Characteristics of brain Kv1 channels tailored to mimic native counterparts by tandem linkage of alpha subunits: implications for K+ channelopathies

New High-Affinity Peptide Ligands for Kv1.2 Channel: Selective Blockers and Fluorescent Probes

Advanced molecular probes are required to study the functional activity of the Kv1.2 potassium channel in normal and pathological conditions. To address this, a fully active Kv1.2 channel fused with fluorescent protein mKate2 (K-Kv1.2) was engineered that has high plasma membrane presentation due to the S371T substitution, and hongotoxin 1 (HgTx1) fused with eGFP at the C-terminus (HgTx-G) was produced. HgTx-G and HgTx1 N-terminally labeled with Atto488 fluorophore were shown to be fluorescent probes of Kv1.2 in cells with dissociation constants (Kd) of 120 and 80 pM, respectively. K-Kv1.2 and HgTx-G were used as components of an analytical system to study peptide blockers of the channel and helped to find out that Ce1 and Ce4 peptides from Centruroides elegans venom possess high affinity (Kd of 10 and 30 pM) and selectivity for Kv1.2. Using molecular docking and molecular modeling techniques, the complexes of Kv1.2 with HgTx1, Ce1, and Ce4 were modeled, and determinants of the high affinity binding were proposed. New fluorescent probes and selective blockers of Kv1.2 can be used to resolve Kv1.2-related challenges in neuroscience and neuropharmacology.

Interactions of the Kv1.1 Channel with Peptide Pore Blockers: A Fluorescent Analysis on Mammalian Cells

The voltage-gated potassium channel Kv1.1, which is abundant in the CNS and peripheral nervous system, controls neuronal excitability and neuromuscular transmission and mediates a number of physiological functions in non-excitable cells. The development of some diseases is accompanied by changes in the expression level and/or activity of the channels in particular types of cells. To meet the requirements of studies related to the expression and localization of the Kv1.1 channels, we report on the subnanomolar affinity of hongotoxin 1 N-terminally labeled with Atto 488 fluorophore (A-HgTx) for the Kv1.1 channel and its applicability for fluorescent imaging of the channel in living cells. Taking into consideration the pharmacological potential of the Kv1.1 channel, a fluorescence-based analytical system was developed for the study of peptide ligands that block the ion conductivity of Kv1.1 and are potentially able to correct abnormal activity of the channel. The system is based on analysis of the competitive binding of the studied compounds and A-HgTx to the mKate2-tagged human Kv1.1 (S369T) channel, expressed in the plasma membrane of Neuro2a cells. The system was validated by measuring the affinities of the known Kv1.1-channel peptide blockers, such as agitoxin 2, kaliotoxin 1, hongotoxin 1, and margatoxin. Peptide pore blocker Ce1, from the venom of the scorpion Centruroides elegans, was shown to possess a nanomolar affinity for the Kv1.1 channel. It is reported that interactions of the Kv1.1 channel with the studied peptide blockers are not affected by the transition of the channel from the closed to open state. The conclusion is made that the structural rearrangements accompanying the channel transition into the open state do not change the conformation of the P-loop (including the selectivity filter) involved in the formation of the binding site of the peptide pore blockers.

An epilepsy-associated KV1.2 charge-transfer-center mutation impairs KV1.2 and KV1.4 trafficking

We report on a heterozygous KCNA2 variant in a child with epilepsy. KCNA2 encodes KV1.2 subunits, which form homotetrameric potassium channels and participate in heterotetrameric channel complexes with other KV1-family subunits, regulating neuronal excitability. The mutation causes substitution F233S at the KV1.2 charge transfer center of the voltage-sensing domain. Immunocytochemical trafficking assays showed that KV1.2(F233S) subunits are trafficking deficient and reduce the surface expression of wild-type KV1.2 and KV1.4: a dominant-negative phenotype extending beyond KCNA2, likely profoundly perturbing electrical signaling. Yet some KV1.2(F233S) trafficking was rescued by wild-type KV1.2 and KV1.4 subunits, likely in permissible heterotetrameric stoichiometries: electrophysiological studies utilizing applied transcriptomics and concatemer constructs support that up to one or two KV1.2(F233S) subunits can participate in trafficking-capable heterotetramers with wild-type KV1.2 or KV1.4, respectively, and that both early and late events along the biosynthesis and secretion pathway impair trafficking. These studies suggested that F233S causes a depolarizing shift of ∼48 mV on KV1.2 voltage dependence. Optical tracking of the KV1.2(F233S) voltage-sensing domain (rescued by wild-type KV1.2 or KV1.4) revealed that it operates with modestly perturbed voltage dependence and retains pore coupling, evidenced by off-charge immobilization. The equivalent mutation in the Shaker K+ channel (F290S) was reported to modestly affect trafficking and strongly affect function: an ∼80-mV depolarizing shift, disrupted voltage sensor activation and pore coupling. Our work exposes the multigenic, molecular etiology of a variant associated with epilepsy and reveals that charge-transfer-center disruption has different effects in KV1.2 and Shaker, the archetypes for potassium channel structure and function.

The Molecular Basis for the Calcium-Dependent Slow Afterhyperpolarization in CA1 Hippocampal Pyramidal Neurons

Neuronal signal transmission depends on the frequency, pattern, and timing of spike output, each of which are shaped by spike afterhyperpolarizations (AHPs). There are classically three post-spike AHPs of increasing duration categorized as fast, medium and slow AHPs that hyperpolarize a cell over a range of 10 ms to 30 s. Intensive early work on CA1 hippocampal pyramidal cells revealed that all three AHPs incorporate activation of calcium-gated potassium channels. The ionic basis for a fAHP was rapidly attributed to the actions of big conductance (BK) and the mAHP to small conductance (SK) or Kv7 potassium channels. In stark contrast, the ionic basis for a prominent slow AHP of up to 30 s duration remained an enigma for over 30 years. Recent advances in pharmacological, molecular, and imaging tools have uncovered the expression of a calcium-gated intermediate conductance potassium channel (IK, KCa3.1) in central neurons that proves to contribute to the slow AHP in CA1 hippocampal pyramidal cells. Together the data show that the sAHP arises in part from a core tripartite complex between Cav1.3 (L-type) calcium channels, ryanodine receptors, and IK channels at endoplasmic reticulum-plasma membrane junctions. Work on the sAHP in CA1 pyramidal neurons has again quickened pace, with identified contributions by both IK channels and the Na-K pump providing answers to several mysteries in the pharmacological properties of the sAHP.

Studying Independent Kcna6 Knock-out Mice Reveals Toxicity of Exogenous LacZ to Central Nociceptor Terminals and Differential Effects of Kv1.6 on Acute and Neuropathic Pain Sensation

The potassium channel Kv1.6 has recently been implicated as a major modulatory channel subunit expressed in primary nociceptors. Furthermore, its expression at juxtaparanodes of myelinated primary afferents is induced following traumatic nerve injury as part of an endogenous mechanism to reduce hyperexcitability and pain-related hypersensitivity. In this study, we compared two mouse models of constitutive Kv1.6 knock-out (KO) achieved by different methods: traditional gene trap via homologous recombination and CRISPR-mediated excision. Both Kv1.6 KO mouse lines exhibited an unexpected reduction in sensitivity to noxious heat stimuli, to differing extents: the Kv1.6 mice produced via gene trap had a far more significant hyposensitivity. These mice (Kcna6^lacZ ) expressed the bacterial reporter enzyme LacZ in place of Kv1.6 as a result of the gene trap mechanism, and we found that their central primary afferent presynaptic terminals developed a striking neurodegenerative phenotype involving accumulation of lipid species, development of "meganeurites," and impaired transmission to dorsal horn wide dynamic range neurons. The anatomic defects were absent in CRISPR-mediated Kv1.6 KO mice (Kcna6 ^-/-) but were present in a third mouse model expressing exogenous LacZ in nociceptors under the control of a Nav1.8-promoted Cre recombinase. LacZ reporter enzymes are thus intrinsically neurotoxic to sensory neurons and may induce pathologic defects in transgenic mice, which has confounding implications for the interpretation of gene KOs using lacZ Nonetheless, in Kcna6 ^-/- mice not affected by LacZ, we demonstrated a significant role for Kv1.6 regulating acute noxious thermal sensitivity, and both mechanical and thermal pain-related hypersensitivity after nerve injury.SIGNIFICANCE STATEMENT In recent decades, the expansion of technologies to experimentally manipulate the rodent genome has contributed significantly to the field of neuroscience. While introduction of enzymatic or fluorescent reporter proteins to label neuronal populations is now commonplace, often potential toxicity effects are not fully considered. We show a role of Kv1.6 in acute and neuropathic pain states through analysis of two mouse models lacking Kv1.6 potassium channels: one with additional expression of LacZ and one without. We show that LacZ reporter enzymes induce unintended defects in sensory neurons, with an impact on behavioral data outcomes. To summarize we highlight the importance of Kv1.6 in recovery of normal sensory function following nerve injury, and careful interpretation of data from LacZ reporter models.

Development of a selective inhibitor for Kv1.1 channels prevalent in demyelinated nerves

Members of the voltage-gated K⁺ channel subfamily (Kv1), involved in regulating transmission between neurons or to muscles, are associated with human diseases and, thus, putative targets for neurotherapeutics. This applies especially to those containing Kv1.1 α subunits which become prevalent in murine demyelinated axons and appear abnormally at inter-nodes, underlying the perturbed propagation of nerve signals. To overcome this dysfunction, akin to the consequential debilitation in multiple sclerosis (MS), small inhibitors were sought that are selective for the culpable hyper-polarising K⁺ currents. Herein, we report a new semi-podand - compound 3 - that was designed based on the modelling of its interactions with the extracellular pore region in a deduced Kv1.1 channel structure. After synthesis, purification, and structural characterisation, compound 3 was found to potently (IC₅₀ = 8 µM) and selectively block Kv1.1 and 1.6 channels. The tested compound showed no apparent effect on native Nav and Cav channels expressed in F-11 cells. Compound 3 also extensively and selectively inhibited MS-related Kv1.1 homomer but not the brain native Kv1.1- or 1.6-containing channels. These collective findings highlight the therapeutic potential of compound 3 to block currents mediated by Kv1.1 channels enriched in demyelinated central neurons.

Sound Localization in Preweanling Mice Was More Severely Affected by Deleting the Kcna1 Gene Compared to Deleting Kcna2, and a Curious Inverted-U Course of Development That Appeared to Exceed Adult Performance Was Observed in All Groups

The submillisecond acuity for detecting rapid spatial and temporal fluctuations in acoustic stimuli observed in humans and laboratory animals depends in part on select groups of auditory neurons that preserve synchrony from the ears to the binaural nuclei in the brainstem. These fibers have specialized synapses and axons that use a low-threshold voltage-activated outward current, IKL, conducted through Kv1 potassium ion channels. These are in turn coupled with HCN channels that express a mixed cation inward mixed current, IH, to support precise synchronized firing. The behavioral evidence is that their respective Kcna1 or HCN1 genes are absent in adult mice; the results are weak startle reflexes, slow responding to noise offsets, and poor sound localization. The present behavioral experiments were motivated by an in vitro study reporting increased IKL in an auditory nucleus in Kcna2-/- mice lacking the Kv1.2 subunit, suggesting that Kcna2-/- mice might perform better than Kcna2+/+ mice. Because Kcna2-/- mice have only a 17-18-day lifespan, we compared both preweanling Kcna2-/- vs. Kcna2+/+ mice and Kcna1-/- vs. Kcna1+/+ mice at P12-P17/18; then, the remaining mice were tested at P23/P25. Both null mutant strains had a stunted physique, but the Kcna1-/- mice had severe behavioral deficits while those in Kcna2-/- mice were relatively few and minor. The in vitro increase of IKL could have resulted from Kv1.1 subunits substituting for Kv1.2 units and the loss of the inhibitory "managerial" effect of Kv1.2 on Kv1.1. However, any increased neuronal synchronicity that accompanies increased IKL may not have been enough to affect behavior. All mice performed unusually well on the early spatial tests, but then, they fell towards adult levels. This unexpected effect may reflect a shift from summated independent monaural pathways to integrated binaural processing, as has been suggested for similar observations for human infants.

Preferred Formation of Heteromeric Channels between Coexpressed SK1 and IKCa Channel Subunits Provides a Unique Pharmacological Profile of Ca2+-Activated Potassium Channels

Three small conductance calcium-activated potassium channel (SK) subunits have been cloned and found to preferentially form heteromeric channels when expressed in a heterologous expression system. The original cloning of the gene encoding the intermediate conductance calcium-activated potassium channel (IKCa) was termed SK4 because of the high homology between channel subtypes. Recent immunovisualization suggests that IKCa is expressed in the same subcellular compartments of some neurons as SK channel subunits. Stochastic optical reconstruction microscopy super-resolution microscopy revealed that coexpressed IKCa and SK1 channel subunits were closely associated, a finding substantiated by measurement of fluorescence resonance energy transfer between coexpressed fluorophore-tagged subunits. Expression of homomeric SK1 channels produced current that displayed typical sensitivity to SK channel inhibitors, while expressed IKCa channel current was inhibited by known IKCa channel blockers. Expression of both SK1 and IKCa subunits gave a current that displayed no sensitivity to SK channel inhibitors and a decreased sensitivity to IKCa current inhibitors. Single channel recording indicated that coexpression of SK1 and IKCa subunits produced channels with properties intermediate between those observed for homomeric channels. These data indicate that SK1 and IKCa channel subunits preferentially combine to form heteromeric channels that display pharmacological and biophysical properties distinct from those seen with homomeric channels.

A channelopathy mutation in the voltage-sensor discloses contributions of a conserved phenylalanine to gating properties of Kv1.1 channels and ataxia

Channelopathy mutations prove informative on disease causing mechanisms and channel gating dynamics. We have identified a novel heterozygous mutation in the KCNA1 gene of a young proband displaying typical signs and symptoms of Episodic Ataxia type 1 (EA1). This mutation is in the S4 helix of the voltage-sensing domain and results in the substitution of the highly conserved phenylalanine 303 by valine (p.F303V). The contributions of F303 towards K⁺ channel voltage gating are unclear and here have been assessed biophysically and by performing structural analysis using rat Kv1.2 coordinates. We observed significant positive shifts of voltage-dependence, changes in the activation, deactivation and slow inactivation kinetics, reduced window currents, and decreased current amplitudes of both Kv1.1 and Kv1.1/1.2 channels. Structural analysis revealed altered interactions between F303V and L339 and I335 of the S5 helix of a neighboring subunit. The substitution of an aromatic phenylalanine with an aliphatic valine within the voltage-sensor destabilizes the open state of the channel. Thus, F303 fine-tunes the Kv1.1 gating properties and contributes to the interactions between the S4 segment and neighboring alpha helices. The resulting channel's loss of function validates the clinical relevance of the mutation for EA1 pathogenesis.

Disruption of myelin leads to ectopic expression of K(V)1.1 channels with abnormal conductivity of optic nerve axons in a cuprizone-induced model of demyelination

The molecular determinants of abnormal propagation of action potentials along axons and ectopic conductance in demyelinating diseases of the central nervous system, like multiple sclerosis (MS), are poorly defined. Widespread interruption of myelin occurs in several mouse models of demyelination, rendering them useful for research. Herein, considerable myelin loss is shown in the optic nerves of cuprizone-treated demyelinating mice. Immuno-fluorescence confocal analysis of the expression and distribution of voltage-activated K⁺ channels (K(V)1.1 and 1.2 α subunits) revealed their spread from typical juxta-paranodal (JXP) sites to nodes in demyelinated axons, albeit with a disproportionate increase in the level of K(V)1.1 subunit. Functionally, in contrast to monophasic compound action potentials (CAPs) recorded in controls, responses derived from optic nerves of cuprizone-treated mice displayed initial synchronous waveform followed by a dispersed component. Partial restoration of CAPs by broad spectrum (4-aminopyridine) or K(V)1.1-subunit selective (dendrotoxin K) blockers of K⁺ currents suggest enhanced K(V)1.1-mediated conductance in the demyelinated optic nerve. Biophysical profiling of K⁺ currents mediated by recombinant channels comprised of different K(V)1.1 and 1.2 stoichiometries revealed that the enrichment of K(V)1 channels K(V)1.1 subunit endows a decrease in the voltage threshold and accelerates the activation kinetics. Together with the morphometric data, these findings provide important clues to a molecular basis for temporal dispersion of CAPs and reduced excitability of demyelinated optic nerves, which could be of potential relevance to the patho-physiology of MS and related disorders.

Pharmacological characteristics of Kv1.1- and Kv1.2-containing channels are influenced by the stoichiometry and positioning of their α subunits

Voltage-sensitive neuronal Kv1 channels composed of four α subunits and four associated auxiliary β subunits control neuronal excitability and neurotransmission. Limited information exists on the combinations of α subunit isoforms (i.e. Kv1.1-1.6) or their positions in the oligomers, and how these affect sensitivity to blockers. It is known that TEA (tetraethylammonium) inhibits Kv1.1 channels largely due to binding a critical tyrosine (Tyr379) in the pore, whereas Val381 at the equivalent location in Kv1.2 makes it insensitive. With the eventual aim of developing blockers for therapeutic purposes, Kv1.1 and 1.2 α subunit genes were concatenated to form combinations representing those in central neurons, followed by surface expression in HEK (human embryonic kidney)-293 cells as single-chain functional proteins. Patch-clamp recordings demonstrated the influences of the ratios and positioning of these α subunits on the biophysical and pharmacological properties of oligomeric K+ channels. Raising the ratio of Kv1.1 to Kv1.2 in Kv1.2-1.2-1.1-1.2 led to the resultant channels being more sensitive to TEA and also affected their biophysical parameters. Moreover, mutagenesis of one or more residues in the first Kv1.2 to resemble those in Kv1.1 increased TEA sensitivity only when it is adjacent to a Kv1.1 subunit, whereas placing a non-interactive subunit between these two diminished susceptibility. The findings of the present study support the possibility of α subunits being precisely arranged in Kv1 channels, rather than being randomly assembled. This is important in designing drugs with abilities to inhibit particular oligomeric Kv1 subtypes, with the goal of elevating neuronal excitability and improving neurotransmission in certain diseases.

Loss of the Kv1.1 potassium channel promotes pathologic sharp waves and high frequency oscillations in in vitro hippocampal slices

In human disease, channelopathies involving functional reduction of the delayed rectifier potassium channel α-subunit Kv1.1 - either by mutation or autoimmune inhibition - result in temporal lobe epilepsy. Kv1.1 is prominently expressed in the axons of the hippocampal tri-synaptic pathway, suggesting its absence will result in widespread effects on normal network oscillatory activity. Here, we performed in vitro extracellular recordings using a multielectrode array to determine the effects of loss of Kv1.1 on spontaneous sharp waves (SPWs) and high frequency oscillations (HFOs). We found that Kcna1-null hippocampi generate SPWs and ripples (80-200Hz bandwidth) with a 50% increased rate of incidence and 50% longer duration, and that epilepsy-associated pathologic HFOs in the fast ripple bandwidth (200-600Hz) are also present. Furthermore, Kcna1-null CA3 has enhanced coupling of excitatory inputs and population spike generation and CA3 principal cells have reduced spike timing reliability. Removing the influence of mossy fiber and perforant path inputs by micro-dissecting the Kcna1-null CA3 region mostly rescued the oscillatory behavior and improved spike timing. We found that Kcna1-null mossy fibers and medial perforant path axons are hyperexcitable and produce greater pre- and post-synaptic responses with reduced paired-pulse ratios suggesting increased neurotransmitter release at these terminals. These findings were recapitulated in wild-type slices exposed to the Kv1.1 inhibitor dendrotoxin-κ. Collectively, these data indicate that loss of Kv1.1 enhances synaptic release in the CA3 region, which reduces spike timing precision of individual neurons leading to disorganization of network oscillatory activity and promotes the emergence of fast ripples.

A defined heteromeric KV1 channel stabilizes the intrinsic pacemaking and regulates the output of deep cerebellar nuclear neurons to thalamic targets

The output of the cerebellum to the motor axis of the central nervous system is orchestrated mainly by synaptic inputs and intrinsic pacemaker activity of deep cerebellar nuclear (DCN) projection neurons. Herein, we demonstrate that the soma of these cells is enriched with K(V)1 channels produced by mandatory multi-merization of K(V)1.1, 1.2 α and KV β2 subunits. Being constitutively active, the K(+) current (IK(V)1) mediated by these channels stabilizes the rate and regulates the temporal precision of self-sustained firing of these neurons. Placed strategically, IK(V)1 provides a powerful counter-balance to prolonged depolarizing inputs, attenuates the rebound excitation, and dampens the membrane potential bi-stability. Somatic location with low activation threshold render IK(V)1 instrumental in voltage-dependent de-coupling of the axon initial segment from the cell body of projection neurons, impeding invasion of back-propagating action potentials into the somato-dendritic compartment. The latter is also demonstrated to secure the dominance of clock-like somatic pacemaking in driving the regenerative firing activity of these neurons, to encode time variant inputs with high fidelity. Through the use of multi-compartmental modelling and retro-axonal labelling, the physiological significance of the described functions for processing and communication of information from the lateral DCN to thalamic relay nuclei is established.

Increased Kv1 channel expression may contribute to decreased sIPSC frequency following chronic inhibition of NR2B-containing NMDAR

Numerous studies have documented the effects of chronic N-methyl-D-aspartate receptor (NMDAR) blockade on excitatory circuits, but the effects on inhibitory circuitry are not well studied. NR2A- and NR2B-containing NMDARs play differential roles in physiological processes, but the consequences of chronic NR2A- or NR2B-containing NMDAR inhibition on glutamatergic and GABAergic neurotransmission are unknown. We investigated altered GABAergic neurotransmission in dentate granule cells and interneurons following chronic treatment with the NR2B-selective antagonist, Ro25,6981, the NR2A-prefering antagonist, NVP-AAM077, or the non-subunit-selective NMDAR antagonist, D-APV, in organotypic hippocampal slice cultures. Electrophysiological recordings revealed large reductions in spontaneous inhibitory postsynaptic current (sIPSC) frequency in both granule cells and interneurons following chronic Ro25,6981 treatment, which was associated with minimally altered sIPSC amplitude, miniature inhibitory postsynaptic current (mIPSC) frequency, and mIPSC amplitude, suggesting diminished action potential-dependent GABA release. Chronic NVP-AAM077 or D-APV treatment had little effect on these measures. Reduced sIPSC frequency did not arise from downregulated GABA(A)R, altered excitatory or inhibitory drive to interneurons, altered interneuron membrane properties, increased failure rate, decreased action potential-dependent release probability, or mGluR/GABA(B) receptor modulation of GABA release. However, chronic Ro25,6981-mediated reductions in sIPSC frequency were occluded by the K+ channel blockers, dendrotoxin, margatoxin, and agitoxin, but not dendrotoxin-K or XE991. Immunohistochemistry also showed increased Kv1.2, Kv1.3, and Kv1.6 in the dentate molecular layer following chronic Ro25,6981 treatment. Our findings suggest that increased Kv1 channel expression/function contributed to diminished action potential-dependent GABA release following chronic NR2B-containing NMDAR inhibition and that these Kv1 channels may be heteromeric complexes containing Kv1.2, Kv1.3, and Kv1.6.

Position-dependent attenuation by Kv1.6 of N-type inactivation of Kv1.4-containing channels

Assembly of distinct α subunits of Kv1 (voltage-gated K(+) channels) into tetramers underlies the diversity of their outward currents in neurons. Kv1.4-containing channels normally exhibit N-type rapid inactivation, mediated through an NIB (N-terminal inactivation ball); this can be over-ridden if associated with a Kv1.6 α subunit, via its NIP (N-type inactivation prevention) domain. Herein, NIP function was shown to require positioning of Kv1.6 adjacent to the Kv1.4 subunit. Using a recently devised gene concatenation, heterotetrameric Kv1 channels were expressed as single-chain proteins on the plasmalemma of HEK (human embryonic kidney)-293 cells, so their constituents could be arranged in different positions. Placing the Kv1.4 and 1.6 genes together, followed by two copies of Kv1.2, yielded a K(+) current devoid of fast inactivation. Mutation of critical glutamates within the NIP endowed rapid inactivation. Moreover, separating Kv1.4 and 1.6 with a copy of Kv1.2 gave a fast-inactivating K(+) current with steady-state inactivation shifted to more negative potentials and exhibiting slower recovery, correlating with similar inactivation kinetics seen for Kv1.4-(1.2)(3). Alternatively, separating Kv1.4 and 1.6 with two copies of Kv1.2 yielded slow-inactivating currents, because in this concatamer Kv1.4 and 1.6 should be together. These findings also confirm that the gene concatenation can generate K(+) channels with α subunits in pre-determined positions.

A bifunctional sea anemone peptide with Kunitz type protease and potassium channel inhibiting properties

Sea anemone venom is a known source of interesting bioactive compounds, including peptide toxins which are invaluable tools for studying structure and function of voltage-gated potassium channels. APEKTx1 is a novel peptide isolated from the sea anemone Anthopleura elegantissima, containing 63 amino acids cross-linked by 3 disulfide bridges. Sequence alignment reveals that APEKTx1 is a new member of the type 2 sea anemone peptides targeting voltage-gated potassium channels (K(V)s), which also include the kalicludines from Anemonia sulcata. Similar to the kalicludines, APEKTx1 shares structural homology with both the basic pancreatic trypsin inhibitor (BPTI), a very potent Kunitz-type protease inhibitor, and dendrotoxins which are powerful blockers of voltage-gated potassium channels. In this study, APEKTx1 has been subjected to a screening on a wide range of 23 ion channels expressed in Xenopus laevis oocytes: 13 cloned voltage-gated potassium channels (K(V)1.1-K(V)1.6, K(V)1.1 triple mutant, K(V)2.1, K(V)3.1, K(V)4.2, K(V)4.3, hERG, the insect channel Shaker IR), 2 cloned hyperpolarization-activated cyclic nucleotide-sensitive cation non-selective channels (HCN1 and HCN2) and 8 cloned voltage-gated sodium channels (Na(V)1.2-Na(V)1.8 and the insect channel DmNa(V)1). Our data show that APEKTx1 selectively blocks K(V)1.1 channels in a very potent manner with an IC(50) value of 0.9nM. Furthermore, we compared the trypsin inhibitory activity of this toxin with BPTI. APEKTx1 inhibits trypsin with a dissociation constant of 124nM. In conclusion, this study demonstrates that APEKTx1 has the unique feature to combine the dual functionality of a potent and selective blocker of K(V)1.1 channels with that of a competitive inhibitor of trypsin.

Arrangement of Kv1 alpha subunits dictates sensitivity to tetraethylammonium

Shaker-related Kv1 channels contain four channel-forming α subunits. Subfamily member Kv1.1 often occurs oligomerized with Kv1.2 α subunits in synaptic membranes, and so information was sought on the influence of their positions within tetramers on the channels’ properties. Kv1.1 and 1.2 α genes were tandem linked in various arrangements, followed by expression as single-chain proteins in mammalian cells. As some concatenations reported previously seemed not to reliably position Kv1 subunits in their assemblies, the identity of expressed channels was methodically evaluated. Surface protein, isolated by biotinylation of intact transiently transfected HEK-293 cells, gave Kv1.1/1.2 reactivity on immunoblots with electrophoretic mobilities corresponding to full-length concatenated tetramers. There was no evidence of protein degradation, indicating that concatemers were delivered intact to the plasmalemma. Constructs with like genes adjacent (Kv1.1-1.1-1.2-1.2 or Kv1.2-1.2-1.1-1.1) yielded delayed-rectifying, voltage-dependent K⁺ currents with activation parameters and inactivation kinetics slightly different from the diagonally positioned genes (Kv1.1-1.2-1.1-1.2 or 1.2–1.1-1.2-1.1). Pore-blocking petidergic toxins, α dendrotoxin, agitoxin-1, tityustoxin-Kα, and kaliotoxin, were unable to distinguish between the adjacent and diagonal concatamers. Unprecedentedly, external application of the pore-blocker tetraethylammonium (TEA) differentially inhibited the adjacent versus diagonal subunit arrangements, with diagonal constructs having enhanced susceptibility. Concatenation did not directly alter the sensitivities of homomeric Kv1.1 or 1.2 channels to TEA or the toxins. TEA inhibition of currents generated by channels made up from dimers (Kv1.1-1.2 and/or Kv1.2-1.1) was similar to the adjacently arranged constructs. These collective findings indicate that assembly of α subunits can be directed by this optimized concatenation, and that subunit arrangement in heteromeric Kv channels affects TEA affinity.

Separation of heteromeric potassium channel Kcv towards probing subunit composition-regulated ion permeation and gating

The chlorella virus-encoded Kcv can form a homo-tetrameric potassium channel in lipid membranes. This miniature peptide can be synthesized in vitro, and the tetramer purified from the SDS-polyacrylamide gel retains the K(+) channel functionality. Combining this capability with the mass-tagging method, we propose a simple, straightforward approach that can generically manipulate individual subunits in the tetramer, thereby enabling the detection of contribution from individual subunits to the channel functions. Using this approach, we showed that the structural change in the selectivity filter from only one subunit is sufficient to cause permanent channel inactivation ("all-or-none" mechanism), whereas the mutation near the extracellular entrance additively modifies the ion permeation with the number of mutant subunits in the tetramer ("additive" mechanism).

Effects of exercise training and hypercholesterolemia on adenosine activation of voltage-dependent K+ channels in coronary arterioles

Coronary arterioles from hypercholesterolemic swine display attenuated adenosine-mediated vasodilatation that is attributable to the elimination of voltage-dependent K(+) (Kv) channel stimulation. For the present study, we tested the hypotheses that exercise training would correct impaired adenosine-induced dilatation in coronary arterioles from hypercholesterolemic pigs through restoration of adenosine activation of Kv channels and that vasodilatation to the receptor-independent adenylyl cyclase activator, forskolin, would also be attenuated in arterioles from hypercholesterolemic pigs. Pigs were randomly assigned to a control (NC) or high-fat, high-cholesterol (HC) diet for 20 wk. Four weeks after the diet was initiated, pigs from both groups were assigned to exercise training (Ex; 5 days/wk for 16 wk) or sedentary (Sed) protocols, resulting in four groups of pigs: NC-Sed, NC-Ex, HC-Sed, and HC-Ex. Arterioles ( approximately 150 mum) from both HC-Sed and HC-Ex pigs displayed impaired adenosine-mediated dilatation that was attributable to the elimination of 4-aminopyridine (4-AP; 1 mM)-sensitive Kv channel activation compared with NC counterparts. Arteriolar smooth muscle whole cell Kv currents were significantly reduced in HC-Sed compared with NC-Sed, although HC-Ex and NC-Ex did not differ. Forskolin-mediated dilatation was attenuated by 4-AP (1 mM) and in a concentration-dependent manner by tetraethylammonium (TEA; 0.1-1 mM) in NC-Sed but not HC-Sed. Further, TEA-sensitive Kv currents were diminished in cells of HC-Sed compared with NC-Sed pigs. Quantitative RT-PCR revealed similar expression levels of Kv3.1 and 3.3 in arterioles of NC-Sed and HC-Sed swine with undetectable expression of Kv1.1, 3.2, and 3.4. Taken together, these results suggest that hypercholesterolemia-mediated attenuation of adenosine-induced vasodilatation in coronary arterioles is not corrected by exercise training and is likely attributable to an impairment in the pathway coupling adenylyl cyclase with a highly TEA-sensitive Kv channel isoform(s).

Seizures and reduced life span in mice lacking the potassium channel subunit Kv1.2, but hypoexcitability and enlarged Kv1 currents in auditory neurons

Genes Kcna1 and Kcna2 code for the voltage-dependent potassium channel subunits Kv1.1 and Kv1.2, which are coexpressed in large axons and commonly present within the same tetramers. Both contribute to the low-voltage-activated potassium current I Kv1, which powerfully limits excitability and facilitates temporally precise transmission of information, e.g., in auditory neurons of the medial nucleus of the trapezoid body (MNTB). Kcna1-null mice lacking Kv1.1 exhibited seizure susceptibility and hyperexcitability in axons and MNTB neurons, which also had reduced I Kv1. To explore whether a lack of Kv1.2 would cause a similar phenotype, we created and characterized Kcna2-null mice (-/-). The -/- mice exhibited increased seizure susceptibility compared with their +/+ and +/- littermates, as early as P14. The mRNA for Kv1.1 and Kv1.2 increased strongly in +/+ brain stems between P7 and P14, suggesting the increasing importance of these subunits for limiting excitability. Surprisingly, MNTB neurons in brain stem slices from -/- and +/- mice were hypoexcitable despite their Kcna2 deficit, and voltage-clamped -/- MNTB neurons had enlarged I Kv1. This contrasts strikingly with the Kcna1-null MNTB phenotype. Toxin block experiments on MNTB neurons suggested Kv1.2 was present in every +/+ Kv1 channel, about 60% of +/- Kv1 channels, and no -/- Kv1 channels. Kv1 channels lacking Kv1.2 activated at abnormally negative potentials, which may explain why MNTB neurons with larger proportions of such channels had larger I Kv1. If channel voltage dependence is determined by how many Kv1.2 subunits each contains, neurons might be able to fine-tune their excitability by adjusting the Kv1.1:Kv1.2 balance rather than altering Kv1 channel density.

Concatemers of brain Kv1 channel alpha subunits that give similar K+ currents yield pharmacologically distinguishable heteromers

At least five subtypes of voltage-gated (Kv1) channels occur in neurons as tetrameric combinations of different alpha subunits. Their involvement in controlling cell excitability and synaptic transmission make them potential targets for neurotherapeutics. As a prerequisite for this, we established herein how the characteristics of hetero-oligomeric K(+) channels can be influenced by alpha subunit composition. Since the three most prevalent Kv1 subunits in brain are Kv1.2, 1.1 and 1.6, new Kv1.6-1.2 and Kv1.1-1.2 concatenated constructs in pIRES-EGFP were stably expressed in HEK cells and the biophysical plus pharmacological properties of their K(+) currents determined relative to those for the requisite homo-tetramers. These heteromers yielded delayed-rectifier type K(+) currents whose activation, deactivation and inactivation parameters are fairly similar although substituting Kv1.1 with Kv1.6 led to a small negative shift in the conductance-voltage relationship, a direction unexpected from the characteristics of the parental homo-tetramers. Changes resulting from swapping Kv1.6 for Kv1.1 in the concatemers were clearly discerned with two pharmacological agents, as measured by inhibition of the K(+) currents and Rb(+) efflux. alphaDendrotoxin and 4-aminopyridine gave a similar blockade of both hetero-tetramers, as expected. Most important for pharmacological dissection of channel subtypes, dendrotoxin(k) and tetraethylammonium readily distinguished the susceptible Kv1.1-1.2 containing oligomers from the resistant Kv1.6-1.2 channels. Moreover, the discriminating ability of dendrotoxin(k) was further confirmed by its far greater ability to displace (125)I-labelled alphadendrotoxin binding to Kv1.1-1.2 than Kv1.6-1.2 channels. Thus, due to the profiles of these two channel subtypes being found to differ, it seems that only multimers corresponding to those present in the nervous system provide meaningful targets for drug development.

Potassium channel blockers in multiple sclerosis: Neuronal Kv channels and effects of symptomatic treatment

Multiple sclerosis (MS) is an inflammatory disease of the central nervous system (CNS) characterized by demyelination, with a relative sparing of axons. In MS patients, many neurologic signs and symptoms have been attributed to the underlying conduction deficits. The idea that neurologic function might be improved if conduction could be restored in CNS demyelinated axons led to the testing of potassium (K(+)) channel blockers as a symptomatic treatment. To date, only 2 broad-spectrum K(+) channel blockers, 4-aminopyridine (4-AP) and 3,4-diaminopyridine (3,4-DAP), have been tested in MS patients. Although both 4-AP and 3,4-DAP produce clear neurologic benefits, their use has been limited by toxicity. Here we review the current status of basic science and clinical research related to the therapeutic targeting of voltage-gated K(+) channels (K(v)) in MS. By bringing together 3 distinct but interrelated disciplines, we aim to provide perspective on a vast body of work highlighting the lengthy and ongoing process entailed in translating fundamental K(v) channel knowledge into new clinical treatments for patients with MS and other demyelinating diseases. Covered are (1) K(v) channel nomenclature, structure, function, and pharmacology; (2) classic and current experimental morphology and neurophysiology studies of demyelination and conduction deficits; and (3) a comprehensive overview of clinical trials utilizing 4-AP and 3,4-DAP in MS patients.

Kv1.1-containing channels are critical for temporal precision during spike initiation

Low threshold, voltage-gated potassium currents (Ikl) are widely expressed in auditory neurons that can fire temporally precise action potentials (APs). In the medial nucleus of the trapezoid body (MNTB), channels containing the Kv1.1 subunit (encoded by the Kcna1 gene) underlie Ikl. Using pharmacology, genetics and whole cell patch-clamp recordings in mouse brain slices, we tested the role of Ikl in limiting AP latency-variability (jitter) in response to trains of single inputs at moderate to high stimulation rates. With dendrotoxin-K (DTX-K, a selective blocker of Kv1.1-containing channels), we blocked Ikl maximally (approximately 80% with 100 nM DTX-K) or partially (approximately 50% with 1-h incubation in 3 nM DTX-K). Ikl was similar in 3 nM DTX-K-treated cells and cells from Kcna1(-/-) mice, allowing a comparison of these two different methods of Ikl reduction. In response to current injection, Ikl reduction increased the temporal window for AP initiation and increased jitter in response to the smallest currents that were able to drive APs. While 100 nM DTX-K caused the largest increases, latency and jitter in Kcna1(-/-) cells and in 3 nM DTX-K-treated cells were similar to each other but increased compared with +/+. The near-phenocopy of the Kcna1(-/-) cells with 3 nM DTX-K shows that acute blockade of a subset of the Kv1.1-containing channels is functionally similar to the chronic elimination of all Kv1.1 subunits. During rapid stimulation (100-500 Hz), Ikl reduction increased jitter in response to both large and small inputs. These data show that Ikl is critical for maintaining AP temporal precision at physiologically relevant firing rates.

Systematic variation of potassium current amplitudes across the tonotopic axis of the rat medial nucleus of the trapezoid body

Many central auditory nuclei preserve the tonotopic organization of their afferent inputs, generating a frequency "map" across the nucleus. In the medial nucleus of the trapezoid body (MNTB) the most medial neurons receive inputs corresponding to the highest frequency sounds and the most lateral neurons have the lowest characteristic frequencies. Whole-cell patch recording from MNTB principal neurons in rat brainstem slices demonstrates a corresponding tonotopic organization of voltage-gated outward potassium currents. Medial MNTB neurons had larger total outward K+ current amplitudes than lateral neurons and similar medial to-lateral gradients were observed for two K+ current subtypes distinguished by their low and high voltage activation thresholds. In contrast, a third K+ conductance with an intermediate voltage threshold and slower kinetics showed an inverse gradient (being smallest in medial MNTB). The orthogonal axes of MNTB did not exhibit potassium current gradients (dorsal-to-ventral, or rostral-to-caudal). The input resistance was unchanged across the MNTB, but a slow capacitative component was enhanced in lateral neurons. These data demonstrate that the intrinsic properties of rat MNTB neurons are tuned across the tonotopic axis so as to promote shorter action potentials, faster firing and therefore greater accuracy in transmission of auditory information in the high characteristic frequency regions.

Kv1 channels selectively prevent dendritic hyperexcitability in rat Purkinje cells

Purkinje cells, the sole output of the cerebellar cortex, encode the timing signals required for motor coordination in their firing rate and activity pattern. Dendrites of Purkinje cells express a high density of P/Q-type voltage-gated calcium channels and fire dendritic calcium spikes. Here we show that dendritic subthreshold Kv1.2 subunit-containing Kv1 potassium channels prevent generation of random spontaneous calcium spikes. With Kv1 channels blocked, dendritic calcium spikes drive bursts of somatic sodium spikes and prevent the cell from faithfully encoding motor timing signals. The selective dendritic function of Kv1 channels in Purkinje cells allows them to effectively suppress dendritic hyperexcitability without hindering the generation of somatic action potentials. Further, we show that Kv1 channels also contribute to dendritic integration of parallel fibre synaptic input. Kv1 channels are often targeted to soma and axon and the data presented support a major dendritic function for these channels.

Involvement of kv1 potassium channels in spreading acidification and depression in the cerebellar cortex

Spreading acidification and depression (SAD) is a form of propagated activity in the cerebellar cortex characterized by acidification and a transient depression in excitability. This study investigated the role of Kv1 potassium channels in SAD using neutral red, flavoprotein autofluorescence, and voltage-sensitive dye optical imaging in the mouse cerebellar cortex, in vivo. The probability of evoking SAD was greatly increased by blocking Kv1.1 as well as Kv1.2 potassium channels by their specific blockers dendrotoxin K (DTX-K) and tityustoxin (TsTX), respectively. DTX-K not only greatly lowered the threshold for evoking SAD but also resulted in multiple cycles of spread and spontaneous SAD. The occurrence of spontaneous SAD originating from spontaneous parallel fiber-like beams of activity suggests that blocking Kv1 channels increased parallel fiber excitability. This was confirmed by the generation of parallel fiber-like beams with the microinjection of glutamate into the upper molecular layer in the presence of DTX-K. The dramatic effects of DTX-K suggest a possible connection between SAD and episodic ataxia type 1 (EA1), a Kv1.1 potassium channelopathy. The threshold for evoking SAD was significantly lowered in the Kv1.1 heterozygous knockout mouse compared with wild-type littermates. Carbamazepine and acetazolamide, both effective in the treatment of EA1, significantly decreased the likelihood of evoking SAD. Blocking GABAergic neurotransmission did not alter the effectiveness of DTX-K. The cyclin D2 null mouse, which lacks cerebellar stellate cells, also exhibited SAD. Therefore blocking Kv1 potassium channels establishes the conditions needed to generate SAD. Furthermore, the results are consistent with the hypothesis that SAD may underlie the transient attacks of ataxia characterizing EA1.

GRIF-1 and OIP106, members of a novel gene family of coiled-coil domain proteins: association in vivo and in vitro with kinesin

Gamma-aminobutyric acid(A) receptor-interacting factor (GRIF-1) is a 913-amino acid protein proposed to function as a GABA(A) receptor beta(2) subunit-interacting, trafficking protein. GRIF-1 shares approximately 44% amino acid sequence identity with O-linked N-acetylglucosamine transferase interacting protein 106, OIP106. Both proteins contain predicted coiled-coil domains and probably constitute a novel gene family. The Drosophila orthologue of this family of proteins may be Milton. Milton shares approximately 44% amino acid homology with GRIF-1. Milton is proposed to function in kinesin-mediated transport of mitochondria to nerve terminals. We report here that GRIF-1 and OIP106 also associate with kinesin and mitochondria. Following expression in human embryonic kidney 293 cells, both GRIF-1 and OIP106 were shown by co-immunoprecipitation to be specifically associated with an endogenous kinesin heavy chain species of 115 kDa and exogenous KIF5C. Association of GRIF-1 with kinesin was also evident in native brain and heart tissue. In the brain, anti-GRIF-1-(8-633) antibodies specifically co-immunoprecipitated two kinesin-immunoreactive species with molecular masses of 118 and 115 kDa, and in the heart, one kinesin-immunoreactive species, 115 kDa, was immunoprecipitated. Further studies revealed that GRIF-1 was predominantly associated with KIF5A in the brain and with KIF5B in both the heart and in HEK 293 cells. Yeast two-hybrid interaction assays and immunoprecipitations showed that GRIF-1 associated directly with KIF5C with the GRIF-1/KIF5C interaction domain localized to GRIF-1-(124-283). These results further support a role for GRIF-1 and OIP106 in protein and/or organelle transport in excitable cells in a manner analogous to glutamate receptor-interacting-protein 1, in the motor-dependent transport of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate glutamate excitatory neurotransmitter receptors to dendrites.

Kv1.1 and Kv1.3 channels contribute to the delayed-rectifying K+conductance in rat choroid plexus epithelial cells

The choroid plexuses secrete, and maintain the composition of, the cerebrospinal fluid. K+channels play an important role in these processes. In this study the molecular identity and properties of the delayed-rectifying K+(Kv) conductance in rat choroid plexus epithelial cells were investigated. Whole cell K+currents were significantly reduced by 10 nM dendrotoxin-K and 1 nM margatoxin, which are specific inhibitors of Kv1.1 and Kv1.3 channels, respectively. A combination of dendrotoxin-K and margatoxin caused a depolarization of the membrane potential in current-clamp experiments. Western blot analysis indicated the presence of Kv1.1 and Kv1.3 proteins in the choroid plexus. Furthermore, the Kv1.3 and Kv1.1 proteins appear to be expressed in the apical membrane of the epithelial cells in immunocytochemical studies. The Kv conductance was inhibited by 1 μM serotonin (5-HT), with maximum inhibition to 48% of control occurring in 8 min ( P < 0.05 by Student's t-test for paired data). Channel inhibition by 5-HT was prevented by the 5-HT2Cantagonist mesulergine (300 nM). It was also attenuated in the presence of calphostin C (a protein kinase C inhibitor). The conductance was partially inhibited by 1,2-dioctanoyl- sn-glycerol and phorbol 12-myristate 13-acetate, both of which activate protein kinase C. These data suggest that 5-HT acts at 5-HT2Creceptors to activate protein kinase C, which inhibits the Kv channels. In conclusion, Kv1.1 and Kv1.3 channels make a significant contribution to K+efflux at the apical membrane of the choroid plexus.

Truncation of the Shaker-like voltage-gated potassium channel, Kv1.1, causes megencephaly

The megencephaly mouse, mceph/mceph, displays dramatically increased brain volume and hypertrophic brain cells. Despite overall enlargement, the mceph/mceph brain appears structurally normal, without oedema, hydrocephaly or leukodystrophy, and with only minor astrocytosis. Furthermore, it presents striking disturbances in expression of trophic and neuromodulating factors within the hippocampus and cortex. Using a positional cloning approach we have identified the mceph mutation. We show that mceph/mceph mice carry an 11-base-pair deletion in the gene encoding the Shaker-like voltage-gated potassium channel subtype 1, Kcna1. The mutation leads to a frame shift and the predicted MCEPH protein is truncated at amino acid 230 (out of 495), terminating with six aberrant amino acids. The expression of Kcna1 mRNA is increased in the mceph/mceph brain. However, the C-terminal domains of the corresponding Kv1.1 protein are absent. The putative MCEPH protein retains only the N-terminal domains for channel assembly and may congregate nonfunctional complexes of multiple Shaker-like subunits. Indeed, whereas Kcna2 and Kcna3 mRNA expression is normal, the mceph/mceph hippocampus displays decreased amounts of Kv1.2 and Kv1.3 proteins, suggesting interactions at the protein level. We show that mceph/mceph mice have disturbed brain electrophysiology and experience recurrent behavioural seizures, in agreement with the abnormal electrical brain activity found in Shaker mutants. However, in contrast to the commonly demonstrated epilepsy-induced neurodegeneration, we find that the mceph mutation leads to seizures with a concomitant increase in brain size, without overt neural atrophy.

Hyperexcitability and reduced low threshold potassium currents in auditory neurons of mice lacking the channel subunit Kv1.1

A low voltage-activated potassium current, IKL, is found in auditory neuron types that have low excitability and precisely preserve the temporal pattern of activity present in their presynaptic inputs. The gene Kcna1 codes for Kv1.1 potassium channel subunits, which combine in expression systems to produce channel tetramers with properties similar to those of IKL, including sensitivity to dendrotoxin (DTX). Kv1.1 is strongly expressed in neurons with IKL, including auditory neurons of the medial nucleus of the trapezoid body (MNTB). We therefore decided to investigate how the absence of Kv1.1 affected channel properties and function in MNTB neurons from mice lacking Kcna1. We used the whole cell version of the patch clamp technique to record from MNTB neurons in brainstem slices from Kcna1-null (-/-) mice and their wild-type (+/+) and heterozygous (+/-) littermates. There was an IKL in voltage-clamped -/- MNTB neurons, but it was about half the amplitude of the IKL in +/+ neurons, with otherwise similar properties. Consistent with this, -/- MNTB neurons were more excitable than their +/+ counterparts; they fired more than twice as many action potentials (APs) during current steps, and the threshold current amplitude required to generate an AP was roughly halved. +/- MNTB neurons had excitability and IKL amplitudes identical to the +/+ neurons. The IKL remaining in -/- neurons was blocked by DTX, suggesting the underlying channels contained subunits Kv1.2 and/or Kv1.6 (also DTX-sensitive). DTX increased excitability further in the already hyperexcitable -/- MNTB neurons, suggesting that -/- IKL limited excitability despite its reduced amplitude in the absence of Kv1.1 subunits.

Smooth muscle membrane potential modulates endothelium-dependent relaxation of rat basilar artery via myo-endothelial gap junctions

The release of endothelium-derived relaxing factors, such as nitric oxide (NO), is dependent on an increase in intracellular calcium levels ([Ca(2+)](i)) within endothelial cells. Endothelial cell membrane potential plays a critical role in the regulation of [Ca(2+)](i) in that calcium influx from the extracellular space is dependent on membrane hyperpolarization. In this study, the effect of inhibition of vascular smooth muscle delayed rectifier K(+) (K(DR)) channels by 4-aminopyridine (4-AP) on endothelium-dependent relaxation of rat basilar artery to acetylcholine (ACh) was assessed. ACh-evoked endothelium-dependent relaxations were inhibited by N-(Omega)-nitro-L-arginine (L-NNA) or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), confirming a role for NO and guanylyl cyclase. 4-AP (300 microM) also suppressed ACh-induced relaxation, with the maximal response reduced from approximately 92 to approximately 33 % (n = 11; P < 0.01). However, relaxations in response to exogenous NO, applied in the form of authentic NO, sodium nitroprusside or diethylamineNONOate (DEANONOate), were not affected by 4-AP treatment (n = 3-11). These data are not consistent with the view that 4-AP-sensitive K(DR) channels are mediators of vascular hyperpolarization and relaxation in response to endothelium-derived NO. Inhibition of ACh-evoked relaxation by 4-AP was reversed by pinacidil (0.5-1 microM; n = 5) or 18beta-glycyrrhetinic acid (18betaGA; 5 microM; n = 5), indicating that depolarization and electrical coupling of the smooth muscle to the endothelium were involved. 4-AP caused depolarization of both endothelial and vascular smooth muscle cells of isolated segments of basilar artery (mean change 11 +/- 1 and 9 +/- 2 mV, respectively; n = 15). Significantly, 18betaGA almost completely prevented the depolarization of endothelial cells (n = 6), but not smooth muscle cells (n = 6) by 4-AP. ACh-induced hyperpolarization of endothelium and smooth muscle cells was also reduced by 4-AP, but this inhibition was not observed in the combined presence of 4-AP and 18betaGA. These data indicate that 4-AP can induce an indirect inhibition of endothelium-dependent relaxation in the rat basilar artery by electrical coupling of smooth muscle membrane depolarization to the endothelium via myo-endothelial gap junctions.