Charybdotoxin, a protein inhibitor of single Ca2+-activated K+ channels from mammalian skeletal muscle

Apamin-sensitive conductance mediates the K(+) current response during chemical ischemia in CA3 pyramidal cells

Pyramidal cells typically respond to ischemia with initial transient hyperpolarization, which may represent a neuroprotective response. To identify the conductance underlying this hyperpolarization in CA3 pyramidal neurons of rat hippocampal organotypic slice cultures, recordings were obtained using the single-electrode voltage-clamp technique. Brief chemical ischemia (2 mM 2-deoxyglucose and 3 mM NaN(3), for 4 min) induced a response mediated by an increase in K(+) conductance. This current was blocked by intracellular application of the Ca(2+) chelator, bis-(o-aminophenoxy)-N,N,N', N'-tetraacetic acid (BAPTA), reduced with low external [Ca(2+)], and inhibited by a selective L-type Ca(2+) channel inhibitor, isradipine, consistent with the activation of a Ca(2+)-dependent K(+) conductance. Experiments with charybdotoxin (10 nM) and tetraethylammonium (TEA; 1 mM), or with the protein kinase C activator, phorbol 12,13-diacetate (PDAc; 3 microM), ruled out an involvement of a large conductance-type or an apamin-insensitive small conductance, respectively. In the presence of apamin (1 microM), however, the outward current was significantly reduced. These results demonstrate that in rat hippocampal CA3 pyramidal neurons an apamin-sensitive Ca(2+)-dependent K(+) conductance is activated in response to brief ischemia generating a pronounced outward current.

Involvement of apamin-sensitive SK channels in spike frequency adaptation of neurons in nucleus tractus solitarii of the rat

We delineated the role of Ca(2+)-activated K(+) channels in the phenomenon of spike frequency adaptation (SFA) exhibited by neurons in the caudal region of nucleus tractus solitarius (cNTS) using intracellular recording coupled with the current-clamp technique in rat brain slices. Intracellular injection of a constant depolarizing current evoked a train of action potentials whose discharge frequency declined rapidly to a lower steady-state level of irregular discharges. This manifested phenomenon of SFA was found to be related to extracellular Ca(2+). Low Ca(2+) (0.25 mM) or Cd(2+) (0.5 mM) in the perfusing medium resulted in a significant increase in the adaptation time constant (tau(adap)) and an appreciable reduction in the percentage adaptation of spike frequency (F(adap)). In addition, the evoked discharges were converted from an irregular to a regular pattern, accompanied by a profound increase in mean firing rate. Intriguingly, similar alterations in tau(adap), F(adap), discharge pattern and discharge rate were elicited by apamin (1 microM), a selective blocker for small-conductance Ca(2+)-activated K(+) (SK) channels. On the other hand, charybdotoxin (0.1 microM), a selective blocker for large-conductance Ca(2+)-activated K(+) channels, was ineffective. Our results suggest that SK channels of cNTS neurons may subserve the generation of both SFA and irregular discharge patterns displayed by action potentials evoked with a prolonged depolarizing current.

Dendritic calcium spike initiation and repolarization are controlled by distinct potassium channel subtypes in CA1 pyramidal neurons

In CA1 pyramidal neurons of the hippocampus, calcium-dependent spikes occur in vivo during specific behavioral states and may be enhanced during epileptiform activity. However, the mechanisms that control calcium spike initiation and repolarization are poorly understood. Using dendritic and somatic patch-pipette recordings, we show that calcium spikes are initiated in the apical dendrites of CA1 pyramidal neurons and drive bursts of sodium-dependent action potentials at the soma. Initiation of calcium spikes at the soma was suppressed in part by potassium channels activated by sodium-dependent action potentials. Low-threshold, putative D-type potassium channels [blocked by 100 microM 4-aminopyridine (4-AP) and 0.5-1 microM alpha-dendrotoxin (alpha-DTX)] played a prominent role in setting a high threshold for somatic calcium spikes, thus restricting initiation to the dendrites. DTX- and 4-AP-sensitive channels were activated during sodium-dependent action potentials and mediated a large component of their afterhyperpolarization. Once initiated, repetitive firing of calcium spikes was limited by activation of putative BK-type calcium-activated potassium channels (blocked by 250 microM tetraethylammonium chloride, 70 nM charybdotoxin, or 100 nM iberiotoxin). Thus, the concerted action of calcium- and voltage-activated potassium channels serves to focus spatially and temporally the membrane depolarization and calcium influx generated by calcium spikes during strong, synchronous network excitation.

Involvement of K(+) channels in the relaxant effects of YC-1 in vascular smooth muscle

This study addresses the question whether K(+) channels are involved in the vasorelaxant effects of 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl-indazole (YC-1 ). In rat aorta, guinea pig aorta, and guinea pig a. carotis, YC-1 inhibited contractions induced by phenylephrine (3 microM) more potently than those induced by K(+)(48 mM). In rat aorta, tetraethylammonium (10 mM), charybdotoxin (0.2 microM), and iberiotoxin (0.1 microM), but not glibenclamide (10 microM), attenuated the relaxant effects of YC-1. In guinea pig a. carotis, YC-1 (30 microM) induced a hyperpolarisation which was antagonised by 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ; 50 microM). In rat aorta, YC-1 (30 microM) increased the rate constant of 86Rb-efflux. The effect of YC-1 was potentiated by zaprinast (10 microM), but inhibited by ODQ (50 microM) or charybdotoxin (0.2 microM). In smooth muscle cells from rat aorta, YC-1 (10 microM) increased BK(Ca) channel activity. It is suggested that YC-1-induced vasorelaxation is partially mediated by the activation of K(+) channels.

Potassium channels in rat prostate epithelial cells

Voltage-dependent K(+) channels were identified and characterized in primary culture of rat prostate epithelial cells. A voltage-dependent, inactivating K(+) channel was the most commonly observed ion channel in both lateral and dorsal cells. The K(+) current exhibited a voltage threshold at -40 mV. Averaged half-inactivation potential (V(1/2)) and the slope factor (k) values were -26 mV and 6, respectively. It showed a monoexponential decay with an inactivation time constant of about 600 ms at +60 mV. The deactivation time constant at -60 mV was 30 ms and the reversal potential was estimated at -80 mV, suggesting that current was carried by potassium ions. The scorpion venom peptides charybdotoxin (5 nM) and margatoxin (1 nM), inhibited K(+) current at all membrane potentials with a rapid and a slow reversibility respectively. Both tetraethylammonium (10 mM) and 4-aminopyridine (50 microM) reduced K(+) current by approximately 40%. We conclude that plasma membranes of lateral and dorsal rat prostate epithelial cells contain Kv K(+) channels that have biophysical and pharmacological properties consistent with those of the Kv1.3 family.

Fast BK-type channel mediates the Ca(2+)-activated K(+) current in crayfish muscle

The role of the Ca(2+)-activated K(+) current (I(K(Ca))) in crayfish opener muscle fibers is functionally important because it regulates the graded electrical activity that is characteristic of these fibers. Using the cell-attached and inside-out configurations of the patch-clamp technique, we found three different classes of channels with properties that matched those expected of the three different ionic channels mediating the depolarization-activated macroscopic currents previously described (Ca(2+), K(+), and Ca(2+)-dependent K(+) currents). We investigated the properties of the ionic channels mediating the extremely fast activating and persistent I(K(Ca)). These voltage- and Ca(2+)-activated channels had a mean single-channel conductance of approximately 70 pS and showed a very fast activation. Both the single-channel open probability and the speed of activation increased with depolarization. Both parameters also increased in inside-out patches, i.e., in high Ca(2+) concentration. Intracellular loading with the Ca(2+) chelator bis(2-aminophenoxy) ethane-N, N,N',N'-tetraacetic acid gradually reduced and eventually prevented channel openings. The channels opened at very brief delays after the pulse depolarization onset (<5 ms), and the time-dependent open probability was constant during sustained depolarization (< or =560 ms), matching both the extremely fast activation kinetics and the persistent nature of the macroscopic I(K(Ca)). However, the intrinsic properties of these single channels do not account for the partial apparent inactivation of the macroscopic I(K(Ca)), which probably reflects temporal Ca(2+) variations in the whole muscle fiber. We conclude that the channels mediating I(K(Ca)) in crayfish muscle are voltage- and Ca(2+)-gated BK channels with relatively small conductance. The intrinsic properties of these channels allow them to act as precise Ca(2+) sensors that supply the exact feedback current needed to control the graded electrical activity and therefore the contraction of opener muscle fibers.

Endothelins activate Ca(2+)-gated K(+) channels via endothelin B receptors in CD-1 mouse erythrocytes

Cell dehydration mediated by Ca(2+)-activated K(+) channels plays an important role in the pathogenesis of sickle cell disease. CD-1 mouse erythrocytes possess a Ca(2+)-activated K(+) channel (Gardos channel) with maximal velocity (V(max)) of 0.154 +/- 0.02 mmol. l cells(-1). min(-1) and an affinity constant (K(0.5)) for Ca(2+) of 286 +/- 83 nM in the presence of A-23187. Cells pretreated with 500 nM endothelin-1 (ET-1) increased their V(max) by 88 +/- 9% (n = 8) and decreased their K(0.5) for Ca(2+) to 139 +/- 63 nM (P < 0.05; n = 4). Activation of the Gardos channel resulted in an EC(50) of 75 +/- 20 nM for ET-1 and 374 +/- 97 nM for ET-3. Analysis of the affinity of unlabeled ET-1 for its receptor showed two classes of binding sites with apparent dissociation constants of 167 +/- 51 and 785 +/- 143 nM and with capacity of binding sites of 298 +/- 38 and 1,568 +/- 211 sites/cell, respectively. The Gardos channel was activated by the endothelin B (ET(B)) receptor agonist IRL 1620 and inhibited by BQ-788, demonstrating the involvement of ET(B) receptors. Calphostin C inhibited 73% of ET-1-induced Gardos activation and 84% of the ET-1-induced membrane protein kinase C activity. Thus endothelins regulate erythrocyte Gardos channels via ET(B) receptors and a calphostin-sensitive mechanism.

Scorpion toxins specific for Na+-channels

Na+-channel specific scorpion toxins are peptides of 60-76 amino acid residues in length, tightly bound by four disulfide bridges. The complete amino acid sequence of 85 distinct peptides are presently known. For some toxins, the three-dimensional structure has been solved by X-ray diffraction and NMR spectroscopy. A constant structural motif has been found in all of them, consisting of one or two short segments of alpha-helix plus a triple-stranded beta-sheet, connected by variable regions forming loops (turns). Physiological experiments have shown that these toxins are modifiers of the gating mechanism of the Na+-channel function, affecting either the inactivation (alpha-toxins) or the activation (beta-toxins) kinetics of the channels. Many functional variations of these peptides have been demonstrated, which include not only the classical alpha- and beta-types, but also the species specificity of their action. There are peptides that bind or affect the function of Na+-channels from different species (mammals, insects or crustaceans) or are toxic to more than one group of animals. Based on functional and structural features of the known toxins, a classification containing 10 different groups of toxins is proposed in this review. Attempts have been made to correlate the presence of certain amino acid residues or 'active sites' of these peptides with Na+-channel functions. Segments containing positively charged residues in special locations, such as the five-residue turn, the turn between the second and the third beta-strands, the C-terminal residues and a segment of the N-terminal region from residues 2-11, seems to be implicated in the activity of these toxins. However, the uncertainty, and the limited success obtained in the search for the site through which these peptides bind to the channels, are mainly due to the lack of an easy method for expression of cloned genes to produce a well-folded, active peptide. Many scorpion toxin coding genes have been obtained from cDNA libraries and from polymerase chain reactions using fragments of scorpion DNAs, as templates. The presence of an intron at the DNA level, situated in the middle of the signal peptide, has been demonstrated.

Calcium-activated potassium conductances contribute to action potential repolarization at the soma but not the dendrites of hippocampal CA1 pyramidal neurons

Evidence is accumulating that voltage-gated channels are distributed nonuniformly throughout neurons and that this nonuniformity underlies regional differences in excitability within the single neuron. Previous reports have shown that Ca2+, Na+, A-type K+, and hyperpolarization-activated, mixed cation conductances have varying distributions in hippocampal CA1 pyramidal neurons, with significantly different densities in the apical dendrites compared with the soma. Another important channel mediates the large-conductance Ca2+-activated K+ current (IC), which is responsible in part for repolarization of the action potential (AP) and generation of the afterhyperpolarization that follows the AP recorded at the soma. We have investigated whether this current is activated by APs retrogradely propagating in the dendrites of hippocampal pyramidal neurons using whole-cell dendritic patch-clamp recording techniques. We found no IC activation by back-propagating APs in distal dendritic recordings. Dendritic APs activated IC only in the proximal dendrites, and this activation decayed within the first 100-150 micrometer of distance from the soma. The decay of IC in the proximal dendrites occurred despite AP amplitude, plus presumably AP-induced Ca2+ influx, that was comparable with that at the soma. Thus we conclude that IC activation by action potentials is nonuniform in the hippocampal pyramidal neuron, which may represent a further example of regional differences in neuronal excitability that are determined by the nonuniform distribution of voltage-gated channels in dendrites.

Quantification and distribution of Ca(2+)-activated maxi K(+) channels in rabbit distal colon

The Ca(2+)-activated maxi K(+) channel is an abundant channel type in the distal colon epithelium, but nothing is known regarding the actual number and precise localization of these channels. The aim of this study has therefore been to quantify the maxi K(+) channels in colon epithelium by binding of iberiotoxin (IbTX), a selective peptidyl ligand for maxi K(+) channels. In isotope flux measurements 75% of the total K(+) channel activity in plasma membranes from distal colon epithelium is inhibited by IbTX (K(0.5) = 4.5 pM), indicating that the maxi K(+) channel is the predominant channel type in this epithelium. Consistent with the functional studies, the radiolabeled double mutant (125)I-IbTX-D19Y/Y36F binds to the colon epithelium membranes with an equilibrium dissociation constant of approximately 10 pM. The maximum receptor concentration values (in fmol/mg protein) for (125)I-IbTX-D19Y/Y36F binding to colon epithelium are 78 for surface membranes and 8 for crypt membranes, suggesting that the maxi K(+) channels are predominantly expressed in the Na(+)-absorbing surface cells, as compared with the Cl(-)-secreting crypt cells. However, aldosterone stimulation of this tissue induced by a low-Na(+) diet does not change the total number of maxi K(+) channels.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Discoordinate regulation of different K channels in cultured rat skeletal muscle by nerve growth factor

We investigated the effects of nerve growth factor (NGF) on expression of K+ channels in cultured skeletal muscle. The channels studied were (1) charybdotoxin (ChTx)-sensitive channels by using a polyclonal antibody raised in rabbits against ChTx, (2) Kv1.5 voltage-sensitive channels, and (3) apamin-sensitive (afterhyperpolarization) channels. Crude homogenates were prepared from cultures made from limb muscles of 1-2-day-old rat pups for identification of ChTx-sensitive and Kv1.5 channels by Western blotting techniques. Apamin-sensitive K+ channels were studied by measurement of specific [125I]-apamin binding by whole cell preparations. ChTx-sensitive channels display a fusion-related increase in expression, and NGF downregulates these channels in both myoblasts and myotubes. Voltage-dependent Kv1.5 channel expression is low in myoblasts and increases dramatically with fusion; NGF induces early expression of these channels and causes expression after fusion to increase even further. NGF downregulates apamin-sensitive channels. NGF increases the rate of fall of the action potential recorded intracellularly from single myotubes with intracellular microelectrodes. The results confirm and extend those of previous studies in showing a functional role for NGF in the regulation of membrane properties of skeletal muscle. Moreover, the findings demonstrate that the different K+ channels in this preparation are regulated in a discoordinate manner. The divergent effects of NGF on expression of different K+ channels, however, do not appear sufficient to explain the NGF-induced increase in the rate of fall of the action potential. The changes during the falling phase may rather be due to increases in channel properties or may result from an increased driving force on the membrane potential secondary to the NGF-induced hyperpolarization.

Contribution of potassium conductances to a time-dependent transition in electrical properties of a cockroach motoneuron soma

Contribution of potassium conductances to a time-dependent transition in electrical properties of a cockroach motoneuron soma. The cell body of the cockroach (Periplaneta americana) fast coxal depressor motoneuron (Df) displays a time-dependent change in excitability. Immediately after dissection, depolarization evokes plateau potentials, but after several hours all-or-none action potentials are evoked. Because K channel blockers have been shown to produce a similar transition in electrical properties, we have used current-clamp, voltage-clamp and action-potential-clamp recording to elucidate the contribution of different classes of K channel to the transition in electrical activity of the neuron. Apamin had no detectable effect on the neuron, but charybdotoxin (ChTX) caused a rapid transition from plateau potentials to spikes in the somatic response of Df to depolarization. In neurons that already produced spikes when depolarized, ChTX increased spike amplitude but did not increase their duration nor decrease the amplitude of their afterhyperpolarization. 4-Aminopyridine (4-AP) (which selectively blocks transient K currents) did not cause a transition from plateau potentials to spikes but did enhance oscillations superimposed on plateau potentials. When applied to neurons that already generated spikes when depolarized, 4-AP could augment spike amplitude, decrease the latency to the first spike, and prolong the afterhyperpolarization. Evidence suggests that the time-dependent transition in electrical properties of this motoneuron soma may result, at least in part, from a fall in calcium-dependent potassium current (IK,Ca), consequent on a gradual reduction in [Ca2+ ]i. Voltage-clamp experiments demonstrated directly that outward K currents in this neuron do fall with a time course that could be significant in the transition of electrical properties. Voltage-clamp experiments also confirmed the ineffectiveness of apamin and showed that ChTX blocked most of IK,Ca. Application of Cd2+ (0.5 mM), however, caused a small additional suppression in outward current. Calcium-insensitive outward currents could be divided into transient (4-AP-sensitive) and sustained components. The action-potential-clamp technique revealed that the ChTX-sensitive current underwent sufficient activation during the depolarizing phase of plateau potentials to enable it to shunt inward conductances. Although the ChTX-sensitive conductance apparently makes little contribution to spike repolarization, the ChTX-resistant IK,Ca does make a significant contribution to this phase of the action potential. The 4-AP-sensitive current began to develop during the rising phase of both action potentials and plateau potentials but had little effect on the electrical activity of the neuron, probably because of its relatively small amplitude.

Ca2+-activated K channel of the BK-type in the inner mitochondrial membrane of a human glioma cell line

A single channel current was recorded from mitoplasts (i.e., inner mitochondrial membrane) of the human glioma cell line LN229 using patch-clamp techniques in the mitoplast-attached mode. We frequently found a 295 +/- 18 pS channel that showed a straight i-E relation in the range +/-60 mV in 150 mM KCl solutions on either side of the mitoplast. If KCl in the bath was exchanged against NaCl, outward currents were undetectable, indicating potassium selectivity. Channel activity determined as open probability increased with increasing Ca2+ concentrations (EC50 = 0.9 microM at 60 mV). Open probability was voltage dependent. An e-fold increase of time spent in the open state was induced by a depolarization of 10.5 mV. Open probability was decreased by charybdotoxin concentration and voltage dependently (EC50 = 1.4 nM). In conclusion, we show for the first time that the inner mitochondrial membrane in human glioma cells contains a calcium-dependent K channel of the BK-type.

TsTX-IV, a short chain four-disulfide-bridged neurotoxin from Tityus serrulatus venom which acts on Ca2+-activated K+ channels

The primary structure of TsTX-IV, a neurotoxin isolated from Tityrus serrulatus scorpion venom, is reported. Its amino acid sequence was determined by automated Edman sequential degradation of the reduced and carboxymethylated toxin and of relevant peptides obtained by digestion with Staphylococcus aureus strain V8 protease or trypsin and cleavage by CNBr. The complete sequence showed 41 amino acid residues, which account for an estimated molecular weight of 4520, and eight half-cystine residues which cross-link the toxin molecule with four disulfide bonds. The molecular weight determined by mass spectrometry was 4518. Comparison of this sequence with those from other scorpion toxins showed a resemblance with toxins which act on different types of K+ channels. TsTx-IV was able to block Ca2+-activated K+ channels of high conductance. TsTX-IV is the first four-disulfide-bridged short toxin from T. serrulatus so far completely sequenced.

Effect of potassium channel modulators in mouse forced swimming test

1. The effect of intracerebroventricular (i.c.v.) administration of different potassium channel blockers (tetraethylammonium, apamin, charybdotoxin, gliquidone), potassium channel openers (pinacidil, minoxidil, cromakalim) and aODN to mKv1.1 on immobility time was evaluated in the mouse forced swimming test, an animal model of depression. 2. Tetraethylammonium (TEA; 5 microg per mouse i.c.v.), apamin (3 ng per mouse i.c.v.), charybdotoxin (1 microg per mouse i.c.v.) and gliquidone (6 microg per mouse i.c.v.) administered 20 min before the test produced anti-immobility comparable to that induced by the tricyclic antidepressants amitriptyline (15 mg kg(-1) s.c.) and imipramine (30 mg kg(-1) s.c.). 3. By contrast pinacidil (10-20 microg per mouse i.c.v.), minoxidil (10-20 microg per mouse i.c.v.) and cromakalim (20-30 microg per mouse i.c.v.) increased immobility time when administered in the same experimental conditions. 4. Repeated administration of an antisense oligonucleotide (aODN) to the mKv1.1 gene (1 and 3 nmol per single i.c.v. injection) produced a dose-dependent increase in immobility time of mice 72 h after the last injection. At day 7, the increasing effect produced by aODN disappeared. A degenerate mKv1.1 oligonucleotide (dODN), used as control, did not produce any effect in comparison with saline- and vector-treated mice. 5. At the highest effective dose, potassium channels modulators and the mKv1.1 aODN did not impair motor coordination, as revealed by the rota rod test, nor did they modify spontaneous motility as revealed by the Animex apparatus. 6. These results suggest that modulation of potassium channels plays an important role in the regulation of immobility time in the mouse forced swimming test.

Molecular cloning and genomic analysis of TsNTxp: an immunogenic protein from Tityus serrulatus scorpion venom

A non-toxic protein (TsNTxP) from Tityus serrulatus scorpion venom has been shown to be an efficient immunogen and anti-TsNTxP antibodies recognize and neutralize the effect of Tityus serrulatus venom [Chávez-Olórtegui et al., 1997. Toxicon 35, 213-221]. With the purpose of studying the organization of the gene that code for this protein, we have isolated a full length cDNA clone for TsNTxP from a cDNA expression library using anti-TsNTxP antibodies. The nucleotide sequence of the gene that encodes TsNTxP was also obtained and it reveals the presence of an intron within the signal peptide sequence. The TsNTxP gene showed high degree of similarity with genes encoding toxins from scorpions of the genus Tityrus.

Solution structure of potassium channel-inhibiting scorpion toxin Lq2

Lq2 is a unique scorpion toxin. Acting from the extracellular side, Lq2 blocks the ion conduction pore in not only the voltage- and Ca2+ -activated channels, but also the inward-rectifier K+ channels. This finding argues that the three-dimensional structures of the pores in these K+ channels are similar. However, the amino acid sequences that form the external part of the pore are minimally conserved among the various classes of K+ channels. Because Lq2 can bind to all the three classes of K+ channels, we can use Lq2 as a structural probe to examine how the non-conserved pore-forming sequences are arranged in space to form similar pore structures. In the present study, we determined the three-dimensional structure of Lq2 using nuclear magnetic resonance (NMR) techniques. Lq2 consists of an alpha-helix (residues S10 to L20) and a beta-sheet, connected by an alphabeta3 loop (residues N22 to N24). The beta-sheet has two well-defined anti-parallel strands (residues G26 to M29 and residues K32 to C35), which are connected by a type I' beta-turn centered between residues N30 and K31. The N-terminal segment (residues Z1 to T8) appears to form a quasi-third strand of the beta-sheet.

Simultaneous binding of basic peptides at intracellular sites on a large conductance Ca2+-activated K+ channel. Equilibrium and kinetic basis of negatively coupled ligand interactions

The homologous Kunitz inhibitor proteins, bovine pancreatic trypsin inhibitor (BPTI) and dendrotoxin I (DTX-I), interact with large conductance Ca2+-activated K+ channels (maxi-KCa) by binding to an intracellular site outside of the pore to produce discrete substate events. In contrast, certain homologues of the Shaker ball peptide produce discrete blocking events by binding within the ion conduction pathway. In this study, we investigated ligand interactions of these positively charged peptide molecules by analysis of single maxi-KCa channels in planar bilayers recorded in the presence of DTX-I and BPTI, or DTX-I and a high-affinity homologue of ball peptide. Both DTX-I (Kd, 16.5 nM) and BPTI (Kd, 1, 490 nM) exhibit one-site binding kinetics when studied alone; however, records in the presence of DTX-I plus BPTI demonstrate simultaneous binding of these two molecules. The affinity of BPTI (net charge, +6) decreases by 11.7-fold (Kd, 17,500 nM) when DTX-I (net charge, +10) is bound and, conversely, the affinity of DTX-I decreases by 10.8-fold (Kd, 178 nM) when BPTI is bound. The ball peptide homologue (BP; net charge, +6) exhibits high blocking affinity (Kd, 7.2 nM) at a single site when studied alone, but has 8.0-fold lower affinity (Kd, 57 nM) for blocking the DTX-occupied channel. The affinity of DTX-I likewise decreases by 8.4-fold (Kd, 139 nM) when BP is bound. These results identify two types of negatively coupled ligand-ligand interactions at distinct sites on the intracellular surface of maxi-KCa channels. Such antagonistic ligand interactions explain how the binding of BPTI or DTX-I to four potentially available sites on a tetrameric channel protein can exhibit apparent one-site kinetics. We hypothesize that negatively coupled binding equilibria and asymmetric changes in transition state energies for the interaction between DTX-I and BP originate from repulsive electrostatic interactions between positively charged peptide ligands on the channel surface. In contrast, there is no detectable binding interaction between DTX-I on the inside and tetraethylammonium or charybdotoxin on the outside of the maxi-KCa channel.

Ca2+-activated outward currents in neostriatal neurons

Whole-cell voltage-clamp recordings of outward currents were obtained from acutely dissociated neurons of the rat neostriatum in conditions in which inward Ca2+ current was not blocked and intracellular Ca2+ concentration was lightly buffered. Na+ currents were blocked with tetrodotoxin. In this situation, about 53 +/- 4% (mean +/- S.E.M.; n = 18) of the outward current evoked by a depolarization to 0 mV was sensitive to 400 microM Cd2+. A similar percentage was sensitive to high concentrations of intracellular chelators or to extracellular Ca2+ reduction (<500 microM); 35+/-4% (n=25) of the outward current was sensitive to 3.0 mM 4-aminopyridine. Most of the remaining current was blocked by 10 mM tetraethylammonium. The results suggest that about half of the outward current is activated by Ca2+ entry in the present conditions. The peptidic toxins charybdotoxin, iberotoxin and apamin confirmed these results, since 34 +/- 5% (n = 14), 29 5% (n= 14) and 28 +/- 6% (n=9) of the outward current was blocked by these peptides, respectively. The effects of charybdotoxin and iberotoxin added to that of apamin, but their effects largely occluded each other. There was additional Cd2+ block after the effect of any combination of toxins. Therefore, it is concluded that Ca2+-activated outward currents in neostriatal neurons comprise several components, including small and large conductance types. In addition, the present experiments demonstrate that Ca2+-activated K+ currents are a very important component of the outward current activated by depolarization in neostriatal neurons.

Components of after-hyperpolarization in magnocellular neurones of the rat supraoptic nucleus in vitro

1. The pharmacological sensitivity of hyperpolarizing components of spike train after-potentials was examined in sixty-one magnocellular neurones of the rat supraoptic nucleus using intracellular recording techniques in a brain slice preparation. 2. In 26 % of all neurones a slow after-hyperpolarization (AHP) was observed in addition to a fast AHP. In 31 % of all neurones a depolarizing after-potential (DAP) was observed. 3. The fast AHP was blocked by apamin whereas the slow AHP was blocked by charybdotoxin (ChTX). The DAP was enhanced by ChTX or a DAP was unmasked if not present during the control period. 4. Low concentrations of TEA (0.15-1.5 mM) induced effects on the slow AHP and the DAP essentially resembling those of ChTX. The same was true for the effects of CoCl2 (1 mM). 5. Spike train after-potentials were not affected by either iberiotoxin (IbTX), a selective high-conductance potassium (BK) channel antagonist, or margatoxin (MgTX), a Kv1.3 alpha-subunit antagonist. 6. Kv1.3 alpha-subunit immunohistochemistry revealed that these units are not expressed in the somato-dendritic region of supraoptic neurones. 7. The effects of ChTX, IbTX, MgTX, TEA, CoCl2 and CdCl2 on spike train after-potentials are interpreted in terms of an induction of the slow AHP by the activation of calcium-dependent potassium channels of intermediate single channel conductance (IK channels). 8. The results suggest that at least the majority of supraoptic magnocellular neurones share the capability of generating both a slow AHP and a DAP. The slow AHP may act to control the expression of the DAP, thus regulating the excitability of magnocellular neurones. The interaction of the slow AHP and the DAP may be important for the control of phasic discharge.

Potassium currents in freshly dissociated uterine myocytes from nonpregnant and late-pregnant rats

In freshly dissociated uterine myocytes, the outward current is carried by K+ through channels highly selective for K+. Typically, nonpregnant myocytes have rather noisy K+ currents; half of them also have a fast-inactivating transient outward current (ITO). In contrast, the current records are not noisy in late pregnant myocytes, and ITO densities are low. The whole-cell IK of nonpregnant myocytes respond strongly to changes in [Ca2+]o or changes in [Ca2+]i caused by photolysis of caged Ca2+ compounds, nitr 5 or DM-nitrophene, but that of late-pregnant myocytes respond weakly or not at all. The Ca2+ insensitivity of the latter is present before any exposure to dissociating enzymes. By holding at -80, -40, or 0 mV and digital subtractions, the whole-cell IK of each type of myocyte can be separated into one noninactivating and two inactivating components with half-inactivation at approximately -61 and -22 mV. The noninactivating components, which consist mainly of iberiotoxin-susceptible large-conductance Ca2+-activated K+ currents, are half-activated at 39 mV in nonpregnant myocytes, but at 63 mV in late-pregnant myocytes. In detached membrane patches from the latter, identified 139 pS, Ca2+-sensitive K+ channels also have a half-open probability at 68 mV, and are less sensitive to Ca2+ than similar channels in taenia coli myocytes. Ca2+-activated K+ currents, susceptible to tetraethylammonium, charybdotoxin, and iberiotoxin contribute 30-35% of the total IK in nonpregnant myocytes, but <20% in late-pregnant myocytes. Dendrotoxin-susceptible, small-conductance delayed rectifier currents are not seen in nonpregnant myocytes, but contribute approximately 20% of total IK in late-pregnant myocytes. Thus, in late-pregnancy, myometrial excitability is increased by changes in K+ currents that include a suppression of the ITO, a redistribution of IK expression from large-conductance Ca2+-activated channels to smaller-conductance delayed rectifier channels, a lowered Ca2+ sensitivity, and a positive shift of the activation of some large-conductance Ca2+-activated channels.

Expression of sperm Ca2+-activated K+ channels in Xenopus oocytes and their modulation by extracellular ATP

Ionic fluxes across the sperm membrane have been shown to be important in the initiating process of sperm activation and gamete interaction; however, electrophysiological investigation of the ion channels involved has been precluded by the small size of the sperm, especially in mammalian species. In the present study sperm ion channels were expressed in Xenopus oocytes by injection of RNAs of spermatogenic cells isolated from the rat testes. The RNA-injected oocytes responded to ATP, a factor known to regulate sperm activation, with the activation of an outwardly rectifying whole-cell current which was dependent on K+ concentrations and inhibitable by K+ channel blockers, charybdotoxin (CTX) and tetraethylammonium (TEA). The ATP-induced current could be mimicked by a Ca2+ ionophore but suppressed by a Ca2+ chelator applied intracellularly, indicating a Ca2+ dependence of the current. Single-channel measurements on RNA-injected oocytes revealed channels of large conductance which could be blocked by CTX and TEA. Co-injection of germ cell RNAs with the antisense RNA for a mouse gene encoding slowpoke 'Maxi' Ca2+-activated K+ channels resulted in significant reduction of the ATP- and ionomycin-induced current. The expression of the 'Maxi' Ca2+-activated K+ channels in sperm collected from the rat epididymis was also confirmed by Western blot analysis. These results suggest that sperm possess Ca2+-activated K+ channels which may be involved in the process of sperm activation.

Chemical basis for alkali cation selectivity in potassium-channel proteins

The determination of the crystal structure of a K+-selective channel protein from Streptomyces lividans reveals how the rapid movement of K+ across membranes is catalyzed by a large family of pore-forming proteins. Many features of the structure mirror hypotheses, predictions and models of K+ channels developed over the past four decades of functional analysis.

Mechanism of maxi-K channel activation by dehydrosoyasaponin-I

Dehydrosoyasaponin-I (DHS-I) is a potent activator of high-conductance, calcium-activated potassium (maxi-K) channels. Interaction of DHS-I with maxi-K channels from bovine aortic smooth muscle was studied after incorporating single channels into planar lipid bilayers. Nanomolar amounts of intracellular DHS-I caused the appearance of discrete episodes of high channel open probability interrupted by periods of apparently normal activity. Statistical analysis of these periods revealed two clearly separable gating modes that likely reflect binding and unbinding of DHS-I. Kinetic analysis of durations of DHS-I-modified modes suggested DHS-I activates maxi-K channels through a high-order reaction. Average durations of DHS-I-modified modes increased with DHS-I concentration, and distributions of these mode durations contained two or more exponential components. In addition, dose-dependent increases in channel open probability from low initial values were high order with average Hill slopes of 2.4-2.9 under different conditions, suggesting at least three to four DHS-I molecules bind to maximally activate the channel. Changes in membrane potential over a 60-mV range appeared to have little effect on DHS-I binding. DHS-I modified calcium- and voltage-dependent channel gating. 100 nM DHS-I caused a threefold decrease in concentration of calcium required to half maximally open channels. DHS-I shifted the midpoint voltage for channel opening to more hyperpolarized potentials with a maximum shift of -105 mV. 100 nM DHS-I had a larger effect on voltage-dependent compared with calcium-dependent channel gating, suggesting DHS-I may differentiate these gating mechanisms. A model specifying four identical, noninteracting binding sites, where DHS-I binds to open conformations with 10-20-fold higher affinity than to closed conformations, explained changes in voltage-dependent gating and DHS-I-induced modes. This model of channel activation by DHS-I may provide a framework for understanding protein structures underlying maxi-K channel gating, and may provide a basis for understanding ligand activation of other ion channels.

Effects of alpha-pompilidotoxin on synchronized firing in networks of rat cortical neurons

We studied the effect of a novel neurotoxin, alpha-pompilidotoxin (alpha-PMTX) on the spontaneously synchronized network firing of cultured rat cortical neurons. Alpha-PMTX acted immediately and irreversibly to disrupt synchronous activity, leaving only residual sparse, uncorrelated firing and was effective at concentrations of 10 nM. In the presence of bicuculline to block inhibitory synaptic transmission, the shutdown in synchronized activity occurred with a significant delay, required a higher concentration of alpha-PMTX (> 100 nM), and was preceded by a transiently increased level of firing. It appears that both inhibitory and excitatory neuronal activity or synaptic transmission are amplified by alpha-PMTX, but that intense activity eventually leads to inactivation or transmitter depletion.

Ionic mechanism of electroresponsiveness in cerebellar granule cells implicates the action of a persistent sodium current

Although substantial knowledge has been accumulated on cerebellar granule cell voltage-dependent currents, their role in regulating electroresponsiveness has remained speculative. In this paper, we have used patch-clamp recording techniques in acute slice preparations to investigate the ionic basis of electroresponsiveness of rat cerebellar granule cells at a mature developmental stage. The granule cell generated a Na+-dependent spike discharge resistant to voltage and time inactivation, showing a linear frequency increase with injected currents. Action potentials arose when subthreshold depolarizing potentials, which were driven by a persistent Na+ current, reached a critical threshold. The stability and linearity of the repetitive discharge was based on a complex mechanism involving a N-type Ca2+ current blocked by omega-CTx GVIA, and a Ca2+-dependent K+ current blocked by charibdotoxin and low tetraethylammonium (TEA; <1 mM); a voltage-dependent Ca2+-independent K+ current blocked by high TEA (>1 mM); and an A current blocked by 2 mM 4-aminopyridine. Weakening TEA-sensitive K+ currents switched the granule cell into a bursting mode sustained by the persistent Na+ current. A dynamic model is proposed in which the Na+ current-dependent action potential causes secondary Ca2+ current activation and feedback voltage- and Ca2+-dependent afterhyperpolarization. The afterhyperpolarization reprimes the channels inactivated in the spike, preventing adaptation and bursting and controlling the duration of the interspike interval and firing frequency. This result reveals complex dynamics behind repetitive spike discharge and suggests that a persistent Na+ current plays an important role in action potential initiation and in the regulation of mossy fiber-granule cells transmission.

Functional expression of two KvLQT1-related potassium channels responsible for an inherited idiopathic epilepsy

Benign familial neonatal convulsions (BFNC), a class of idiopathic generalized epilepsy, is an autosomal dominantly inherited disorder of newborns. BFNC has been linked to mutations in two putative K+ channel genes, KCNQ2 and KCNQ3. Amino acid sequence comparison reveals that both genes share strong homology to KvLQT1, the potassium channel encoded by KCNQ1, which is responsible for over 50% of inherited long QT syndrome. Here we describe the cloning, functional expression, and characterization of K+ channels encoded by KCNQ2 and KCNQ3 cDNAs. Individually, expression of KCNQ2 or KCNQ3 in Xenopus oocytes elicits voltage-gated, rapidly activating K+-selective currents similar to KCNQ1. However, unlike KCNQ1, KCNQ2 and KCNQ3 currents are not augmented by coexpression with the KCNQ1 beta subunit, KCNE1 (minK, IsK). Northern blot analyses reveal that KCNQ2 and KCNQ3 exhibit similar expression patterns in different regions within the brain. Interestingly, coexpression of KCNQ2 and KCNQ3 results in a substantial synergistic increase in current amplitude. Coexpression of KCNE1 with the two channels strongly suppressed current amplitude and slowed kinetics of activation. The pharmacological and biophysical properties of the K+ currents observed in the coinjected oocytes differ somewhat from those observed after injecting either KCNQ2 or KCNQ3 by itself. The functional interaction between KCNQ2 and KCNQ3 provides a framework for understanding how mutations in either channel can cause a form of idiopathic generalized epilepsy.

The endothelium-dependent, substance P relaxation of porcine coronary arteries resistant to nitric oxide synthesis inhibition is partially mediated by 4-aminopyridine-sensitive voltage-dependent K+ channels

We examined the role of K+ channels in the endothelium-dependent relaxation which is resistant to nitric oxide (NO) synthase inhibition in porcine coronary artery. In the presence of 0.2 mM NG-nitro-L-arginine (L-NNA), a potent inhibitor of NO synthase, 10 nM substance P (SP) added to 9,11-dideoxy-11alpha,9alpha-epoxymethano-prostaglandin F2alpha (U46619) contractures elicited a relaxation. The L-NNA-resistant relaxation induced by SP was strongly inhibited by 5 mM tetrabutylammonium chloride (TBA), a non-specific inhibitor of K+ channels. Interestingly, 4-aminopyridine (4-AP, 1 mM), a relatively specific inhibitor of voltage-sensitive K+ channels, shortened the duration of SP response, but it had no effect on the peak of SP response. Although 4-AP has also been shown to inhibit Ca2+-activated K+ channels, the shortening effect of 4-AP in SP response was observed in the presence of 1 microM apamin, an inhibitor of small conductance Ca2+-activated K+ channels, or 100 nM charybdotoxin, and inhibitor of large conductance Ca2+-activated K+ channels. Moreover, although SP stimulates both L-NNA-resistant relaxation and endothelium-derived NO-dependent relaxation (EDNO) in porcine coronary arteries, a low concentration of 4-AP (1 mM) affected only the L-NNA-resistant response, but not the EDNO response. These are the first results to show that the L-NNA-resistant relaxation induced by SP, probably, endothelium-derived hyperpolarizing factor(s) (EDHF) response, is dependent on voltage-dependent K+ channels in porcine coronary artery.

Hydroxylamine-induced relaxation inhibited by K+ channel blockers in rat aortic rings

Hydroxylamine, a putative endogenous nitric oxide donor, relaxed rat aorta in a concentration-dependent manner (0.01-30 microM). Removal of endothelium or pretreatment of aortic tissue with N(G)-nitro-L-arginine (L-NOARG, 100 microM) did not affect the relaxant effect of hydroxylamine but L-NOARG at 100 microM abolished the acetylcholine-induced relaxation. Methylene blue (10 microM) significantly reduced the relaxant effect of hydroxylamine in endothelium-denuded arteries. Tetrapentylammonium ions (0.3-3 microM), tetraethylammonium ions (1-3 mM) and charybdotoxin (100 nM) reduced the relaxant effect of hydroxylamine in the endothelium-denuded arteries while glibenclamide (3 microM) had no effect. Neither tetrapentylammonium nor tetraethylammonium ions affected relaxations induced by forskolin and verapamil. The effects of tetrapentylammonium ions (3 microM) and charybdotoxin (100 nM) were additive. Tetrapentylammonium ions (3 microM), tetraethylammonium ions (3 mM) and charybdotoxin (100 nM) decreased the relaxation induced by sodium nitroprusside in the endothelium-denuded arteries while glibenclamide (3 microM) had no effect. The concentration-relaxation curve for the relaxant effect of hydroxylamine was shifted to the right in the presence of high extracellular K+ (15-60 mM). Neither tetrapentylammonium ions (3 microM) nor charybdotoxin (100 nM) affected hydroxylamine-induced relaxation of the endothelium-denuded aorta precontracted with 60 mM K+. These results indicate that hydroxylamine relaxes the rat aorta partially through activation of tetrapentylammonium-, tetraethylammonium- and charybdotoxin-sensitive K+ channels and its action is comparable with that of sodium nitroprusside, an exogenous nitric oxide donor. The endothelium is not involved in the aortic response to hydroxylamine.

Effect of nitric oxide on calcium-activated potassium channels in colonic smooth muscle of rabbits

Nitric oxide (NO) hyperpolarizes intestinal smooth muscle cells. This study was designed to determine the mechanism whereby NO activates KCa channels of circular smooth muscle of the rabbit colon. Transmural biopsies of the rabbit colon were stained for NADPH-diaphorase. Freshly dispersed circular smooth muscle cells were studied in the whole cell configuration, as well as in on-cell and excised inside-out patch recording configurations, while KCa current and the activity of KCa channels, respectively, were monitored. NADPH-diaphorase-positive nerve fibers were found in both muscle layers. NO (1%) increased whole cell net outward current by 79% and hyperpolarized resting membrane voltage from -59 to -73 mV (n = 8 cells, P < 0.01). In the on-cell patch recording configuration. NO (0.5% or 1%) in the bath increased NPo of KCa channels; charybdotoxin (125 nM) in the pipette solution blocked this effect. In the excised inside-out patch recording configuration, NO (1%) had no effect on NPo of KCa channels. In the on-cell patch recording configuration, methylene blue (1 microM) or cystamine (5 mM) in the bath solution decreased the effect of NO (1%) on NPo of KCa channels. NPo was increased by 8-bromo-cGMP (8-BrcGMP; 1 mM), a cGMP analog, and zaprinast (100 microM), an inhibitor of cGMP phosphodiesterase. These data suggest that NO increased whole cell outward K+ current by activating KCa channels through a cGMP pathway.

Neuroregulation of mucus secretion by opioid receptors and K(ATP) and BK(Ca) channels in ferret trachea in vitro

1. Opioid agonists inhibit neurogenic mucus secretion in the airways. The mechanism of the inhibition is unknown but may be via opening of potassium (K+) channels. We studied the effect on neurogenic secretion in ferret trachea in vitro of the OP1 receptor (formerly known as delta opioid receptor) agonist [D-Pen2,5]enkephalin (DPDPE), the OP2 receptor (formely kappa) agonist U-50,488H, the OP3 receptor (formerly micro) agonist [D-Ala2, N-Me-Phe, Gly-ol5]enkephalin (DAMGO), the ATP-sensitive K+ (K(ATP)) channel inhibitor glibenclamide, the large conductance calcium activated K+ (BK(Ca)) channel blocker iberiotoxin, the small conductance K(Ca) (SK(Ca)) channel blocker apamin, the K(ATP) channel opener levcromakalim, a putative K(ATP) channel opener RS 91309, and the BK(Ca) channel opener NS 1619. Secretion was quantified by use of 35SO4 as a mucus marker. 2. Electrical stimulation increased tracheal secretion by up to 40 fold above sham-stimulated levels. DAMGO or DPDPE (10 microm each) significantly inhibited neurogenic secretion by 85% and 77%, respectively, effects which were reversed by naloxone. U-50,488H had no significant inhibitory effect on neurogenic secretion, and none of the opioids had any effect on ACh-induced or [Sar9]substance P-induced secretion. 3. Inhibition of neurogenic secretion by DAMGO or DPDPE was reversed by iberiotoxin (3 microM) but not by either glibenclamide or apamin (0.1 microM each). Iberiotoxin alone did not affect the neurogenic secretory response. 4. Levcromakalim, RS 91309 or NS 1619 (3 nM-3 microM) inhibited neurogenic secretion with maximal inhibitions at 3 microM of 68%, 72% and 96%, respectively. Neither levcromakalim nor RS 91309 at any concentration tested significantly inhibited acetylcholine (ACh)-induced secretion, whereas inhibition (60%) was achieved at the highest concentration of NS 1619, a response which was blocked by iberiotoxin. 5. Inhibition of neurogenic secretion by levcromakalim (3 microM) or RS 91309 (30 nM) was inhibited by glibenclamide but not by iberiotoxin. In contrast, inhibition by NS 1619 (30 nM and 3 microM) was blocked by iberiotoxin but not by glibenclamide. 6. We conclude that, in ferret trachea in vitro, OP1 or OP3 opioid receptors inhibit neurogenic mucus secretion at a prejunctional site and that the mechanism of the inhibition is via opening of BK(Ca) channels. Direct opening of BK(Ca) channels or K(ATP) channels also inhibits neurogenic mucus secretion. In addition, opening of BK(Ca) channels inhibits ACh-evoked secretion of mucus. Drugs which open BK(Ca) channels may have therapeutic anti-secretory activity in bronchial diseases in which neurogenic mechanisms and mucus hypersecretion are implicated in pathophysiology, for example asthma and chronic bronchitis.

Ryanodine receptors regulate arterial diameter and wall [Ca2+] in cerebral arteries of rat via Ca2+-dependent K+ channels

1. The effects of inhibitors of ryanodine-sensitive calcium release (RyR) channels in the sarcoplasmic reticulum (SR) and Ca2+-dependent potassium (KCa) channels on the membrane potential, intracellular [Ca2+], and diameters of small pressurized (60 mmHg) cerebral arteries (100-200 micron) were studied using digital fluorescence video imaging of arterial diameter and wall [Ca2+], combined with microelectrode measurements of arterial membrane potential. 2. Ryanodine (10 microM), an inhibitor of RyR channels, depolarized by 9 mV, increased intracellular [Ca2+] by 46 nM and constricted pressurized (to 60 mmHg) arteries with myogenic tone by 44 micron (approximately 22 %). Iberiotoxin (100 nM), a blocker of KCa channels, under the same conditions, depolarized the arteries by 10 mV, increased arterial wall calcium by 51 nM, and constricted by 37 micron (approximately 19 %). The effects of ryanodine and iberiotoxin were not additive and were blocked by inhibitors of voltage-dependent Ca2+ channels. 3. Caffeine (10 mM), an activator of RyR channels, transiently increased arterial wall [Ca2+] by 136 +/- 9 nM in control arteries and by 158 +/- 12 nM in the presence of iberiotoxin. Caffeine was relatively ineffective in the presence of ryanodine, increasing [calcium] by 18 +/- 5 nM. 4. In the presence of blockers of voltage-dependent Ca2+ channels (nimodipine, diltiazem), ryanodine and inhibitors of the SR calcium ATPase (thapsigargin, cyclopiazonic acid) were without effect on arterial wall [Ca2+] and diameter. 5. These results suggest that local Ca2+ release originating from RyR channels (Ca2+ sparks) in the SR of arterial smooth muscle regulates myogenic tone in cerebral arteries solely through activation of KCa channels, which regulate membrane potential through tonic hyperpolarization, thus limiting Ca2+ entry through L-type voltage-dependent Ca2+ channels. KCa channels therefore act as a negative feedback control element regulating arterial diameter through a reduction in global intracellular free [Ca2+].

Sea anemone peptides with a specific blocking activity against the fast inactivating potassium channel Kv3.4

Sea anemone venom is known to contain toxins that are active on voltage-sensitive Na+ channels, as well as on delayed rectifier K+ channels belonging to the Kv1 family. This report describes the properties of a new set of peptides from Anemonia sulcata that act as blockers of a specific member of the Kv3 potassium channel family. These toxins, blood depressing substance (BDS)-I and BDS-II, are 43 amino acids long and differ at only two positions. They share no sequence homologies with other K+ channel toxins from sea anemones, such as AsKS, AsKC, ShK, or BgK. In COS-transfected cells, the Kv3.4 current was inhibited in a reversible manner by BDS-I, with an IC50 value of 47 nM. This inhibition is specific because BDS-I failed to block other K+ channels in the Kv1, Kv2, Kv3, and Kv4 subfamilies. Inward rectifier K+ channels are also insensitive to BDS-I. BDS-I and BDS-II share the same binding site on brain synaptic membranes, with K0.5 values of 12 and 19 nM, respectively. We observed that BDS-I and BDS-II have some sequence homologies with other sea anemone Na+ channels toxins, such as AsI, AsII, and AxI. However, they had a weak effect on tetrodotoxin-sensitive Na+ channels in neuroblastoma cells and no effect on Na+ channels in cardiac and skeletal muscle cells. BDS-I and BDS-II are the first specific blockers identified so far for the rapidly inactivating Kv3.4 channel.

Influence of potassium channel modulators on cognitive processes in mice

1. The effect of i.c.v. administration of different potassium channel openers (minoxidil, pinacidil, cromakalim) and potassium channel blockers (tetraethylammonium, apamin, charybdotoxin, gliquidone, glibenclamide) on memory processes was evaluated in the mouse passive avoidance test. 2. The administration of minoxidil (10 microg per mouse i.c.v.), pinacidil (5-25 microg per mouse i.c.v.) and cromakalim (10-25 microg per mouse i.c.v.) immediately after the training session produced an amnesic effect. 3. Tetraethylammonium (TEA; 1-5 microg per mouse i.c.v.), apamin (10 ng per mouse i.c.v.), charybdotoxin (1 microg per mouse i.c.v.), gliquidone (3 microg per mouse i.c.v.) and glibenclamide (1 microg per mouse i.c.v.), administered 20 min before the training session, prevented the potassium channel opener-induced amnesia. 4. At the highest effective doses, none of the drugs impaired motor coordination, as revealed by the rota rod test, or modified spontaneous motility and inspection activity, as revealed by the hole board test. 5. These results suggest that the modulation of potassium channels plays an important role in the regulation of memory processes. On this basis, the potassium channel blockers could be useful in the treatment of cognitive deficits.

Charybdotoxin block of Ca(2+)-activated K+ channels in colonic muscle depends on membrane potential dynamics

Charybdotoxin (ChTX) is a specific blocker of Ca(2+)-activated K+ channels. The voltage- and time-dependent dynamics of ChTX block were investigated using canine colonic myocytes and the whole cell patch-clamp technique with step and ramp depolarization protocols. During prolonged step depolarizations, K+ current slowly increased in the continued presence of ChTX (100 nM). The rate of increase depended on membrane potential with an e-fold change for every 60 mV. During ramp depolarizations, the effectiveness of ChTX block depended significantly on the rate of the ramp (50% at 0.01 V/s to 80% at 0.5 V/s). Results are consistent with a mechanism in which ChTX slowly "unbinds" in a voltage-dependent manner. A simple kinetic model was developed in which ChTX binds to both open and closed states. Slow unbinding is consistent with ChTX having little effect on electrical slow waves recorded from circular muscle while causing depolarization and contraction of longitudinal muscle, which displays more rapid "spikes." Resting membrane potential and membrane potential dynamics are important determinants of ChTX action.

The effect of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and charybdotoxin (CTX) on relaxations of isolated cerebral arteries to nitric oxide

The mechanism underlying smooth muscle relaxations of cerebral arteries in response to nitric oxide is still not completely understood. The present study was designed to determine the role of soluble guanylate cyclase in the relaxations to a nitric oxide/nucleophile complex, diethylaminodiazen-1-ium-1,2-dioate (DEA-NONOate). Rings of canine middle cerebral arteries without endothelium were suspended in Krebs-Ringer bicarbonate solution for isometric tension recording. The levels of guanosine 3',5'-cyclic monophosphate (cyclic GMP) were measured by radioimmunoassay technique. During contractions to uridine 5'-triphosphate (UTP), DEA-NONOate (10(-10) to 10(-5) M) caused concentration-dependent relaxations. Measurements of cyclic GMP levels in cerebral arterial wall demonstrated that DEA-NONOate is a potent stimulator of guanylate cyclase and subsequent formation of cyclic GMP. Increasing concentrations of a selective soluble guanylate cyclase inhibitor, 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), caused concentration-dependent reduction of both cyclic GMP production and relaxations to DEA-NONOate. Interestingly, in the presence of the highest concentration (3 x 10(-6) M) of ODQ, production of cyclic GMP in response to 10(-6) M of DEA-NONOate was abolished, whereas the same concentration of DEA-NONOate caused almost complete relaxation, suggesting that mechanisms independent of cyclic GMP production may mediate relaxing effect of high concentration of a nitric oxide donor. A selective Ca2+-activated potassium channel blocker charybdotoxin (CTX) significantly reduced relaxations to DEA-NONOate resistant to ODQ, supporting the idea that in cerebral arteries nitric oxide may activate potassium channels independently of cyclic GMP. The results of our study suggest that under physiological conditions, guanylate cyclase is a key mediator of cerebral arterial relaxations to nitric oxide. However, under pathological conditions associated with induction of nitric oxide synthase and increased biosynthesis of nitric oxide (e.g., cerebral ischemia, inflammation, sepsis), mechanisms other than formation of cyclic GMP may be activated.

Slo3, a novel pH-sensitive K+ channel from mammalian spermatocytes

Potassium channels have evolved to play specialized roles in both excitable and inexcitable tissues. Here we describe the cloning and expression of Slo3, a novel potassium channel abundantly expressed in mammalian spermatocytes. Slo3 represents a new and unique type of potassium channel regulated by both intracellular pH and membrane voltage. Reverse transcription-polymerase chain reaction, Northern analysis, and in situ hybridization show that Slo3 is primarily expressed in testis in both mouse and human. Because of its sensitivity to both pH and voltage, Slo3 could be involved in sperm capacitation and/or the acrosome reaction, essential steps in fertilization where changes in both intracellular pH and membrane potential are known to occur. The protein sequence of mSlo3 (the mouse Slo3 homologue) is similar to Slo1, the large conductance, calcium- and voltage-gated potassium channel. These results suggest that Slo channels comprise a multigene family, defined by a combination of sensitivity to voltage and a variety of intracellular factors. Northern analysis from human testis indicates that a Slo3 homologue is present in humans and conserved with regard to sequence, transcript size, and tissue distribution. Because of its high testis-specific expression, pharmacological agents that target human Slo3 channels may be useful in both the study of fertilization as well as in the control or enhancement of fertility.

Subarachnoid hemorrhage and the role of potassium channels in relaxations of canine basilar artery to nitrovasodilators

This study was designed to determine the effect of subarachnoid hemorrhage (SAH) on potassium (K+) channels involved in relaxations of cerebral arteries to nitrovasodilators. The effects of K+ channel inhibitors on relaxations to 3-morpholinosydnonimine (SIN-1) and sodium nitroprusside (SNP) were studied in rings of basilar arteries obtained from untreated dogs and dogs exposed to SAH. The levels of cyclic GMP were measured by radioimmunoassay. In rings without endothelium, concentration-dependent relaxations to SIN-1 (10(-9)-10(-4) mol/L) and SNP (10(-9)-10(-4) mol/L) were not affected by SAH, whereas increase in cyclic GMP production stimulated by SIN-1 (10(-6) mol/L) was significantly suppressed after SAH. The relaxations to SIN-1 and SNP were reduced by charybdotoxin (CTX: 10(-7) mol/L), a selective Ca(2+)-activated K+ channel inhibitor, in both normal and SAH arteries; however, the reduction of relaxations by CTX was significantly greater in SAH arteries. By contrast, the relaxations to these nitrovasodilators were not affected by glyburide (10(-5) mol/L), an ATP-sensitive K+ channel inhibitor, in both normal and SAH arteries. These findings suggest that in cerebral arteries exposed to SAH, CA(2+)-activated K+ channels may play a compensatory role in mediation of relaxations to nitric oxide. This may help to explain mechanisms of relaxations to nitrovasodilators in arteries with impaired production of cyclic GMP.

The effects of levcromakalim and pinacidil on the human internal mammary artery

The present study was undertaken to examine the effects of pinacidil and levcromakalim, two potassium, channel openers, on human internal mammary artery (HIMA) obtained from patients undergoing coronary artery bypass surgery, and to clarify the contribution of different K+ channel subtypes in pinacidil and levcromakalim action in this blood vessel. Pinacidil and levcromakalim induced a concentration-dependent relaxation of the precontracted arterial segments (pEC50 = 5.77 +/- 0.05 and 6.89 +/- 0.03, respectively), 4-Aminopyridine (3 mM), a non-selective blocker of K+ channels, induced significant shifts to the right of the concentration-response curves for pinacidil and levcromakalim. Tetraethylammonium (6 mM), charybdotoxin (0.4 microM) and apamin (0.1 microM), blockers of Ca(2+)-sensitive K+ channels, had no effect on the pinacidil- and levcromakalim-evoked relaxation. Glibenclamide (0.1-10 microM), a selective blocker of adenosine triphosphate (ATP)-sensitive K+ channels, competitively antagonized the response to levcromakalim (pKB = 7.92 +/- 0.07). In contrast, glibenclamide, in significantly higher concentrations (3-30 microM), non-competitively antagonized the response to pinacidil. High concentrations of pinacidil (> 10 microM) relaxed arterial rings bathed by a medium containing 100 mM K+ with maximum response 83 +/- 6%. Under the same conditions, the maximum levcromakalim-induced relaxation on HIMA was almost abolished (15 +/- 2%). It is concluded that pinacidil and levcromakalim do not relax the HIMA through the same subtype of K+ channel. ATP-sensitive K+ channels are probably involved in levcromakalim- but not in a pinacidil-induced relaxation in the HIMA. In addition, in pinacidil-induced relaxation of the HIMA, K+ channel-independent mechanisms seem to be involved.

Pandinus imperator scorpion venom blocks voltage-gated K+ channels in human lymphocytes

Using the patch-clamp technique, we determined that Pandinus imperator scorpion venom blocked whole-cell n-type K+ currents in human peripheral blood lymphocytes in a dose-dependent manner with Kd = 0.02 microgram/ml. K+ channel block was instantaneous and removable by washing with venom-free extracellular solution. The venom-induced block was independent of membrane potential. The venom did not influence activation and inactivation kinetics of the K+ channels, however, accelerated recovery from inactivation. Purified peptides Pi1, Pi2, and Pi3 from the P. imperator venom powerfully blocked Kv1.3 channels in human lymphocytes with Kd values of 9.7 nM, 50 pM, and 0.5 nM, respectively. Flow cytometric membrane potential measurements with the oxonol dye showed that Pi2, the most effective peptide toxin of the P. imperator venom, depolarizes human lymphocytes in accordance with its K+ channel blocking effect.

Inactivating BK channels in rat chromaffin cells may arise from heteromultimeric assembly of distinct inactivation-competent and noninactivating subunits

Inactivating and noninactivating variants of large-conductance, Ca2+-dependent, voltage-dependent BK-type channels are found in rat chromaffin cells and are largely segregated into different cells. Here we test the hypothesis that, within the population of cells that express inactivating BK current (BKi current), the BKi channels are largely heteromultimers composed of inactivation-competent subunits (bk(i)) and noninactivating subunits (bk(s)). Several independent types of evidence support this view. The gradual removal of inactivation by trypsin is consistent with the idea that in most cells and patches there are, on average, about two to three inactivation domains per channel. In addition, several aspects of blockade of BKi current by charybdotoxin (CTX) are consistent with the idea that BKi channels contain differing numbers (one to four) of relatively CTX-resistant bk(i) subunits. Finally, the frequency of occurrence of noninactivating BKs channels in patches with predominantly inactivating BKi channels is consistent with the binomial expectations of random, independent assembly of two distinct subunits, if most cells have, on average, about two to three bk(i) subunits per channel. These results suggest that the phenotypic properties of BKi currents and the resulting cellular electrical excitability may exhibit a continuum of behavior that arises simply from the differential expression of two distinct subunits.

Activation of Ca(2+)-dependent K+ channels by cyanide in guinea pig adrenal chromaffin cells

The effects of cyanide (CN) on whole cell current measured with the perforated-patch method were studied in adrenal medullary cells. Application of CN produced initially inward and then outward currents at -52 mV or more negative. As the membrane potential was hyperpolarized, amplitude and latency of the outward current (Io) by CN became small and long, respectively. A decrease in the external Na+ concentration did not affect the latency for CN-induced Io but enhanced the amplitude markedly. The CN Io reversed polarity at -85 mV, close to the Nernst potential for K+, and was suppressed by the K+ channel blockers curare and apamin but not by glibenclamide, suggesting that Io is due to the activation of Ca(2+)-dependent K+ channels. Consistent with this notion, the Ca(2+)-mobilizing agents, muscarine and caffeine, also produced Io. Exposure to CN in a Ca(2+)-deficient medium for 4 min abolished caffeine- or muscarine-induced Io without development of Io, and addition of Ca2+ to the CN-containing solution induced Io. We conclude that exposure to CN produces Ca(2+)-dependent K+ currents in an external Ca(2+)-dependent manner, probably via facilitation of Ca2+ influx.

Role of potassium channels in the nitrergic nerve stimulation-induced vasodilatation in the guinea-pig isolated basilar artery

1. We studied the effects of various K+ channel blockers on the vasodilator responses of guinea-pig isolated basilar arteries to nitrergic nerve stimulation, the nitric oxide (NO) donor sodium nitroprusside (SNP), and the membrane permeable guanosine-3',5'-cyclic monophosphate (cyclic GMP) analogue 8-bromo-cyclic GMP (8-Br-cyclic GMP). 2. In endothelium-denuded preparations which were contracted with prostaglandin F2alpha (1 microM), electrical field stimulation (EFS, 10 Hz for 30 s) produced a vasodilatation which was totally blocked by the nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester L-NAME; 100 microM) (n=3) and by the selective NO-sensitive guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ; 1 microM) (n=4). The vasodilator response to SNP (100 nM) was not reduced by L-NAME but was abolished by ODQ (1 microM) (n=4). 3. EFS-elicited vasodilatation was partly but significantly reduced by the non-selective K+ channel blockers tetraethylammonium (TEA, 1 and 3 mM) and 4-aminopyridine (4-AP, 3 mM), and by the large-conductance calcium-activated K+ channel (K(Ca) channel) blockers charybdotoxin (ChTX, 150 nM) and iberiotoxin (IbTX, 30 and 100 nM). In contrast, the ATP-sensitive K+ channel (K(ATP) channel) blocker glibenclamide (1-10 microM) and the small-conductance K(Ca) channel blocker apamin (100-500 nM) did not affect EFS-induced vasodilatation. 4. The vasodilator response elicited by SNP (10-100 nM) was significantly reduced by TEA (3 mM) and ChTX (150 nM) but not by apamin (500 nM) or glibenclamide (1 microM). The vasodilatation elicited by 8-Br-cyclic GMP (100 microM) was also reduced by TEA (3 mM) and ChTX (150 nM). 5. The results indicate that the vasodilatations induced by nitrergic nerve stimulation and the NO donor SNP in endothelium-denuded guinea-pig basilar artery depend on the formation of intracellular cyclic GMP. The increased cyclic GMP level activates large-conductance K(Ca) channels which partly mediate the vasodilator response. Neither K(ATP) channels nor apamin-sensitive small-conductance K(Ca) channels are involved in nitrergic transmitter-mediated vasodilatation.

Alpha-pompilidotoxin (alpha-PMTX), a novel neurotoxin from the venom of a solitary wasp, facilitates transmission in the crustacean neuromuscular synapse

A new neurotoxin, named alpha-pompilidotoxin (alpha-PMTX) has been found in the venom of the solitary wasp Anoplius safnariensis. In the neuromuscular synapse of the lobster walking leg preparation, alpha-PMTX (10-100 micro/M) caused great enhancement of both the excitatory and inhibitory postsynaptic potentials. Recordings of the excitatory post synaptic currents (EPSCs) at the synaptic sites showed that alpha-PMTX reversibly and dose-dependently potentiates EPSCs. Alpha-PMTX may act primarily on the presynaptic membrane but the mode of action of the toxin is clearly different from other known facilitatory neurotoxins, such as alpha-latrotoxin, apamin or charybdotoxin. This novel toxin will serve as a useful tool in the research field of neuroscience.