Inhibitory effects of dronedarone on muscarinic K+ current in guinea pig atrial cells

Dronedarone (SR33589), an amiodarone-like noniodinated antiarrhythmic agent, is undergoing clinical trials in atrial fibrillation. Because vagal activation plays a role in the pathophysiology of supraventricular arrhythmias, we have assessed the ability of dronedarone (0.01, 0.1, and 1 microM), compared with amiodarone (0.1, 1, and 10 microM) to inhibit the muscarinic acetylcholine receptor-operated K+ current (I(K(ACh))) in single cells isolated from guinea pig atria (patch-clamp technique). I(K(ACh)) was activated by extracellular application of carbachol (10 microM) or by intracellular loading with GTP-gamma-S (100 microM). Dronedarone and amiodarone reduced the carbachol-induced I(K(ACh)) with an IC50 (concentration required for 50% inhibition) slightly above 10 nM and 1 microM, respectively. Dronedarone also inhibited the GTP-gamma-S induced K+ current by 28% and 58% at 0.01 and 0.1 microM, respectively. These data suggest that dronedarone inhibits I(K(ACh)) by depressing the function of K(ACh) channel itself or associated GTP-binding proteins. Compared with amiodarone, dronedarone is approximately 100 times more potent on I(K(ACh)) and seems more selective in inhibiting I(K(ACh)) with respect to its antagonism of other inward and outward currents reported in the literature. This relative high potency of dronedarone to reduce I(K(ACh)) may be involved, at least in part, in the antiarrhythmic action of dronedarone against atrial fibrillation.

Molecular mechanism and functional significance of the MinK control of the KvLQT1 channel activity

The very slowly activating delayed rectifier K+ channel IKs is essential for controlling the repolarization phase of cardiac action potentials and K+ homeostasis in the inner ear. The IKs channel is formed via the assembly of two transmembrane proteins, KvLQT1 and MinK. Mutations in KvLQT1 are associated with a long QT syndrome that causes syncope and sudden death and also with deafness. Here, we show a new mode of association between ion channel forming subunits in that the cytoplasmic C-terminal end of MinK interacts directly with the pore region of KvLQT1. This interaction reduces KvLQT1 channel conductance from 7.6 to 0.58 picosiemens. However, because MinK also reveals a large number of previously silent KvLQT1 channels (x 60), the overall effect is a large increase (x 4) in the macroscopic K+ current. Conformational changes associated with the KvLQT1/MinK association create very slow and complex activation kinetics without much alteration in the deactivation process. Changes induced by MinK have an essential regulatory role in the development of this K+ channel activity upon repetitive electrical stimulation with a particular interest in tachycardia.

Modes of regulation of shab K+ channel activity by the Kv8.1 subunit

The Kv8.1 subunit is unable to generate K+ channel activity in Xenopus oocytes or in COSm6 cells. The Kv8.1 subunit expressed at high levels acts as a specific suppressor of the activity of Kv2 and Kv3 channels in Xenopus oocytes (Hugnot, J. P., Salinas, M., Lesage, F., Guillemare, E., Weille, J., Heurteaux, C., Mattéi, M. G., and Lazdunski, M. (1996) EMBO J. 15, 3322-3331). At lower levels, Kv8.1 associates with Kv2.1 and Kv2.2 to form hybrid Kv8.1/Kv2 channels, which have new biophysical properties and more particularly modified properties of the inactivation process as compared with homopolymers of Kv2.1 or Kv2.2 channels. The same effects have been seen by coexpressing the Kv8.1 subunit and the Kv2.2 subunit in COSm6 cells. In these cells, Kv8.1 expressed alone remains in intracellular compartments, but it can reach the plasma membrane when it associates with Kv2.2, and it then also forms new types of Kv8.1/Kv2. 2 channels. Present results indicate that Kv8.1 when expressed at low concentrations acts as a modifier of Kv2.1 and Kv2.2 activity, while when expressed at high concentrations in oocytes it completely abolishes Kv2.1, Kv2.2, or Kv3.4 K+ channel activity. The S6 segment of Kv8.1 is atypical and contains the structural elements that modify inactivation of Kv2 channels.

Effects of SR 48692 on neurotensin-induced calcium-activated chloride currents in the Xenopus oocyte expression system: agonist-like activity on the levocabastine-sensitive neurotensin receptor and absence of antagonist effect on the levocabastine insensitive neurotensin receptor

The effect of the drug SR 48692 on the Ca(2+)-activated Cl- current induced by neurotensin on Xenopus oocytes injected with cRNAs encoding rodent high and low affinity neurotensin receptors, was examined. In this receptor expression system, SR 48692 failed to antagonize electrophysiological measurement of neurotensin-evoked current via the rat high affinity neurotensin receptor, whereas its application onto oocytes expressing the mouse low affinity neurotensin receptor triggered an inward current, as well as neurotensin itself. However, no current activation was observed after application of the drug on oocytes expressing the rat high affinity neurotensin receptor. These observations in the oocyte expression system did not reflect typical antagonist properties of SR 48692 drug.

Dimerization of TWIK-1 K+ channel subunits via a disulfide bridge

TWIK-1 is a new type of K+ channel with two P domains and is abundantly expressed in human heart and brain. Here we show that TWIK-1 subunits can self-associate to give dimers containing an interchain disulfide bridge. This assembly involves a 34 amino acid domain that is localized to the extracellular M1P1 linker loop. Cysteine 69 which is part of this interacting domain is implicated in the formation of the disulfide bond. Replacing this cysteine with a serine residue results in the loss of functional K+ channel expression. This is the first example of a covalent association of functional subunits in voltage-sensitive channels via a disulfide bridge.

K(V)LQT1 and lsK (minK) proteins associate to form the I(Ks) cardiac potassium current

In mammalian cardiac cells, a variety of transient or sustained K+ currents contribute to the repolarization of action potentials. There are two main components of the delayed-rectifier sustained K+ current, I(Kr) (rapid) and I(Ks), (slow). I(Kr) is the product of the gene HERG, which is altered in the long-QT syndrome, LQT2. A channel with properties similar to those of the I(Ks) channel is produced when the cardiac protein IsK is expressed in Xenopus oocytes. However, it is a small protein with a very unusual structure for a cation channel. The LQT1 gene is another gene associated with the LQT syndrome, a disorder that causes sudden death from ventricular arrhythmias. Here we report the cloning of the full-length mouse K(V)LQT1 complementary DNA and show that K(V)LQT1 associates with IsK to form the channel underlying the I(Ks) cardiac current, which is a target of class-III anti-arrhythmic drugs and is involved in the LQT1 syndrome.

Structure, functional expression, and cerebral localization of the levocabastine-sensitive neurotensin/neuromedin N receptor from mouse brain

This work describes the cloning and expression of the levocabastine-sensitive neurotensin (NT) receptor from mouse brain. The receptor protein comprises 417 amino acids and bears the characteristics of G-protein-coupled receptors. This new NT receptor (NTR) type is 39% homologous to, but pharmacologically distinct from, the only other NTR cloned to date from the rat brain and the human HT29 cell line. When the receptor is expressed in Xenopus laevis oocytes, the H1 antihistaminic drug levocabastine, like NT and neuromedin N, triggers an inward current. The pharmacological properties of this receptor correspond to those of the low-affinity, levocabastine-sensitive NT binding site described initially in membranes prepared from rat and mouse brain. It is expressed maximally in the cerebellum, hippocampus, piriform cortex, and neocortex of adult mouse brain.

Kv8.1, a new neuronal potassium channel subunit with specific inhibitory properties towards Shab and Shaw channels

Outward rectifier K+ channels have a characteristic structure with six transmembrane segments and one pore region. A new member of this family of transmembrane proteins has been cloned and called Kv8.1. Kv8.1 is essentially present in the brain where it is located mainly in layers II, IV and VI of the cerebral cortex, in hippocampus, in CA1-CA4 pyramidal cell layer as well in granule cells of the dentate gyrus, in the granule cell layer and in the Purkinje cell layer of the cerebellum. The Kv8.1 gene is in the 8q22.3-8q24.1 region of the human genome. Although Kv8.1 has the hallmarks of functional subunits of outward rectifier K+ channels, injection of its cRNA in Xenopus oocytes does not produce K+ currents. However Kv8.1 abolishes the functional expression of members of the Kv2 and Kv3 subfamilies, suggesting that the functional role of Kv8.1 might be to inhibit the function of a particular class of outward rectifier K+ channel types. Immunoprecipitation studies have demonstrated that inhibition occurs by formation of heteropolymeric channels, and results obtained with Kv8.1 chimeras have indicated that association of Kv8.1 with other types of subunits is via its N-terminal domain.

TWIK-1, a ubiquitous human weakly inward rectifying K+ channel with a novel structure

A new human weakly inward rectifying K+ channel, TWIK-1, has been isolated. This channel is 336 amino acids long and has four transmembrane domains. Unlike other mammalian K+ channels, it contains two pore-forming regions called P domains. Genes encoding structural homologues are present in the genome of Caenorhabditis elegans. TWIK-1 currents expressed in Xenopus oocytes are time-independent and present a nearly linear I-V relationship that saturated for depolarizations positive to O mV in the presence of internal Mg2+. This inward rectification is abolished in the absence of internal Mg2+. TWIK-1 has a unitary conductance of 34 pS and a kinetic behaviour that is dependent on the membrane potential. In the presence of internal Mg2+, the mean open times are 0.3 and 1.9 ms at -80 and +80 mV, respectively. The channel activity is up-regulated by activation of protein kinase C and down-regulated by internal acidification. Both types of regulation are indirect. TWIK-1 channel activity is blocked by Ba2+(IC50=100 microM), quinine (IC50=50 microM) and quinidine (IC50=95 microM). This channel is of particular interest because its mRNA is widely distributed in human tissues, and is particularly abundant in brain and heart. TWIK-1 channels are probably involved in the control of background K+ membrane conductances.

A pH-sensitive yeast outward rectifier K+ channel with two pore domains and novel gating properties

YORK is a newly cloned K+ channel from yeast. Unlike all other cloned K+ channels, it has two pore domains instead of one. It displays eight transmembrane segments arranged like a covalent assembly of a Shaker-type voltage-dependent K+ channel (without S4 transmembrane segments) with an inward rectifier K+ channel. When expressed in Xenopus oocytes, YORK does not pass inward currents; it conducts only K+-selective outward currents. However, the mechanism responsible for this strict outward rectification is unusual. Like inward rectifiers, its activation potential threshold closely follows the K+ equilibrium potential. Unlike inward rectifiers, the rectification is not due to a voltage-dependent Mg2+ block. The blocking element is probably intrinsic to the YORK protein itself. YORK activity is decreased at acidic internal pH, with a pKa of 6.5. Pharmacological and regulation properties were analyzed. Ba2+ ions and quinine block YORK currents through high and low affinity sites, while tetraethylammonium displays only one affinity for blocking. Activation of protein kinase C indirectly produces an increase of the current, while protein kinase A activation has no effect.

Susceptibility of cloned K+ channels to reactive oxygen species

Free radical-induced oxidant stress has been implicated in a number of physiological and pathophysiological states including ischemia and reperfusion-induced dysrhythmia in the heart, apoptosis of T lymphocytes, phagocytosis, and neurodegeneration. We have studied the effects of oxidant stress on the native K+ channel from T lymphocytes and on K+ channels cloned from cardiac, brain, and T-lymphocyte cells and expressed in Xenopus oocytes. The activity of three Shaker K+ channels (Kv1.3, Kv1.4, and Kv1.5), one Shaw channel (Kv3.4), and one inward rectifier K+ channel (IRK3) was drastically inhibited by photoactivation of rose bengal, a classical generator of reactive oxygen species. Other channel types (such as Shaker K+ channel Kv1.2, Shab channels Kv2.1 and Kv2.2, Shal channel Kv4.1, inward rectifiers IRK1 and ROMK1, and hIsK) were completely resistant to this treatment. On the other hand tert-butyl hydroperoxide, another generator of reactive oxygen species, removed the fast inactivation processes of Kv1.4 and Kv3.4 but did not alter other channels. Xanthine/xanthine oxidase system had no effect on all channels studied. Thus, we show that different types of K+ channels are differently modified by reactive oxygen species, an observation that might be of importance in disease states.

Molecular properties of neuronal G-protein-activated inwardly rectifying K+ channels

Four cDNA-encoding G-activated inwardly rectifying K+ channels have been cloned recently (Kubo, Y., Reuveny, E., Slesinger, P. A., Jan, Y. N., and Jan, L. Y. (1993) Nature 364, 802-806; Lesage, F., Duprat, F., Fink, M., Guillemare, E., Coppola, T., Lazdunski, M., and Hugnot, J. P. (1994) FEBS Lett. 353, 37-42; Krapivinsky, G., Gordon, E. A., Wickman, K., Velimirovic, B., Krapivinsky, L., and Clapham, D. E. (1995) Nature 374, 135-141). We report the cloning of a mouse GIRK2 splice variant, noted mGIRK2A. Both channel proteins are functionally expressed in Xenopus oocytes upon injection of their cRNA, alone or in combination with the GIRK1 cRNA. Three GIRK channels, mGIRK1-3, are shown to be present in the brain. Colocalization in the same neurons of mGIRK1 and mGIRK2 supports the hypothesis that native channels are made by an heteromeric subunit assembly. GIRK3 channels have not been expressed successfully, even in the presence of the other types of subunits. However, GIRK3 chimeras with the amino- and carboxyl-terminal of GIRK2 are functionally expressed in the presence of GIRK1. The expressed mGIRK2 and mGIRK1, -2 currents are blocked by Ba2+ and Cs+ ions. They are not regulated by protein kinase A and protein kinase C. Channel activity runs down in inside-out excised patches, and ATP is required to prevent this rundown. Since the nonhydrolyzable ATP analog AMP-PCP is also active and since addition of kinases A and C as well as alkaline phosphatase does not modify the ATP effect, it is concluded that ATP hydrolysis is not required. An ATP binding process appears to be essential for maintaining a functional state of the neuronal inward rectifier K+ channel. A Na+ binding site on the cytoplasmic face of the membrane acts in synergy with the ATP binding site to stabilize channel activity.

Kalicludines and kaliseptine. Two different classes of sea anemone toxins for voltage sensitive K+ channels

New peptides have been isolated from the sea anemone Anemonia sulcata which inhibit competitively the binding of 125I-dendrotoxin I (a classical ligand for K+ channel) to rat brain membranes and behave as blockers of voltage-sensitive K+ channels. Sea anemone kalicludines are 58-59-amino acid peptides cross-linked with three disulfide bridges. They are structurally homologous both to dendrotoxins which are snake venom toxins and to the basic pancreatic trypsin inhibitor (Kunitz inhibitor) and have the unique property of expressing both the function of dendrotoxins in blocking voltage-sensitive K+ channels and the function of the Kunitz inhibitor in inhibiting trypsin. Kaliseptine is another structural class of peptide comprising 36 amino acids with no sequence homology with kalicludines or with dendrotoxins. In spite of this structural difference, it binds to the same receptor site as dendrotoxin and kalicludines and is as efficient as a K+ channel inhibitor as the most potent kalicludine.

Heterologous multimeric assembly is essential for K+ channel activity of neuronal and cardiac G-protein-activated inward rectifiers

The family of G-protein-activated inward-rectifiers K+ channels presently comprise at least 3 cloned members called GIRK1, GIRK2 and GIRK3. A close structural parent of GIRK channels has recently been described as being an ATP-sensitive K+ channel. This paper shows that Xenopus expression of this new channel that we call GIRK4 does not produce an ATP-inhibitable activity with a pharmacological activation by pinacidil as previously described but instead a G-protein activated inward-rectifier. While oocyte expression of single subunits is infrequent and relatively modest in intensity, expression of GIRK1,2, GIRK1,4 and GIRK2,4 mixtures leads to routine and robust expression of K+ channels indicating that heterologous subunit assembly is necessary for activity. Activity of GIRK1,2, GIRK1,4 and GIRK2,4 channels required the presence of ATP acting as an activator at the cytoplasmic face and is further activated by the beta gamma subunits.

Glibenclamide opens ATP-sensitive potassium channels in Xenopus oocyte follicular cells during metabolic stress

Follicular cells from Xenopus oocytes offer a particularly interesting system to study ATP-sensitive K+ channels (KATP channels). In these cells, as in many other cell types, glibenclamide is a classical blocker of KATP channels. Metabolic inhibition with dinitrophenol (DNP) converts this inhibitory effect into an activation. Follicular cells treated with DNP keep their sensitivity to the KATP channel opener P1060, but this opening effect becomes insensitive to glibenclamide inhibition. Glibenclamide activation of KATP channels in DNP-treated follicular cells occurs with an EC50 of 3 microM. Glibenclamide activation is antagonized by blockers of KATP channels that do not belong to the sulfonylurea family, such as U-37883A, tedisamil, and LH 35. Other sulfonylureas display the same activating behavior as does glibenclamide in DNP-treated cells. Two of the properties of KATP channels in follicular cells are activation by cAMP through protein kinase A and inhibition by muscarinic effectors through protein kinase C activation. The stimulating effects of cAMP and glibenclamide in DNP-treated cells seem to be synergistic as are the cAMP and P1060 effects in control follicular cells. Glibenclamide-activated KATP channels in DNP-treated cells (conductance of 15 pS) are also inhibited by acetylcholine and by phorbol esters. The internal acidosis produced by metabolic exhaustion with DNP appears to be the key element in the conversion of glibenclamide from a blocker to an activator of KATP channels.

Cloning provides evidence for a family of inward rectifier and G-protein coupled K+ channels in the brain

MbIRK3, mbGIRK2 and mbGIRK3 K+ channels cDNAs have been cloned from adult mouse brain. These cDNAs encode polypeptides of 445, 414 and 376 amino acids, respectively, which display the hallmarks of inward rectifier K+ channels, i.e. two hydrophobic membrane-spanning domains M1 and M2 and a pore-forming domain H5. MbIRK3 shows around 65% amino acid identity with IRK1 and rbIRK2 and only 50% with ROMK1 and GIRK1. On the other hand, mbGIRK2 and mbGIRK3 are more similar to GIRK1 (60%) than to ROMK1 and IRK1 (50%). Northern blot analysis reveals that these three novel clones are mainly expressed in the brain. Xenopus oocytes injected with mbIRK3 and mbGIRK2 cRNAs display inward rectifier K(+)-selective currents very similar to IRK1 and GIRK1, respectively. As expected from the sequence homology, mbGIRK2 cRNA directs the expression of G-protein coupled inward rectifier K+ channels which has been observed through their functional coupling with co-expressed delta-opioid receptors. These results provide the first evidence that the GIRK family, as the IRK family, is composed of multiple genes with members specifically expressed in the nervous system.

CGRP-induced activation of KATP channels in follicular Xenopus oocytes

The two-microelectrode voltage-clamp technique was used to monitor K+ channel activity in Xenopus oocyte follicular cells, which are electrically coupled to the oocyte itself by gap junctions. Endogenous vasodilators such as calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP), prostaglandin E2 (PGE2) and adenosine activate glibenclamide-ATP-sensitive K+ (KATP) channels in Xenopus oocyte follicular cells. The mechanism of action of CGRP was studied in detail. CGRP effects undergo a rapid desensitization. CGRP acts via CGRPI receptors. Its effects are antagonised by the amino-truncated CGRP analog hCGRP(8-37). The second messenger for CGRP activation of KATP channels is cAMP. Phosphodiesterase inhibition by 3-isobutyl-1-methylxanthine enhances the CGRP response while adenyl cyclase inhibition by either 2',5'-dideoxyadenosine or progesterone nearly completely depresses the CGRP response. Vasoconstrictors such as ACh and angiotensin II also have receptors in follicular cells. ACh strongly inhibits the CGRP activation of K+ channels as it inhibits the activation of KATP channels by P1060, but angiotensin II does not. It is concluded that as in vascular smooth muscle cells, CGRP and probably other hyperpolarizing vasodilators open KATP channels in follicular cells by protein kinase A activation.

Injection of a K+ channel (Kv1.3) cRNA in fertilized eggs leads to functional expression in cultured myotomal muscle cells from Xenopus embryos

The synthetic cRNA encoding for the major T lymphocyte K+ channel (Kv1.3) was injected into Xenopus fertilized eggs. Somites from embryos of stage 20-22 (about 40 h post-fertilization at 19 degrees C) were dissociated and myotomal muscle cells were cultured in vitro for 2 days. The whole cell configuration of the tight seal patch-clamp technique was used to record K+ channel activity in cultured myocytes. These myocytes have two endogenous delayed-rectifiers (sustained and transient) and an inward-rectifier K+ currents, all of which are insensitive to the scorpion toxin charybdotoxin. Cultured myocytes dissociated from embryos injected with the Kv1.3 cRNA expressed the exogenous Kv1.3 channel. The Kv1.3 channel was identified by its physiological (a very low recovery from inactivation) and its pharmacological properties (a high sensitivity to charybdotoxin). This work demonstrates that Xenopus cultured myotomal muscle cells represent a very efficient and practical assay system for the functional expression of cloned ion channels.

Functional receptors in Xenopus oocytes for U-37883A, a novel ATP-sensitive K+ channel blocker: comparison with rat insulinoma cells

Follicle-enclosed Xenopus oocytes were used to describe the ATP-sensitive K+ (KATP) channel-blocking properties of U-37883A (4-morpholinecarboximidine-N-1-adamantyl-N'-cyclohexyl), in comparison with glibenclamide. In follicular oocytes, the KATP channel opener P1060 (30 microM), a pinacidil analog, activated a large outward K+ current that was blocked by glibenclamide (IC50 = 0.33 microM) and U-37883A (IC50 = 0.26 microM). P1060 activation was inhibited by both U-37883A and glibenclamide in a noncompetitive manner. U-37883A also blocked the KATP channel activation by cAMP (300 microM) and adenosine (10 microM). Single-channel studies on isolated follicular cells showed that U-37883A (10 microM) reduced the open probability of the KATP channel by 76%, without significantly modifying the single-channel current amplitude. Receptor binding studies with [3H]U-37883 in membranes from follicle-enclosed oocytes demonstrated a single class of low affinity binding sites, with a Kd of 450 nM and a Bmax of 17 pmol/mg of protein. Studies with analogs of U-37883A showed that U-52090A inhibited KATP current and displaced [3H]U-37883 from its binding site with similar potencies. In contrast, U-42069D neither inhibited KATP current nor competed with [3H]U-37883 binding. In RINm5F cells (an insulinoma cell line), U-37883A, unlike glibenclamide, failed to inhibit KATP current. Furthermore, there was no significant specific binding of [3H]U-37883 in RINm5F cell membranes, which displayed high levels of specific binding of [3H]glibenclamide. These data demonstrate the presence of a receptor for U-37883A-type guanidines that controls the activity of the endogenous KATP channels in follicle-enclosed oocytes. The available data collectively suggest that U-37883A is a more selective blocker of the follicular KATP channel, which is very similar to that in smooth muscle, than of the pancreatic beta cell KATP channel.

Multiple mRNA isoforms encoding the mouse cardiac Kv1-5 delayed rectifier K+ channel

The mouse Kv1-5 K+ channel cDNA has been cloned from heart. This channel was highly expressed in heart and, to a lesser extent, in other tissues, including brain and thymus. Two alternatively spliced isoforms were found. The longer form encoded a 602-amino acid protein, while in the short form (Kv1-5 delta 5'), the first 200 amino acids lying upstream the transmembrane segment S1 were deleted. RNase protection experiments showed that both Kv1-5 mRNA isoforms are present in the mouse tissues examined, the longer form being predominant. The short mRNA (Kv1-5 delta 5') arose by an unusual splicing event within the exonic sequence. An additional short cDNA clone (Kv1-5 delta 3') that codes for a carboxyl-terminal truncated protein has been isolated. The gene coding sequence contained a single exon and has been mapped on human chromosome 12 (p13) and on mouse chromosome 6 (band F). Expression in Xenopus oocytes revealed that the long (Kv1-5) and the amino-terminal deleted (Kv1-5 delta 5') isoforms elicited similar K+ currents with a drastically decreased efficacy for Kv1-5 delta 5'. The carboxyl-terminal truncated Kv1-5 delta 3' clone was not functional but inhibited the expression of the long isoform.

The protein IsK is a dual activator of K+ and Cl- channels

The protein IsK (M(r) 14,500) is present in epithelial cells, heart, uterus and lymphocytes and induces slowly activating K+ currents when expressed in Xenopus oocytes. The finding that mutations of its single transmembrane segment altered channel gating or selectivity has suggested that IsK is a channel-forming protein. But IsK does not exhibit the K+ channel hallmarks (a conserved K+ selective pore (H5) flanked by either six or two membrane-spanning regions). Here we report that IsK expression in Xenopus oocytes also induces a Cl- selective current very similar to the Cl- current produced by phospholemman expression and with biophysical, pharmacological and regulation characteristics very different from those of the IsK-induced K+ channel activity. IsK mutagenesis identifies amino- and carboxy-terminal domains as critical for the induction of Cl- and K+ channel activities, respectively. Our data lead to a model in which the IsK protein (now called IsK, Cl) acts as a potent activator of endogenous and otherwise silent K+ or Cl- channels.

Effects of the level of mRNA expression on biophysical properties, sensitivity to neurotoxins, and regulation of the brain delayed-rectifier K+ channels Kv1.2

Injection of 0.2 ng of cRNA encoding the brain Kv1.2 channel into Xenopus oocytes leads to the expression of a very slowly inactivating K+ current. Inactivation is absent in oocytes injected with 20 ng of cRNA although activation remains unchanged. Low cRNA concentrations generate a channel which is sensitive to dendrotoxin I (IC50 = 2 nM at 0.2 ng of cRNA/oocyte) and to less potent analogs of this toxin from Dendroaspis polylepis venom. A good correlation is found between blockade of the K+ current and binding of the different toxins to rat brain membranes. High cRNA concentrations generate another form of the K+ channel which is largely insensitive to dendrotoxin I (IC50 = 200 nM at 20 ng of cRNA per oocyte). At low cRNA concentrations, the expressed Kv1.2 channel is also blocked by other polypeptide toxins such as MCD peptide (IC50 = 20 nM), charybdotoxin (IC50 = 50 nM), and beta-bungarotoxin (IC50 = 50 nM), which bind to distinct and allosterically related sites on the channel protein. The pharmacologically distinct type of K+ channel expressed at high cRNA concentrations (20 ng of cRNA/oocyte) is nearly totally resistant to 100 nM MCD peptide and hardly altered by charybdotoxin and beta-bungarotoxin at concentrations as high as 1 microM. Both at low and at high cRNA concentrations, the expressed Kv1.2 channel is blocked by an increase in intracellular Ca2+ from the inositol trisphosphate sensitive pools and by the phorbol ester PMA that activates protein kinase C.