holding potential) through the nicotinic receptor channels and reduced the inward calcium current (during a step depolarization to 0 mV). After the ACh application was discontinued the holding current rapidly returned to pre-ACh levels (20 s) whereas the calcium current showed a slow, partial recovery to pre-ACh levels. Outward potassium current was also reduced during the application of ACh but recovered completely after ACh was discontinued. The effect of ACh on the calcium current was not mimicked by muscarine (100 M) and was absent when 10 g/ml -bungarotoxin was added to the bath suggesting that the decrease in calcium current was mediated by current through the nicotinic receptor.     

Activation by intracellular calcium of a potassium channel in cardiac sarcoplasmic reticulum

The effects of low (pCa 7.5 to 3) concentrations of intracellular calcium ion on a single potassium channel in the sarcoplasmic reticulum of canine heart ventricular muscle were investigated using a planar lipid bilayer technique. The low concentrations were obtained by mixing EGTA and calcium chloride. By varying the pCa of the cytoplasmic face between 3 to 7.5, two novel effects were observed. First, an increase in the intracellular Ca²⁺ concentration produced an increase in the unit current amplitude of open states; the voltage-current relationship was ohmic at these concentrations. Second, an increase in the Ca²⁺ concentration increased the open probability. Both these effects of Ca²⁺ were dose-dependent, and were consistently observed in all channels tested. Thus, the SR potassium channel observed appears to belong to the class of Ca²⁺ -activated potassium channels.     

Inhibition of the slow inward current and the time-dependent outward current of mammalian ventricular muscle by gentamicin

The aminoglycoside antibiotic, gentamicin (GM), depressed the plateau phase and shortened the duration of the action potential in guinea pig papillary muscle. Its effect on the membrane currents was studied by a single sucrose gap voltage clamp method. The slow inward current (i_s) was remarkably diminished by GM with little change in its time course, in the voltage-dependency of the steady-state inactivation and activation or in its reversal potential. The maximal amplitude of i_s, obtained by subtracting the Co²⁺-resistant current, was reduced to 57% by 0.1 mmol/l GM and almost reduced to zero by 1 mmol/l GM. The efficacy of GM in inhibiting i_s was reduced by increasing the external Ca²⁺ concentration from 1.8 to 5.4 or 10.8 mmol/l, but not by the application of adrenaline. The time-dependent outward current (i_K) was also decreased by GM but only at higher concentrations. It is proposed that the depressant action of GM on i_s was due to a blockade of slow channels, whereby GM may have dislocated Ca from the binding sites at slow channels on the external surface of the membrane.     

钙通道阻滞剂对大鼠肾上腺嗜铬细胞烟碱受体通道电流的影响

目的:研究钙通道阻滞剂(CCB)对大鼠肾上腺嗜铬细胞(RAMC) 烟碱受体通道电流(I_NIC)的作用。方法: 利用全细胞膜片钳技术,用烟碱(NIC)诱发RAMC的NIC受体通道电流,分别观察硝苯吡啶(NIF)、ω-conotoxin GVIA和ω-agatoxin IVA急性灌洗细胞前后I_NIC的变化。结果: 不同类型和浓度的CCB分别灌洗RAMC ,均能明显抑制NIC所诱发的细胞I_NIC,灌洗5 min时 10 μmol/L NIF、400 nmol/L ω-conotoxin GVIA和100 nmol/L ω-agatoxin IVA对I_NIC的峰值抑制率分别为(61.7±5.1)%、(29.3±7.4)%和(17.6±7.5)%。结论: 不同类型的CCB急性作用于RAMC能明显阻滞NIC所诱发的I_NIC,提示CCB可能通过直接阻滞NIC受体通道来抑制RAMC儿茶酚胺的分泌过程。     

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.     

Enhancement of Ca2+-dependent outward current in sheep bladder myocytes by Evans blue dye

 Whole-cell and inside-out patch-clamp techniques were used to assess the action of a well-known dye, Evans blue, on membrane currents in bladder isolated smooth muscle cells from sheep. In whole cells Evans blue dose-dependently increased the outward current by up to fivefold. In contrast, Evans blue had no effect on inward Ca²⁺ current. The effect on outward current was abolished or reduced if the cells were bathed in Ca²⁺-free solution, iberiotoxin (5 × 10^–8 M), or charybdotoxin (5 × 10^–8 M), but was unaffected by externally applied caffeine (5 mM) or in cells exposed to heparin (1 mg/ml) via the patch pipette. In inside-out patches bathed in a Ca²⁺ concentration of 5 × 10^–7 M, Evans blue (10^–4 M) increased the open probability of large-conductance (298-pS) Ca²⁺-dependent K⁺ channels (BK channels), shifting the half maximal-activation voltage by –70 mV. We conclude that Evans blue dye acts as an opener of BK channels. Received: 1 September 1997 / Received after revision: 18 November 1997 / Accepted: 20 November 1997     

Possible treatment of pre-eclampsia with calcium channel blocking agents

A review of the pathophysiology and treatment of pre-eclampsia indicates that vasospasm and vascular hyperreactivity are important elements in the disease process and its control. Five requirements for a therapeutic agent in pre-eclampsia are identified: control of generalized vasospasm, hypertension, and eclamptic convulsions, maintenance of placental perfusion, and lack of adverse effects on the fetus/neonate. Calcium channel blocking agents are well known arteriolar vasodilators and inhibitors of coronary artery vasospasm, and appear to possess qualities relevant to these therapeutic requirements for pre-eclampsia. It is proposed that calcium channel blocking agents may be effective agents in the management of pre-eclampsia.     

Single Ca channel currents in mouse pancreatic B-cells

Barium currents flowing through single Ca²⁺ channels were recorded from outside-out patches isolated from mouse pancreatic B-cells. Only one type of Ca²⁺ channel was observed. In 110 mM Ba²⁺, the single channel conductance was 24 pS (at negative membrane potentials) and the current amplitude at 0 mV was–0.7 pA. Channel openings were activated by depolarisations more positive than –30 mV and showed little inactivation during 200 ms pulses. Open times were increased by BAY K 8644 an decreased by micromolar Cd²⁺. Channel activity was subject to rundown in excised patches and little activity remained after 10 min. These properties resemble those of L-type Ca²⁺ channels in other tissues. It is suggested that this Ca²⁺ channel participates in the generation of the B-cell action potential and mediates the increase in Ca²⁺ influx required for insulin secretion.     

Fast decrease of the peak current carried by barium ions through calcium channels in the somatic membrane of mollusc neurons

In experiments on nonidentified intracellularly perfused snail neurons the effects of replacing external divalent cations on the function of potential-dependent Ca channels have been studied Ba substitution for Ca in the external medium caused a rapid decline (with half-times of about 2–3 min) in peak inward current amplitude when the current was activated from holding potential levels close to the resting potential. The decline could be reversed by membrane hyperpolarization. Barium current declined to a steady-state level which resembled in both relative amplitude (10–30% of the initial current amplitude) and insensitivity to intracellular introduction of exogenous cAMP the steady-state Ca current reached during the wash out process. It is suggested that two populations of Ca channels exist in snail neuronal membrane, one of which is dependent on cAMP metabolism and is reversibly switched off by the passage of Ba ions.     

Helothermine,a lizard venom toxin,inhibits calcium current in cerebellar granules

Helothermine (HLTx), a 25.5-kDa peptide toxin isolated from the venom of the Mexican beaded lizard (Heloderma horridum horridum), was found to be an inhibitor of Ca²⁺ channels in cerebellar granule cells of newborn rats. Macroscopic currents, carried by 10 mM Ba²⁺, were measured in whole-cell configuration. The toxin at the saturating dose of 2.5 M reversibly produced an 67% block of the voltage-dependent Ca²⁺ current by a fast mechanism of action. The current inhibition and recovery were reached in less than 1 min. Inhibition was concentration-dependent, with a half-effective dose of 0.25 M. The current block was practically voltage-independent, whereas the steady-state inactivation h was significantly affected by HLTx (10 mV). The toxin did not affect the activation and inactivation kinetics of the Ca²⁺ current. Experiments with other Ca²⁺ channel blockers showed that HLTx abolished -conotoxin GVIA-sensitive Ca²⁺ currents, as well as -AgaIVA- and dihydropyridine-sensitive Ca²⁺ currents. These drugs had virtually no effect when HLTx was applied first. The present results indicate that HLTx produce a high-potency blockage of the three pharmacologically distinct Ca²⁺ current components.     

Tetrandrine blocks a slow,large-conductance,Ca2+-activated potassium channel besides inhibiting a non-inactivating Ca2+ current in isolated nerve terminals of the rat neurohypophysis

The effects of tetrandrine, a bis-benzyl-isoquinoline alkaloid, on voltage-gated Ca²⁺ currents (I _Ca) and on Ca²⁺-activated K⁺ current (I _K(Ca)) and channels in isolated nerve terminals of the rat neurohypophysis were investigated using patch-clamp techniques. The non-inactivating component of I _Ca was inhibited by external tetrandrine in a voltage- and dose-dependent manner, with an IC₅₀=10.1 M. I _K(Ca) was elicited by depolarizations when approximately 10 M Ca²⁺ was present on the cytoplasmic side. Only externally applied tetrandrine, at 1 M, decreased the amplitude of I _K(Ca), whereas the fast inward Na⁺ current and transient outward K⁺ current were not affected. Tetrandrine, applied to the extracellular side of outside-out patches excised from the nerve terminals, induced frequent and short closures of single type II, maxi-Ca²⁺-activated K⁺channels. Tetrandrine decreased the channel-open probability, within bursts, with an IC₅₀=0.21 M. Kinetic analysis of the channel activity showed that the open-time constant decreased linearly with increasing tetrandrine concentrations (0.01–3 M), giving an association rate constant of 8.8×10⁸ M^–1s^–1, whereas the arithmetic mean closed time did not change, giving a dissociation rate constant of 136.6s^–1. These results show that tetrandrine is a high-affinity blocker of the type II, maxi-Ca²⁺-activated K⁺ channel of the rat neurohypophysial terminals.     

Ultra-slow voltage-dependent inactivation of the calcium current in guinea-pig and ferret ventricular myocytes

L-type Ca²⁺ current, i _Ca, has been recorded in guinea-pig ventricular myocytes at 36° C using the whole cell patch clamp technique. Intracellular Ca²⁺ was buffered with ethylenebis(oxonitrilo)tetraacetate (EGTA). An increase in the rate of stimulation from 0.5 to 3 Hz resulted in an abrupt decrease in i _Ca in the first beat at the high rate, followed by a progressive decrease ( approx. 7 s) over the next 30 s. The changes were not the result of Ca²⁺-dependent inactivation, because similar changes occurred with either Ba²⁺ or Na⁺ as the charge carrier. During 20-s voltage clamp pulses there was an ultra-slow phase of inactivation of Ba²⁺ or Na⁺ current through the Ca²⁺ channel ( approx. 6 s at 0 mV). This was confirmed by applying test pulses after conditioning pulses of different duration: the Ba²⁺ current during the test pulse decreased progressively when the duration of the conditioning pulse was increased progressively to 20 s. Ultra-slow inactivation of Ba²⁺ current was voltage dependent and increased monotonically at more positive potentials. Recovery of Ba²⁺ current from ultra-slow inactivation occurred with a time constant of 3.7 s at –40 mV and 0.7 s at –80 mV. The gradual decrease in i _Ca on increasing the rate to 3 Hz may have been the result of the development of ultra-slow voltage-dependent inactivation.     

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.     

Cation channel blocked by extracellular Ca2+ in the apical membrane of the chick embryonic ectoderm

In the chick embryo (20 h incubation, gastrula stage), the apical membrane of the ectodermal cells shows a high density of a non-selective cation channel which is blocked by very low extracellular Ca²⁺ concentrations. Properties of this channel were studied at the single-channel level using the patch-clamp technique in the cell-attached mode.With 1 mmol/l Ca²⁺ in the pipette, only outward current was present and the channel conductance measured at +120 mV was 25.5 pS. In the absence of Ca²⁺, also inward current through the channel was observed. The conductances measured at –50 mV were 49.5 pS with Na⁺ as the charge carrier, 72.5 pS with K⁺, 49.1 pS with Cs⁺, and 18.5 pS with Li⁺. The conductance measured at +80 mV was around 23 pS in all four cases. The reversal potential was similar (around 25 mV) for all four ions, which indicates a poor selectivity of the channel. In the absence of Ca²⁺ and the presence of 1 mmol/l ethylene-bis(oxonitrilo) tetraacetate (EGTA), the kinetics of the channel were characterized by bursts of the order of seconds. During a burst, the channel flickered between one open and one closed level. The open time was constant between –30 mV and –80 mV, while the closed time decreased with hyperpolarization. The open time varied according to the permeant ion (K⁺+=Cs⁺+).Extracellular Ca²⁺ blocked the inward current in a voltage-dependent manner. The K _d values, 1 mol/l at –30 mV and 3.2 mol/l at –80 mV, indicate that Ca²⁺ ions exit the channel toward the intracellular side. A weak voltage dependency of the association rate constant suggests that the Ca²⁺-binding site is close to the outside mouth. Extracellular Ca²⁺ was much less efficient at blocking the outward current (K _d about 1 mmol/l at 80 mV). Tetracaine, but not uraniumdioxide, decreased the opening probability of the channel. The embryonic channel shows similarities with the Ca²⁺-blockable, poorly selective channel described in the epithelium of toad urinary bladder.     

Voltage-dependent properties of macroscopic and elementary calcium channel currents in guinea pig ventricular myocytes

Whole-cell Ca channel currents were recorded from guinea pig ventricular myocytes that were internally perfused with Cs solution and bathed in solutions containing 3.6 mM Ca, 3.6 mM Ba or 90 mM Ba (34° C). Single Ca channel currents were recorded from cell-attached membrane patches of similar myocytes; the patch pipettes contained a 90 mM Ba solution. 1. Although the shape of the whole-cellI–V relation was independent of the bathing solution, this was not the case with the location of the inward current maximum (V _peak);V _peak in 90 mM Ba was about 30 mV positive toV _peak in 3.6 mM Ba. 2. The activation and inactivation of whole-cell currents were voltage dependent. Compared to the voltage dependencies in 3.6 mM Ba, those in 90 mM Ba were shifted by about 30 mV to the right, suggesting a neutralization of surface charges. 3. Observations compatible with the ion permeation model proposed by Hess and Tsien (1984) included (a) a depression of current during Ca/Ba solution exchange, (b) a high divalent to monovalent ion permeability, and (c) rectification of the outward limb of theI–V relation. 4. Estimated current densities atV _peak were similar for myocytes in 3.6 mM Ca and 3.6 mM Ba, and about 10 times larger in 90 mM Ba. 5. Average currents (I ^*) calculated from ensembles of records of single Ca channel current had voltage-dependent time courses resembling those of whole-cellI _Ba (90 mM). 6. Single-channelI ^*–V relations were superimposable on whole-cellI–V curves suggesting that voltage-dependent single-channel parameters (probability of opening, elementary current amplitude) can be related to the voltage-dependent macroscopic current parameters (activation, instantaneousI–V relation) when scaled by channel number. 7. The density of Ca channels in myocytes was calculated from whole-cellI _Ba (90 mM) and average current through single channels. The outcome, 3–5 channels/m², agrees with two other recent estimates (Tsien et al. 1983; Lux and Brown 1984). However, it is difficult to reconcile with the much lower density that one would forecast from the frequency of functional channel observation in myocyte membrane patches (Pelzer et al. 1985c).Supported by the Deutsche Forschungsgemeinschaft (DFG), SFB 38 (Membranforschung), project G1, and the Medical Research Council (Canada)     

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.