The erg-like potassium current in rat lactotrophs

Inhibitory Perturbations of Fluvastatin on Afterhyperpolarization Current, Erg-mediated K+ Current, and Hyperpolarization-activated Cation Current in Both Pituitary GH3 Cells and Primary Embryonic Mouse Cortical Neurons

Fluvastatin (FLV), the first synthetically derived 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor, is a potent inhibitor of cholesterol biosynthesis. While its primary mechanism of action is to reduce cholesterol levels, there is some evidence suggesting that it may also have effects on K+ channels. However, the overall effects of fluvastatin on ionic currents are not yet well understood. The whole-cell clamp recordings were applied to evaluate the ionic currents and action potentials of cells. Here, we have demonstrated that FLV can effectively inhibit the amplitude of erg-mediated K+ current (IK(erg)) in pituitary tumor (GH3) cells, with an IC50 of approximately 3.2 µM. In the presence of FLV, the midpoint in the activation curve of IK(erg) was distinctly shifted to a less negative potential by 10 mV, with minimal modification of the gating charge. However, the magnitude of hyperpolarization-activated cation current (Ih) elicited by long-lasting membrane hyperpolarization was progressively decreased, with an IC50 value of 8.7 µM, upon exposure to FLV. More interestingly, we also found that FLV (5 µM) could regulate the action potential and afterhyperpolarization properties in primary embryonic mouse cortical neurons. Our study presents compelling evidence indicating that FLV has the potential to impact both the amplitude and gating of the ion channels IK(erg) and Ih. We also provide credible evidence suggesting that this drug has the potential to modify the properties of action potentials and the afterhyperpolarization current in electrically excitable cells. However, the assumption that these findings translate to similar in-vivo results remains unclear.

The ERG1 K+ Channel and Its Role in Neuronal Health and Disease

The ERG1 potassium channel, encoded by KCNH2, has long been associated with cardiac electrical excitability. Yet, a growing body of work suggests that ERG1 mediates physiology throughout the human body, including the brain. ERG1 is a regulator of neuronal excitability, ERG1 variants are associated with neuronal diseases (e.g., epilepsy and schizophrenia), and ERG1 serves as a potential therapeutic target for neuronal pathophysiology. This review summarizes the current state-of-the-field regarding the ERG1 channel structure and function, ERG1’s relationship to the mammalian brain and highlights key questions that have yet to be answered.

The Effectiveness of Isoplumbagin and Plumbagin in Regulating Amplitude, Gating Kinetics, and Voltage-Dependent Hysteresis of erg-mediated K+ Currents

Isoplumbagin (isoPLB, 5-hydroxy-3-methyl-1,4-naphthoquinone), a naturally occurring quinone, has been observed to exercise anti-inflammatory, antimicrobial, and antineoplastic activities. Notably, whether and how isoPLB, plumbagin (PLB), or other related compounds impact transmembrane ionic currents is not entirely clear. In this study, during GH3-cell exposure to isoPLB, the peak and sustained components of an erg (ether-à-go-go related gene)-mediated K+ current (IK(erg)) evoked with long-lasting-step hyperpolarization were concentration-dependently decreased, with a concomitant increase in the decaying time constant of the deactivating current. The presence of isoPLB led to a differential reduction in the peak and sustained components of deactivating IK(erg) with effective IC50 values of 18.3 and 2.4 μM, respectively, while the KD value according to the minimum binding scheme was estimated to be 2.58 μM. Inhibition by isoPLB of IK(erg) was not reversed by diazoxide; however, further addition of isoPLB, during the continued exposure to 4,4′-dithiopyridine, did not suppress IK(erg) further. The recovery of IK(erg) by a two-step voltage pulse with a geometric progression was slowed in the presence of isoPLB, and the decaying rate of IK(erg) activated by the envelope-of-tail method was increased in its presence. The strength of the IK(erg) hysteresis in response to an inverted isosceles-triangular ramp pulse was diminished by adding isoPLB. A mild inhibition of the delayed-rectifier K+ current (IK(DR)) produced by the presence of isoPLB was seen in GH3 cells, while minimal changes in the magnitude of the voltage-gated Na+ current were demonstrated in its presence. Moreover, the IK(erg) identified in MA-10 Leydig tumor cells was blocked by adding isoPLB. Therefore, the effects of isoPLB or PLB on ionic currents (e.g., IK(erg) and IK(DR)) demonstrated herein would be upstream of our previously reported perturbations on mitochondrial morphogenesis or respiration. Taken together, the perturbations of ionic currents by isoPLB or PLB demonstrated herein are likely to contribute to the underlying mechanism through which they, or other structurally similar compounds, result in adjustments in the functional activities of different neoplastic cells (e.g., GH3 and MA-10 cells), presuming that similar in vivo observations occur.

Kv11 (ether-à-go-go-related gene) voltage-dependent K+ channels promote resonance and oscillation of subthreshold membrane potentials

Key points

Some ion channels are known to behave as inductors and make up the parallel resonant circuit in the plasma membrane of neurons, which enables neurons to respond to current inputs with a specific frequency (so‐called ‘resonant properties’).

Here, we report that heterologous expression of mouse Kv11 voltage‐dependent K⁺ channels generate resonance and oscillation at depolarized membrane potentials in HEK293 cells; expressions of individual Kv11 subtypes generate resonance and oscillation with different frequency properties.

Kv11.3‐expressing HEK293 cells exhibited transient conductance changes that opposed the current changes induced by voltage steps; this probably enables Kv11 channels to behave like an inductor.

The resonance and oscillation of inferior olivary neurons were impaired at the resting membrane potential in Kv11.3 knockout mice.

This study helps to elucidate basic ion channel properties that are crucial for the frequency responses of neurons.

Abstract

The plasma membranes of some neurons preferentially respond to current inputs with a specific frequency, and output as large voltage changes. This property is called resonance, and is thought to be mediated by ion channels that show inductor‐like behaviour. However, details of the candidate ion channels remain unclear. In this study, we mainly focused on the functional roles of Kv11 potassium (K⁺) channels, encoded by ether‐á‐go‐go‐related genes, in resonance in mouse inferior olivary (IO) neurons. We transfected HEK293 cells with long or short splice variants of Kv11.1 (Merg1a and Merg1b) or Kv11.3, and examined membrane properties using whole‐cell recording. Transfection with Kv11 channels reproduced resonance at membrane potentials depolarized from the resting state. Frequency ranges of Kv11.3‐, Kv11.1(Merg1b)‐ and Kv11.1(Merg1a)‐expressing cells were 2–6 Hz, 2–4 Hz, and 0.6–0.8 Hz, respectively. Responses of Kv11.3 currents to step voltage changes were essentially similar to those of inductor currents in the resistor–inductor–capacitor circuit. Furthermore, Kv11 transfections generated membrane potential oscillations. We also confirmed the contribution of HCN1 channels as a major mediator of resonance at more hyperpolarized potentials by transfection into HEK293 cells. The Kv11 current kinetics and properties of Kv11‐dependent resonance suggested that Kv11.3 mediated resonance in IO neurons. This finding was confirmed by the impairment of resonance and oscillation at –30 to –60 mV in Kcnh7 (Kv11.3) knockout mice. These results suggest that Kv11 channels have important roles in inducing frequency‐dependent responses in a subtype‐dependent manner from resting to depolarized membrane potentials.

Some ion channels are known to behave as inductors and make up the parallel resonant circuit in the plasma membrane of neurons, which enables neurons to respond to current inputs with a specific frequency (so‐called ‘resonant properties’).

Here, we report that heterologous expression of mouse Kv11 voltage‐dependent K⁺ channels generate resonance and oscillation at depolarized membrane potentials in HEK293 cells; expressions of individual Kv11 subtypes generate resonance and oscillation with different frequency properties.

Kv11.3‐expressing HEK293 cells exhibited transient conductance changes that opposed the current changes induced by voltage steps; this probably enables Kv11 channels to behave like an inductor.

The resonance and oscillation of inferior olivary neurons were impaired at the resting membrane potential in Kv11.3 knockout mice.

This study helps to elucidate basic ion channel properties that are crucial for the frequency responses of neurons.

Calcium signaling properties of a thyrotroph cell line, mouse TαT1 cells

TαT1 cells are mouse thyrotroph cell line frequently used for studies on thyroid-stimulating hormone beta subunit gene expression and other cellular functions. Here we have characterized calcium-signaling pathways in TαT1 cells, an issue not previously addressed in these cells and incompletely described in native thyrotrophs. TαT1 cells are excitable and fire action potentials spontaneously and in response to application of thyrotropin-releasing hormone (TRH), the native hypothalamic agonist for thyrotrophs. Spontaneous electrical activity is coupled to small amplitude fluctuations in intracellular calcium, whereas TRH stimulates both calcium mobilization from intracellular pools and calcium influx. Non-receptor-mediated depletion of intracellular pool also leads to a prominent facilitation of calcium influx. Both receptor and non-receptor stimulated calcium influx is substantially attenuated but not completely abolished by inhibition of voltage-gated calcium channels, suggesting that depletion of intracellular calcium pool in these cells provides a signal for both voltage-independent and -dependent calcium influx, the latter by facilitating the pacemaking activity. These cells also express purinergic P2Y1 receptors and their activation by extracellular ATP mimics TRH action on calcium mobilization and influx. The thyroid hormone triiodothyronine prolongs duration of TRH-induced calcium spikes during 30-min exposure. These data indicate that TαT1 cells are capable of responding to natively feed-forward TRH signaling and intrapituitary ATP signaling with acute calcium mobilization and sustained calcium influx. Amplification of TRH-induced calcium signaling by triiodothyronine further suggests the existence of a pathway for positive feedback effects of thyroid hormones probably in a non-genomic manner.

ERK and RSK are necessary for TRH-induced inhibition of r-ERG potassium currents in rat pituitary GH3 cells

The transduction pathway mediating the inhibitory effect that TRH exerts on r-ERG channels has been thoroughly studied in GH3 rat pituitary cells but some elements have yet to be discovered, including those involved in a phosphorylation event(s). Using a quantitative phosphoproteomic approach we studied the changes in phosphorylation caused by treatment with 1μM TRH for 5min in GH3 cells. The activating residues of Erk2 and Erk1 undergo phosphorylation increases of 5.26 and 4.87 fold, respectively, in agreement with previous reports of ERK activation by TRH in GH3 cells. Thus, we studied the possible involvement of ERK pathway in the signal transduction from TRH receptor to r-ERG channels. The MEK inhibitor U0126 at 0.5μM caused no major blockade of the basal r-ERG current, but impaired the TRH inhibitory effect on r-ERG. Indeed, the TRH effect on r-ERG was also reduced when GH3 cells were transfected with siRNAs against either Erk1 or Erk2. Using antibodies, we found that TRH treatment also causes activating phosphorylation of Rsk. The TRH effect on r-ERG current was also impaired when cells were transfected with any of two different siRNAs mixtures against Rsk1. However, treatment of GH3 cells with 20nM EGF for 5min, which causes ERK and RSK activation, had no effect on the r-ERG currents. Therefore, we conclude that in the native GH3 cell system, ERK and RSK are involved in the pathway linking TRH receptor to r-ERG channel inhibition, but additional components must participate to cause such inhibition.

Multiple interactions between cytoplasmic domains regulate slow deactivation of Kv11.1 channels

The intracellular domains of many ion channels are important for fine-tuning their gating kinetics. In Kv11.1 channels, the slow kinetics of channel deactivation, which are critical for their function in the heart, are largely regulated by the N-terminal N-Cap and Per-Arnt-Sim (PAS) domains, as well as the C-terminal cyclic nucleotide-binding homology (cNBH) domain. Here, we use mutant cycle analysis to probe for functional interactions between the N-Cap/PAS domains and the cNBH domain. We identified a specific and stable charge-charge interaction between Arg(56) of the PAS domain and Asp(803) of the cNBH domain, as well an additional interaction between the cNBH domain and the N-Cap, both of which are critical for maintaining slow deactivation kinetics. Furthermore, we found that positively charged arginine residues within the disordered region of the N-Cap interact with negatively charged residues of the C-linker domain. Although this interaction is likely more transient than the PAS-cNBD interaction, it is strong enough to stabilize the open conformation of the channel and thus slow deactivation. These findings provide novel insights into the slow deactivation mechanism of Kv11.1 channels.

C-terminal β9-strand of the cyclic nucleotide-binding homology domain stabilizes activated states of Kv11.1 channels

Kv11.1 potassium channels are important for regulation of the normal rhythm of the heartbeat. Reduced activity of Kv11.1 channels causes long QT syndrome type 2, a disorder that increases the risk of cardiac arrhythmias and sudden cardiac arrest. Kv11.1 channels are members of the KCNH subfamily of voltage-gated K⁺ channels. However, they also share many similarities with the cyclic nucleotide gated ion channel family, including having a cyclic nucleotide-binding homology (cNBH) domain. Kv11.1 channels, however, are not directly regulated by cyclic nucleotides. Recently, crystal structures of the cNBH domain from mEAG and zELK channels, both members of the KCNH family of voltage-gated potassium channels, revealed that a C-terminal β9-strand in the cNBH domain occupied the putative cyclic nucleotide-binding site thereby precluding binding of cyclic nucleotides. Here we show that mutations to residues in the β9-strand affect the stability of the open state relative to the closed state of Kv11.1 channels. We also show that disrupting the structure of the β9-strand reduces the stability of the inactivated state relative to the open state. Clinical mutations located in this β9-strand result in reduced trafficking efficiency, which suggests that binding of the C-terminal β9-strand to the putative cyclic nucleotide-binding pocket is also important for assembly and trafficking of Kv11.1 channels.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

Cytoplasmic domains and voltage-dependent potassium channel gating

The basic architecture of the voltage-dependent K⁺ channels (Kv channels) corresponds to a transmembrane protein core in which the permeation pore, the voltage-sensing components and the gating machinery (cytoplasmic facing gate and sensor–gate coupler) reside. Usually, large protein tails are attached to this core, hanging toward the inside of the cell. These cytoplasmic regions are essential for normal channel function and, due to their accessibility to the cytoplasmic environment, constitute obvious targets for cell-physiological control of channel behavior. Here we review the present knowledge about the molecular organization of these intracellular channel regions and their role in both setting and controlling Kv voltage-dependent gating properties. This includes the influence that they exert on Kv rapid/N-type inactivation and on activation/deactivation gating of Shaker-like and eag-type Kv channels. Some illustrative examples about the relevance of these cytoplasmic domains determining the possibilities for modulation of Kv channel gating by cellular components are also considered.

Cell type influences the molecular mechanisms involved in hormonal regulation of ERG K+ channels

While the thyrotropin-releasing hormone (TRH) effect of raising intracellular Ca(2+) levels has been shown to rely on G(q/11) and PLC activation, the molecular mechanisms involved in the regulation of ERG K(+) channels by TRH are still partially unknown. We have analysed the effects of βγ scavengers, Akt/PKB inactivation, and TRH receptor (TRH-R) overexpression on such regulation in native and heterologous expression cell systems. In native rat pituitary GH(3) cells β-ARK/CT, Gα(t), and phosducin significantly reduced TRH inhibition of rERG currents, whereas in HEK-H36/T1 cells permanently expressing TRH-R and hERG, neither of the βγ scavengers affected the TRH-induced shift in V (1/2). Use of specific siRNAs to knock Akt/PKB expression down abolished the TRH effect on HEK-H36/T1 cell hERG, but not on rERG from GH(3) cells. Indeed, wortmannin or long insulin pretreatment also blocked TRH regulation of ERG currents in HEK-H36/T1 but not in GH(3) cells. To determine whether these differences could be related to the amount of TRH-Rs in the cell, we studied the TRH concentration dependence of the Ca(2+) and ERG responses in GH(3) cells overexpressing the receptors. The data indicated that independent of the receptor number additional cellular factor(s) contribute differently to couple the TRH-R to hERG channel modulation in HEK-H36/T1 cells. We conclude that regulation of ERG currents by TRH and its receptor is transduced in GH(3) and HEK-H36/T1 cell systems through common and different elements, and hence that the cell type influences the signalling pathways involved in the TRH-evoked responses.

Molecular mechanisms of pituitary endocrine cell calcium handling

Endocrine pituitary cells express numerous voltage-gated Na(+), Ca(2+), K(+), and Cl(-) channels and several ligand-gated channels, and they fire action potentials spontaneously. Depending on the cell type, this electrical activity can generate localized or global Ca(2+) signals, the latter reaching the threshold for stimulus-secretion coupling. These cells also express numerous G-protein-coupled receptors, which can stimulate or silence electrical activity and Ca(2+) influx through voltage-gated Ca(2+) channels and hormone release. Receptors positively coupled to the adenylyl cyclase signaling pathway stimulate electrical activity with cAMP, which activates hyperpolarization-activated cyclic nucleotide-regulated channels directly, or by cAMP-dependent kinase-mediated phosphorylation of K(+), Na(+), Ca(2+), and/or non-selective cation-conducting channels. Receptors that are negatively coupled to adenylyl cyclase signaling pathways inhibit spontaneous electrical activity and accompanied Ca(2+) transients predominantly through the activation of inwardly rectifying K(+) channels and the inhibition of voltage-gated Ca(2+) channels. The Ca(2+)-mobilizing receptors activate inositol trisphosphate-gated Ca(2+) channels in the endoplasmic reticulum, leading to Ca(2+) release in an oscillatory or non-oscillatory manner, depending on the cell type. This Ca(2+) release causes a cell type-specific modulation of electrical activity and intracellular Ca(2+) handling.

Inhibition of HERG1 K+ channel protein expression decreases cell proliferation of human small cell lung cancer cells

HERG (human ether-à-go-go-related gene) K⁺ currents fulfill important ionic functions in cardiac and other excitable cells. In addition, HERG channels influence cell growth and migration in various types of tumor cells. The mechanisms underlying these functions are still not resolved. Here, we investigated the role of HERG channels for cell growth in a cell line (SW2) derived from small cell lung cancer (SCLC), a malignant variant of lung cancer. The two HERG1 isoforms (HERG1a, HERG1b) as well as HERG2 and HERG3 are expressed in SW2 cells. Inhibition of HERG currents by acute or sustained application of E-4031, a specific ERG channel blocker, depolarized SW2 cells by 10–15 mV. This result indicated that HERG K⁺ conductance contributes considerably to the maintenance of the resting potential of about −45 mV. Blockage of HERG channels by E-4031 for up to 72 h did not affect cell proliferation. In contrast, siRNA-induced inhibition of HERG1 protein expression decreased cell proliferation by about 50%. Reduction of HERG1 protein expression was confirmed by Western blots. HERG current was almost absent in SW2 cells transfected with siRNA against HERG1. Qualitatively similar results were obtained in three other SCLC cell lines (OH1, OH3, H82), suggesting that the HERG1 channel protein is involved in SCLC cell growth, whereas the ion-conducting function of HERG1 seems not to be important for cell growth.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

New Trends in Cancer Therapy: Targeting Ion Channels and Transporters

The expression and activity of different channel types mark and regulate specific stages of cancer establishment and progression. Blocking channel activity impairs the growth of some tumors, both in vitro and in vivo, which opens a new field for pharmaceutical research. However, ion channel blockers may produce serious side effects, such as cardiac arrhythmias. For instance, Kv11.1 (hERG1) channels are aberrantly expressed in several human cancers, in which they control different aspects of the neoplastic cell behaviour. hERG1 blockers tend to inhibit cancer growth. However they also retard the cardiac repolarization, thus lengthening the electrocardiographic QT interval, which can lead to life-threatening ventricular arrhythmias. Several possibilities exist to produce less harmful compounds, such as developing specific drugs that bind hERG1 channels in the open state or disassemble the ion channel/integrin complex which appears to be crucial in certain stages of neoplastic progression. The potential approaches to improve the efficacy and safety of ion channel targeting in oncology include: (1) targeting specific conformational channel states; (2) finding ever more specific inhibitors, including peptide toxins, for channel subtypes mainly expressed in well-identified tumors; (3) using specific ligands to convey traceable or cytotoxic compounds; (4) developing channel blocking antibodies; (5) designing new molecular tools to decrease channel expression in selected cancer types. Similar concepts apply to ion transporters such as the Na⁺/K⁺ pump and the Na⁺/H⁺ exchanger. Pharmacological targeting of these transporters is also currently being considered in anti-neoplastic therapy.

Gonadotropin-releasing hormone inhibits ether-à-go-go-related gene K+ currents in mouse gonadotropes

Secretion of LH from gonadotropes is initiated by a GnRH-induced increase in intracellular Ca(2+) concentration ([Ca(2+)](i)). This increase in [Ca(2+)](i) is the result of Ca(2+) release from intracellular stores and Ca(2+) influx through voltage-dependent Ca(2+) channels. Here we describe an ether-à-go-go-related gene (erg) K(+) current in primary mouse gonadotropes and its possible function in the control of Ca(2+) influx. To detect gonadotropes, we used a knock-in mouse strain, in which GnRH receptor-expressing cells are fluorescently labeled. Erg K(+) currents were recorded in 80-90% of gonadotropes. Blockage of erg currents by E-4031 depolarized the resting potential by 5-8 mV and led to an increase in [Ca(2+)](i), which was abolished by nifedipine. GnRH inhibited erg currents by a reduction of the maximal erg current and in some cells additionally by a shift of the activation curve to more positive potentials. In conclusion, the erg current contributes to the maintenance of the resting potential in gonadotropes, thereby securing a low [Ca(2+)](i) by restricting Ca(2+) influx. In addition, the erg channels are modulated by GnRH by an as-yet unknown signal cascade.

Loss of functional K+ channels encoded by ether-à-go-go-related genes in mouse myometrium prior to labour onset

There is a growing appreciation that ion channels encoded by the ether-à-go-go-related gene family have a functional impact in smooth muscle in addition to their accepted role in cardiac myocytes and neurones. This study aimed to assess the expression of ERG1-3 (KCNH1-3) genes in the murine myometrium (smooth muscle layer of the uterus) and determine the functional impact of the ion channels encoded by these genes in pregnant and non-pregnant animals. Quantitative RT-PCR did not detect message for ERG2 and 3 in whole myometrial tissue extracts. In contrast, message for two isoforms of mERG1 were readily detected with mERG1a more abundant than mERG1b. In isometric tension studies of non-pregnant myometrium, the ERG channel blockers dofetilide (1 microM), E4031 (1 microM) and Be-KM1 (100 nM) increased spontaneous contractility and ERG activators (PD118057 and NS1643) inhibited spontaneous contractility. In contrast, neither ERG blockade nor activation had any effect on the inherent contractility in myometrium from late pregnant (19 days gestation) animals. Moreover, dofetilide-sensitive K(+) currents with distinctive 'hooked' kinetics were considerably smaller in uterine myocytes from late pregnant compared to non-pregnant animals. Expression of mERG1 isoforms did not alter throughout gestation or upon delivery, but the expression of genes encoding auxillary subunits (KCNE) were up-regulated considerably. This study provides the first evidence for a regulation of ERG-encoded K(+) channels as a precursor to late pregnancy physiological activity.

The role of ether-a-go-go-related gene K(+) channels in glucocorticoid inhibition of adrenocorticotropin release by rat pituitary cells

The present study investigated the role of K(+) channels in the inhibitory effect of glucocorticoid on adrenocorticotropin (ACTH) release induced by corticotropin-releasing hormone (CRH) using cultured rat anterior pituitary cells. Apamin and charybdotoxin (CTX) did not have a significant effect on ACTH release induced by CRH (1 nM). Tetraethylammonium (TEA), a broad spectrum K(+) channel blocker, increased the ACTH response to CRH only at the highest concentration (10 mM). The exposure to 100 nM corticosterone for 60 min inhibited the CRH-induced ACTH release. Neither TEA, apamin, nor CTX affected the inhibitory effect of corticosterone. In contrast, astemizole (Ast) and E-4031, ether-a-go-go-related gene (erg) K(+) channel blockers, abolished the inhibitory effect of corticosterone on CRH-induced ACTH release (1.25+/-0.10 vs. 1.45+/-0.11 ng/well at 10 microM Ast, p>0.05, 1.71+/-0.16 vs. 1.91+/-0.32 ng/well at 10 microM E-4031, p>0.05, vehicle vs. corticosterone). RT-PCR demonstrated all three subtypes of rat-erg mRNAs in the pituitary and corticosterone increased only erg1 mRNA up to 2.47+/-0.54 fold. In conclusion, erg K(+) channels were up-regulated by glucocorticoid, and have indispensable roles in delayed glucocorticoid inhibition of CRH-induced ACTH release by rat pituitary cells.

Thermodynamic and kinetic properties of amino-terminal and S4-S5 loop HERG channel mutants under steady-state conditions

Gating kinetics and underlying thermodynamic properties of human ether-a-go-go-related gene (HERG) K(+) channels expressed in Xenopus oocytes were studied using protocols able to yield true steady-state kinetic parameters. Channel mutants lacking the initial 16 residues of the amino terminus before the conserved eag/PAS region showed significant positive shifts in activation voltage dependence associated with a reduction of z(g) values and a less negative DeltaG(o), indicating a deletion-induced displacement of the equilibrium toward the closed state. Conversely, a negative shift and an increased DeltaG(o), indicative of closed-state destabilization, were observed in channels lacking the amino-terminal proximal domain. Furthermore, accelerated activation and deactivation kinetics were observed in these constructs when differences in driving force were considered, suggesting that the presence of distal and proximal amino-terminal segments contributes in wild-type channels to specific chemical interactions that raise the energy barrier for activation. Steady-state characteristics of some single point mutants in the intracellular loop linking S4 and S5 helices revealed a striking parallelism between the effects of these mutations and those of the amino-terminal modifications. Our data indicate that in addition to the recognized influence of the initial amino-terminus region on HERG deactivation, this cytoplasmic region also affects activation behavior. The data also suggest that not only a slow movement of the voltage sensor itself but also delaying its functional coupling to the activation gate by some cytoplasmic structures possibly acting on the S4-S5 loop may contribute to the atypically slow gating of HERG.

Activation of ERG2 potassium channels by the diphenylurea NS1643

Three members of the ERG potassium channel family have been described (ERG1-3 or Kv 11.1-3). ERG1 is by far the best characterized subtype and it constitutes the molecular component of the cardiac I(Kr) current. All three channel subtypes are expressed in neurons but their function remains unclear. The lack of functional information is at least partly due to the lack of specific pharmacological tools. The compound NS1643 has earlier been reported as an ERG1 channel activator. We found that NS1643 also activates the ERG2 channel; however, the molecular mechanism of the activation differs between the ERG1 and ERG2 channels. This is surprising since ERG1 and ERG2 channels have very similar biophysical and structural characteristics. For ERG2, NS1643 causes a left-ward shift of the activation curve, a faster time-constant of activation and a slower time-constant of inactivation as well as an increased relative importance for the fast component of deactivation to the total deactivation. In contrast, for ERG1, NS1643 causes a right-ward shift in the voltage-dependent release from inactivation but does not affect time-constants of deactivation. Because of these differences in the responses of ERG1 and ERG2 to NS1643, NS1643 can be used as a pharmacological tool to address ERG channel function. It may be useful for revealing physiological functions of ERG channels in neuronal tissue as well as to elucidate the structure-function relationships of the ERG channels.

Toxins interacting with ether-à-go-go-related gene voltage-dependent potassium channels

The critical role that ether-à-go-go-related gene (erg) K(+) channels play in mating in Caenorhabditis elegans, neuronal seizures in Drosophila and cardiac action potential repolarization in humans has been well documented. Three erg genes (erg1, erg2 and erg3) have been identified and characterized. A structurally diverse number of compounds block these channels, but do not display specificity among the different channel isoforms. In this review we describe the blocking properties of several peptides, purified from scorpion, sea anemone and spider venoms, which are selective for certain members of the ERG family of channels. These peptides do not behave as classical pore blockers and appear to modify the gating properties of the channel. Genomic studies predict the existence of many other novel peptides with the potential of being more selective for ERG channels than those discussed here.

Evidence for TREK-like tandem-pore domain channels in intrapulmonary chemoreceptor chemotransduction

Intrapulmonary chemoreceptors (IPC) are carbon dioxide sensing neurons that innervate the lungs of birds, control breathing pattern, and are inhibited by halothane and intracellular acidosis. TASK and TREK are subfamilies of tandem-pore domain potassium leak channels, important in setting resting membrane potential, that are affected by volatile anesthetics and acidosis. We hypothesized that such channels might underlie signal transduction in IPC. We treated mallard ducks with four volatile anesthetics in increasing concentrations to test their effects on IPC discharge through single cell, extracellular recording from vagal fibers. Isoflurane inhalation attenuated IPC discharge only at 8.25% inspired (alpha=0.05). Halothane attenuated IPC discharge significantly (alpha=0.05) at all treatment levels. Chloroform at 3.8%, 5.6%, and 8.25% significantly attenuated IPC discharge (alpha=0.05). Ether at 1.9%, 2.9%, and 3.8% significantly attenuated IPC discharge (alpha=0.05), abolishing IPC discharge at 3.8% inspired. The pharmacological signature of IPC discharge attenuation suggests that IPC express tandem-pore domain leak channels similar to TREK channels, which are inhibited by intracellular acidosis.

Dependence of electrical activity and calcium influx-controlled prolactin release on adenylyl cyclase signaling pathway in pituitary lactotrophs

Pituitary lactotrophs in vitro fire extracellular Ca2+-dependent action potentials spontaneously through still unidentified pacemaking channels, and the associated voltage-gated Ca2+influx (VGCI) is sufficient to maintain basal prolactin (PRL) secretion high and steady. Numerous plasma membrane channels have been characterized in these cells, but the mechanism underlying their pacemaking activity is still not known. Here we studied the relevance of cyclic nucleotide signaling pathways in control of pacemaking, VGCI, and PRL release. In mixed anterior pituitary cells, both VGCI-inhibitable and -insensitive adenylyl cyclase (AC) subtypes contributed to the basal cAMP production, and soluble guanylyl cyclase was exclusively responsible for basal cGMP production. Inhibition of basal AC activity, but not soluble guanylyl cyclase activity, reduced PRL release. In contrast, forskolin stimulated cAMP and cGMP production as well as pacemaking, VGCI, and PRL secretion. Elevation in cAMP and cGMP levels by inhibition of phosphodiesterase activity was also accompanied with increased PRL release. The AC inhibitors attenuated forskolin-stimulated cyclic nucleotide production, VGCI, and PRL release. The cell-permeable 8-bromo-cAMP stimulated firing of action potentials and PRL release and rescued hormone secretion in cells with inhibited ACs in an extracellular Ca2+-dependent manner, whereas 8-bromo-cGMP and 8-(4-chlorophenylthio)-2'-O-methyl-cAMP were ineffective. Protein kinase A inhibitors did not stop spontaneous and forskolin-stimulated pacemaking, VGCI, and PRL release. These results indicate that cAMP facilitates pacemaking, VGCI, and PRL release in lactotrophs predominantly in a protein kinase A- and Epac cAMP receptor-independent manner.

Postnatal development of carotid body glomus cell response to hypoxia

This study examines developmental changes in CB glomus cell depolarization, intracellular calcium ([Ca(2+)](i)) and the magnitude of an O(2)-sensitive background ionic conductance that may play roles in the postnatal increase in oxygen sensitivity of glomus cells isolated from rats of 1-3 days and 11-14 days postnatal age. Using fura-2 and perforated patch whole cell recordings, we simultaneously measured [Ca(2+)](i) and membrane potential (E(m)) during normoxia and hypoxia. Resting E(m) in normoxia was similar at both ages. Hypoxia caused a larger E(m) depolarization and correspondingly larger [Ca(2+)](i) response in glomus cells from 11- to 14-day-old rats compared to 1-3-day-old rats. E(m) and [Ca(2+)](i) responses to 40mM K(+) were identical between the two age groups. Under normoxic conditions both age groups had similar background conductances. Under anoxic conditions (at resting membrane potential) background K(+) conductance decreased significantly more in cells from 11- to 14-day-old rats compared to cells from 1- to 3-day-old rats. Glomus cells from newborns therefore have less O(2)-sensitive background K(+) conductance. These results support the hypothesis that postnatal maturation of glomus cell O(2) sensitivity involves developmental regulation of the expression and/or O(2)-sensitivity of background ionic conductances.

Effects of TRH on heteromeric rat erg1a/1b K+ channels are dominated by the rerg1b subunit

The erg1a (HERG) K+ channel subunit and its N-terminal splice variant erg1b are coexpressed in several tissues and both isoforms have been shown to form heteromultimeric erg channels in heart and brain. The reduction of erg1a current by thyrotropin-releasing hormone (TRH) is well studied, but no comparable data exist for erg1b. Since TRH and TRH receptors are widely expressed in the brain, we have now studied the different TRH effects on the biophysical properties of homomeric rat erg1b as well as heteromeric rat erg1a/1b channels. The erg channels were overexpressed in the clonal somatomammotroph pituitary cell line GH3/B6, which contains TRH receptors and endogenous erg channels. Compared to rerg1a, homomeric rerg1b channels exhibited not only faster deactivation kinetics, but also considerably less steady-state inactivation, and half-maximal activation occurred at about 10 mV more positive potentials. Coexpression of both isoforms resulted in erg currents with intermediate properties concerning the deactivation kinetics, whereas rerg1a dominated the voltage dependence of activation and rerg1b strongly influenced steady-state inactivation. Application of TRH induced a reduction of maximal erg conductance for all tested erg1 currents without effects on the voltage dependence of steady-state inactivation. Nevertheless, homomeric rerg1b channels significantly differed in their response to TRH from rerg1a channels. The TRH-induced shift in the activation curve to more positive potentials, the dramatic slowing of activation and the acceleration of deactivation typical for rerg1a modulation were absent in rerg1b channels. Surprisingly, most effects of TRH on heteromeric rerg1 channels were dominated by the rerg1b subunit.

Downregulation of the HERG (KCNH2) K+ channel by ceramide: evidence for ubiquitin-mediated lysosomal degradation

The HERG (KCNH2) potassium channel underlies the rapid component of the delayed rectifier current (Ikr), a current contributing to the repolarisation of the cardiac action potential. Mutations in HERG can cause the hereditary forms of the short-QT and long-QT syndromes, predisposing to ventricular arrhythmias and sudden cardiac death. HERG is expressed mainly in the cell membrane of cardiac myocytes, but has also been identified in cell membranes of a range of other cells, including smooth muscle and neurones. The mechanisms regulating the surface expression have however not yet been elucidated. Here we show, using stable HERG-expressing HEK 293 cells, that ceramide evokes a time-dependent decrease in HERG current which was not attributable to a change in gating properties of the channel. Surface expression of the HERG channel protein was reduced by ceramide as shown by biotinylation of surface proteins, western blotting and immunocytochemistry. The rapid decline in HERG protein after ceramide stimulation was due to protein ubiquitylation and its association with lysosomes. The results demonstrate that the surface expression of HERG is strictly regulated, and that ceramide modifies HERG currents and targets the protein for lysosomal degradation.

Postnatal development of carotid body glomus cell O2 sensitivity

In mammals, the main sensors of arterial oxygen level are the carotid chemoreceptors, which exhibit low sensitivity to hypoxia at birth and become more sensitive over the first few days or weeks of life. This postnatal increase in hypoxia sensitivity of the arterial chemoreceptors, termed "resetting", remains poorly understood. In the carotid body, hypoxia is transduced by glomus cells, which are secretory sensory neurons that respond to hypoxia at higher P(O2) levels than non-chemoreceptor cell types. Maturation or resetting of carotid body O2 sensitivity potentially involves numerous aspects of the O2 transduction cascade at the glomus cell level, including glomus cell neurotransmitter secretion, neuromodulator function, neurotransmitter receptor expression, glomus cell depolarization in response to hypoxia, [Ca2+]i responses to hypoxia, K+ and Ca2+ channel O2 sensitivity and K+ channel expression. However, although progress has been made in the understanding of carotid body development, the precise mechanisms underlying postnatal maturation of these numerous aspects of chemotransduction remain obscure.

Expression of ether-a-go-go-related gene in gastrointestinal tract of normal CD1 mice

AIM: To investigate the expression of ether-a-go-go-related gene (ERG) mRNA and protein and its different distributions in the gastrointestinal tract of CD1 mice.

METHODS: The expression of ERG K+ current related protein was examined by immunohistochemistry and the expression of ERG mRNA was detected by reverse transcription polymerase chain reaction (RT-PCR) and in situ hybridization in the tissues of stomach, jejunum, ileum and colon from adult CD1 mice. Then the distribution of ERG was analyzed.

RESULTS: ERG was positively expressed in all the tissues of stomach, jejunum, ileum and colon. The positive cells mainly distributed in the muscular layer. A small number of positive cells distributed in the mucosa and chorion. The levels of ERG expression in stomach and colon were significantly higher than those in jejunum and ileum (4.30±0.95, 3.60±0.70 vs2.70±0.82, 2.30±1.06; P<0.05).

CONCLUSION: ERG is expressed in the gastrointestinal tract of CD1 mice. Furthermore, the expression is differential at different sites.

Specificity of TRH receptor coupling to G-proteins for regulation of ERG K+ channels in GH3 rat anterior pituitary cells

The identity of the G-protein coupling thyrotropin-releasing hormone (TRH) receptors to rat ether-à-go-go related gene (r-ERG) K+ channel modulation was studied in situ using perforated-patch clamped adenohypophysial GH(3) cells and dominant-negative variants (Galpha-QL/DN) of G-protein alpha subunits. Expression of dominant-negative Galpha(q/11) that minimizes the TRH-induced Ca2+ signal had no effect on r-ERG current inhibition elicited by the hormone. In contrast, the introduction of dominant-negative variants of Galpha13 and the small G-protein Rho caused a significant loss of the inhibitory effect of TRH on r-ERG. A strong reduction of this TRH effect was also obtained in cells expressing either dominant-negative Galpha(s) or transducin alpha subunits, an agent known to sequester free G-protein betagamma dimers. As a further indication of specificity of the dominant-negative effects, only the dominant-negative variants of Galpha13 and Rho (but not Galpha(s)-QL/DN or Galpha(t)) were able to reduce the TRH-induced shifts of human ERG (HERG) activation voltage dependence in HEK293 cells permanently expressing HERG channels and TRH receptors. Our results demonstrate that whereas the TRH receptor uses a G(q/11) protein for transducing the Ca2+ signal during the initial response to TRH, this G-protein is not involved in the TRH-induced inhibition of endogenous r-ERG currents in pituitary cells. They also identify G(s) (or a G(s)-like protein) and G13 as important contributors to the hormonal effect in these cells and suggest that betagamma dimers released from these proteins may participate in modulation of ERG currents triggered by TRH.

Fast erg K+ currents in rat embryonic serotonergic neurones

Ether-á-go-go-related gene (erg) channels form one subfamily of the ether-á-go-go (EAG) K(+) channels and all three erg channels (erg1-3) are expressed in the brain. In the present study we characterize a fast erg current in neurones in primary culture derived from the median part of rat embryonic rhombencephala (E15-16). The relatively uniform erg current was regularly found in large multipolar serotonergic neurones, and occurred also in other less well characterized neurones. The erg current was blocked by the antiarrhythmic substance E-4031. Single-cell RT-PCR revealed the expression of erg1a, erg1b, erg2 and erg3 mRNA in different combinations in large multipolar neurones. These cells also contained neuronal tryptophan hydroxylase, a key enzyme for serotonin production. To characterize the molecular properties of the channels mediating the native erg current, we compared the voltage and time dependence of activation and deactivation of the neuronal erg current to erg1a, erg1b, erg2 and erg3 currents heterologously expressed in CHO cells. The biophysical properties of the neuronal erg current were well within the range displayed by the different heterologously expressed erg currents. Activation and deactivation kinetics of the neuronal erg current were fast and resembled those of erg3 currents. Our data suggest that the erg channels in rat embryonic rhombencephalon neurones are heteromultimers formed by different erg channel subunits.

Postnatal development of E-4031-sensitive potassium current in rat carotid chemoreceptor cells

The O2 sensitivity of dissociated type I cells from rat carotid body increases with age until approximately 14-16 days. Hypoxia-induced depolarization appears to be mediated by an O2-sensitive K+ current, but other K+ currents may modulate depolarization. We hypothesized that membrane potential may be stabilized in newborn type I cells by human ether-a-go-go-related gene (HERG)-like K+ currents that inhibit hypoxia-induced depolarization and that a decrease in this current with age could underlie, in part, the developmental increase in type I cell depolarization response to hypoxia. In dissociated type I cells from 0- to 1- and 11- to 16-day-old rats, using perforated patch-clamp and 70 mM K+ extracellular solution, we measured repolarization-induced inward K+ tail currents in the absence and presence of E-4031, a specific HERG channel blocker. This allowed isolation of the E-4031-sensitive HERG-like current. E-4031-sensitive peak currents in type I cells from 0- to- 1-day-old rats were 2.5-fold larger than in cells from 11- to 16-day-old rats. E-4031-sensitive current density in newborn type I cells was twofold greater than in cells from 11- to 16-day-old rats. Under current clamp conditions, E-4031 enhanced hypoxia-induced depolarization in type I cells from 0- to- 1-day-old but not 11- to 16-day-old rats. With use of fura 2 to measure intracellular Ca2+, E-4031 increased the cytosolic Ca2+ concentration response to anoxia in cells from 0- to- 1-day-old but not cells from 11- to 16-day-old rats. E-4031-sensitive K+ currents are present in newborn carotid body type I cells and decline with age. These findings are consistent with a role for E-4031-sensitive K+ current, and possibly HERG-like K+ currents, in the type I cell hypoxia response maturation.

Functions of erg K+ channels in excitable cells

Ether-à-go-go-related gene (erg) channels are voltage-dependent K+ channels mediating inward-rectifying K+ currents because of their peculiar gating kinetics. These characteristics are essential for repolarization of the cardiac action potential. Inherited and acquired malfunctioning of erg channels may lead to the long QT-syndrome. However, erg currents have also been recorded in many other excitable cells, like smooth muscle fibres of the gastrointestinal tract, neuroblastoma cells or neuroendocrine cells. In these cells erg currents contribute to the maintenance of the resting potential. Changes in the resting potential are related to cell-specific functions like increase in hormone secretion, frequency adaptation or increase in contractility.

Inhibition of BK channels contributes to the second phase of the response to TRH in clonal rat anterior pituitary cells

AIM

Thyrotropin-releasing hormone (TRH) induces biphasic changes in the electrical activity, the cytosolic free Ca2+ concentration ([Ca2+]i), and prolactin secretion from both GH cells and native lactotrophs. It is well established that inhibition of erg channels contributes to the second phase of the TRH response. We have investigated if BK channels are also involved.

RESULTS

The BK channels may be active at the resting membrane potential (open probability, Po=0.01) in clonal rat anterior pituitary cells (GH4), which makes it possible that inhibition of these channels may contribute to the reduced K+ conductance during the TRH response. The specific BK channel blocker iberiotoxin (IbTx, 100 nm) had no effect on the resting conductance at holding potentials negative to -40 mV, but significantly reduced the conductance at shallower membrane potentials. This corresponds to the voltage dependency of the sustained [Ca2+]i. Furthermore, IbTx increased the action potential frequency by 36% in spontaneously firing cells. During the second phase of the TRH response, the action potential frequency increased by 34%, concomitantly with 61% reduction of the Po of single BK channels. The protein kinase C (PKC)-activating phorbol ester TPA had no significant effect on BK channel Po within the normal range of the resting potential.

CONCLUSION

The BK channels may contribute to the resting membrane conductance, and they are partially inhibited by TRH during the second phase. This modulation seems not to depend on PKC. We propose that inhibition of erg and BK channels acts in concert to enhance the cell excitability during the second phase of the response to TRH.

Expression pattern of the ether-a-gogo-related (ERG) K+ channel-encoding genes ERG1, ERG2, and ERG3 in the adult rat central nervous system

Voltage-dependent K(+) channels play a pivotal role in controlling cellular excitability within the nervous system. The aim of the present study was to investigate the expression in the adult rat brain of the three ether-a-gogo-related gene (ERG) family members ERG1, ERG2, and ERG3, encoding for K(+) channel subunits. To this aim, the distribution of ERG transcripts was studied by means of reverse-transcription polymerase chain reaction (RT-PCR) and nonradioactive in situ hybridization histochemistry (NR-ISH). Furthermore, ERG1 subunit distribution was studied by immunohistochemical analysis. RT-PCR analysis revealed ERG1, ERG2, and ERG3 expression in the olfactory bulb, cerebral cortex, hippocampus, hypothalamus, and cerebellum. NR-ISH experiments detected transcripts encoded by all three ERG genes in the cerebral cortex and in all CA subfields and in the granular cell layer of the dentate gyrus of the hippocampus; strong ERG1 signals were also detected in scattered large elements throughout the oriens, pyramidal, and radiatum layers, and in the hilus of the dentate gyrus. In the thalamus, positively labeled neurons were detected in the reticular nucleus with ERG1 and ERG3 and in the anterodorsal nucleus with ERG2 riboprobes. Transcripts for ERG1 and, to a lesser degree, also for ERG3, were detected in the basal ganglia and in several brainstem nuclei. All three ERG genes appeared to be expressed in cerebellar Purkinje cells. Finally, ERG1 expression was also revealed in non-neuronal elements such as ependymal and subependymal cells along the ventricular walls and hippocampal astrocytes. These results suggest that the K(+) channel isoforms of the ERG family appear to be expressed in different central nervous system regions where they might differentially control the firing of neurons engaged in several networks.

Role of K(+) channels in frequency regulation of spontaneous action potentials in rat pituitary GH(3) cells

The frequency of spontaneous action potentials (SAP) is important in the regulation of hormone secretion. The decrease in K(+) conductance is known as a primary mechanism for increasing SAP frequency. To investigate the nature of K(+) channels that contribute to the frequency regulation of the SAP in rat clonal pituitary GH(3) cells, the effect of various K(+) channel blockers on the SAP and membrane currents were recorded using the patch-clamp technique. A classical inward rectifying K(+) channel blocker, Cs(+) (5 mM), caused an increase in firing frequency and depolarization in after-hyperpolarization (AHP) voltage. An ETHER-A-GO-GO(ERG) type K(+) channel blocker, E-4031 (5 microM), caused no significant effect on the SAP. Tetraethylammonium (TEA, 10 mM) decreased firing frequency and increased the duration of SAP. These effects were not changed by the presence of high concentration of Ca(2+) buffer (10 mM EGTA or BAPTA) in pipette solutions. In voltage-clamp experiments, Cs(+) and E-4031 did not affect outwardly rectifying K(+) currents, but significantly inhibited inwardly rectifying K(+) currents recorded in isotonic K(+) solution. However, the kinetics of Cs(+)-sensitive current and E-4031-sensitive current were distinctive: the time to peak was more immediate and the decay rate was slower in Cs(+)-sensitive current than in E-4031-sensitive current. These results imply that Cs(+) and E-4031 inhibit the distinct components of inwardly rectifying K(+) currents, and that the contribution of the Cs(+)-sensitive current can be immediate on repolarization and can last more effectively over pacemaking potential range than E-4031-sensitive current.

Protein kinase C is necessary for recovery from the thyrotropin-releasing hormone-induced r-ERG current reduction in GH3 rat anterior pituitary cells

The biochemical cascade linking activation of phospholipase C-coupled thyrotropin-releasing hormone (TRH) receptors to rat ERG (r-ERG) channel modulation was studied in situ using perforated-patch clamped adenohypophysial GH3 cells and pharmacological inhibitors. To check the recent suggestion that Rho kinase is involved in the TRH-induced r-ERG current suppression, the hormonal effects were studied in cells pretreated with the Rho kinase inhibitors Y-27632 and HA-1077. The TRH-induced r-ERG inhibition was not significantly modified in the presence of the inhibitors. Surprisingly, the hormonal effects became irreversible in the presence of HA-1077 but not in the presence of the more potent Rho kinase inhibitor Y-27632. Further experiments indicated that the effect of HA-1077 correlated with its ability to inhibit protein kinase C (PKC). The hormonal effects also became irreversible in cells in which PKC activity was selectively impaired with GF109203X, Gö6976 or long-term incubation with phorbol esters. Furthermore, the reversal of the effects of TRH, but not its ability to suppress r-ERG currents, was blocked if diacylglycerol generation was prevented by blocking phospholipase C activity with U-73122. Our results suggest that a pathway involving an as yet unidentified protein kinase is the main cause of r-ERG inhibition in perforated-patch clamped GH3 cells. Furthermore, they demonstrate that although not necessary to trigger the ERG current reductions induced by TRH, an intracellular signal cascade involving phosphatidylinositol-4,5-bisphosphate hydrolysis by phospholipase C, activation of an alpha/betaII conventional PKC and one or more dephosphorylation steps catalysed by protein phosphatase 2A, mediates recovery of ERG currents following TRH withdrawal.

Role of BK potassium channels shaping action potentials and the associated [Ca(2+)](i) oscillations in GH(3) rat anterior pituitary cells

Measurements of electrical activity and intracellular Ca(2+) levels were performed in perforated-patch clamped GH(3) cells to determine the contribution of large-conductance calcium-activated K(+) (BK) channels to action potential repolarization and size of the associated Ca(2+) oscillations. By examining the dependence of action potential (AP) duration on extracellular Ca(2+) levels in the presence and the absence of the specific BK channel blocker paxilline, it is observed that plateau-like action potentials are associated to low densities of paxilline-sensitive currents. Extracellular Ca(2+) increases or paxilline additions are not able to largely modify action potential duration in cells showing a reduced expression of BK currents. Furthermore, specific blockade of these currents with paxilline systematically elongates AP duration, but only under conditions in which short APs and/or prominent BK currents recorded under voltage-clamp mode are present in the same cells. Our data indicate that in GH(3) cells, BK channels act primarily ending the action potential and suggest that by contributing to fine-tuning cellular electrical properties and hence intracellular Ca(2+) variations, BK channels may play an important role on time- and cell-dependent modulation of physiological outputs in adenohypophyseal cells.

Dependence of prolactin release on coupling between Ca(2+) mobilization and voltage-gated Ca(2+) influx pathways in rat lactotrophs

Two Ca(2+)-mobilizing receptors expressed in lactotrophs, endothelin-A (ET(A)) and thyrotropin-releasing hormone (TRH), induce a rapid Ca(2+) release from intracellular stores and prolactin (PRL) secretion but differ in their actions during the sustained stimulation; TRH facilitates and ET-1 inhibits voltage-gated calcium influx (VGCI) and PRL secretion. In pertussis toxin (PTX) treated cells, ET-1-induced inhibition of VGCI was abolished and the pattern of Ca(2+) signaling was highly comparable with that observed in TRH-stimulated cells. The addition of Cs(+), a relatively specific blocker of inward rectifier K(+) channels, mimicked the effect of PTX on the pattern of ET-1-induced sustained Ca(2+) signaling, but only in about 50% of cells, and did not affect agonist-induced inhibition of PRL secretion. Extracellular Cs(+) was also ineffective in altering the TRH-induced facilitation of VGCI and PRL secretion. Furthermore, apamin and paxilline, specific blockers of Ca(2+)-activated SKand BK-type K(+) channels, respectively; E-4031, a blocker of ether a-go-go K(+) channel; and linopirdine, a blocker of M-type K(+) channel, did not affect the agonist-specific patterns of calcium signaling and PRL secretion. These results suggest that ET-1 inhibits VGCI through activation of Cs(+)-sensitive channels, presumably the Gi/o-controlled inward rectifier K(+) channels, and that this agonist also inhibits PRL release, but downstream of Ca(2+) influx. Further studies are required to identify the mechanism of sustained TRH-induced facilitation of VGCI and PRL secretion.

Relevance of the proximal domain in the amino-terminus of HERG channels for regulation by a phospholipase C-coupled hormone receptor

We used Xenopus oocytes co-expressing thyrotropin-releasing hormone (TRH) receptors and human ether-a-go-go-related gene (HERG) K+ channel variants carrying different amino-terminal modifications to check the relevance of the proximal domain for hormonal regulation of the channel. Deletion of the whole proximal domain (Delta 138-373) eliminates TRH-induced modifications in activation and deactivation parameters. TRH effects on activation are also suppressed with channels lacking the second half of the proximal domain or only residues 326-373. However, normal responses to TRH are obtained with Delta 346-373 channels. Thus, whereas residues 326-345 are required for the hormonal modulation of HERG activation, different proximal domain sequences contribute to set HERG gating characteristics and its regulation by TRH.

Cloning and functional characterization of the smooth muscle ether-a-go-go-related gene K+ channel. Potential role of a conserved amino acid substitution in the S4 region

The human ether-a-go-go-related gene (HERG) product forms the pore-forming subunit of the delayed rectifier K(+) channel in the heart. Unlike the cardiac isoform, the erg K(+) channels in native smooth muscle demonstrate gating properties consistent with a role in maintaining resting potential. We have cloned the smooth muscle isoform of HERG, denoted as erg1-sm, from human and rabbit colon. erg1-sm is truncated by 101 amino acids in the C terminus due to a single nucleotide deletion in the 14th exon. Sequence alignment against HERG showed a substitution of alanine for valine in the S4 domain. When expressed in Xenopus oocytes, erg1-sm currents had much faster activation and deactivation kinetics compared with HERG. Step depolarization positive to -20 mV consistently produced a transient outward component. The threshold for activation of erg1-sm was -60 mV and steady-state conductance was approximately 10-fold greater than HERG near the resting potential of smooth muscle. Site-directed mutagenesis of alanine to valine in the S4 region of erg1-sm converted many of the properties to that of the cardiac HERG, including shifts in the voltage dependence of activation and slowing of deactivation. These studies define the functional role of a novel isoform of the ether-a-go-go-related gene K(+) channel in smooth muscle.

Block of erg current by linoleoylamide, a sleep-inducing agent, in pituitary GH3 cells

Linoleoylamide is physiological constituent of neurons. The effects of this agent, also a sleep-inducing agent, on ion currents in pituitary GH(3) cells were investigated. Hyperpolarization-elicited K(+) currents in GH(3) cells bathed in a high-K(+), Ca(2+)-free solution were studied to determine the effects of linoleoylamide and other related compounds on the I(K(IR)) that was sensitive to inhibition by E-4031 and identified as an erg (ether-à-go-go-related-gene) current. Linoleoylamide suppressed the amplitude of I(K(IR)) in a concentration-dependent manner with an IC(50) value of 5 microM. Oleamide (20 microM) inhibited the amplitude of I(K(IR)), while neither arachidonic acid (20 microM) nor 14,15-epoxyeicosatrienoic acid (20 microM) had an effect on it. In GH(3) cells incubated with anandamide (20 microM) or arachidonic acid (20 microM), the linoleoylamide-induced inhibition of I(K(IR)) remained unaltered. In inside-out patches, arachidonic acid (20 microM) and 14,15-epoxyeicosatrienoic acid (20 microM) stimulated large-conductance Ca(2+)-activated K(+) channels; however, linoleoylamide (20 microM) had little or no effect on them. Under current-clamp mode, linoleoylamide (20 microM) increased the firing rate. In IMR-32 neuroblastoma cells, linoleoylamide also suppressed I(K(IR)). This study provides the evidence that linoleoylamide has a depressant effect on the erg current, and suggests that this effect may affect hormonal secretion.

Growth hormone-releasing peptide-2 reduces inward rectifying K+ currents via a PKA-cAMP-mediated signalling pathway in ovine somatotropes

Inward-rectifying potassium (Kir) channels are essential for maintaining the resting membrane potential near the K(+) equilibrium and they are responsible for hyperpolarisation-induced K(+) influx. We characterised the Kir current in primary cultured ovine somatotropes and examined the effect of growth hormone-releasing peptide-2 (GHRP-2) on this current and its related intracellular signalling pathways. The Kir current was, in most cases, isolated using nystatin-perforated patch-clamp techniques. In bath solution containing 5 mM K(+), the Kir current was composed of both transient (fast activated) and delayed (slowly activated) components. An increase in the external K(+) concentration from 5 to 25 mM induced an augmentation of approximately 4-fold in the delayed part of the Kir current and both BaCl(2) and CsCl dose-dependently inhibited this current, confirming the presence of the Kir current in ovine somatotropes. Moreover, this specific effect of high K(+) on the Kir current was only observed in the cells that showed positive staining with anti-growth hormone (GH) antibodies, or in GC cells that belong to a rat somatotrope cell line. Application of GHRP-2 (100 nM) reversibly and significantly reduced the Kir current in bath solutions with 5 or 25 mM K(+) in ovine somatotropes. In addition, we found that the reduction in the Kir current mediated by GHRP-2 was totally abolished by the pretreatments with H89 (1 microM) or Rp-cAMP (100 microM) or by intracellular dialysis of a specific protein kinase A (PKA) inhibitory peptide PKI (10 microM). The specific PKC blocker chelerythrine (1 microM) or inhibitory peptide PKC(19-36) (10 microM) did not show any effects on the GHRP-2-induced decrease in the Kir current. These results suggest that the inhibition of Kir current through PKA-cAMP pathways may play an integral role in GHRP-2-induced depolarisation and GH release in ovine somatotropes.

Functional and molecular identification of ERG channels in murine portal vein myocytes

Ion channels encoded by ether-à-go-go-related genes (ERG) have been implicated in repolarization of the cardiac action potential and also as components of the resting membrane conductance in various cells. The aim of the present study was to determine whether ERG channels were expressed in smooth muscle cells isolated from portal vein. RT-PCR demonstrated the expression of murine ERG (mERG), and real-time quantitative PCR showed that the mERG1b isoform predominated over the mERG1a, mERG2, and mERG3 in portal vein. Single myocytes from portal vein displayed membrane staining with an ERG1-specific antibody. Whole cell voltage-clamp experiments were performed to determine whether portal vein myocytes expressed functional ERG channels. Large inward currents with distinctive kinetics were elicited that were inhibited rapidly by E-4031 (mean amplitude of the E-4031-sensitive current at -120 mV was -205 +/- 24 pA; n = 14). Deactivation of the E-4031-sensitive current was voltage dependent (mean time constants at -80 and -120 mV were 103 +/- 9 and 33 +/- 2 ms, respectively; n = 13). Because of the rapid kinetics of mERG currents at more negative potentials, there was a substantial noninactivating "window" current that reached a maximum of -66 +/- 10 pA at -70 mV. Complete portal veins exhibited spontaneous contractile activity in isometric tension experiments, and this activity was modified significantly by E-4031. These data show that ERG channels are expressed in murine portal vein myocytes that may contribute to the resting membrane conductance.

Ca(2+)-mobilizing endothelin-A receptors inhibit voltage-gated Ca(2+) influx through G(i/o) signaling pathway in pituitary lactotrophs

In excitable cells, receptor-induced Ca(2+) release from intracellular stores is usually accompanied by sustained depolarization of cells and facilitated voltage-gated Ca(2+) influx (VGCI). In quiescent pituitary lactotrophs, however, endothelin-1 (ET-1) induced rapid Ca(2+) release without triggering Ca(2+) influx. Furthermore, in spontaneously firing and depolarized lactotrophs, the Ca(2+)-mobilizing action of ET-1 was followed by inhibition of spontaneous VGCI caused by prolonged cell hyperpolarization and abolition of action potential-driven Ca(2+) influx. Agonist-induced depolarization of cells and enhancement of VGCI upon Ca(2+) mobilization was established in both quiescent and firing lactotrophs treated overnight with pertussis toxin (PTX). Activation of adenylyl cyclase by forskolin and addition of cell-permeable 8-bromo-cAMP did not affect ET-1-induced sustained inhibition of VGCI, suggesting that the cAMP-protein kinase A signaling pathway does not mediate the inhibitory action of ET-1 on VGCI. Consistent with the role of PTX-sensitive K(+) channels in ET-1-induced hyperpolarization of control cells, but not PTX-treated cells, ET-1 decreased the cell input resistance and activated a 5 mM Cs(+)-sensitive K(+) current. In the presence of Cs(+), ET-1 stimulated VGCI in a manner comparable with that observed in PTX-treated cells, whereas E-4031, a specific blocker of ether-a-go-go-related gene-like K(+) channels, was ineffective. Similar effects of PTX and Cs(+) were also observed in GH(3) immortalized cells transiently expressing ET(A) receptors. These results indicate that signaling of ET(A) receptors through the G(i/o) pathway in lactotrophs and the subsequent activation of inward rectifier K(+) channels provide an effective and adenylyl cyclase-independent mechanism for a prolonged uncoupling of Ca(2+) mobilization and influx pathways.

Functional up-regulation of HERG K+ channels in neoplastic hematopoietic cells

Kv1.3 channels regulate proliferation of normal lymphocytes, but the role of voltage-gated potassium channels in transformed hematopoietic cells is not known. We examined transcripts for Kv1.3, h-erg, h-eag, and BEC1 genes in primary lymphocytes and leukemias and in several hematopoietic cell lines. Surprisingly, BEC1, formerly thought to be brain-specific, was present in all the primary leukemias examined, in resting peripheral blood lymphocytes, and in proliferating activated tonsillar cells, lymphocytes from Sjögren's patients, and Epstein-Barr virus-transformed B-cells. Only h-erg mRNA was up-regulated in the cancer cells, but this was not due to proliferation per se, because it was not elevated in any of the proliferating noncancerous lymphocyte types examined. Nor did h-erg transcript levels correlate with the B-cell subset, because it was elevated in immature neoplastic B-CLL cells (CD5(+)) and in a CD5(-) Burkitt's lymphoma cell line (Raji) but not in Sjögren's syndrome cells (enriched in CD5(+) B-cells) or Epstein-Barr virus-transformed B-cells, which are mature CD5(-) B-cells. The protein and whole cell current levels roughly corresponded with the amount of mRNA expressed in three hematopoietic cell lines: CEM (an acute lymphoblastic leukemic line), K562 (a chronic myelogenous leukemic line), and U937 (an acute promyelocytic leukemic line). The selective HERG channel blocker, E-4031, reduced proliferation of CEM, U937, and K562 cells, and this appears to be the first direct evidence of a functional role for the HERG current in cancer cells. Selective up-regulation of h-erg appears to occur in neoplastic hematopoietic cells, thus providing a marker and potential therapeutic target.

Isolation of a long-lasting eag-related gene-type K+ current in MMQ lactotrophs and its accommodating role during slow firing and prolactin release

Native rat lactotrophs express thyrotrophin-releasing hormone-dependent K+ currents consisting of fast and slow deactivating components that are both sensitive to the class III anti-arrhythmic drugs that block the eag-related gene (ERG) K+ current (I(ERG)). Here we describe in MMQ prolactin-releasing pituitary cells the isolation of the slowly deactivating long-lasting component (I(ERGS)), which, unlike the fast component (I(ERGF)), is insensitive to verapamil 2 microm but sensitive to a novel scorpion toxin (ErgTx-2) that hardly affects I(ERGF). The time constants of I(ERGS) activation, deactivation, and recovery from inactivation are more than one order of magnitude greater than in I(ERGF), and the voltage-dependent inactivation is left-shifted by approximately 25 mV. The very slow MMQ firing frequency (approximately 0.2 Hz) investigated in perforated patch is increased approximately four times by anti-arrhythmic agents, by ErgTx-2, and by the abrupt I(ERGS) deactivation. Prolactin secretion in the presence of anti-arrhythmics is three- to fourfold higher in comparison with controls. We provide evidence from I(ERGS) and I(ERGF) simulations in a firing model cell to indicate that only I(ERGS) has an accommodating role during the experimentally observed very slow firing. Thus, we suggest that I(ERGS) potently modulates both firing and prolactin release in lactotroph cells.