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

Two currents activated by epidermal growth factor in EGFR-T17 fibroblasts

Application of 10 nM Epidermal Growth Factor (EGF) to single EGFR-T17 fibroblasts induced a marked hyperpolarization that could last for tens of minutes; in many cases the first transient was followed by a series of oscillations of the membrane potential. The outward current responsible for the hyperpolarizing response could be recorded simultaneously to an increase in the intracellular calcium concentration, as measured with the fluorescent indicator fura-2. The conductance was nearly linear in the voltage range from -100 to +50 mV. While the EGF-induced current had many characteristics of a K+ current and was strongly reduced by 50 nM charybdotoxin (ChTx), its reversal potential was apparently more negative than the potassium equilibrium potential (VK). The application of 2 microM ouabain prior to EGF stimulation produced responses that were similar to those obtained without ouabain; however, under these conditions the EGF-induced current showed a reversal potential of -96.6 +/- 3.2 mV, very close to VK. Simultaneous application of both 2 microM ouabain and 50 nM ChTx completely abolished the response. It can be concluded that the response to EGF stimulation in EGFR-T17 cells consists of two components: the first is a current carried through Ca(2+)-activated K+ channels; the second is due to the acceleration of the operation of the Na+/K(+)-ATPase.

Amino acid sequence and immunological characterization with monoclonal antibodies of two toxins from the venom of the scorpion Centruroides noxius Hoffmann

Two toxins, which we propose to call toxins 2 and 3, were purified to homogeneity from the venom of the scorpion Centruroides noxius Hoffmann. The full primary structures of both peptides (66 amino acid residues each) was determined. Sequence comparison indicates that the two new toxins display 79% identity and present a high similarity to previously characterized Centruroides toxins, the most similar toxins being Centruroides suffusus toxin 2 and Centruroides limpidus tecomanus toxin 1. Six monoclonal antibodies (mAb) directed against purified fraction II-9.2 (which contains toxins 2 and 3) were isolated in order to carry out the immunochemical characterization of these toxins. mAb BCF2, BCF3, BCF7 and BCF9 reacted only with toxin 2, whereas BCF1 and BCF8 reacted with both toxins 2 and 3 with the same affinity. Simultaneous binding of mAb pairs to the toxin and cross-reactivity of the venoms of different scorpions with the mAb were examined. The results of these experiments showed that the mAb define four different epitopes (A-D). Epitope A (BCF8) is topographically unrelated to epitopes B (BCF2 and BCF7), C (BCF3) and D (BCF9) but the latter three appear to be more closely related or in close proximity to each other. Epitope A was found in all Centruroides venoms tested as well as on four different purified toxins of C. noxius, and thus seems to correspond to a highly conserved structure. Based on the cross-reactivity of their venoms with the mAb, Centruroides species could be classified in the following order: Centruroides elegans, Centruroides suffusus suffusus = Centruroides infamatus infamatus, Centruroides limpidus tecomanus, Centruroides limpidus limpidus, and Centruroides limpidus acatlanensis, according to increasing immunochemical relatedness of their toxins to those of Centruroides noxius. All six mAb inhibited the binding of toxin 2 to rat brain synaptosomal membranes, but only mAb BCF2, which belongs to the IgG2a subclass, displayed a clear neutralizing activity in vivo.

Identification of RBK1 potassium channels in C6 astrocytoma cells

Ionic currents in C6 astrocytoma cells were studied using the patch clamp technique under the whole cell configuration. A delayed rectifier K+ current with an amplitude of approximately 1 nA at +50 mV was observed in 86% (92/107) of the cells examined. This K+ current resembled the delayed rectifier present in type-1 and type-2 astrocytes in vitro and could be inhibited by a variety of K+ channel blockers, including TEA (IC50:0.5 mM), 4-aminopyridine (IC50:0.2 mM), MCD peptide (IC50:52 nM), dendrotoxin I (IC50:9 nM), and charybdotoxin (74% inhibition at 50 nM). Northern blot analysis, cloning of cDNA and subsequent sequencing showed that the C6 cell delayed rectifier K+ channel is equivalent to the RBK1 K+ channel derived from a rat brain cDNA library. The level of RBK1 transcripts in C6 cells was comparable to that reported in rat brain. The C6 delayed rectifier K+ channel is probably a homomeric RBK1 K+ channel judging from its pharmacological properties which are similar to the RBK1 channel expressed in Xenopus oocytes. Some C6 cells also expressed a transiently activated outward K+ current (IA). This current was found in less than 50% of the cells and in general contributed no more than 8% of the total outward current. No voltage-dependent inward Na+ or Ca2+ currents or inwardly rectifying K+ currents were observed in over 100 C6 cells examined. The present results show that the dominant voltage gated ionic current in C6 cells is the RBK1 delayed rectifier K+ channel.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of ion transport pathways by changes in cell volume

Swelling-activated K+ and Cl- channels, which mediate RVD, are found in most cell types. Prominent exceptions to this rule include red cells, which together with some types of epithelia, utilize electroneutral [K(+)-Cl-] cotransport for down-regulation of volume. Shrinkage-activated Na+/H+ exchange and [Na(+)-K(+)-2 Cl-] cotransport mediate RVI in many cell types, although the activation of these systems may require special conditions, such as previous RVD. Swelling-activated K+/H+ exchange and Ca2+/Na+ exchange seem to be restricted to certain species of red cells. Swelling-activated calcium channels, although not carrying sufficient ion flux to contribute to volume changes may play an important role in the activation of transport pathways. In this review of volume-activated ion transport pathways we have concentrated on regulatory phenomena. We have listed known secondary messenger pathways that modulate volume-activated transporters, although the evidence that volume signals are transduced via these systems is preliminary. We have focused on several mechanisms that might function as volume sensors. In our view, the most important candidates for this role are the structures which detect deformation or stretching of the membrane and the skeletal filaments attached to it, and the extraordinary effects that small changes in concentration of cytoplasmic macromolecules may exert on the activities of cytoplasmic and membrane enzymes (macromolecular crowding). It is noteworthy that volume-activated ion transporters are intercalated into the cellular signaling network as receptors, messengers and effectors. Stretch-activated ion channels may serve as receptors for cell volume itself. Cell swelling or shrinkage may serve a messenger function in the communication between opposing surfaces of epithelia, or in the regulation of metabolic pathways in the liver. Finally, these transporters may act as effector systems when they perform regulatory volume increase or decrease. This review discusses several examples in which relatively simple methods of examining volume regulation led to the discovery of transporters ultimately found to play key roles in the transmission of information within the cell. So, why volume? Because it's functionally important, it's relatively cheap (if you happened to have everything else, you only need some distilled water or concentrated salt solution), and since it involves many disciplines of experimental biology, it's fun to do.

Refined structure of charybdotoxin: common motifs in scorpion toxins and insect defensins

Conflicting three-dimensional structures of charybdotoxin (Chtx), a blocker of K+ channels, have been previously reported. A high-resolution model depicting the tertiary structure of Chtx has been obtained by DIANA and X-PLOR calculations from new proton nuclear magnetic resonance (NMR) data. The protein possesses a small triple-stranded antiparallel beta sheet linked to a short helix by two disulfides and to an extended fragment by one disulfide, respectively. This motif also exists in all known structures of scorpion toxins, irrespective of their size, sequence, and function. Strikingly, antibacterial insect defensins also adopt this folding pattern.

Two outward K+ currents in bovine pigmented ciliary epithelial cells: IK(Ca) and IK(V)

Pigmented ciliary epithelial cells were studied using the whole cell voltage-clamp technique. Depolarizing steps from a holding potential of -80 mV resulted in a small initial inward current followed by a large outward current. Prolonged depolarizing voltage steps revealed inactivating and noninactivating components of outward current. Outward current was sensitive to the level of Ca2+ in the pipette and was increased by the calcium ionophore A23187; it was blocked by tetraethylammonium (TEA+), quinine, and 4-aminopyridine (4-AP). 4-AP blocked 70% of the outward current with a Ki of 7 x 10(-5) M, and part of the remaining current was abolished by Ni2+. Ni2+ caused a reduction in outward current by blocking IK(Ca) indirectly via decreasing Ca2+ entry through T-type Ca2+ channels. Separating Ni(2+)-sensitive from -insensitive outward conductance gives components that correspond notionally to IK(Ca) and IK(V), respectively. On this basis IK(Ca) represents approximately 28% of K+ outward current. Charybdotoxin blocked 26% of the outward conductance at very depolarized voltage steps as calculated from the slope of the current-voltage curve in this region. It is concluded that there are two major components to the outward current: IK(V), an inactivating voltage-sensitive K+ current, and IK(Ca), which is dependent on the entry of Ca2+ through T-type Ca2+ channels and comprises approximately a quarter of the total K+ outward current under the conditions described. Because of their relative voltage-activation properties, IK(Ca) will be the more important in terms of K+ transport and the secretion of aqueous humor by the ciliary epithelium.

Cyclic GMP regulation of a voltage-activated K channel in dissociated enterocytes

Enterocytes from the intestinal epithelium of the winter flounder were isolated by collagenase digestion and incubated in flounder Ringer solution. Conventional whole-cell and amphotericin-perforated whole-cell recording techniques were used to characterize the properties of a voltage-activated K current present in dissociated cells. Resting membrane potentials and series resistances were significantly lower (from -23 to -39 mV and 29 to 13 M omega, respectively) when amphotericin was used to achieve the whole-cell configuration. When cells were placed in flounder Ringer solution, held at -80 mV and subsequently stepped to a series of depolarizing voltages (from -70 to 0 mV), an outward current was observed that exhibited inactivation at voltages above -20 mV. This current was sensitive to holding potential and was not activated when the cells were held at -40 mV or above. When cells were bathed in symmetric K Ringer solution and the same voltage protocol was applied to the cell, inward currents were observed in response to the negative intracellular potentials. Reversal potentials at two different extracellular K concentrations were consistent with K as the current-carrying ion. BaCl2 (2 mM) and CsCl (0.5 mM) both produced voltage-dependent blockade of the current when added to the bathing solution. Charybdotoxin (300 nM extracellular concentration) completely blocked the current. The IC50 for charybdotoxin was 50 nM. Cyclic GMP inhibited the voltage-activated current in flounder Ringer and in symmetric K Ringer solution. The cyclic GMP analog, 8-Br cGMP, lowered the threshold for voltage activation and potentiated inactivation of the current at voltages above -40 mV.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of multiple Ca(2+)-activated K+ channels in acutely dissociated chick ciliary-ganglion neurones

1. Whole-cell and single-channel recordings were used to characterize Ca(2+)-activated K+ channels (IK(Ca)) in acutely dissociated chick-ganglion neurones. 2. Application of depolarizing voltage steps resulted in outward currents that could be separated according to their dependence on external Ca2+ and/or holding potential. IK(Ca) was the only outward current that could be evoked from holding potentials of -50 mV or less. IK(Ca) was eliminated by bath application of Ca(2+)-free salines. A voltage-dependent outward current (IK(V)) could be evoked from more negative holding potentials in Ca(2+)-free salines. IK(V) was only partially blocked by as much as 30 mM-tetraethylammonium (TEA). 3. Tail currents associated with IK(Ca) reversed close to the K+ equilibrium potential (EK). IK(Ca) tail currents appeared sigmoidal, but the falling phase of the tail currents could be fitted with exponential curves that decayed faster at more negative membrane potentials. 4. IK(Ca) was blocked completely and reversibly by 10 mM-TEA. IK(Ca) was substantially reduced (80-90%) by as little as 1 mM-TEA. 5. Total IK(Ca) was reduced but not eliminated by saturating concentrations of apamin (200 nM). This blockade was not reversible with up to 30 min of washing. Application of 100 microM-d-tubocurare (dTC) also produced a partial blockade of total IK(Ca). 6. Whole-cell current-clamp recordings showed that IK(Ca) contributed to the late phases of spike repolarization and was the dominant current flowing during the spike after-hyperpolarization (AHP). Application of 200 nM-apamin caused a reduction in the duration of the AHP. This reduction was best seen when multiple spikes were evoked by prolonged (20-50 ms) injections of depolarizing current. 7. Three distinct types of IK(Ca) channels could be observed in inside-out patches in the presence of free Ca2+ concentrations of 2 x 10(-7) M, but not in the presence of free Ca2+ at concentrations of less than 10(-9) M. These had unitary chord conductances of 190 pS (i1), 110 pS (i2), and 45 pS (i3) with [K+]o = 150 mM and [K+]i = 75 mM. Each of these three channels had distinct kinetic properties. The 45 pS channel was most sensitive to activation by Ca2+ and could be detected at free Ca2+ concentrations as low as 10(-8) M. 8. All three IK(Ca) channels could be observed in inside-out patches held at membrane potentials where IK(V) was fully inactivated. Application of 10 mM-TEA caused a complete block of IK(Ca) channels in outside-out patches.(ABSTRACT TRUNCATED AT 400 WORDS)

Inositol trisphosphate-dependent periodic activation of a Ca(2+)-activated K+ conductance in glucose-stimulated pancreatic beta-cells

Glucose-stimulated insulin secretion is associated with the appearance of electrical activity in the pancreatic beta-cell. At intermediate glucose concentrations, beta-cell electrical activity follows a characteristic pattern of slow oscillations in membrane potential on which bursts of action potentials are superimposed. The electrophysiological background of the bursting pattern remains unestablished. Activation of Ca(2+)-activated large-conductance K+ channels (KCa channel) has been implicated in this process but seems unlikely in view of recent evidence demonstrating that the beta-cell electrical activity is unaffected by the specific KCa channel blocker charybdotoxin. Another hypothesis postulates that the bursting arises as a consequence of two components of Ca(2+)-current inactivation. Here we show that activation of a novel Ca(2+)-dependent K+ current in glucose-stimulated beta-cells produces a transient membrane repolarization. This interrupts action potential firing so that action potentials appear in bursts. Spontaneous activity of this current was seen only rarely but could be induced by addition of compounds functionally related to hormones and neurotransmitters present in the intact pancreatic islet. K+ currents of the same type could be evoked by intracellular application of GTP, the effect of which was mediated by mobilization of Ca2+ from inositol 1,4,5-trisphosphate (InsP3)-sensitive intracellular Ca2+ stores. These observations suggest that oscillatory glucose-stimulated electrical activity, which is correlated with pulsatile release of insulin, results from the interaction between the beta-cell and intraislet hormones and neurotransmitters. Our data also provide evidence for a close interplay between ion channels in the plasma membrane and InsP3-induced mobilization of intracellular Ca2+ in an excitable cell.

Characterization of voltage-gated and calcium-activated potassium currents in toadfish saccular hair cells

Patch clamp methods were used to study calcium activated (IKCa) and voltage-gated (IK) potassium currents in enzymatically disassociated hair cells from the saccule of the toadfish Opsanus tau. In one population of hair cells, tetraethylammonium bromide (TEA) blocked all outward current, leaving only an inward calcium current (ICa). This current blocked by TEA was also blocked by barium (5 mM) and cadmium (0.2 mM) but only partially blocked by zero external calcium. In the majority of the cells, after TEA (25 mM) was used to block IKCa, a second outward current remained. This current was resistant to block by apamin, barium (5 mM) and cadmium (0.2 mM). Its kinetics of activation and deactivation were considerably slower than those of IKCa. Because of the current/voltage characteristics, its resistance to block by the above agents and voltage-gated activation, this current was termed IK. Study of the rates of activation and deactivation of the two currents in hair cells exhibiting either fast or slow total outward current activation showed that these two kinetic parameters were linked in a cell, i.e., cells with fast IKCa kinetics exhibit faster IKCa kinetics than cells with slower IKCa kinetics. Cell attached and inside out recordings showed a high conductance channel with short open times and a lower conductance channel with longer open times active over the same voltage ranges as those seen in whole cell recordings. Since these two currents with quite different but linked kinetics are active over the same voltage range, their co-existence may be of some importance to sensory coding in the hair cells.

A component of calcium-activated potassium channels encoded by the Drosophila slo locus

Calcium-activated potassium channels mediate many biologically important functions in electrically excitable cells. Despite recent progress in the molecular analysis of voltage-activated K+ channels, Ca(2+)-activated K+ channels have not been similarly characterized. The Drosophila slowpoke (slo) locus, mutations of which specifically abolish a Ca(2+)-activated K+ current in muscles and neurons, provides an opportunity for molecular characterization of these channels. Genomic and complementary DNA clones from the slo locus were isolated and sequenced. The polypeptide predicted by slo is similar to voltage-activated K+ channel polypeptides in discrete domains known to be essential for function. Thus, these results indicate that slo encodes a structural component of Ca(2+)-activated K+ channels.

Calcium-activated potassium channels: regulation by calcium

A wide variety of calcium-activated K channels has been described and can be conveniently separated into three classes based on differences in single-channel conductance, voltage dependence of channel opening, and sensitivity to blockers. Large-conductance calcium-activated K channels typically require micromolar concentrations of calcium to open, and their sensitivity to calcium increases with membrane depolarization, suggesting that they may be involved in repolarization events. Small-conductance calcium-activated K channels are generally more sensitive to calcium at negative membrane potentials, but their sensitivity to calcium is independent of membrane potential, suggesting that they may be involved in regulating membrane properties near the resting potential. Intermediate-conductance calcium-activated K channels are a loosely defined group, where membership is determined because a channel does not fit in either of the other two groups. Within each broad group, variations in calcium sensitivity and single-channel conductance have been observed, suggesting that there may be families of closely related calcium-activated K channels. Kinetic studies of the gating of calcium-activated potassium channels have revealed some basic features of the mechanisms involved in activation of these channels by calcium, including the number of calcium ions participating in channel opening, the number of major conformations of the channels involved in the gating process, and the number of transition pathways between open and closed states. Methods of analysis have been developed that may allow identification of models that give accurate descriptions of the gating of these channels. Although such kinetic models are likely to be oversimplifications of the behavior of a large macromolecule, these models may provide some insight into the mechanisms that control the gating of the channel, and are subject to falsification by new data.

Use of toxins to study potassium channels

Potassium channels comprise groups of diverse proteins which can be distinguished according to each member's biophysical properties. Some types of K+ channels are blocked with high affinity by specific peptidyl toxins. Three toxins, charybdotoxin, iberiotoxin, and noxiustoxin, which display a high degree of homology in their primary amino acid sequences, have been purified to homogeneity from scorpion venom. While charybdotoxin and noxiustoxin are known to inhibit more than one class of channel (i.e., several Ca(2+)-activated and voltage-dependent K+ channels), iberiotoxin appears to be a selective blocker of the high-conductance, Ca(2+)-activated K+ channel that is present in muscle and neuroendocrine tissue. A distinct class of small-conductance Ca(2+)-activated K+ channel is blocked by two other toxins, apamin and leiurotoxin-1, that share no sequence homology with each other. A family of homologous toxins, the dendrotoxins, have been purified from venom of various related species of snakes. These toxins inhibit several inactivating voltage-dependent K+ channels. Although molecular biology approaches have been employed to identify and characterize several species of voltage-gated K+ channels, toxins directed against a particular channel can still be useful in defining the physiological role of that channel in a particular tissue. In addition, for those K+ channels which are not yet successfully probed by molecular biology techniques, toxins can be used as biochemical tools with which to purify the target protein of interest.

Potassium currents in freshly dispersed myometrial cells

K+ currents in freshly dispersed cells from rat myometrium at estrus were studied with the patch-clamp technique (whole cell). Three types of K+ currents were identified: 1) a fast-activating current (IKf), 2) a slowly activating current (IKs), and 3) a transient current (IKt). IKf had a half-activation voltage of 12 mV and a time constant of activation (tau on) of approximately 3 ms at +50 mV. IKs had a tau on of approximately 9 ms at +50 mV and a half-activation potential of 33 mV. Both IKf and IKs were sustained and became potentiated by the entrance of Ca2+ from the patch pipette. These two Ca(2+)-activated K+ currents were inhibited by 100 nM external charybdotoxin and were blocked by external tetraethylammonium (TEA, 2-20 mM). The third current (IKt) was transient, had a faster tau on (approximately 1 ms), and a decay phase with a time constant of approximately 8 ms at +50 mV. This current had a half-activation potential of 22 mV. IKt was not potentiated by intracellular Ca2+, was sensitive to 4-aminopyridine (1 mM), was insensitive to external charybdotoxin (100 nM) and TEA (2 mM), and spontaneously decreased with time.

Using mutagenesis to study potassium channel mechanisms

The voltage-activated K+ channels are members of an ion channel family that includes the voltage-activated Na+ and Ca2+ channels. These ion channels mediate the transmembrane ionic currents that are responsible for the electrical signals produced by cells. The recent cloning of numerous voltage-activated K+ channels has made it possible to combine molecular-genetic and biophysical methods to study K+ channel mechanisms. These mutagenesis-function studies are beginning to provide new information about the architecture of K+ channel proteins and how they form a voltage-gated, K(+)-selective pore.

Charybdotoxin blocks cation-channels in the vacuolar membrane of suspension cells of Chenopodium rubrum L

Using the patch-clamp technique, we studied the action of charybdotoxin which blocks Ca(2+)-activated large-conductance K+ channels in animal tissue on the slow-activating (SV), Ca(2+)-activated cation channel in the vacuolar membrane of suspension-cells of Chenopodium rubrum L. The toxin reversibly reduced the vacuolar current with EC50 approximately 20 nM suggesting structural similarities between ion channels in animal and plant membranes.

Somatostatin stimulates Ca(2+)-activated K+ channels through protein dephosphorylation

The neuropeptide somatostatin inhibits secretion from electrically excitable cells in the pituitary, pancreas, gut and brain. In mammalian pituitary tumour cells somatostatin inhibits secretion through two distinct pertussis toxin-sensitive mechanisms. One involves inhibition of adenylyl cyclase, the other an unidentified cyclic AMP-independent mechanism that reduces Ca2+ influx by increasing membrane conductance to potassium. Here we demonstrate that the predominant electrophysiological effect of somatostatin on metabolically intact pituitary tumour cells is a large, sustained increase in the activity of the large-conductance Ca(2+)- and voltage-activated K+ channels (BK). This action of somatostatin does not involve direct effects of Ca2+, cAMP or G proteins on the channels. Our results indicate instead that somatostatin stimulates BK channel activity through protein dephosphorylation.

Acute electrophysiological responses of bradykinin-stimulated human fibroblasts

1. Acute responses to bradykinin in human dermal fibroblasts were studied at 20-24 degrees C using both the patch-clamp technique to monitor ion currents and Fura-2 fluorescence to monitor [Ca2+]i. 2. During subconfluent culture, human dermal fibroblasts can express a diversity of ion channels as described in the preceding paper. 3. When GTP (1 mM) was included in the pipette solution, two additional ion channel populations were transiently augmented in response to bradykinin stimulation. 4. The first is a component of outwardly rectifying current which reached maximal induction within 10-15 s after bradykinin addition (1 microM) and then decayed back to near baseline over 60 s. 5. Ion substitution experiments combined with tail current analysis indicate that the outward current is carried predominantly by K+. 6. Video imaging of single-cell Fura-2 fluorescence from both intact cells and patch-clamped cells showed temporal correlation of the K+ current modulation and the Ca2+ transients in response to bradykinin stimulation. 7. The calcium ionophore, ionomycin, caused both an increase in intracellular calcium and the augmentation of the outward K+ current. The amount of additional K+ current was correlated with [Ca2+]i levels and could be elicited even without the presence of GTP in the pipette. 8. Apamin, a blocker of Ca(2+)-activated K+ channels, inhibited (at 1 microM) the ionomycin-induced modulation of K+ current. 9. In addition, an inward current was transiently induced in response to bradykinin. This current was strictly dependent on the presence of GTP in the pipette solution. This current showed little voltage dependence, as evidenced by a linear current vs. voltage relation, and a reversal potential near but measurably more positive than 0 mV. 10. This current could be decoupled from the Ca2+ transient and be irreversibly induced by including GTP gamma S (100 microM) in the pipette solution. 11. Ion substitution experiments show that this is a non-specific cation channel. This current prefers monovalents but exhibits a small permeability to divalents. 12. GTP gamma S-induced single channels from isolated outside-out patches showed similar ion selectivity and voltage dependence. These channels are 32 pS in size with an estimated reversal potential of 17 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Determination of the subunit stoichiometry of a voltage-activated potassium channel

The voltage-activated K+, Na+ and Ca2+ channels are responsible for the generation and propagation of electrical signals in cell membranes. The K+ channels are multimeric membrane proteins formed by the aggregation of an unknown number of independent subunits. By studying the interaction of a scorpion toxin with coexpressed wild-type and toxin-insensitive mutant Shaker K+ channels, the subunit stoichiometry can be determined. The Shaker K+ channel is found to have a tetrameric structure. This is consistent with the sequence relationship between a K+ channel and each of the four internally homologous repeats of Na+ and Ca2+ channels.

Design, synthesis, and functional expression of a gene for charybdotoxin, a peptide blocker of K+ channels

A gene encoding charybdotoxin (CTX), a K+ channel blocker from scorpion venom, was designed, synthesized, and expressed as a cleavable fusion protein in Escherichia coli. A sequence-specific protease, factor Xa, was used to cleave the fusion protein and thus release the toxin peptide. The recombinant toxin was purified, oxidized to form disulfide bonds, and treated to form N-terminal pyroglutamate. Recombinant CTX is identical to the native venom CTX with respect to high-performance liquid chromatography mobility, amino acid composition, and N-terminal modification. With single Ca2(+)-activated K+ channels as an assay system, recombinant CTX shows blocking and dissociation kinetics identical to the native venom toxin. The synthetic gene and high-level expression of functionally active CTX make it possible to study the fundamental mechanism of the toxin-ion channel interaction.

K+ and Cl- currents in enterocytes isolated from guinea-pig small intestinal villi

1. The whole-cell configuration of the patch-clamp technique has been used to investigate the conductance properties of villus enterocytes isolated from guinea-pig small intestinal epithelium. 2. With near physiological ionic gradients inward and outward rectification was observed in the hyperpolarizing and depolarizing voltage domains respectively. 3. Replacement of intra- and extracellular K+ with N-methyl-D-glucamine (NMG) eliminated inward rectification but did not alter outward currents. In symmetrical low Cl- solutions outward currents were reduced but inward rectification was not affected. Under these conditions increases in extracellular K+ shifted both the current-voltage relation and the extrapolated reversal potential as expected for a K(+)-selective current. 4. The inwardly rectifying nature of the K+ current observed here remained unaltered after chelation of internal Mg2+ with ATP or EDTA. 5. Extracellular application of 5 mM-Ba2+ or 50 micrograms ml-1 of the venom of the scorpion Leiurus quinquestriatus abolished the inward K+ current, while 5 mM-extracellular tetraethylammonium (TEA) had little effect. 6. The current remaining in the presence of symmetrical Cl- solutions and in the complete absence of K+ rectified outwardly and reversed at 0 mV. The anionic nature of this current was confirmed by replacing Cl- with different anions. SCN- and Br- carried more current than Cl-, while F- and gluconate were less permeant. 7. Anionic currents of villus guinea-pig enterocytes were not stimulated by cyclic AMP and were strongly and reversibly inhibited by the Cl- channel blocker 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB, 10(-5) M). 8. The inwardly rectifying K+ current described here shares some, but not all, characteristics with others previously described. It is postulated that this conductance might function to couple K+ permeability and the Na(+)-K+ pump rate in enterocytes. Absorption of chloride may be mediated by the Cl- channels.

Three-dimensional structure of natural charybdotoxin in aqueous solution by 1H-NMR. Charybdotoxin possesses a structural motif found in other scorpion toxins

A 600-MHz proton NMR study of natural charybdotoxin, a toxin acting on K+ channels, is reported. The unambiguous sequential assignment of all the protons of the toxin was achieved. The analysis of NOEs and of backbone coupling constants showed the existence of an alpha-helix (residues 10-19) and of an antiparallel beta-sheet in the 26-35 part. Three-dimensional structures were generated by distance geometry, using a set of 114 interresidual calibrated constraints (63 sequential, 47 medium and long range, 4 hydrogen bonds) and 29 phi angles. These structures show that charybdotoxin is composed of a beta-sheet linked to an alpha-helix by two disulphide bridges and to an extended fragment by the third disulphide bridge. Comparison with the other known structures of long and short scorpion toxins shows that this structural motif is common to all these proteins.

Role of K+ channels in spontaneous electrical and mechanical activity of smooth muscle in the guinea-pig mesotubarium

1. The spontaneous electrical and mechanical activity and the efflux rate of 86Rb+ in the guinea-pig mesotubarium were studied in the presence of agents interacting with K+ channels. 2. Tetraethylammonium (TEA, 10 mM) increased the amplitude of the action potentials while having no consistent effect on the frequency or amplitude of spontaneous contractions. 3. 4-Aminopyridine (4-AP, 1-5 mM) caused a graded increase in the duration of the contractions and of the electrical slow waves, and a decrease in the duration of the relaxed period between contractions. At 4 mM-4-AP or more the cell was unable to repolarize from the slow wave and the membrane depolarized to -26 mV from the normal resting potential of -63 mV. The rate of 86Rb+ efflux in the presence of 5 mM-4-AP was higher than that at 60 mM-K+, where the membrane potential is -24 mV. 4. 4-AP (5 mM) evoked a contracture in Ca(2+)-free solution, containing 1 mM-EGTA, both at the normal [K+]o of 5.9 mM and at 60 mM-K+, suggesting release of intracellular Ca2+. 5. Apamin (0.1-1 microM) and charybdotoxin (1-10 nM), blockers of Ca(2+)-dependent K+ channels, were without effects on the spontaneous electrical and mechanical activity. 6. The K+ channel opener pinacidil (10 microM) inhibited the spontaneous contractions and hyperpolarized the membrane by about 7 mV. The permeability to 86Rb+ was increased by a factor of 1.4. 7. It is concluded that different K+ channels are involved in the generation of spikes and slow waves: one sensitive to TEA and responsible for repolarization of the individual action potential, and another sensitive to 4-AP and responsible for repolarization of the slow wave. The duration of the relaxed period can be influenced by activation of K+ channels sensitive to pinacidil.

Electrophysiological characterization of a new member of the RCK family of rat brain K+ channels

A novel member of the RCK family of rat brain K+ channels, called RCK2, has been sequenced and expressed in Xenopus oocytes. The K+ currents were voltage-dependent, activated within 20 ms (at 0 mV), did not inactivate in 5 s, and had a single channel conductance in frog Ringers of 8.2 pS. Compared to other members of the RCK family the pharmacological profile of RCK2 was unique in that the channel was resistant to block (IC50 = 3.3 microM) by charybdotoxin [(1988) Proc. Natl. Acad. Sci. USA 85, 3329-3333] but relatively sensitive to 4-aminopyridine (0.3 mM), tetraethylammonium (1.7 mM), alpha-dendrotoxin (25 nM), noxiustoxin (200 nM), and mast cell degranulating peptide (200 nM). Thus, RCK2 is a non-inactivating delayed rectifier K+ channel with interesting pharmacological properties.

A fast, transient K+ current in neurohypophysial nerve terminals of the rat

1. Nerve terminals of the rat posterior pituitary were acutely dissociated and identified using a combination of morphological and immunohistochemical techniques. Macroscopic terminal membrane currents and voltages were studied using the whole-cell patch clamp technique. 2. In physiological solutions, depolarizing voltage clamp steps, from a holding potential (-80 mV) similar to the normal terminal resting potential, elicited a fast, inward followed by a fast, transient, outward current. 3. The threshold of activation for the outward current was -60 mV. The outward current quickly reached a peak and then decayed more slowly. The decay was fitted by two exponentials with time constants of 21 +/- 2.9 and 143 +/- 36 ms. These decay constants did not show a dependence on voltage. The time to peak of the outward current decreased and the amplitude increased with increasingly depolarized potential steps. 4. The outward current was blocked by the substitution of K+ with Cs+ and its reversal potential was consistent with a potassium current. 5. The transient outward current showed steady-state inactivation at more depolarized (than -80 mV) holding potentials with 50% inactivation occurring at -47.9 mV. The time course of recovery from inactivation was complex with full recovery taking greater than 16 s. 6. 4-Aminopyridine (4-AP) blocked the transient outward current in a dose-dependent manner (approximately IC50 = 3 mM), while charybdotoxin (4 micrograms/ml) and tetraethylammonium (100 mM) had no effect on the current amplitude. 7. Lowering external [Ca2+] had no effect on the fast, transient outward current nor did the calcium channel blocker Cd2+ (2 mM). 8. The neurohypophysial outward current reported here corresponds most closely to IA, and not to the delayed rectifier or Ca2(+)-activated K+ currents. Neurohypophysial IA, however, appears to be different from the outward currents found in the cell bodies in the hypothalamus which project their axons to the posterior pituitary. 9. Under current clamp, evoked action potential duration increased (122%) upon application of 5 mM-4-AP, indicating that IA is involved in neurohypophysial spike repolarization. 10. The existence of this current could help explain why maximal peptide release only occurs in response to bursts of electrical activity invading the nerve terminals.

Charybdotoxin-sensitive K(Ca) channel is not involved in glucose-induced electrical activity in pancreatic beta-cells

The effects of charybdotoxin (CTX) on single [Ca2+]-activated potassium channel (K(Ca)) activity and whole-cell K+ currents were examined in rat and mouse pancreatic beta-cells in culture using the patch-clamp method. The effects of CTX on glucose-induced electrical activity from both cultured beta-cells and beta-cells in intact islets were compared. K(Ca) activity was very infrequent at negative patch potentials (-70 less than Vm less than 0 mV), channel activity appearing at highly depolarized Vm. K(Ca) open probability at these depolarized Vm values was insensitive to glucose (10 and 20 mM) and the metabolic uncoupler 2,4 dinitrophenol (DNP). However, DNP blocked glucose-evoked action potential firing and reversed glucose-induced inhibition of the activity of K+ channels of smaller conductance. The venom from Leiurus quinquestriatus hebreus (LQV) and highly purified CTX inhibited K(Ca) channel activity when applied to the outer aspect of the excised membrane patch. CTX (5.8 and 18 nM) inhibited channel activity by 50 and 100%, respectively. Whole-cell outward K+ currents exhibited an early transient component which was blocked by CTX, and a delayed component which was insensitive to the toxin. The individual spikes evoked by glucose, recorded in the perforated-patch modality, were not affected by CTX (20 nM). Moreover, the frequency of slow oscillations in membrane potential, the frequency of action potentials and the rate of repolarization of the action potentials recorded from pancreatic islet beta-cells in the presence of glucose were not affected by CTX. We conclude that the K(Ca) does not participate in the steady-state glucose-induced electrical activity in rodent pancreatic islets.

Oscillating activity of a calcium-activated K+ channel in normal and cancerous mammary cells in culture

Calcium-activated potassium channels were the channels most frequently observed in primary cultured normal mammary cell and in the established mammary tumor cell, MMT060562. In both cells, single-channel and whole-cell clamp recordings sometimes showed slow oscillations of the Ca2(+)-gated K+ current. The characteristics of the Ca2(+)-activated K+ channels in normal and cancerous mammary cells were quite similar. The slope conductances changed from 8 to 70 pS depending on the mode of recording and the ionic composition in the patch electrode. The open probability of this channel increased between 0.1 to 1 microM of the intracellular Ca2+, but it was independent of the membrane potential. Charybdotoxin reduced the activity of the Ca2(+)-activated K+ channel and the oscillation of the membrane current, but apamin had no apparent effect. The application of tetraethylammonium (TEA) from outside and BaCl2 from inside of the cell diminished the activity of the channel. The properties of this channel were different from those of both the large conductance (BK or MAXI K) and small conductance (SK) type Ca2(+)-activated K+ channels.

Neuropeptide modulation of single calcium and potassium channels detected with a new patch clamp configuration

Calcium channel activity is crucial for secretion and synaptic transmission, but it has been difficult to study Ca2+ channel modulation because survival and regulation of some of these channels require cytoplasmic constituents that are lost with the formation of cell-free patches. Here we report a new patch clamp configuration in which activity and regulation of channels are maintained after removal from cells. A pipette containing the pore-forming agent nystatin is sealed onto a cell and withdrawn to form an enclosed vesicle. The resulting perforated vesicle, formed from pituitary tumour cells, contains Ca2+ and K+ channels. Ca2(+)-activated K+ channels in the vesicle are activated by cyclic AMP analogues, and by a neuropeptide (thyrotropin-releasing hormone) that stimulates phosphatidylinositol turnover and inositol trisphosphate-gated Ca2+ release from intracellular organelles. Thus, the perforated vesicle retains signal transduction systems necessary for ion channel modulation. Functional dihydropyridine-sensitive Ca2+ channels (L-type) are maintained in the vesicle, and their gating is inhibited by thyrotropin-releasing hormone. Hence, this new patch clamp configuration has allowed a direct detection of the single-channel basis of transmitter-induced inhibition of L-type Ca2+ channels. The modulation of Ca2(+)-channel gating may be an important mechanism for regulating hormone secretion from pituitary cells.

Mapping the receptor site for charybdotoxin, a pore-blocking potassium channel inhibitor

The Shaker K+ channel belongs to a family of structurally related voltage-activated cation channels that play a central role in cellular electrical signaling. By studying multiple site-directed mutants of the Shaker K+ channel, a region that forms the binding site for a pore-blocking scorpion toxin has been identified. The region contains a sequence that is highly conserved among cloned K+ channels and may contribute to the formation of the ion conduction pore.

Mutations affecting TEA blockade and ion permeation in voltage-activated K+ channels

Voltage-dependent ion channels are responsible for electrical signaling in neurons and other cells. The main classes of voltage-dependent channels (sodium-, calcium-, and potassium-selective channels) have closely related molecular structures. For one member of this superfamily, the transiently voltage-activated Shaker H4 potassium channel, specific amino acid residues have now been identified that affect channel blockade by the small ion tetraethylammonium, as well as the conduction of ions through the pore. Furthermore, variation at one of these amino acid positions among naturally occurring potassium channels may account for most of their differences in sensitivity to tetraethylammonium.

Ion channel expression by white matter glia: the type-1 astrocyte

We describe the electrophysiological properties of acutely isolated type-1 astrocytes using a new "tissue print" dissociation procedure. Because the enzymes used did not destroy or modify the ion channels, and the cells retained many processes, the properties may reflect those in vivo. The types of ion channels in type-1 astrocytes changed rapidly during the first 10 postnatal days, when they attained their adult phenotype. This change was dependent on the presence of neurons. In culture, most of these channel types were not expressed, but a phenotype more typical of that in vivo could be induced by co-culture with neurons. The electrophysiological properties of astrocytes make some existing hypotheses of astrocyte function less likely.

Voltage-gated potassium channels and the control of membrane potential in human platelets

1. Human platelets were studied using a combination of patch-clamp and fluorescent indicators of membrane potential and [Ca2+]i. 2. Whole-cell and cell-attached patch recordings showed voltage-gated channels selective for K+ (IK(V]. These channels were activated by depolarization at a threshold close to the platelet resting potential and were blocked by the venom charybdotoxin (CTX; 10-20 nM). Several different conductance states were observed, ranging from 5 to 34 pS, with isotonic KCl in the patch pipette and bath. 3. Measurements with the potential-sensitive dye 3,3'-dipropylthia-dicarbocyanine, diS-C3-(5), in platelet suspensions showed that CTX depolarized the resting potential by approximately 25 mV. Thus, CTX-sensitive, voltage-gated K+ channels appear to play a major part in setting the resting potential. 4. ADP-evoked Ca2+ influx, monitored with Fura-2, was reduced by 10 nM-CTX. Restoration of a large negative membrane potential with valinomycin reversed this effect of CTX. These results suggest that the Ca2+ influx depends on the negative membrane potential and that K+ channels may be important in maintaining this potential during activation.

Molecular structure of charybdotoxin, a pore-directed inhibitor of potassium ion channels

The three-dimensional structure of charybdotoxin, a high-affinity peptide blocker of several potassium ion channels, was determined by two-dimensional nuclear magnetic resonance (2-D NMR) spectroscopy. Unambiguous NMR assignments of backbone and side chain hydrogens were made for all 37 amino acids. The structure was determined by distance geometry and refined by nuclear Overhauser and exchange spectroscopy back calculation. The peptide is built on a foundation of three antiparallel beta strands to which other parts of the sequence are attached by three disulfide bridges. The overall shape is roughly ellipsoidal, with axes of approximately 2.5 and 1.5 nanometers. Nine of the ten charged groups are located on one side of the ellipsoid, with seven of the eight positive residues lying in a stripe 2.5 nanometers in length. The other side displays three hydrophobic residues projecting prominently into aqueous solution. The structure rationalizes several mechanistic features of charybdotoxin block of the high-conductance Ca2(+)-activated K+ channel.