KCNQ channels mediate IKs, a slow K+ current regulating excitability in the rat node of Ranvier

Mutations that reduce the function of KCNQ2 channels cause neuronal hyperexcitability, manifested as epileptic seizures and myokymia. These channels are present in nodes of Ranvier in rat brain and nerve and have been proposed to mediate the slow nodal potassium current I(Ks). We have used immunocytochemistry, electrophysiology and pharmacology to test this hypothesis and to determine the contribution of KCNQ channels to nerve excitability in the rat. When myelinated nerve fibres of the sciatic nerve were examined by immunofluorescence microscopy using antibodies against KCNQ2 and KCNQ3, all nodes showed strong immunoreactivity for KCNQ2. The nodes of about half the small and intermediate sized fibres showed labelling for both KCNQ2 and KCNQ3, but nodes of large fibres were labelled by KCNQ2 antibodies only. In voltage-clamp experiments using large myelinated fibres, the selective KCNQ channel blockers XE991 (IC50 = 2.2 microm) and linopirdine (IC50 = 5.5 microm) completely inhibited I(Ks), as did TEA (IC50 = 0.22 mm). The KCNQ channel opener retigabine (10 microm) shifted the activation curve to more negative membrane potentials by -24 mV, thereby increasing I(Ks). In isotonic KCl 50% of I(Ks) was activated at -62 mV. The activation curve shifted to more positive potentials as [K+]o was reduced, so that the pharmacological and biophysical properties of I(Ks) were consistent with those of heterologously expressed homomeric KCNQ2 channels. The ability of XE991 to selectively block I(Ks) was further exploited to study I(Ks) function in vivo. In anaesthetized rats, the excitability of tail motor axons was indicated by the stimulus current required to elicit a 40% of maximal compound muscle action potential. XE991 (2.5 mg kg(-1) i.p.) eliminated all nerve excitability functions previously attributed to I(Ks): accommodation to 100 ms subthreshold depolarizing currents, the post-depolarization undershoot in excitability, and the late subexcitability after a single impulse or short trains of impulses. Due to reduced spike-frequency adaptation after XE991 treatment, 100 ms suprathreshold current injections generated long trains of action potentials. We conclude that the nodal I(Ks) current is mediated by KCNQ channels, which in large fibres of rat sciatic nerve appear to be KCNQ2 homomers.

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.

Activation of T cell calcium influx by the second messenger ADP-ribose

Stimulation of Jurkat T cells by high concentrations of concanavalin A (ConA) induced an elevation of the endogenous adenosine diphosphoribose (ADPR) concentration and an inward current significantly different from the Ca2+ release-activated Ca2+ current (I(CRAC)). Electrophysiological characterization and activation of a similar current by infusion of ADPR indicated that the ConA-induced current is carried by TRPM2. Expression of TRPM2 in the plasma membrane of Jurkat T cells was demonstrated by reverse transcription-PCR, Western blot, and immunofluorescence. Inhibition of ADPR formation reduced ConA-mediated, but not store-operated, Ca2+ entry and prevented ConA-induced cell death of Jurkat cells. Moreover, gene silencing of TRPM2 abolished the ADPR- and ConA-mediated inward current. Thus, ADPR is a novel second messenger significantly involved in ConA-mediated cell death in T cells.

Extracellular potassium effects are conserved within the rat erg K+ channel family

The biophysical properties of native cardiac erg1 and recombinant HERG1 channels have been shown to be influenced by the extracellular K(+) concentration ([K(+)](o)). The erg1 conductance, for example, increases dramatically with a rise in [K(+)](o). In the brain, where local [K(+)](o) can change considerably with the extent of physiological and pathophysiological neuronal activity, all three erg channel subunits are expressed. We have now investigated and compared the effects of an increase in [K(+)](o) from 2 to 10 mm on the three rat erg channels heterologously expressed in CHO cells. Upon increasing [K(+)](o), the voltage dependence of activation was shifted to more negative potentials for erg1 (DeltaV(0.5) = -4.0 +/- 1.1 mV, n = 28) and erg3 (DeltaV(0.5) = -8.4 +/- 1.2 mV, n = 25), and was almost unchanged for erg2 (DeltaV(0.5) = -2.0 +/- 1.3 mV, n = 6). For all three erg channels, activation kinetics were independent of [K(+)](o), but the slowing of inactivation by increased [K(+)](o) was even more pronounced for erg2 and erg3 than for erg1. In addition, with increased [K(+)](o), all three erg channels exhibited significantly slower time courses of recovery from inactivation and of deactivation. Whole-cell erg-mediated conductance was determined at the end of 4 s depolarizing pulses as well as with 1 s voltage ramps starting from the fully activated state. The rise in [K(+)](o) resulted in increased conductance values for all three erg channels which were more pronounced for erg2 (factor 3-4) than for erg1 (factor 2.5-3) and erg3 (factor 2-2.5). The data demonstrate that most [K(+)](o)-dependent changes in the biophysical properties are well conserved within the erg K(+) channel family, despite gradual differences in the magnitude of the effects.

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.

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.

Nitric oxide donor sodium nitroprusside dilates rat small arteries by activation of inward rectifier potassium channels

The role of vascular smooth muscle inward rectifier K+ (K(IR)) channels in the mechanisms underlying vasodilation is still unclear. The hypothesis that K(IR) channels are involved in sodium nitroprusside (SNP)-induced dilation of rat-tail small arteries was tested. SNP relaxed tail small arteries with an EC50 of 2.6x10(-8) mol/L. Endothelium removal did not attenuate this effect. Vessel pretreatment with hydroxocobalamin, a nitric oxide (NO) scavenger, but not with rhodanese and sodium thiosulfate, inactivators of cyanide (CN), abolished the SNP effect. Vessel pretreatment with 10(-5) mol/L Ba2+, a specific blocker of K(IR) channels at micromolar concentrations, reduced the SNP effect. Low concentrations of K+ dilated the vessels; this effect was attenuated largely after pretreatment with 3x10(-5) mol/L Ba2+. In freshly isolated smooth muscle cells, a barium-sensitive current was observed at potentials negative to the potassium equilibrium potential. Application of 10(-4) mol/L SNP increased the barium-sensitive current 1.79+/-0.23-fold at -100 mV and hyperpolarized the membrane potential by 8.6+/-0.5 mV. In tissue from freshly dissected vessels, transcripts for K(IR) 2.1 and 2.2, but not for K(IR) 2.3 and 2.4, were found. However, only K(IR) 2.1 antibodies immunostained the tunica media of the vessel. These data suggest that vascular smooth muscle K(IR) 2.1 channels are involved in the SNP-induced dilation of rat-tail small arteries.

HERG K(+) currents in human prolactin-secreting adenoma cells

To investigate the presence and possible function of ether-à-go-go-related gene (erg) K(+) channels in human lactotroph cells (HERG channels), primary cultures were prepared from human prolactinoma tissue. In almost all primary cultures, HERG currents could be recorded in identified prolactin cells using an external high-K(+) solution. The antiarrhythmic agent E-4031, a specific blocker of erg channels, served to isolate HERG currents as the drug-sensitive currents. In cells of two tumours tested, thyrotropin-releasing hormone significantly reduced the amplitude of the HERG currents. The potential dependence of HERG current availability and the deactivation kinetics differed significantly even between prolactin cells derived from one adenoma. For comparison, corresponding values were obtained for heterologously expressed rat erg1, erg2 and erg3 channels. The expression of the three HERG channel subunits was investigated in nine human adenomas using RT-PCR. Transcripts for HERG1 were present in all adenomas and although transcripts for HERG2 and HERG3 were also detected, their expression level was more variable. The results demonstrate the functional expression of HERG channels in human prolactin-secreting tumours and are compatible with a physiological role for these channels in the control of prolactin secretion, as has been shown in normal rat lactotroph cells.

Biophysical properties of heteromultimeric erg K+ channels

The three ether-à-go-go-related gene (erg) K(+) channel subunits are able to form heteromultimers within their subfamily. The functional importance of this finding is indicated by in situ hybridization experiments showing that the different erg subunits have overlapping expression patterns in several regions of the brain. To investigate the biophysical properties of heteromultimeric erg channels, concatemers of two erg subunits were constructed and expressed heterologously in Chinese hamster ovary (CHO) cells. The resulting currents were measured using the patch-clamp technique. The heteromultimers exhibited an intermediate potential dependence of activation compared with the corresponding wild-type (WT) erg channels. In contrast, the time course of activation was clearly dominated by the faster activating subunit. The kinetics of recovery from inactivation and the deactivation kinetics of all heteromultimers were similar to those of WT erg1 channels, the rat homologue of the human erg1 K(+) channel (HERG), even if erg1 was not part of the concatemer. Taken together, the biophysical properties of heteromultimeric erg channels result in larger current amplitudes upon both depolarization and repolarization. Thus, through heteromeric assembly erg channels may contribute significantly to different physiological functions such as setting and stabilizing the resting membrane potential and modulation of action potential frequency.

The neuropeptide head activator induces activation and translocation of the growth-factor-regulated Ca2+-permeable channel GRC

The neuropeptide head activator stimulates cell proliferation of neuronal precursor and neuroendocrine cells. The mitogenic signaling cascade requires Ca2+ influx for which, as we show in this paper, the growth-factor-regulated Ca2+-permeable cation channel, GRC, is responsible. GRC is a member of the transient receptor potential channel family. In uninduced cells only low amounts of GRC are present on the plasma membrane but, upon stimulation with head activator, GRC translocates from an intracellular compartment to the cell surface. Head activator functions as an inducer of GRC translocation in neuronal and neuroendocrine cells, which express GRC endogenously, and also in COS-7 cells after transfection with GRC. Head activator is no direct ligand for GRC, but its action requires the presence of a receptor coupled to a pertussis-toxin inhibitable G-protein. Heterologously expressed GRC becomes activated by head activator, which results in opening of the channel and Ca2+ influx. SK&F 96365, an inhibitor specific for TRP-like channels, blocks Ca2+ entry and, consequently, translocation of GRC is prevented. Head activator-induced GRC activation and translocation are also inhibited by wortmannin and KN-93, blockers of the phosphatidylinositol 3-kinase and of the Ca2+/calmodulin-dependent kinase, respectively, which implies a role for both kinases in head-activator signaling to GRC.

Ca2+ channels in clonal rat anterior pituitary cells (GH3/B6)

In clonal rat somatomammotroph cells (GH3/ B6) Ca2+ influx through voltage-dependent Ca2+ channels is important for regulating the Ca2+ concentration that mediates hormone secretion. To study the Ca2+ channel subtypes in GH3/B6 cells, Ca2+ channel currents were recorded with the whole-cell configuration of the patch-clamp technique using Ba2+ as the charge carrier. Forty-nine percent of the total Ba2+ current amplitude was mediated by a nifedipine-sensitive current (L-type). In addition, three other high-voltage-activated Ca2+ channel current components could be distinguished pharmacologically: 10 nM omega-agatoxin-IVA-sensitive current (22%; P-type), omega-conotoxin-MVIIC-sensitive current (18%; Q-type), and toxin-resistant current (24%). Since omega-conotoxin GVIA (2 microM) had no blocking effect, N-type Ca2+ channels are assumed not to be present in GH3/B6 cells. The T-type Ca2+ channel current was either absent or very small. Different pore-forming alpha1 subunits of Ca2+ channels were found to be expressed in GH3/B6 cells, which could be the molecular correlates of the different Ba2+ current subtypes: alpha1G of T-type, alpha1C, alpha1D and alpha1S of L-type, and alpha1A of P/Q-type current. In addition, transcripts for beta1, beta2 and beta3 subunits were detected. Blockage of L-type channels with 10 microM nifedipine or P/Q-type channels with 10 nM omega-agatoxin MVIIC + 200 nM omega-conotoxin blocked action potential firing in GH3/B6 cells and decreased basal prolactin secretion.

Modulation of rat erg1, erg2, erg3 and HERG K+ currents by thyrotropin-releasing hormone in anterior pituitary cells via the native signal cascade

The mechanism of thyrotropin-releasing hormone (TRH)-induced ether-a-go-go-related gene (erg) K+ current modulation was investigated with the perforated-patch whole-cell technique in clonal somatomammotroph GH3/B6 cells. These cells express a small endogenous erg current known to be reduced by TRH. GH3/B6 cells were injected with cDNA coding for rat erg1, erg2, erg3 and HERG K+ channels. The corresponding erg currents were isolated with the help of the specific erg channel blockers E-4031 and dofetilide and their biophysical properties were determined. TRH (1 M) was able to significantly reduce the different erg currents. The voltage dependence of activation was shifted by 15 mV (erg1), 10 mV (erg2) and 6 mV (erg3) to more positive potentials without strongly affecting erg inactivation. TRH reduced the maximal available erg current amplitude by 12% (erg1), 13% (erg2) and 39% (erg3) and accelerated the time course of erg1 and erg2 channel deactivation, whereas erg3 deactivation kinetics were not significantly altered. The effects of TRH on HERG currents did not differ from those on its rat homologue erg1. In addition, coinjection of rat MiRP1 with HERG cDNA did not influence the TRH-induced modulation of HERG channels. Rat erg1 currents recorded in the cell-attached configuration were reduced by application of TRH to the extra-patch membrane in the majority of the experiments, confirming the involvement of a diffusible second messenger. Application of the phorbol ester phorbol 12-myristate 13-acetate (PMA; 1 M) shifted the voltage dependence of erg1 activation in the depolarizing direction, but it did not reduce the maximal current amplitude. The voltage shift could not be explained by a selective effect on protein kinase C (PKC) since the PKC inhibitor bisindolylmaleimide I did not block the effects of TRH and PMA on erg1. In addition, cholecystokinin, known to activate the phosphoinositol pathway similarly to TRH, did not significantly affect the erg1 current. Various agents interfering with different known TRH-elicited cellular responses were not able to completely mimic or inhibit the TRH effects on erg1. Tested substances included modulators of the cAMP-protein kinase A pathway, arachidonic acid, inhibitors of tyrosine kinase and mitogen-activated protein kinase, sodium nitroprusside and cytochalasin D. The results demonstrate that all three members of the erg channel subfamily are modulated by TRH in GH3/B6 cells. In agreement with previous studies on the TRH-induced modulation of the endogenous erg current in prolactin-secreting anterior pituitary cells, the TRH effects on overexpressed erg1 channels are not mediated by any of the tested signalling pathways.

Erg1, erg2 and erg3 K channel subunits are able to form heteromultimers

Clonal somato-mammotroph GH3/B6 cells and lactotroph MMQ cells express two (ergl, erg2) of the three cloned rat ether-à-go-go-related gene (erg) K channel subunits. To study whether the erg subunits form heteromultimers, dominant-negative mutants of erg and erg2 were constructed by point mutation (erg1G630S, erg2G480S). After co-expression of these mutants with the wild-type erg1, erg2, or erg3 in Chinese hamster ovary (CHO) cells no erg currents could be detected. In contrast, in co-expression experiments with members of the other ether-à-go-go (EAG) subfamilies (eagl, elkl) the mutant erg1G630S had no effect. These results strongly suggest that erg channel subunits are able to form heteromultimers within the erg channel subfamily. Suppression of the endogenous E-4031-sensitive currents in GH3/B6 and MMQ cells by erg1G630S confirms that they are mediated by erg channels despite the differences in gating kinetics in these cells. Reduction of the erg current in GH3/B6 cells by erg2G480S indicates that erg heteromultimers can also be formed in these cells.

Expression of mRNA for voltage-dependent and inward-rectifying K channels in GH3/B6 cells and rat pituitary

The expression of mRNA for voltage-dependent (Kv) and inward-rectifying K channels (Kir) was studied in clonal rat somato-mammotroph cells (GH3/B6 cells) and rat pituitary using reverse transcription-polymerase chain reaction (RT-PCR). In GH3/B6 cells transcripts for 16 different Kv channel alpha-subunits (seven Shaker-related: Kv1.2, Kv1.4, Kv1.5, Kv2.1, Kv3.2, Kv4.1, Kv5.1; six EAG: eag1, erg1, erg2, elk1-elk3; three KCNQ: KCNQ1-KCNQ3) and for five different Kir channel alpha-subunits (Kir1.1, Kir2.3, Kir3.2, Kir3.3, Kir6.2) were found. In addition, transcripts for a short isoform of Kvbeta2 and transcripts for Kvbeta3 subunits were present. In rat pituitary transcripts for 21 different Kv channel alpha-subunits (11 Shaker-related: Kv1.3, Kv1.4, Kv1.6, Kv2.1, Kv2.2, Kv3.2, Kv3.4, Kv4.1, Kv4.2, Kv4.3, Kv6.1; seven EAG: eag1, erg1-erg3, elk1-elk3; three KCNQ: KCNQ1-KCNQ3) and nine Kir channel alpha-subunits (Kir1.1, Kir2.2, Kir3.1-Kir3.4, Kir4.1, Kir6.1, Kir6. 2) were found. In addition, all tested auxiliary subunits (Kvbeta1-Kvbeta3, minK, SUR1, SUR2) are expressed in the pituitary. The results indicate that the macroscopic K currents in GH3/B6 and pituitary cells are presumably mediated by K channels constructed by a larger number of K channel alpha-subunits and auxiliary beta-subunits than previously distinguished electrophysiologically and pharmacologically.

The ether-à-go-go-Related Gene K(+) Current: Functions of a Strange Inward Rectifier

The erg channels mediate an inward-rectifying K(+) current because of their peculiar gating kinetics. They are involved in repolarization of the cardiac action potential, frequency adaptation, and maintenance of the resting potential. Reduction of erg currents via an intracellular signal cascade underlies the thyrotropin-releasing hormone-induced increase in prolactin secretion.

The erg-like potassium current in rat lactotrophs

1. The ether-à-go-go-related gene (erg)-like K+ current in rat lactotrophs from primary culture was characterized and compared with that in clonal rat pituitary cells (GH3/B6). The class III antiarrhythmic E-4031 known to block specifically erg K+ channels was used to isolate the erg-like current as the E-4031-sensitive current. The experiments were performed in 150 mM K+ external solution using the patch-clamp technique. 2. The erg-like K+ current elicited with hyperpolarizing pulses negative to -100 mV consisted of a fast and a pronounced slowly deactivating current component. The contribution of the slow component to the total current amplitude was potential dependent and varied from cell to cell. At -100 mV it ranged from 50 to 85% and at -140 mV from 21 to 45%. 3. The potential-dependent channel availability curves determined with 2 s prepulses were fitted with the sum of two Boltzmann functions. The function related to the slowly deactivating component of the erg-like current was shifted by more than 40 mV to more negative membrane potentials compared with that of the fast component. 4. In contrast to that of native lactotrophs studied under identical conditions, the erg-like K+ current of GH3/B6 cells was characterized by a predominant fast deactivating current component, with similar kinetic and steady-state properties to the fast deactivating current component of native lactotrophs. 5. Thyrotrophin-releasing hormone reduced the erg-like current in native lactotrophs via an intracellular signal cascade which seemed to involve a pathway independent from protein kinase A and protein kinase C. 6. RT-PCR studies on cytoplasm from single lactotrophs revealed the presence of mRNA of the rat homologue of the human ether-à-go-go-related gene HERG (r-erg1) as well as mRNA of the two other cloned r-erg cDNAs (r-erg2 and r-erg3) in different combinations. In GH3/B6 cells, only the transcripts of r-erg1 and r-erg2 were found.

Separation of M-like current and ERG current in NG108-15 cells

Differentiated NG108-15 neuroblastoma x glioma hybrid cells were whole-cell voltage-clamped. Hyperpolarizing pulses, superimposed on a depolarized holding potential (-30 or -20 mV), elicited deactivation currents which consisted of two components, distinguishable by fitting with two exponential functions. Linopirdine [DuP 996, 3,3-bis(4-pyridinylmethyl)-1-phenylindolin-2-one), a neurotransmitter-release enhancer known as potent and selective blocker of the M-current of rat sympathetic neurons, in concentrations of 5 or 10 microM selectively inhibited the fast component (IC50 = 14.7 microM). The slow component was less sensitive to linopirdine (IC50>20 microM). The class III antiarrhythmics [(4-methylsulphonyl)amido]benzenesulphonamide (WAY-123.398) and 1-[2-(6-methyl-2-pyrydinil)ethyl]-4-(4-methylsulphonylaminobenz oyl) piperidine (E-4031), selective inhibitors of the inwardly rectifying ERG (ether-à-go-go-related gene) potassium channel, inhibited predominantly the slow component (IC50 = 38 nM for E-4031). The time constant of the WAY-123.398-sensitive current resembled the time constant of the slow component in size and voltage dependence. Inwardly rectifying ERG currents, recorded in K+ -rich bath at strongly negative pulse potentials, resembled the slow component of the deactivation current in their low sensitivity to linopirdine (28% inhibition at 50 microM). The size of the slow component varied greatly between cells. Accordingly, varied the effect of WAY-123.398 on deactivation current and holding current. RNA transcripts for the following members of the ether-à-go-go gene (EAG) K+ channel family were found in differentiated NG108-15 cells: ERG1, ERG2, EAGI, EAG-like (ELK)1, ELK2; ERG3 was only present in non-differentiated cells. In addition, RNA transcripts for KCNQ2 and KCNQ3 were found in differentiated and non-differentiated cells. We conclude that the fast component of the deactivation current is M-like current and the slow component is deactivating ERG current. The molecular correlates are probably KCNQ2/KCNQ3 and ERG1/ERG2, respectively.

A functional role of the erg-like inward-rectifying K+ current in prolactin secretion from rat lactotrophs

The functional role of the inward-rectifying erg-like K+ current in rat lactotrophs was studied by the use of a selective blocker, the class III antiarrhythmic agent E-4031. The erg-like current was measured as drug-sensitive current in physiological K+ gradient. In the range of the normal resting membrane potential of rat lactotrophs (around -45 mV) the erg-like current constituted a steady outward current. A selective block of this current by E-4031 resulted in a moderate (5 mV) depolarization of the membrane potential in 64% of the lactotroph cells. Measurements of basal prolactin secretion with the reverse hemolytic plaque assay showed that the number of prolactin secreting cells and the amount of prolactin secreted from single lactotrophs was significantly increased in the presence of E-4031. The data show that the contribution of the erg-like K+ current to the maintenance of the resting membrane potential is functionally important for the regulation of prolactin secretion.

Ionic mechanisms underlying TRH-induced prolactin secretion in rat lactotrophs

Whole cell patch-clamp experiments were performed in clonal rat pituitary cells (GH3/B6 cells) to further analyze an inward-rectifying K current (IK, IR) which is suggested to be involved in the thyrotropin-releasing hormone (TRH)-induced increase in prolactin secretion from these cells. Using the class III antiarrhythmic agent E-4031 which is known as specific blocker of ether-á-go-go-related gene (ERG) K channels, the inward-rectifying K current could be isolated as the drug-sensitive current. To elucidate in molecular basis of this current, comparative experiments were performed in CHO cells which served as heterologous expression system for RERG, the rat homologue of the human ERG (HERG). It is shown that the inward-rectifying K current has properties identical to those mediated by channels encoded by RERG. In external 5 mM K+ solution, the ERG-like current IK, IR was an outward current in the physiological potential range, and this outward current could be strongly reduced by TRH. A specific block of IK, IR was able to mimick the second phase of the TRH-response which is characterized by a depolarization and/or by an increase in the frequency of Ca action potentials. These data show, that the ERG-like current in GH3/B6 cells contributes to the maintainance of the resting membrane potential and that it plays an important part in the mechanisms of the effects of TRH leading to an increase in prolactin secretion.

RERG is a molecular correlate of the inward-rectifying K current in clonal rat pituitary cells

The rat homologue of the human ether-ä-go-go-related gene (r-erg) was cloned from rat brain using homology screening. RERG has a 96% amino acid identify to HERG. Membrane currents recorded in CHO cells after previous injection of r-erg showed that the voltage- and time-dependent properties are indistinguishable from h-erg-induced currents expressed in the same system. RT-PCR revealed the presence of r-erg mRNA in clonal rat pituitary cells (GH3/B6 cells). These cells exhibit a voltage-dependent inward-rectifying K current (IK, IR) which is highly sensitive to the class III antiarrhythmic E-4031. IK, IR recorded in GH3/B6 cells and ERG currents in CHO cells were compared using similar experimental conditions (same pulse protocols and isotonic KCl as extracellular solution). The voltage- and time-dependent properties of both currents were found to be almost identical. These results strongly suggest that RERG channels mediate IK, IR in GH3/B6 cells.

The class III antiarrhythmic agent E-4031 selectively blocks the inactivating inward-rectifying potassium current in rat anterior pituitary tumor cells (GH3/B6 cells)

Hyperpolarization-elicited potassium currents in GH3/B6 cells bathed in high-potassium external solution were recorded to assess effects of the class III antiarrhythmic agent E-4031 on the inactivating inward-rectifying potassium current (IK,IR). E-4031 potently blocked IK,IR with an IC50 value of 10 nM. The complete block of IK,IR achieved with concentrations >/= 1 microM revealed the presence of a non-inactivating outward-rectifying current which contributed to the membrane currents recorded under control conditions. The time dependence of the IK,IR block depended on the concentration of E-4031. Two other methanesulfonanilides were investigated: WAY-123,398 (10 microM) also totally blocked IK,IR, while sotalol (100 microM) was almost ineffective. Also lanthanum (100 microM) had only a very small effect on IK,IR. E-4031 did not affect sodium, calcium and voltage-dependent outward-rectifying potassium currents, suggesting a selective block of IK,IR in GH3/B6 cells. In an external solution containing 16 mM potassium, the E-4031-sensitive current was present as a steady outward current within a broad potential range positive to the potassium equilibrium potential, EK. In many, but not all, cells E-4031 induced an increase in the frequency of action potentials suggesting an important role of IK,IR in controlling cell excitability. Our experiments show that E-4031 is a valuable tool in characterizing IK,IR and its physiological function.

Most lactotrophs from lactating rats are able to respond to both thyrotropin-releasing hormone and dopamine

Intracellular free calcium concentration ([Ca2+]i) was measured with video imaging in lactotrophs from lactating rats. The median resting [Ca2+]i was 24 nM (85 cells). The great majority of cells responded to thyrotropin-releasing hormone (TRH) with an increase in [Ca2+]i, (median peak [Ca2+]i after TRH = 298 nM; n = 73). In 77% of these cells this [Ca2+]i increase was biphasic, with [Ca2+]i remaining high after the initial peak (median [Ca2+]i 90 s after TRH application = 104 nM; n = 56); the second phase depended on calcium influx. Most cells also responded to dopamine (DA), after TRH had been applied. DA reduced or abolished TRH-induced calcium influx and also reduced resting [Ca2+]i if this was above its initial value. A few lactotrophs responded to TRH only after DA application and withdrawal. We conclude that the population of lactotrophs in lactating rats is heterogeneous, but is not composed of two distinct sub-groups defined by their responsiveness to TRH or DA.

An endogenous inactivating inward-rectifying potassium current in oocytes of Xenopus laevis

An endogenous inward-rectifying K+ current is described, which is present in native oocytes of some Xenopus laevis donors. Experiments were performed using defolliculated oocytes from donor frogs obtained from two different suppliers. In all oocytes from animals from one source, an inward-rectifying K+ current could be elicited with negative pulses from a holding potential of -20 mV in external solutions with a high K+ concentration. Increasing external K+ concentrations increased the amplitude of this current and shifted the reversal potential towards more positive potentials. In 118 mM KCl, the inward-rectifying K+ current partially inactivated between -20 and -80 mV and completely inactivated at more negative membrane potentials; 50% steady-state inactivation occurred near -50 mV. The time course of inactivation of the inward-rectifying current could be well fitted with two exponentials. The slow time constant had values of about 500 ms and was voltage independent. In contrast, the fast time constant and the time to reach the peak inward current decreased with more negative membrane potentials. Ba2+, Cs+, quinine (all 5 mM) and 50 mM tetraethylammonium partially blocked the inward-rectifying K+ current, whereas 10 mM 4-aminopyridine was without blocking effect. The oxidant chloramine-T blocked the inward-rectifying K+ current without slowing its inactivation.

An inactivating inward-rectifying K current present in prolactin cells from the pituitary of lactating rats

Primary cultures containing a high percentage of lactotrophs were obtained by dissociating the pituitary of rats following 14-18 days of lactation. Lactotrophs with a distinctive appearance were recorded after 1-35 days in vitro and identified by immunocytochemical staining for prolactin. Whole-cell voltage clamp measurements in isotonic KCl solution from a holding potential of -40 mV revealed the presence of inward-rectifying K currents with a time-dependent, Na(+)-independent inactivation at potentials negative to -60 mV. The time for complete inactivation was strikingly different between lactotrophs, varying between 1 sec and more than 5 sec at -120 mV, and was not related to time in culture. The reversal potential shifted 59 mV (25 degree C) for a tenfold change in external K+ concentration, demonstrating the selectivity of the channel for K+ over Na+. The inward-rectifying K current was blocked by 5 mM Ba2+ and partially blocked by 10 mM TEA. Chloramine-T (1 and 2 mM) produced a total block of the inward-rectifying K current in lactotrophs. Thyrotropin-releasing hormone (500 nM) significantly reduced the inward-rectifying K current in about half of the lactotrophs. This current is similar to the inward-rectifying K current previously characterized in clonal somatomammotrophic pituitary cells (GH3B6). The variability of the rate of inactivation of this current in lactotrophs and its responsiveness to TRH is discussed.