Inhibitory effects of N-methyl-D-aspartate (NMDA) and α1-adrenergic receptor antagonist ifenprodil on human Kv1.5 channel

Ifenprodil has been known to reduce cardiac contractility and cerebral vasodilation by antagonizing α1-adrenergic and N-methyl D-aspartate receptor-mediated intracellular signals. This study aimed to investigate the direct effect of ifenprodil on the human voltage-gated Kv1.5 channel (hKv1.5) by using a Xenopus oocyte expression system and a two-microelectrode voltage clamp technique. The amplitudes of hKv1.5 currents, including peak and steady state, were suppressed in a concentration-dependent manner (IC50; 43.1 and 35.5 μM, respectively) after 6 min of ifenprodil treatment. However, these effects were  ~ 80% reversed by washout, suggesting that ifenprodil directly inhibited the hKv1.5 independent of membrane receptors or intracellular signals. The inhibition rate of steady state showed voltage dependence, wherein the rates increased according to test voltage depolarization. Ifenprodil reduced the time constants of hKv1.5 inactivation but has higher effects on activation. hKv1.5 inhibition by ifenprodil showed use dependency because the drug more rapidly reduced the current at the higher activation frequencies, and subsequent reduction in frequency after high activation frequency caused a partial channel block relief. Therefore, ifenprodil directly blocked the hKv1.5 in an open state and accelerated the time course of the channel inactivation, which provided a biophysical mechanism for the hKv1.5 blocking effects of ifenprodil.

The voltage-gated potassium channel KV1.3 regulates neutrophil recruitment during inflammation

AIMS

Neutrophil trafficking within the vasculature strongly relies on intracellular calcium signalling. Sustained Ca2+ influx into the cell requires a compensatory efflux of potassium to maintain membrane potential. Here, we aimed to investigate whether the voltage-gated potassium channel KV1.3 regulates neutrophil function during the acute inflammatory process by affecting sustained Ca2+ signalling.

METHODS AND RESULTS

Using in vitro assays and electrophysiological techniques, we show that KV1.3 is functionally expressed in human neutrophils regulating sustained store-operated Ca2+ entry through membrane potential stabilizing K+ efflux. Inhibition of KV1.3 on neutrophils by the specific inhibitor 5-(4-Phenoxybutoxy)psoralen (PAP-1) impaired intracellular Ca2+ signalling, thereby preventing cellular spreading, adhesion strengthening, and appropriate crawling under flow conditions in vitro. Using intravital microscopy, we show that pharmacological blockade or genetic deletion of KV1.3 in mice decreased neutrophil adhesion in a blood flow dependent fashion in inflamed cremaster muscle venules. Furthermore, we identified KV1.3 as a critical component for neutrophil extravasation into the inflamed peritoneal cavity. Finally, we also revealed impaired phagocytosis of Escherichia coli particles by neutrophils in the absence of KV1.3.

CONCLUSION

We show that the voltage-gated potassium channel KV1.3 is critical for Ca2+ signalling and neutrophil trafficking during acute inflammatory processes. Our findings do not only provide evidence for a role of KV1.3 for sustained calcium signalling in neutrophils affecting key functions of these cells, they also open up new therapeutic approaches to treat inflammatory disorders characterized by overwhelming neutrophil infiltration.

Evidence for proteasomal degradation of Kv1.5 channel protein

BACKGROUND

The voltage-gated potassium channel Kv1.5 plays a critical role in the maintenance of the membrane potential. While protein degradation is one of the major mechanisms for the regulation of channel functions, little is known on the degradation mechanism of Kv1.5.

METHODS AND RESULTS

Kv1.5 was expressed in COS cells and its degradation, intracellular localization, and channel activities were assessed by pulse-chase analysis, immunofluorescence, and patch clamp techniques, respectively. Expressed Kv1.5 had a half-life time of approximately 6.7 h, which was prolonged by the proteasome inhibitors of MG132, ALLN, proteasomal inhibitor 1, or lactacystine, but not by a lysosomal inhibitor chloroquine. MG132 increased the protein level of Kv1.5, as well as the level of its ubiquitinated form in a dose-dependent manner. Similar effects of MG132 on endogenous Kv1.5 were seen in cultured rat atrial cells. Within a cell, Kv1.5 was mainly localized in both the endoplasmic reticulum and Golgi apparatus. MG132 increased the immunoreactivity of Kv1.5 in these compartments and also increased Ik(ur) currents through the cell-surface Kv1.5. Pretreatment with either brefeldin A or colchicine abolished MG132-induced increase in Ik(ur) currents.

CONCLUSION

Kv1.5 is degraded by the proteasome. The inhibition of the proteasome increased Ik(ur) currents secondary to stabilization of the channel protein in the endoplasmic reticulum/Golgi apparatus.

Constitutive inactivation of the hKv1.5 mutant channel, H463G, in K+-free solutions at physiological pH

Extracellular acidification and reduction of extracellular K(+) are known to decrease the currents of some voltage-gated potassium channels. Although the macroscopic conductance of WT hKv1.5 channels is not very sensitive to [K(+)](o) at pH 7.4, it is very sensitive to [K(+)](o) at pH 6.4, and in the mutant, H463G, the removal of K(+)(o) virtually eliminates the current at pH 7.4. We investigated the mechanism of current regulation by K(+)(o) in the Kv1.5 H463G mutant channel at pH 7.4 and the wild-type channel at pH 6.4 by taking advantage of Na(+) permeation through inactivated channels. Although the H463G currents were abolished in zero [K(+)](o), robust Na(+) tail currents through inactivated channels were observed. The appearance of H463G Na(+) currents with a slow rising phase on repolarization after a very brief depolarization (2 ms) suggests that channels could activate directly from closed-inactivated states. In wild-type channels, when intracellular K(+) was replaced by NMG(+) and the inward Na(+) current was recorded, addition of 1 mM K(+) prevented inactivation, but changing pH from 7.4 to 6.4 reversed this action. The data support the idea that C-type inactivation mediated at R487 in Kv1.5 channels is influenced by H463 in the outer pore. We conclude that both acidification and reduction of [K(+)](o) inhibit Kv1.5 channels through a common mechanism (i.e., by increasing channel inactivation, which occurs in the resting state or develops very rapidly after activation).

Inhibition of the cloned delayed rectifier K+ channels, Kv1.5 and Kv3.1, by riluzole

The action of riluzole, a neuroprotective drug, on cloned delayed rectifier K+ channels (Kv1.5 and Kv3.1) was examined using the whole-cell patch-clamp technique. Riluzole reversibly inhibited Kv1.5 currents in a concentration-dependent manner with an IC50 of 39.69+/-2.37 microM. G-protein inhibitors (pertussis toxin and GDPbetaS) did not prevent this inhibition of riluzole on Kv1.5. No voltage-dependent inhibition by riluzole was found over the voltage range in which channels are fully activated. Riluzole shifted the steady-state inactivation curves of Kv1.5 in a hyperpolarizing direction in a concentration-dependent manner. It accelerated the deactivation kinetics of Kv1.5 in a concentration dependent-manner, but had no effect on the steady-state activation curve. Riluzole exhibited a use-independent inhibition of Kv1.5. The effects of riluzole on Kv3.1, the Shaw-type K+ channel were also examined. Riluzole caused a concentration-dependent inhibition of Kv3.1 currents with an IC50 of 120.98+/-9.74 microM and also shifted the steady-state inactivation curve of Kv3.1 in the hyperpolarizing direction. Thus, riluzole inhibits both Kv1.5 and Kv3.1 currents in a concentration-dependent manner and interacts directly with Kv1.5 by preferentially binding to the inactivated and to the closed states of the channel.

Cibenzoline attenuates upregulation of Kv1.5 channel gene expression by experimental paroxysmal atrial fibrillation

Antiarrhythmic drugs exert their effects by inhibiting the ion channels of cardiomyocytes. However, these effects could also modify the ionic environment around them, and thereby affect the expression of ion channels, leading to biochemical enhancement or attenuation of the antiarrhythmic effects. To test this hypothesis, the physiological and biochemical effects of cibenzoline were evaluated in a rapid atrial pacing model in rats. In rats with rapid atrial pacing, pretreatment with cibenzoline significantly inhibited the increases in Kv1.5 mRNA at 2 hours and immunoreactive protein at 4 hours by 35 +/- 15% and 30 +/- 10%, respectively. These effects were observed only in the rapid atrial pacing group, not in the sham-operated group. With cibenzoline pretreatment, 4-hour rapid atrial pacing resulted in significant prolongation of the atrial refractory period compared to the untreated group even after removal of cibenzoline. In contrast, the sham and rapid atrial pacing model with and without cibenzoline pretreatment showed similar acute physiological responses to cibenzoline. In conclusion, in addition to the acute physiological effects, pretreatment with cibenzoline exerted pleiotropic effects of inhibition of Kv1.5 channel upregulation by rapid pacing, implying differences in the cibenzoline effects when administered before and after onset of paroxysmal atrial fibrillation.

Direct block of cloned hKv1.5 channel by cytochalasins, actin-disrupting agents

The action of cytochalasins, actin-disrupting agents on human Kv1.5 channel (hKv1.5) stably expressed in Ltk−cells was investigated using the whole cell patch-clamp technique. Cytochalasin B inhibited hKv1.5 currents rapidly and reversibly at +60 mV in a concentration-dependent manner with an IC50of 4.2 μM. Cytochalasin A, which has a structure very similar to cytochalasin B, inhibited hKv1.5 (IC50of 1.4 μM at +60 mV). Pretreatment with other actin filament disruptors cytochalasin D and cytochalasin J, and an actin filament stabilizing agent phalloidin had no effect on the cytochalasin B-induced inhibition of hKv1.5 currents. Cytochalasin B accelerated the decay rate of inactivation for the hKv1.5 currents. Cytochalasin B-induced inhibition of the hKv1.5 channels was voltage dependent with a steep increase over the voltage range of the channel's opening. However, the inhibition exhibited voltage independence over the voltage range in which channels are fully activated. Cytochalasin B produced no significant effect on the steady-state activation or inactivation curves. The rate constants for association and dissociation of cytochalasin B were 3.7 μM/s and 7.5 s−1, respectively. Cytochalasin B produced a use-dependent inhibition of hKv1.5 current that was consistent with the slow recovery from inactivation in the presence of the drug. Cytochalasin B (10 μM) also inhibited an ultrarapid delayed rectifier K+current ( IK,ur) in human atrial myocytes. These results indicate that cytochalasin B primarily blocks activated hKv1.5 channels and endogenous IK,urin a cytoskeleton-independent manner as an open-channel blocker.

Kv1.5 surface expression is modulated by retrograde trafficking of newly endocytosed channels by the dynein motor

In this article we have investigated the mechanisms by which retrograde trafficking regulates the surface expression of the voltage-gated potassium channel, Kv1.5. Overexpression of p50/dynamitin, known to disrupt the dynein-dynactin complex responsible for carrying vesicle cargo, substantially increased outward K+ currents in HEK293 cells stably expressing Kv1.5 (0.57+/-0.07 nA/pF, n=12; to 1.18+/-0.2 nA/pF, n=12, P<0.01), as did treatment of the cells with a dynamin inhibitory peptide, which blocks endocytosis. Nocodazole pretreatment, which depolymerizes the microtubule cytoskeleton along which dynein tracks, also doubled Kv1.5 currents in HEK cells and sustained K+ currents in isolated rat atrial myocytes. These increased currents were blocked by 1 mmol/L 4-aminopyridine, and the specific Kv1.5 antagonist, DMM (100 nM). Confocal imaging of both HEK cells and myocytes, as well as experiments testing the sensitivity of the channel in living cells to external Proteinase K, showed that this increase of K+ current density was caused by a redistribution of channels toward the plasma membrane. Coimmunoprecipitation experiments demonstrated a direct interaction between Kv1.5 and the dynein motor complex in both heterologous cells and rat cardiac myocytes, supporting the role of this complex in Kv1.5 trafficking, which required an intact SH3-binding domain in the Kv1.5 N terminus to occur. These experiments highlight a pathway for Kv1.5 internalization from the cell surface involving early endosomes, followed by later trafficking by the dynein motor along microtubules. This work has significant implications for understanding the way Kv channel surface expression is regulated.

Structural basis for competition between drug binding and Kvbeta 1.3 accessory subunit-induced N-type inactivation of Kv1.5 channels

Kvbeta subunits are accessory proteins that modify gating of Kv1 channels. Kvbeta1.3 subunits bind to the N termini of Kv1.5 alpha-subunits and induce fast N-type inactivation, slow the rate of deactivation, and alter the voltage dependence and kinetics of channel activation. The N terminus of a Kvbeta subunit and quaternary ammonium compounds bind to the inner pore of Kv1 channels; however, it is unknown to what extent the pore binding sites for drugs and Kvbeta subunits overlap. Here, we used site-directed Ala mutagenesis to scan residues of the Kv1.5 pore to define the binding site for Kvbeta1.3 subunits. Individual mutations of five residues in the S6 domain (Val505, Ile508, Leu510, Val512, and Val516) greatly retarded or prevented Kvbeta1.3 induced inactivation, and reduced effects on Kv1.5 deactivation. Mutation of Thr479 and Thr480 enhanced Kvbeta1.3-induced N-type inactivation. None of the Ala mutations prevented the Kvbeta1.3-induced negative shifts in the voltage dependence of activation or slow C-type inactivation, suggesting that these gating effects are mediated by an interaction other than the one for N-type inactivation. Thr479, Thr480, Val505, Ile508, and Val512, of Kv1.5 channels are also important interaction sites for the anthranilic acid S0100176 (N-benzyl-N-pyridin-3-ylmethyl-2-(toluene-4-sulfonylamino)-benzamide hydrochloride). Leu510 and V516A prevented Kvbeta1.3-induced inactivation but did not alter drug block. Block of Kv1.5 by S0100176 was reduced and voltage-dependent in the presence of Kvbeta1.3 but not in the presence of an N-truncated form of the Kvbeta subunit. Thus, residues in the pore of Kv1.5 required for N-type inactivation overlap with but are not identical to the drug binding site.

Human cardiac potassium channel DNA polymorphism modulates access to drug-binding site and causes drug resistance

Expression of voltage-gated K channel, shaker-related subfamily, member 5 (KCNA5) underlies the human atrial ultra-rapid delayed rectifier K current (I(Kur)). The KCNA5 polymorphism resulting in P532L in the C terminus generates I(Kur) that is indistinguishable from wild type at baseline but strikingly resistant to drug block. In the present study, truncating the C terminus of KCNA5 generated a channel with wild-type drug sensitivity, which indicated that P532 is not a drug-binding site. Secondary structure prediction algorithms identified a probable alpha-helix in P532L that is absent in wild-type channels. We therefore assessed drug sensitivity of I(Kur) generated in vitro in CHO and HEK cells by channels predicted to exhibit or lack this C-terminal alpha-helix. All constructs displayed near-identical I(Kur) in the absence of drug challenge. However, those predicted to lack the C-terminal alpha-helix generated quinidine-sensitive currents (43-51% block by 10 microM quinidine), while the currents generated by those constructs predicted to generate a C-terminal alpha-helix were inhibited less than 12%. Circular dichroism spectroscopy revealed an alpha-helical signature with peptides derived from drug-resistant channels and no organized structure in those associated with wild-type drug sensitivity. In conclusion, we found that this secondary structure in the KCNA5 C terminus, absent in wild-type channels but generated by a naturally occurring DNA polymorphism, does not alter baseline currents but renders the channel drug resistant. Our data support a model in which this structure impairs access of the drug to a pore-binding site.

KChIP2 modulates the cell surface expression of Kv1.5-encoded K channels

The Kv channel interacting proteins (KChIPs) were identified in a yeast two hybrid screen using the N terminus of Kv4.3 as bait. Previous studies have demonstrated that KChIP2 associates with voltage-gated K(+) (Kv) pore-forming (alpha) subunits of the Kv4 subfamily and contributes to the formation of the rapidly inactivating and recovering Kv4-encoded cardiac transient outward K(+) channels, I(to,f). Here, we report that co-expression of KChIP2 (or KChIP1) also modulates the functional cell surface expression of Kv1.5-encoded K(+) channels in transiently transfected HEK-293 cells. In contrast to the effects of KChIP2 on Kv4 channels, however, co-expression of KChIP2 (or KChIP1) decreases Kv1.5-encoded K(+) currents. Although current densities are reduced, KChIP2 (or KChIP1) co-expression does not affect the time- or voltage-dependent properties of heterologously expressed Kv1.5-encoded K(+) currents. Immunohistochemical and cell surface biotinylation experiments demonstrate that KChIP2 reduces the cell surface expression of Kv1.5, likely by inhibiting forward trafficking from the endoplasmic reticulum. In addition, biochemical experiments reveal that KChIP2 co-immunoprecipitates with Kv1.5 (as well as Kv4.2/Kv4.3) from adult mouse ventricles, demonstrating that, similar to other Kv accessory subunits, KChIP2 is a multifunctional Kv channel accessory subunit. Taken together, the results here suggest that KChIP2 contributes to the formation of functional mouse ventricular (Kv1.5-encoded) I(K,slow1) channels as well, perhaps, as other Kv1.5-encoded K(+) currents, including I(Kur) (I(K,ultrarapid)), in human atria.

Coupling of voltage sensing to channel opening reflects intrasubunit interactions in kv channels

Voltage-gated K(+) channels play a central role in the modulation of excitability. In these channels, the voltage-dependent movement of the voltage sensor (primarily S4) is coupled to the (S6) gate that opens the permeation pathway. Because of the tetrameric structure, such coupling could occur within each subunit or between adjacent subunits. To discriminate between these possibilities, we analyzed various combinations of a S4 mutation (R401N) and a S6 mutation (P511G) in hKv1.5, incorporated into tandem constructs to constrain subunit stoichiometry. R401N shifted the voltage dependence of activation to negative potentials while P511G did the opposite. When both mutations were introduced in the same alpha-subunit of the tandem, the positive shift of P511G was compensated by the negative shift of R401N. With each mutation in a separate subunit of a tandem, this compensation did not occur. This suggests that for Kv channels, the coupling between voltage sensing and gating reflects primarily an intrasubunit interaction.

Effects of hyperthyroidism on delayed rectifier K+ currents in left and right murine atria

Hyperthyroidism has been associated with atrial fibrillation (AF); however, hyperthyroidism-induced ion channel changes that may predispose to AF have not been fully elucidated. To understand the electrophysiological changes that occur in left and right atria with hyperthyroidism, the patch-clamp technique was used to compare action potential duration (APD) and whole cell currents in myocytes from left and right atria from both control and hyperthyroid mice. Additionally, RNase protection assays and immunoblotting were performed to evaluate the mRNA and protein expression levels of K(+) channel alpha-subunits in left and right atria. The results showed that 1) in control mice, the APD was shorter and the ultra-rapid delayed rectifier K(+) conductance (I(Kur)) and the sustained delayed rectifier K(+) conductance (I(ss)) were larger in the left than in the right atrium; also, mRNA and protein expression levels of Kv1.5 and Kv2.1 were higher in the left atrium; 2) in hyperthyroid mice, the APD was shortened and I(Kur) and I(ss) were increased in both left and right atrial myocytes, and the protein expression levels of Kv1.5 and Kv2.1 were increased significantly in both atria; and 3) the influence of hyperthyroidism on APD and delayed rectifier K(+) currents was more prominent in right than in left atrium, which minimized the interatrial APD difference. In conclusion, hyperthyroidism resulted in more significant APD shortening and greater delayed rectifier K(+) current increases in the right vs. the left atrium, which can contribute to the propensity for atrial arrhythmia in hyperthyroid heart.

A Single Residue in the S6 Transmembrane Domain Governs the Differential Flecainide Sensitivity of Voltage-Gated Potassium Channels

Flecainide has been used to differentiate Kv4.2-based transient-outward K(+)-currents (flecainide-sensitive) from Kv1.4-based (flecainide-insensitive). We found that flecainide also inhibits ultrarapid delayed rectifier (I(Kur)) currents in Xenopus laevis oocytes carried by Kv3.1 subunits (IC(50), 28.3 +/- 1.3 microM) more strongly than Kv1.5 currents corresponding to human I(Kur) (IC(50), 237.1 +/- 6.2 microM). The present study examined molecular motifs underlying differential flecainide sensitivity. An initial chimeric approach pointed to a role for S6 and/or carboxyl-terminal sites in Kv3.1/Kv1.5 sensitivity differences. We then looked for homologous amino acid residues of the two sensitive subunits (Kv4.2 and Kv3.1) different from homologous residues for insensitive subunits (Kv1.4 and Kv1.5). Three candidate sites were identified: two in the S5-S6 linker and one in the S6 segment. Mutation of the proximal S5-S6 linker site failed to alter flecainide sensitivity. Mutation at the more distal site in Kv1.5 (V481L) modestly increased sensitivity, but the reciprocal Kv3.1 mutation (L401V) had no effect. S6 mutants caused marked changes: flecainide sensitivity decreased approximately 8-fold for Kv3.1 L422I (IC(50), 213 +/- 9 microM) and increased approximately 7-fold for Kv1.5 I502L (IC(50), 35.6 +/- 1.9 microM). Corresponding mutations reversed flecainide sensitivity of Kv1.4 and Kv4.2; L392I decreased Kv4.2 sensitivity by approximately 17-fold (IC(50) of 37.4 +/- 6.9 to 628 +/- 36 microM); I547L increased Kv1.4 sensitivity by approximately 15-fold (IC(50) of 706 +/- 37 to 40.9 +/- 7.3 microM). Our observations indicate that the flecainide sensitivity differences among these four voltage-gated K(+)-channels are determined by whether an isoleucine or a leucine is present at a specific amino acid location.

The Effects of Putative K+ Channel Blockers on Volume Regulation of Murine Spermatozoa1

Volume regulation is a necessary task for spermatozoa as the osmolarity of female tract fluids is lower than that in the epididymis and because the disruption of it in transgenic mice results in infertility. As the specific mechanisms behind this phenomenon are unknown, spermatozoa from mice were screened for sensitivities to inhibitors known to affect specific channels involved in volume regulation of somatic cells. Spermatozoa from the cauda epididymidis were exposed to physiological hypotonic conditions with and without inhibitor. Flow cytometric forward scatter measurements were taken to indicate relative sperm size at 5 and 75 min of incubation. The presence of quinine (0.8 mM), cadmium (0.2 mM), flecainide (100 microM), 4-aminopyridine (4 mM), barium (1 mM), clofilium (10 microM), and phrixotoxin (100 nM) for 75 min resulted in significantly higher forward scatter values than sperm incubated in medium without an inhibitor. These results imply that channels potentially involved in volume regulation of murine spermatozoa include the voltage-dependent Kv1.4 (also known as KCNA1), Kv1.5 (KCNA5), Kv4.1 (KCND1), Kv4.2 (KCND2), Kv4.3 (KCND3), mink (KCNE1), and acid-sensitive TASK2 (KCNK5) and TASK3 (KCNK9). Western blots confirmed the presence of Kv1.5 and TASK2 proteins in sperm plasma membranes at similar (Kv1.5) or higher (TASK2) molecular weight than in somatic cells. Incubation in a different pH did not reveal acid sensitivity of volume regulation. Volume regulation of spermatozoa may involve novel voltage-gated and pH-sensitive potassium channels, which could be valuable targets for the development of a posttesticular male contraceptive.

Diverse phenotypes of outward currents in cells that have survived in the 5-day-infarcted heart

We have shown reduced density and altered kinetics in slowly activating K+ currents (I(Ks)) in epicardial border zone (EBZ) cells (IZs) of the 5-day-infarcted canine heart (Jiang M, Cabo, C, Yao J-A, Boyden PA, and Tseng G-N. Cardiovasc Res 48: 34-43, 2000). beta-Adrenergic stimulation with isoproterenol increases I(Ks) in normal cells (NZs). In this study, we used a voltage-clamp protocol with an external solution to isolate I(Ks) from contaminating currents to determine the effects of 1 muM isoproterenol on I(Ks) in IZs and NZs. Under our recording conditions, 10 microM azimilide-sensitive currents were stimulated with isoproterenol to compare responsiveness of I(Ks) to isoproterenol in the two cell groups. I(Ks) tail density was reduced 67% in IZs (group I, n = 26) compared with NZs (n = 24, P < 0.05). Isoproterenol-stimulated azimilide-sensitive tail currents were increased 1.72 +/- 0.2-fold in NZs and 2.2 +/- 0.3-fold in IZs (P > 0.05). In 33% of IZs (group II, n = 13), native currents showed no tail currents; however, isoproterenol-stimulated azimilide-sensitive currents were voltage dependent, fast activating, and large in amplitude compared with group I IZs, similar to "lone" KCNQ1 currents. Using short clamp pulses, we also found an increase in sustained currents sensitive to tetraethylammonium chloride (TEA) and no change in C-9356-sensitive currents in IZs with little or no transient outward current. In some IZs where I(Ks) is downregulated, the effect of isoproterenol on I(Ks) was similar to that on I(Ks) in NZs. In others, the existence of lone KCNQ1-type currents, which are sensitive to beta-adrenergic stimulation, is consistent with our findings of an increased KCNQ1-to-KCNE1 mRNA ratio (Jiang et al.). Accompanying altered I(Ks) in IZs are an enhanced TEA-sensitive current and a normal C-9356-sensitive current.

Polymorphism screening in the cardiac K+ channel gene KCNA5

BACKGROUND

Common deoxyribonucleic acid polymorphisms that modulate normal cardiac electrophysiologic characteristics have previously been identified in long QT syndrome disease genes. In this study we screened an additional gene encoding the cardiac potassium channel KCNA5 (underlying I(Kur)) in 3 ethnic groups and evaluated the functional consequences of the variants identified.

METHODS

The coding region was screened by single-stranded conformational polymorphism analysis and direct sequencing, and nonsynonymous variants were studied by patch-clamping transfected Chinese hamster ovary cells. Results Five synonymous and 6 nonsynonymous polymorphisms were found in KCNA5. None of these polymorphisms was present in greater than 7% of alleles screened or in all 3 ethnic groups. Expression of the nonsynonymous KCNA5 variants revealed normal gating. However, 2 variants (P532L and R578K, both in the C-terminus) were resistant to block by the prototypical inhibitor quinidine; the concentration required to block I(Kur) by 50% (IC(50)) was 8.4 micromol/L for wild type versus 54 micromol/L for R578K and 133 micromol/L for P532L (both P < .0001, versus wild type).

CONCLUSION

KCNA5 displays little variability in its coding region. C-terminal KCNA5 variants displayed near-normal gating but striking resistance to drug block; thus these pharmacogenomic studies have identified a heretofore-unappreciated role of this region as a modulator of channel sensitivity to drugs. Resistance to I(Kur) blockers may be genetically determined.

Open channel block of hKv1.5 by psoralen fromHeracleum moellendorffii hance

A furocoumarin derivative, psoralen (7H-furo[3,2-g][1]benzopyran-7-one), was isolated from the n-hexane fraction of Heracleum moellendorffii Hance. We examined the effects of psoralen on a human Kv1.5 potassium channel (hKv1.5) cloned from human heart and stably expressed in Ltk- cells. We found that psoralen inhibited the hKv1.5 current in a concentration-, use- and voltage-dependent manner with an IC50 value of 180 +/- 21 nM at +60 mV. Psoralen accelerated the inactivation kinetics of the hKv1.5 channel, and it slowed the deactivation kinetics of the hKv1.5 current resulting in a tail crossover phenomenon. These results indicate that psoralen acts on the hKv1.5 channel as an open channel blocker. Furthermore, psoralen prolonged the action potential duration of rat atrial muscles in a dose-dependent manner. Taken together, the present results strongly suggest that psoralen may be an ideal antiarrhythmic drug for atrial fibrillation.

Galantamine blocks cloned Kv2.1, but not Kv1.5 potassium channels

Galantamine is a cholinesterase inhibitor (AChEI) currently used in treatment of Alzheimer's disease (AD). In the present study, the effects of galantamine on currents of cloned Kv2.1 and Kv1.5 potassium channels were investigated by using patch-clamp whole cell recording techniques. Kv2.1 and Kv1.5 were stably expressed in HEK293 cells. Galantamine blocked Kv2.1 current in a concentration-dependent manner. When depolarizing from -50 to +40 mV, the IC50 of galantamine for inhibition of Kv2.1 was 5.6 microM. Galantamine 10 microM shifted the activation curve of Kv2.1 to negative potential by 4.0 mV. At the same concentration, galantamine shifted the inactivation curve to negative potential by 25.2 mV. While Kv1.5 was not sensitive to galantamine, Kv1.5 current was not changed by galantamine at concentration of 10 microM. Our data suggest that galantamine potently blocks Kv2.1, but not Kv1.5 channels.

Accessory Kvbeta1 subunits differentially modulate the functional expression of voltage-gated K+ channels in mouse ventricular myocytes

Voltage-gated K+ (Kv) channel accessory (beta) subunits associate with pore-forming Kv alpha subunits and modify the properties and/or cell surface expression of Kv channels in heterologous expression systems. There is very little presently known, however, about the functional role(s) of Kv beta subunits in the generation of native cardiac Kv channels. Exploiting mice with a targeted disruption of the Kvbeta1 gene (Kvbeta1-/-), the studies here were undertaken to explore directly the role of Kvbeta1 in the generation of ventricular Kv currents. Action potential waveforms and peak Kv current densities are indistinguishable in myocytes isolated from the left ventricular apex (LVA) of Kvbeta1-/- and wild-type (WT) animals. Analysis of Kv current waveforms, however, revealed that mean+/-SEM I(to,f) density is significantly (P< or =0.01) lower in Kvbeta1-/- (21.0+/-0.9 pA/pF; n=68), than in WT (25.3+/-1.4 pA/pF; n=42), LVA myocytes, and that mean+/-SEM I(K,slow) density is significantly (P< or =0.01) higher in Kvbeta1-/- (19.1+/-0.9 pA/pF; n=68), compared with WT (15.9+/-0.7 pA/pF; n=42), LVA cells. Pharmacological studies demonstrated that the TEA-sensitive component of I(K,slow), I(K,slow2,) is selectively increased in Kvbeta1-/- LVA myocytes. In parallel with the alterations in I(to,f) and I(K,slow2) densities, Kv4.3 expression is decreased and Kv2.1 expression is increased in Kvbeta1-/- ventricles. Taken together, these results demonstrate that Kvbeta1 differentially regulates the functional cell surface expression of myocardial I(to,f) and I(K,slow2) channels.

Synergistic Inhibition of the Maximum Conductance of Kv1.5 Channels by Extracellular K+ Reduction and Acidification

Voltage-gated potassium (Kv) channels exist in the membranes of all living cells. Of the functional classes of Kv channels, the Kv1 channels are the largest and the best studied and are known to play essential roles in excitable cell function, providing an essential counterpoint to the various inward currents that trigger excitability. The serum potassium concentration [K(+)(o)] is tightly regulated in mammals and disturbances can cause significant functional alterations in the electrical behavior of excitable tissues in the nervous system and the heart. At least some of these changes may be mediated by Kv channels that are regulated by changes in the extracellular K(+) concentration. As well as changes in serum [K(+)(o)], tissue acidification is a frequent pathological condition known to inhibit Shaker and Kv1 voltage-gated potassium channels. In recent studies, it has become recognized that the acidification-induced inhibition of some Kv1 channels is K(+)(o)-dependent, and the suggestion has been made that pH and K(+)(o) may regulate the channels via a common mechanism. Here we discuss P/C type inactivation as the common pathway by which some Kv channels become unavailable at acid pH and lowered K(+)(o). It is suggested that binding of protons to a regulatory site in the outer pore mouth of some Kv channels favors transitions to the inactivated state, whereas K(+) ions exert countereffects. We suggest that modulation of the number of excitable voltage-gated K(+) channels in the open vs inactivated states of the channels by physiological H(+) and K(+) concentrations represents an important pathway to control Kv channel function in health and disease.

Sex and strain differences in adult mouse cardiac repolarization: importance of androgens

OBJECTIVE

Gender differences in mouse cardiac repolarization have been reported to be due to the stimulatory action of androgens on the ultrarapid delayed rectifier K(+) current (I(Kur)) and its underlying Kv1.5 channel. To confirm the regulation of ventricular repolarization by androgens, the present study compared two strains of mice (CD-1 and C57BL/6) that present different androgen levels.

METHODS AND RESULTS

Measurement of testosterone levels in different strains of mice (CD-1, C57BL/6, C3H and FVB) revealed that male C57BL/6 mice had very low levels of testosterone, whereas males of the other strains displayed normal testosterone levels. Furthermore, whole-cell voltage clamp recordings in isolated ventricular myocytes showed that the current density of I(Kur) in male C57BL/6 mice was similar to that in female mice but smaller with respect to male CD-1 mice. Androgen replacement in male C57BL/6 mice as well as in castrated male CD-1 mice shortened ventricular repolarization, increased I(Kur) current density, and increased expression of Kv1.5 channels.

CONCLUSION

Strain and gender differences observed in mouse cardiac repolarization can be explained by different androgen levels. As a consequence, androgens are major regulatory factors in cardiac repolarization and special attention should be paid to the hormonal status of the animal when studying hormonal regulation of cardiac repolarization.

Delayed rectifier potassium currents and Kv2.1 mRNA increase in hippocampal neurons of scopolamine-induced memory-deficient rats

To explore the ionic mechanisms of memory deficits induced by cholinergic lesion, whole-cell patch clamp recording techniques in combination with single-cell RT-PCR were used to characterize delayed rectifier potassium currents (IK) in acutely isolated hippocampal pyramidal neurons of scopolamine-induced cognitive impairment rats. Scopolamine could induce deficits in spatial memory of rats. The peak amplitude and current density of IK measured in hippocampal pyramidal neurons were increased from 1.2+/-0.6 nA and 38+/-19 pA/pF of the control group (n=12) to 1.8+/-0.5 nA and 62+/-24 pA/pF (n=48, P<0.01) of the scopolamine-treated group. The steady-state activation curve of IK was shifted about 8 mV (P<0.01) in the direction of hyperpolarization in scopolamine-treated rats. The mRNA level of Kv2.1 was increased (P<0.01) in the scopolamine-treated group, but there was no significant change of Kv1.5 mRNA level. The present study demonstrated for the first time that IK was enhanced significantly in hippocampal pyramidal neurons of scopolamine-induced cognitive impairment rats. The increase of Kv2.1 mRNA expression in hippocampal pyramidal cells might be responsible for the enhancement of IK and could be the ionic basis of the memory deficits induced by scopolamine.

Heteromultimeric Kv1 channels contribute to myogenic control of arterial diameter

Inhibition of vascular smooth muscle (VSM) delayed rectifier K+ channels (K(DR)) by 4-aminopyridine (4-AP; 200 micromol/L) or correolide (1 micromol/L), a selective inhibitor of Kv1 channels, enhanced myogenic contraction of rat mesenteric arteries (RMAs) in response to increases in intraluminal pressure. The molecular identity of K(DR) of RMA myocytes was characterized using RT-PCR, real-time PCR, and immunocytochemistry. Transcripts encoding the pore-forming Kvalpha subunits, Kv1.2, Kv1.4, Kv1.5, and Kv1.6, were identified and confirmed at the protein level with subunit-specific antibodies. Kvbeta transcript (beta1.1, beta1.2, beta1.3, and beta2.1) expression was also identified. Kv1.5 message was approximately 2-fold more abundant than that for Kv1.2 and Kv1.6. Transcripts encoding these three Kv1alpha subunits were approximately 2-fold more abundant in 1st/2nd order conduit compared with 4th order resistance RMAs, and Kvbeta1 was 8-fold higher than Kvbeta2 message. RMA K(DR) activated positive to -50 mV, exhibited incomplete inactivation, and were inhibited by 4-AP and correolide. However, neither alpha-dendrotoxin or kappa-dendrotoxin affected RMA K(DR), implicating the presence of Kv1.5 in all channels and the absence of Kv1.1, respectively. Currents mediated by channels because of coexpression of Kv1.2, Kv1.5, Kv1.6, and Kvbeta1.2 in human embryonic kidney 293 cells had biophysical and pharmacological properties similar to those of RMA K(DR). It is concluded that K(DR) channels composed of heteromultimers of Kv1 subunits play a critical role in myogenic control of arterial diameter.

Diltiazem inhibits hKv1.5 and Kv4.3 currents at therapeutic concentrations

OBJECTIVE

In the present study we examined the effects of diltiazem, an L-type Ca(2+) channel blocker widely used for the control of the ventricular rate in patients with supraventricular arrhythmias, on hKv1.5 and Kv4.3 channels that generate the cardiac ultrarapid delayed rectifier (I(Kur)) and the 4-aminopyridine sensitive transient outward (I(to)) K(+) currents, respectively.

METHODS

hKv1.5 and Kv4.3 channels were stably and transiently expressed in mouse fibroblast and Chinese hamster ovary cells, respectively. Currents were recorded using the whole-cell patch clamp.

RESULTS

Diltiazem (0.01 nM-500 muM) blocked hKv1.5 channels, in a frequency-dependent manner exhibiting a biphasic dose-response curve (IC(50)=4.8+/-1.5 nM and 42.3+/-3.6 muM). Diltiazem delayed the initial phase of the tail current decline and shifted the midpoint of the activation (Vh=-16.5+/-2.1 mV vs -20.4+/-2.6 mV, P<0.001) and inactivation (Vh=-22.4+/-0.7 mV vs. -28.2+/-1.9 mV, P<0.001) curves to more negative potentials. The analysis of the development of the diltiazem-induced block yielded apparent association (k) and dissociation (P) rate constants of (1.6+/-0.2) x 10(6) M(-1)s(-1) and 46.8+/-4.8 s(-1), respectively. Diltiazem (0.1 nM-100 muM) also blocked Kv4.3 channels in a frequency-dependent manner exhibiting a biphasic dose-response curve (IC(50)=62.6+/-11.1 nM and 109.9+/-12.8 muM). Diltiazem decreased the peak current and, at concentrations > or =0.1 microM, accelerated the inactivation time course. The apparent association and dissociation rate constants resulted (1.7+/-0.2) x 10(6) M(-1)s(-1) and 258.6+/-38.1 s(-1), respectively. Diltiazem, 10 nM, shifted to more negative potentials the voltage-dependence of Kv4.3 channel inactivation (Vh=-33.1+/-2.3 mV vs -38.2+/-3.5 mV, n=6, Plt;0.05) the blockade increasing at potentials at which the amount of inactivated channels increased.

CONCLUSION

The results demonstrated for the first time that diltiazem, at therapeutic concentrations, decreased hKv1.5 and Kv4.3 currents by binding to the open and the inactivated state of the channels.

The external K+ concentration and mutations in the outer pore mouth affect the inhibition of kv1.5 current by Ni2+

By examining the consequences both of changes of [K+]o and of point mutations in the outer pore mouth, our goal was to determine if the mechanism of the block of Kv1.5 ionic currents by external Ni2+ is similar to that for proton block. Ni2+ block is inhibited by increasing [K+]o, by mutating a histidine residue in the pore turret (H463Q) or by mutating a residue near the pore mouth (R487V) that is the homolog of Shaker T449. Aside from a slight rightward shift of the Q-V curve, Ni2+ had no effect on gating currents. We propose that, as with Ho+, Ni2+ binding to H463 facilitates an outer pore inactivation process that is antagonized by Ko+ and that requires R487. However, whereas Ho+ substantially accelerates inactivation of residual currents, Ni2+ is much less potent, indicating incomplete overlap of the profiles of these two metal ions. Analyses with Co2+ and Mn2+, together with previous results, indicate that for the first-row transition metals the rank order for the inhibition of Kv1.5 in 0 mM Ko+ is Zn2+ (KD approximately 0.07 mM) > or = Ni2+) (KD approximately 0.15 mM) > Co2+ (KD approximately 1.4 mM) > Mn2+ (KD > 10 mM).

Effect of passive sensitization by serum from allergic asthmatic patients on the activity and expression of voltage-dependent delayed rectifier potassium channel in human bronchial smooth muscle cells

BACKGROUND

Potassium (K+) channels are important in regulating cell membrane potential and excitability. Although bronchial myocytes from asthmatic rats show a significant reduction in voltage-dependent delayed rectifier potassium channel (Kv) current density and higher excitability, the activity and expression of Kv in human bronchial smooth muscle cells (HBSMCs) have never been studied. The objective of this study was to investigate the effect of passive sensitization by asthmatic serum on the activity of Kv and the expression of Kv isoform Kv1.5 in HBSMCs.

METHODS

HBSMCs were randomly divided into two groups: control group (containing 10% serum from nonatopic individuals) and sensitized group (containing 10% asthmatic serum), then cultured for 24 hours. Whole-cell patch clamp, immunofluorescence staining, reverse transcription-polymerase chain reaction and Western blot techniques were used to study the effect of passive sensitization on the activity of Kv and the expression of Kv1.5 in HBSMCs.

RESULTS

The membrane potential in passively sensitized HBSMCs was significantly depolarized to -(26.7 +/- 5.2) mV compared with -(41.3 +/- 6.4) mV in the control group (P < 0.01). Passive sensitization caused a significant inhibition of Kv currents in HBSMCs, resulting in a downward shift in the current-voltage (I-V) relationship curve. At +50 mV, the peak Kv current density of passively sensitized HBSMCs was significantly decreased from (54.6 +/- 8.7) picoamperes per picofarad (pA/pF) to (32.1 +/- 7.1) pA/pF (P < 0.01). The expression level of Kv1.5 mRNA in passively sensitized HBSMCs was significantly lower than that in the control group (0.76 +/- 0.07 vs 1.04 +/- 0.13, P < 0.05). The expression of Kv1.5 protein of passively sensitized HBSMCs was also significantly reduced compared to that from the control group (984 +/- 168 vs 2200 +/- 380, P < 0.05).

CONCLUSIONS

The activity and expression of Kv were all decreased in HBSMCs passively sensitized by asthmatic serum compared with nonsensitized cells. These changes might be involved in the mechanisms of formation and development of asthma.

Effect of chronic cigarette smoking on large-conductance calcium-activated potassium channel and Kv1.5 expression in bronchial smooth muscle cells of rats

To investigate the role of potassium channels in the pathogenesis of airway hyperresponsiveness induced by cigarette smoking, the alteration in expression of large-conductance calcium-activated potassium channel (BKca) and voltage-dependent delayed rectifier potassium channel (Kv1.5) in bronchial smooth muscle cells were investigated in chronic cigarette smoking rats. Airway responsiveness was determined, hematoxylin and eosin staining, immuno-histochemistry, in-situ hybridization and western blot techniques were used. The results showed: (1) Chronic cigarette smoking down-regulated the protein synthesis and mRNA expression of BKca and Kv1.5 in bronchial and bronchiolar smooth muscles. (2) BKca decreased more markedly than Kv1.5 in bronchi, but there was no difference between them in bronchioli. (3) No changes in the expression of these two potassium channel proteins were found in extracted cell membrane protein from lung tissue. The results suggest that chronic cigarette smoking can down-regulate the levels of BKca and Kv1.5 in rat bronchial smooth muscle cells in vivo, which might contribute to the mechanism of airway hyperresponsiveness induced by cigarette smoking.

Solid support membranes for ion channel arrays and sensors: application to rapid screening of pharmacological compounds

The use of solid supported membranes (SSM) was investigated for reconstitution of ion channels and for potential application to screen pharmacological reagents affecting ion channel function. The voltage-gated Kv1.5 K+ channel was reconstituted on an SSM and a current was measured. This current was dependent on the presence of K+, but not Na+, indicating that the Kv1.5 K+ channel maintained cation specificity when reconstituted on SSM. Two pharmacological reagents applied to Kv1.5 K+ channels reconstituted on SSM had similar inhibitory effects as those measured using Kv1.5 in biological membranes. SSM-mounted ion channels were stable enough to be washed with buffer solution and reused many times, allowing solution exchange essential for pharmacological drug screening.

Blockers of the Kv1.5 channel for the treatment of atrial arrhythmias

Atrial arrhythmias are a common problem in cardiological practice. Despite the availability of several antiarrhythmic drugs, there is a medical need for safer and more efficient antiarrhythmic treatment. Compounds that act atrial selectively without prolonging the QTc-time and without negative inotropy to terminate and/or prevent atrial arrhythmias would be of high interest. In this context, the voltage-gated potassium channel Kv1.5 is regarded as a promising target to achieve atrial selectivity, which in turn would be associated with fewer side effects than classical antiarrhythmics. This review summarizes patents and other publications on compounds which show this novel mode of action. The chemistry, selectivity and structure-activity data disclosed in the literature are discussed in light of recent work demonstrating the antiarrhythmic efficacy of Kv1.5 blockers in vivo. Several studies in pig, dog or goat models have confirmed their proposed atrial selective antiarrhythmic effect in vivo. Most of the more intensively characterized Kv1.5 blockers have turned out not to be selective but also block other ion channels. Based on the currently available data it seems that additional inhibition of Kv4.3 and KACh is beneficial for the desired antiarrhythmic effect or at least does not hamper the atrial selectivity of a Kv1.5 blocker. Significant block of IK1, HERG or sodium channels, however, clearly leads to loss of atrial selectivity and increases the risk of lethal ventricular proarrhythmia.

Dichloroacetate prevents and reverses pulmonary hypertension by inducing pulmonary artery smooth muscle cell apoptosis

The pulmonary arteries (PA) in pulmonary arterial hypertension (PAH) are constricted and remodeled;. They have suppressed apoptosis, partly attributable to suppression of the bone morphogenetic protein axis and selective downregulation of PA smooth muscle cell (PASMC) voltage-gated K+ channels, including Kv1.5. The Kv downregulation-induced increase in [K+]i, tonically inhibits caspases, further suppressing apoptosis. Mitochondria control apoptosis and produce activated oxygen species like H2O2, which regulate vascular tone by activating K+ channels, but their role in PAH is unknown. We show that dichloroacetate (DCA), a metabolic modulator that increases mitochondrial oxidative phosphorylation, prevents and reverses established monocrotaline-induced PAH (MCT-PAH), significantly improving mortality. Compared with MCT-PAH, DCA-treated rats (80 mg/kg per day in drinking water on day 14 after MCT, studied on day 21) have decreased pulmonary, but not systemic, vascular resistance (63% decrease, P<0.002), PA medial thickness (28% decrease, P<0.0001), and right ventricular hypertrophy (34% decrease, P<0.001). DCA is similarly effective when given at day 1 or day 21 after MCT (studied day 28) but has no effect on normal rats. DCA depolarizes MCT-PAH PASMC mitochondria and causes release of H2O2 and cytochrome c, inducing a 10-fold increase in apoptosis within the PA media (TUNEL and caspase 3 activity) and decreasing proliferation (proliferating-cell nuclear antigen and BrdU assays). Immunoblots, immunohistochemistry, laser-captured microdissection-quantitative reverse-transcription polymerase chain reaction and patch-clamping show that DCA reverses the Kv1.5 downregulation in resistance PAs. In summary, DCA reverses PA remodeling by increasing the mitochondria-dependent apoptosis/proliferation ratio and upregulating Kv1.5 in the media. We identify mitochondria-dependent apoptosis as a potential target for therapy and DCA as an effective and selective treatment for PAH.

Oxygen-sensitive Kv channel gene transfer confers oxygen responsiveness to preterm rabbit and remodeled human ductus arteriosus: implications for infants with patent ductus arteriosus

BACKGROUND

Oxygen (O2)-sensitive K+ channels mediate acute O2 sensing in many tissues. At birth, initial functional closure of the ductus arteriosus (DA) results from O2-induced vasoconstriction. This mechanism often fails in premature infants, resulting in persistent DA, a common form of congenital heart disease. We hypothesized that the basis for impaired O2 constriction in preterm DA is reduced expression and function of O2-sensitive, voltage-gated (Kv) channels.

METHODS AND RESULTS

Preterm rabbit DA rings have reduced O2 constriction (even after inhibition of prostaglandin and nitric oxide synthases), and preterm DA smooth muscle cells (DASMCs) display reduced O2-sensitive K+ current. This is associated with decreased mRNA and protein expression of certain O2-sensitive Kv channels (Kv1.5 and Kv2.1) but equivalent expression of the L-type calcium channel. Transmural Kv1.5 or Kv2.1 gene transfer "rescues" the developmental deficiency, conferring O2 responsiveness to preterm rabbit DAs. Targeted SMC Kv1.5 gene transfer also enhances O2 constriction in human DAs.

CONCLUSIONS

These data demonstrate a central role for developmentally regulated DASMC O2-sensitive Kv channels in the functional closure of the DA. Modulation of Kv channels may have therapeutic potential in diseases associated with impaired O2 responsiveness, including persistent DA.

Caveolin-3 and SAP97 form a scaffolding protein complex that regulates the voltage-gated potassium channel Kv1.5

The targeting of ion channels to particular membrane microdomains and their organization in macromolecular complexes allow excitable cells to respond efficiently to extracellular signals. In this study, we describe the formation of a complex that contains two scaffolding proteins: caveolin-3 (Cav-3) and a membrane-associated guanylate kinase (MAGUK), SAP97. Complex formation involves the association of Cav-3 with a segment of SAP97 localized between its PDZ2 and PDZ3 domains. In heterologous expression systems, this scaffolding complex can recruit Kv1.5 to form a tripartite complex in which each of the three components interacts with the other two. These interactions regulate the expression of currents encoded by a glycosylation-deficient mutant of Kv1.5. We conclude that the association of Cav-3 with SAP97 may constitute the nucleation site for the assembly of macromolecular complexes containing potassium channels.

Effects of the atrial antiarrhythmic drug AVE0118 on cardiac ion channels

Previous studies in pigs and goats have demonstrated that AVE0118 prolongs atrial refractoriness without any effect on the QT-interval. The purpose of the present study was to investigate the effect of the compound on various cardiac ion channels. AVE0118 blocked the pig Kv1.5 and the human Kv1.5 expressed in Xenopus oocytes with IC(50) values of 5.4+/-0.7 microM and 6.2+/-0.4 microM respectively. In Chinese hamster ovary (CHO) cells, AVE0118 decreased the steady-state hKv1.5 current with an IC(50) of 1.1+/-0.2 microM. The hKv4.3/KChIP2.2 current in CHO cells was blocked by AVE0118 by accelerating the apparent time-constant of inactivation ( tau(inact)), and the integral current was inhibited with an IC(50) of 3.4+/-0.5 microM. At 10 microM AVE0118 tau(inact) decreased from 9.3+/-0.6 ms ( n=8, control) to 3.0+/-0.3 ms ( n=8). The K(ACh) current was investigated in isolated pig atrial myocytes by application of 10 microM carbachol. At a clamp potential of -100 mV the I(KACh) was half-maximally blocked by 4.5+/-1.6 microM AVE0118. In the absence of carbachol, AVE0118 had no effect on the inward current recorded at -100 mV. Effects on the I(Kr) current were investigated on HERG channels expressed in CHO cells. AVE0118 blocked this current half-maximally at approximately 10 microM. Comparable results were obtained in isolated guinea pig ventricular myocytes, where half-maximal inhibition of the I(Kr) tail current occurred at a similar concentration of AVE0118. Other ionic currents, like the I(Ks), I(KATP) (recorded in guinea pig ventricular myocytes), and L-type Ca(2+) (recorded in pig atrial myocytes) were blocked by 10 microM AVE0118 by 10+/-3% ( n=6), 28+/-7% ( n=4), and 22+/-13% ( n=5) respectively. In summary, AVE0118 preferentially inhibits the atrial K(+) channels I(Kur), I(to) and I(KACH). This profile may explain the selective prolongation of atrial refractoriness described previously in pigs and goats.

Effects of intracellular magnesium on Kv1.5 and Kv2.1 potassium channels

We characterized the effects of intracellular Mg(2+) (Mg(2+) (i)) on potassium currents mediated by the Kv1.5 and Kv2.1 channels expressed in Xenopus oocytes. Increase in Mg(2+) (i) caused a voltage-dependent block of the current amplitude, apparent acceleration of the current kinetics (explained by a corresponding shift in the steady-state activation) and leftward shifts in activation and inactivation dependencies for both channels. The voltage-dependent block was more potent for Kv2.1 [dissociation constant at 0 mV, K(d)(0), was approximately 70 mM and the electric distance of the Mg(2+) binding site, delta, was 0.2] than for the Kv1.5 channel [K(d)(0) approximately 40 mM and delta = 0.1]. Similar shifts in the voltage-dependent parameters for both channels were described by the Gouy-Chapman formalism with the negative charge density of 1 e(-)/100 A(2). Additionally, Mg(2+) (i) selectively reduced a non-inactivating current and increased the accumulation of inactivation of the Kv1.5, but not the Kv2.1 channel. A potential functional role of the differential effects of Mg(2+) (i) on the Kv channels is discussed.

Preferential expression and function of voltage-gated, O2-sensitive K+ channels in resistance pulmonary arteries explains regional heterogeneity in hypoxic pulmonary vasoconstriction: ionic diversity in smooth muscle cells

Hypoxic pulmonary vasoconstriction (HPV) is initiated by inhibition of O2-sensitive, voltage-gated (Kv) channels in pulmonary arterial smooth muscle cells (PASMCs). Kv inhibition depolarizes membrane potential (E(M)), thereby activating Ca2+ influx via voltage-gated Ca2+ channels. HPV is weak in extrapulmonary, conduit pulmonary arteries (PA) and strong in precapillary resistance arteries. We hypothesized that regional heterogeneity in HPV reflects a longitudinal gradient in the function/expression of PASMC O2-sensitive Kv channels. In adult male Sprague Dawley rats, constrictions to hypoxia, the Kv blocker 4-aminopyridine (4-AP), and correolide, a Kv1.x channel inhibitor, were endothelium-independent and greater in resistance versus conduit PAs. Moreover, HPV was dependent on Kv-inhibition, being completely inhibited by pretreatment with 4-AP. Kv1.2, 1.5, Kv2.1, Kv3.1b, Kv4.3, and Kv9.3. mRNA increased as arterial caliber decreased; however, only Kv1.5 protein expression was greater in resistance PAs. Resistance PASMCs had greater K+ current (I(K)) and a more hyperpolarized E(M) and were uniquely O2- and correolide-sensitive. The O2-sensitive current (active at -65 mV) was resistant to iberiotoxin, with minimal tityustoxin sensitivity. In resistance PASMCs, 4-AP and hypoxia inhibited I(K) 57% and 49%, respectively, versus 34% for correolide. Intracellular administration of anti-Kv1.5 antibodies inhibited correolide's effects. The hypoxia-sensitive, correolide-insensitive I(K) (15%) was conducted by Kv2.1. Anti-Kv1.5 and anti-Kv2.1 caused additive depolarization in resistance PASMCs (Kv1.5>Kv2.1) and inhibited hypoxic depolarization. Heterologously expressed human PASMC Kv1.5 generated an O2- and correolide-sensitive I(K) like that in resistance PASMCs. In conclusion, Kv1.5 and Kv2.1 account for virtually all the O2-sensitive current. HPV occurs in a Kv-enriched resistance zone because resistance PASMCs preferentially express O2-sensitive Kv-channels.

A set of homology models of pore loop domain of six eukaryotic voltage-gated potassium channels Kv1.1-Kv1.6

Homology models of the pore loop domain of six eukaryotic potassium channels, Kv1.1-Kv1.6, were generated based on the crystallographic structure of KcsA. The results of amino acid sequence alignment indicate that these Kv channels are composed of two structurally and functionally independent domains: the N-terminal 'voltage sensor' domain and the C-terminal 'pore loop' domain. The homology models reveal that the pore loop domains of these Kv channels exhibit similar folds to those of KcsA. The structural features and specific packing of aromatic residues around the selectivity filter of these Kv channels are nearly identical to those of KcsA, whereas most of the structural variations occur in the turret as well as in the inner and outer helices. The distribution of polar and nonpolar side chains on the surfaces of the KcsA and Kv channels reveals that they exhibit a segregation of side chains common to most integral membrane proteins. As the hydrogen bond between Glu71 and Asp80 in KcsA plays an important role in stabilizing the channel, the substituted Val residue in the Kv family corresponding to Glu71 of KcsA stabilizes the channel by making hydrophobic contact with Tyr residue from the signature sequence of the selectivity filter. The homology models of these Kv channels provide particularly attractive subjects for further structure-based studies.

Pharmacophore-based search, synthesis, and biological evaluation of anthranilic amides as novel blockers of the Kv1.5 channel

The search for novel, potent Kv1.5 blockers based on an anthranilic amide scaffold employing a pharmacophore-based virtual screening approach is described. The synthesis and structure-activity relationships (SAR) with respect to inhibition of the Kv1.5 channel are discussed. The most potent compounds display sub-micromolar inhibition of Kv1.5 and no significant effect on the HERG channel. In addition, good oral bioavailability is demonstrated for compound 3i in rats.

Subacute hypoxia decreases voltage-activated potassium channel expression and function in pulmonary artery myocytes

Chronic hypoxia results in both structural changes in the pulmonary artery and a sustained increase in pulmonary vascular tone. This study investigated the effects of subacute moderate hypoxia on expression and function of potassium (K+) channels in rat pulmonary artery myocytes (PASMCs). The rats were kept at 0.67 atmospheres for 6, 12, or 24 h. We found that the expression of mRNA for voltage-activated K+ channels (Kv)1.2, Kv1.5, and Kv2.1 is reduced after less than 24 h of this moderate hypoxia. K+ current (Ik) is significantly inhibited in PASMCs from rats hypoxic for 24 h, resting membrane potential is depolarized and cytosolic [Ca2+] is increased in these cells. In addition, antibodies to Kv1.2, Kv1.5, and Kv2.1 inhibit Ik, cause membrane depolarization and attenuate both hypoxia- and 4-AP-induced elevation in [Ca2+]i in PASMCs from normoxic rats but not from 24 h hypoxic rats. Subacute hypoxia does not completely remove the mRNA for Kv1.2, Kv1.5, and Kv2.1, but antibodies against these channels no longer alter Ik or cytosolic calcium, suggesting that subacute hypoxia may inactivate the channels as well as reduce expression. As the expression of mRNA for Kv1.2, Kv1.5, and Kv2.1 is sensitive to subacute hypoxia and decreased expression/function of these channels has physiologic effects on membrane potential and cytosolic calcium, it seems likely that these Kv channels may also be involved in the mechanism of high-altitude pulmonary edema and possibly in the signaling of chronic hypoxic pulmonary hypertension.

NH2-terminal inactivation peptide binding to C-type-inactivated Kv channels

In many voltage-gated K⁺ channels, N-type inactivation significantly accelerates the onset of C-type inactivation, but effects on recovery from inactivation are small or absent. We have exploited the Na⁺ permeability of C-type–inactivated K⁺ channels to characterize a strong interaction between the inactivation peptide of Kv1.4 and the C-type–inactivated state of Kv1.4 and Kv1.5. The presence of the Kv1.4 inactivation peptide results in a slower decay of the Na⁺ tail currents normally observed through C-type–inactivated channels, an effective blockade of the peak Na⁺ tail current, and also a delay of the peak tail current. These effects are mimicked by addition of quaternary ammonium ions to the pipette-filling solution. These observations support a common mechanism of action of the inactivation peptide and intracellular quaternary ammonium ions, and also demonstrate that the Kv channel inner vestibule is cytosolically exposed before and after the onset of C-type inactivation. We have also examined the process of N-type inactivation under conditions where C-type inactivation is removed, to compare the interaction of the inactivation peptide with open and C-type–inactivated channels. In C-type–deficient forms of Kv1.4 or Kv1.5 channels, the Kv1.4 inactivation ball behaves like an open channel blocker, and the resultant slowing of deactivation tail currents is considerably weaker than observed in C-type–inactivated channels. We present a kinetic model that duplicates the effects of the inactivation peptide on the slow Na⁺ tail of C-type–inactivated channels. Stable binding between the inactivation peptide and the C-type–inactivated state results in slower current decay, and a reduction of the Na⁺ tail current magnitude, due to slower transition of channels through the Na⁺-permeable states traversed during recovery from inactivation.

Atrial effects of the novel K(+)-channel-blocker AVE0118 in anesthetized pigs

OBJECTIVES

AVE0118 is a novel blocker of the K(+) channels K(v)1.5 and K(v)4.3 which are the molecular basis for the human cardiac ultrarapid delayed rectifier potassium current (I(Kur)) and the transient outward current (I(to)). The objective of this study was to investigate the effect of AVE0118 on atrial refractoriness (ERP), left atrial vulnerability (LAV) and on left atrial monophasic action potentials (MAP) in pentobarbital anesthetized pigs in comparison to the selective I(Kr) blocker dofetilide in order to assess the therapeutic potential of the novel K(+) channel blocker for atrial fibrillation.

METHODS

Atrial ERP was determined with the S1-S2-stimulus method in the free walls of left and right atrium at 240, 300 and 400 ms basic cycle length (BCL). The inducibility of mostly nonsustained atrial tachyarrhythmias by the premature S2 extrastimulus, which is very high in the left pig atrium and referred to as LAV, was evaluated before and after drugs. Left atrial epicardial MAP was recorded to study the influence of the potassium channel blockers on the time course of repolarization. Left ventricular epicardial MAP, ERP and QT interval were measured to investigate a possible effect of AVE0118 on ventricular repolarization.

RESULTS

ERPs determined at 240, 300 and 400 ms BCL were significantly shorter in the left vs. right atrium (99+/-3, 106+/-4 and 113+/-3 ms vs. 133+/-4 ms, 142+/-4 and 149+/-5, respectively; p<0.001; n=21). AVE0118 administered i.v. dose-dependently prolonged the atrial ERP independent from rate and inhibited LAV (100% at 0.5 and 1 mg/kg) while having no effect at all on the corrected QT (QTc) interval. At 1 mg/kg (n=5) AVE0118 prolonged left vs. right atrial ERP by 49.6+/-4.1 ms vs. 37.7+/-9.7 ms (means+/-SEM of changes at 240, 300, and 400 ms BCL), respectively, corresponding to a relative increase of 53.2+/-6.2% vs. 27.6+/-6.8% (p<0.05 for percent increase of left vs. right atrial ERP). In a separate group of pigs (n=5) AVE0118 had no effect on left ventricular ERP at 333, 400 and 500 ms BCL and no effect on MAP duration and QT at 600 ms BCL. After 1 mg/kg of AVE0118 the atrial MAP was significantly prolonged already at 10% repolarization (P<0.05; n=7) reaching the maximum at 40% repolarization. In contrast to AVE0118 the effect of dofetilide (10 microg/kg) on atrial MAP started to become significant only at 60% repolarization (n=6) with a maximum increase at 90%. Dofetilide, which prolonged the QTc interval by 16.9% (P<0.001), had a significantly stronger effect on right (34.7+/-5 ms) vs. left atrial ERP (23.5+/-7 ms) at 300 ms BCL, respectively, but did not significantly inhibit LAV (14%; n=6).

CONCLUSION

The novel K(+) channel blocker AVE0118 prolonged atrial ERP and showed strong atrial antiarrhythmic efficacy with no apparent effect on ventricular repolarization in pigs in vivo.

Theoretical Possibilities for the Development of Novel Antiarrhythmic Drugs

One possible mechanism of action of the available K-channel blocking agents used to treat arrhythmias is to selectively inhibit the HERG plus MIRP channels, which carry the rapid delayed rectifier outward potassium current (I(Kr)). These antiarrhythmics, like sotalol, dofetilide and ibutilide, have been classified as Class III antiarrhythmics. However, in addition to their beneficial effect, they substantially lengthen ventricular repolarization in a reverse-rate dependent manner. This latter effect, in certain situations, can result in life-threatening polymorphic ventricular tachycardia (torsades de pointes). Selective blockers (chromanol 293B, HMR-1556, L-735,821) of the KvLQT1 plus minK channel, which carriy the slow delayed rectifier potassium current (I(Ks)), were also considered to treat arrhythmias, including atrial fibrillation (AF). However, I(Ks) activates slowly and at a more positive voltage than the plateau of the action potential, therefore it remains uncertain how inhibition of this current would result in a therapeutically meaningful repolarization lengthening. The transient outward potassium current (I(to)), which flows through the Kv 4.3 and Kv 4.2 channels, is relatively large in the atrial cells, which suggests that inhibition of this current may cause substantial prolongation of repolarization predominantly in the atria. Although it was reported that some antiarrhythmic drugs (quinidine, disopyramide, flecainide, propafenone, tedisamil) inhibit I(to), no specific blockers for I(to) are currently available. Similarly, no specific inhibitors for the Kir 2.1, 2.2, 2.3 channels, which carry the inward rectifier potassium current (I(kl)), have been developed making difficult to judge the possible beneficial effects of such drugs in both ventricular arrhythmias and AF. Recently, a specific potassium channel (Kv 1.5 channel) has been described in human atrium, which carries the ultrarapid, delayed rectifier potassium current (I(Kur)). The presence of this current has not been observed in the ventricular muscle, which raises the possibility that by specific inhibition of this channel, atrial repolarization can be lengthened without similar effect in the ventricle. Therefore, AF could be terminated and torsades de pointes arrhythmia avoided. Several compounds were reported to inhibit I(Kur)(flecainide, tedisamil, perhexiline, quinidine, ambasilide, AVE 0118), but none of them can be considered as specific for Kv 1.5 channels. Similarly to Kv 1.5 channels, acetylcholine activated potassium channels carry repolarizing current (I(KAch)) in the atria and not in the ventricle during normal vagal tone and after parasympathetic activation. Specific blockers of I(KAch) can, therefore, also be a possible candidate to treat AF without imposing proarrhythmic risk on the ventricle. At present several compounds (amiodarone, dronedarone, aprindine, pirmenol, SD 3212) were shown to inhibit I(KAch) but none of them proved to be selective. Further research is needed to develop specific K-channel blockers, such as I(Kur)and I(KAch) inhibitors, and to establish their possible therapeutic value.

Messenger RNA and protein expression analysis of voltage-gated potassium channels in the brain of A?25-35-treated rats

Potassium channel dysfunction has been implicated in Alzheimer's disease. In the present study, the expression of voltage-gated potassium channel (Kv) subunits in rat brain was measured after a single intracerebroventricular injection of beta-amyloid peptide 25-35 (Abeta(25-35)). After injection of Abeta, the spatial memory of rats was significantly impaired in the Morris water maze. Expression of five main Kv channel subunits (Kv1.5, Kv2.1, Kv1.4, Kv4.2, and Kv4.3) in mRNA level was assessed by using reverse transcription-polymerase chain reaction (RT-PCR). The mRNA levels of Kv2.1 and Kv1.4 were increased by 72% and 67%, respectively, in hippocampus, and Kv4.2 mRNA was increased by 58% in cortex. No other significant mRNA expression changes were found in Abeta-treated rats. The protein expression of Kv2.1, Kv1.4, and Kv4.2 was detected by using Western blotting. Kv2.1 and Kv1.4 protein levels were increased by 48% and 50%, respectively, in hippocampus of Abeta-treated rats, and Kv4.2 protein was increased by 42% in cerebral cortex. This study indicates that the expression up-regulation of Kv1.4, Kv2.1, and Kv4.2 in Abeta-induced cognitive impairment might play an important role in the pathogenesis of Alzheimer's disease.

Different isoforms of synapse-associated protein, SAP97, are expressed in the heart and have distinct effects on the voltage-gated K+ channel Kv1.5

The SAP97 isoforms differ by alternatively spliced insertion domains that regulate protein localization and oligomerization. We used reverse transcription-PCR to identify SAP97 isoforms of human and rat myocardium. In Chinese hamster ovary cells, cloned protein expression was studied using Western blot, confocal imaging of green fluorescent protein-tagged proteins, and patch clamp technique. The two main cardiac SAP97 isoforms contained both I3 and I1B inserts and differed by the I1A insert. Both isoforms co-precipitated with hKv1.5 channels. Only the isoform lacking I1A increased the current (by 215 +/- 22%), whatever the level of channel expression. To examine the involvement of the proline-rich I1A insert in the effect of SAP97, a W623F mutation in the Src homology 3 domain was created, and that restored the effect of the SAP97 on current. SAP97 isoform containing an I1A and I2 domain instead of the I3 domain stimulated the current, whereas SAP97 after deletion of the Src homology 3 and guanylate kinase-like domains did not. In cells co-expressing I3(+I1A) or I3(-I1A), green fluorescent protein-tagged Kv1.5 channels were organized in plaque-like structures at the plasma membrane level, whereas intracellular aggregates of channels predominated with the I2 isoform. The two cardiac SAP97 isoforms have different effects on the hKv1.5 current, depending on their capacity to form channel clusters.

Hydrogen peroxide modulates the Kv1.5 channel expressed in a mammalian cell line

Reactive oxygen species have been implicated in different types of cardiac arrhythmias including human atrial fibrillation. Kv1.5, the presumed molecular correlate of I(Kur), is an important determinant of human atrial repolarization. The aim of this study was to assess the effects of H(2)O(2), at pathophysiologically relevant concentrations (20-1,000 microM), on Kv1.5 expressed in Chinese hamster ovary cell line. Kv1.5 cDNA in pcDNA3 expression vector and CD8, a surface marker protein, were cotransfected in cells by calcium phosphate precipitation. Kv1.5 activation kinetics were significantly accelerated while the activation curve was negatively shifted by 10 mV (V(1/2) changed from -9.3 to -19.0 mV) in the presence of 100 microM H(2)O(2). The shift in Kv1.5 peak current I-V curve was voltage-dependent, the current amplitude being increased for voltages <+20 mV but decreased for high depolarizing voltages. The rapid activation time constant obtained from a bi-exponential fitting was decreased from 16.1+/-3.4 ms to 8.8+/-1.5 ms for a -20 mV depolarization ( n=9; P=0.01) and from 4.3+/-2.1 ms to 2.3+/-0.4 ms when cells were depolarized to +20 mV ( P<0.05). Kv1.5 steady-state inactivation was not modified by H(2)O(2). Intracellular application of SOD or catalase reduced the H(2)O(2) induced shift of activation I-V curve and SOD significantly decreased Kv1.5 amplitude at +40 mV ( n=9; P<0.05). In conclusion, H(2)O(2) increased Kv1.5 current amplitude at voltages corresponding to the action potential repolarization phase and accelerated Kv1.5 channel opening. These changes can reduce the action potential duration, leading to a shortening of the atrial effective refractory period. H(2)O(2)-induced changes in Kv1.5 properties could thus be involved in initiation or perpetuation of AF.

Molecular basis for Kv1.5 channel block: conservation of drug binding sites among voltage-gated K+ channels

Kv1.5 channels conduct the ultrarapid delayed rectifier current (IKur) that contributes to action potential repolarization of human atrial myocytes. Block of these channels has been proposed as a treatment for atrial arrhythmias. Here we report a novel and potent inhibitor of Kv1.5 potassium channels, N-benzyl-N-pyridin-3-yl-methyl-2-(toluene-4-sulfonylamino)-benzamide hydrochloride (S0100176), which exhibits features consistent with preferential block of the open state. The IC50 of S0100176 for Kv1.5 expressed in Xenopus oocytes was 0.7 microm. Ala-scanning mutagenesis within the pore helix and the S6 segment, regions that form the walls of the central cavity, was combined with voltage clamp analysis to identify point mutations that altered drug affinity. This approach identified Thr-479, Thr-480, Val-505, Ile-508, and Val-512 as the most important residues for block by S0100176. Mutations of these key residues to Ala or other amino acids caused marked changes in the IC50 of S0100176 (p<0.01). For example, the IC50 of S0100176 increased 362-fold for T480A, 26-fold for V505A, 150-fold for I508A, and 99-fold for V512A. We used modeling to dock S0100176 into the inner cavity of a Kv1.5 pore homology model that was generated based on the crystal structure of KcsA. The docking predicted that the five residues identified by the Ala scan were positioned less than 4.5 A from the compound. Based on the homology models, the positions of the five amino acids identified to interact with S0100176 face toward the central cavity and overlap with putative binding sites for other blockers and voltage-gated potassium channels.

Selective elimination of I(K,slow1) in mouse ventricular myocytes expressing a dominant negative Kv1.5alpha subunit

Although previous studies have revealed a role for the voltage-gated K+ channel alpha-subunit Kv1.5 (KCNA5) in the generation of the 4-aminopyridine (4-AP)-sensitive component of delayed rectification in mouse ventricles (IK,slow1), the phenotypic consequences of manipulating IK,slow1 expression in vivo in different (mouse) models are distinct. In these experiments, point mutations were introduced in the pore region of Kv1.5 to change the tryptophan (W) at position 461 to phenylalanine (F) to produce a nonconducting subunit, Kv1.5W461F, that is shown to function as a Kv1 subfamily-specific dominant negative (Kv1.5DN). With the use of the alpha-myosin heavy chain promoter to direct cardiac-specific expression, three lines of Kv1.5DN-expressing (C57BL6) transgenic mice were generated and characterized. Electrophysiological recordings from Kv1.5-DN-expressing left ventricular myocytes revealed that the micromolar 4-AP sensitive IK,slow1 is selectively eliminated. The attenuation of IK,slow1 is accompanied by increased ventricular action potential durations and marked QT prolongation. In contrast to previous findings in mice expressing a truncated (DN) Kv1.1 transgene; however, no electrical remodeling is evident in Kv1.5DN-expressing ventricular myocytes, and the (Kv1.5DN-induced) elimination of IK,slow1 does not result in spontaneous ventricular arrhythmias.

Kv1.5 is an important component of repolarizing K+ current in canine atrial myocytes

Although the canine atrium has proven useful in several experimental models of atrial fibrillation and for studying the effects of rapid atrial pacing on atrial electrical remodeling, it may not fully represent the human condition because of reported differences in functional ionic currents and ion channel subunit expression. In this study, we reassessed the molecular components underlying one current, the ultrarapid delayed rectifier current in canine atrium [IKur(d)], by evaluating the mRNA, protein, immunofluorescence, and currents of the candidate channels. Using reverse transcriptase-polymerase chain reaction, we found that Kv1.5 mRNA was expressed in canine atrium whereas message for Kv3.1 was not detected. Western analysis on cytosolic and membrane fractions of canine tissues, using selective antibodies, showed that Kv3.1 was only detectable in the brain preparations, whereas Kv1.5 was expressed at high levels in both atrial and ventricular membrane fractions. Confocal imaging performed on isolated canine atrial myocytes clearly demonstrated the presence of Kv1.5 immunostaining, whereas that of Kv3.1 was equivocal. Voltage- and current-clamp studies showed that 0.5 mmol/L tetraethylammonium had variable effects on sustained K+ currents, whereas a compound with demonstrated selectivity for hKv1.5 versus Kv3.1, hERG or the sodium channel, fully suppressed canine atrial IKur tail currents and depressed sustained outward K+ current. This agent also increased action potential plateau potentials and action potential duration at 20% and 50% repolarization. These results suggest that in canine atria, as in other species including human, Kv1.5 protein is highly expressed and contributes to IKur.

Voltage-gated K+ channels in rat small cerebral arteries: molecular identity of the functional channels

Voltage-gated potassium (KV) channels represent an important dilator influence in the cerebral circulation, but the composition of these tetrameric ion channels remains unclear. The goals of the present study were to evaluate the contribution of KV1 family channels to the resting membrane potential and diameter of small rat cerebral arteries, and to identify the alpha-subunit composition of these channels using patch-clamp, molecular and immunological techniques. Initial studies indicated that 1 micromol l(-1) correolide (COR), a specific antagonist of KV1 channels, depolarized vascular smooth muscle cells (VSMCs) in pressurized (60 mmHg) cerebral arteries from -55 +/- 1 mV to -34 +/- 1 mV, and reduced the resting diameter from 152 +/- 15 microm to 103 +/- 20 microm. In patch clamped VSMCs from these arteries, COR-sensitive KV1 current accounted for 65 % of total outward KV current and was observed at physiological membrane potentials. RT-PCR identified mRNA encoding each of the six classical KV1 alpha-subunits, KV1.1-1.6, in rat cerebral arteries. However, only the KV1.2 and 1.5 proteins were detected by Western blot. The expression of these proteins in VSMCs was confirmed by immunocytochemistry and co-immunoprecipitation of KV1.2 and 1.5 from VSMC membranes suggested KV1.2/1.5 channel assembly. Subsequently, the pharmacological and voltage-sensitive properties of KV1 current in VSMCs were found to be consistent with a predominant expression of KV1.2/1.5 heterotetrameric channels. The findings of this study suggest that KV1.2/1.5 heterotetramers are preferentially expressed in rat cerebral VSMCs, and that these channels contribute to the resting membrane potential and diameter of rat small cerebral arteries.