The correlation between the increase in slow outward current and in contraction induced by caffeine, ryanodine, and rapid cooling in voltage-clamped frog muscle fibers

The effects of caffeine, ryanodine, and rapid cooling were tested on the depolarization-induced contraction and the apamin-insensitive slow outward current (Iso) of voltage-clamped (double mannitol gap) single frog muscle fibers. Subthreshold caffeine concentrations (0.5-2 mM) induced a monotonic increase in contractile and Iso amplitude. Whatever the concentration, the increase in contraction was roughly twice the one in current. Similar results were obtained upon rapid cooling (20-4 degrees C) in the presence of 0.5 mM caffeine. In the absence of external Na+ (choline-substituted) 10(-5) M ryanodine induced a delayed increase (approximately 30 min) in contraction and in current, shortly before the development of a drastic and irreversible contracture. Here again, the increase in contraction was twice that in current. In the presence of 5 mM tetraethylammonium (TEA) and (or) 25 nM charybdotoxin, 2 mM caffeine still induced a strong facilitating effect on contraction but the parallel increase in current was strongly reduced. The linear relationship between the increase in current and contractile amplitude has a slope approximately 0.5 (whatever the drug used to increase contractility); it is approximately 0.1 in the presence of TEA and (or) charybdotoxin. In conclusion, provided the changes in contractile amplitude are caused by parallel changes in depolarization-induced sarcoplasmic reticulum Ca2+ release, about 50% of the apamin-insensitive Iso is controlled by internal Ca2+ release. The main part of this current corresponds to the TEA- and charybdotoxin-sensitive component of Iso.

Ionic events induced by epidermal growth factor. Evidence that hyperpolarization and stimulated cation influx play a role in the stimulation of cell growth

Charybdotoxin, a blocker of K+ channels, and the imidazole drug SC38249, a blocker of both voltage- and second messenger-operated Ca2+ channels, were employed in mouse NIH-3T3 fibroblasts overexpressing the epidermal growth factor (EGF) receptor 1) to characterize the ionic events activated by EGF; and 2) to establish the role of those events in cell growth. The [Ca2+]i response by EGF was little changed by charybdotoxin while the parallel hyperpolarization was inhibited in a dose-dependent manner. At high toxin concentrations (greater than 3 x 10(-8) M), the effect of EGF on membrane potential was turned into a persistent depolarization sustained by both Na+ and Ca2+. Pretreatment with 10 microM SC38249 induced only minor changes of the intracellular Ca2+ release by EGF (the process responsible for the initial phase of the [Ca2+]i and membrane potential responses) and blocked the persistent, second phase [Ca2+]i and the hyperpolarization responses, both dependent on Ca2+ influx, as well as the depolarization in the charybdotoxin-pretreated cells. Long term (up to 2-day) treatment with either charybdotoxin or SC38249 failed to affect the viability and growth of unstimulated EGFR-T17 cells. Moreover, in these cells, the ionic responses to EGF were restored after a 30-min incubation in fresh medium. In contrast, growth stimulated by EGF was inhibited, moderately (-20%) by charybdotoxin and markedly (-60%) by SC38249. These results indicate for the first time that both hyperpolarization and, especially, the persistent increase of [Ca2+]i sustained by Ca2+ influx play a role in the activity of EGF, ultimately cooperating with other intracellular events in mitogenesis.

A patch-clamp study of K(+)-channel activity in bovine isolated tracheal smooth muscle cells

1. Single smooth muscle cells were isolated from bovine trachealis by enzymic digestion. The properties of large conductance plasmalemmal K(+)-channels in these cells were studied by the patch-clamp recording technique. 2. Recordings were made from inside-out plasmalemmal patches when [K+] was symmetrically high (140 mM) and when [Ca2+] on the cytosolic side of the patch was varied from nominally zero to 10 microM. Large unitary currents of both Ca(2+)-dependent and -independent types were observed. Measured between + 20 and + 40 mV, the slope conductances of the channels carrying these currents were 249 +/- 18 pS and 268 +/- 14 pS respectively. 3. Lowering [K+] on the cytosolic side of the patches from 140 to 6 mM, shifted the reversal potentials of the two types of unitary current from approximately zero to much greater than + 40 mV, suggesting that both currents were carried by K(+)-channels. 4. The Ca(2+)-dependent and -independent K(+)-channels detected in inside-out plasmalemmal patches could also be distinguished on the basis of their sensitivity to inhibitors (tetraethylammonium (TEA), 1-10 mM; Cs+, 10 mM; Ba2+, 1-10 mM; quinidine, 100 microM) applied to the cytosolic surface of the patches. 5. Recordings were made from outside-out plasmalemmal patches when [K+] was symmetrically high (140 mM) and when [Ca2+] on the cytosolic side of the patch was varied from nominally zero to 1 microM. Ca(2+)-dependent unitary currents were observed and the slope conductance of the channel carrying these currents was 229 +/- 5 pS. 6. Activity of the Ca2+-dependent K+-channel detected in outside-out patches could be inhibited by application of TEA (1 mM), Cs+ (10mM), Ba2(+210mM) or quinidine (100 microM) to the external surface of the patch. 4-Aminopyridine (4-AP; 1 mM) was ineffective as an inhibitor. 7. The activity of the Ca2+-dependent K+-channel recorded from outside-out patches was reversibly inhibited by charybdotoxin (100 nM). 8. When whole-cell recording was performed, the application of a depolarizing voltage ramp evoked outward current which was dependent on the [Ca2 +] in the recording pipette and which could be reversibly inhibited by charybdotoxin (50 nM-I microM) applied to the external surface of the cell.9. We conclude that bovine trachealis cells are richly endowed with charybdotoxin-sensitive, large conductance, Ca2 +-dependent K+-channels. These channels carry most of the outward current evoked by a depolarizing ramp and could play a major role in determining the outward rectifying properties of the trachealis cells. The role of the large Ca2 + -independent K+ -channels remains unclear.

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.

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.

Characterization of high affinity binding sites for charybdotoxin in human T lymphocytes. Evidence for association with the voltage-gated K+ channel

Charybdotoxin (ChTX) inhibits with high affinity a voltage-gated K+ channel that is present in human T lymphocytes. In this system, 125I-ChTX binds specifically and reversibly to a single class of sites which display a Kd of 8-14 pM, as measured by either equilibrium or kinetic binding protocols. The maximum density of sites, 542 sites/cell, correlates well with the density of K+ channel as determined by electrophysiological experiments. Binding of 125I-ChTX is modulated by the ionic strength of the incubation media and by Ca2+. Increasing concentrations of either K+, Na+, or Ca2+ cause inhibition of toxin binding. Inhibition of binding by Ca2+ is due, primarily, to an effect on toxin dissociation rates. Increasing the pH of the external media from 6.8 to 8.5 enhances toxin binding, due to an increase in affinity with no significant effect on the maximum density of receptor sites. Different agents that block the voltage-gated K+ channel in human T lymphocytes, inhibit toxin binding. Mitogen-stimulated T cells display 2.5-3-fold increase in toxin binding as compared with unstimulated control cells. These data, taken together, suggest that 125I-ChTX binding sites identified in this study, represent the predominant voltage-gated K+ channel present in peripheral human T lymphocytes. Therefore, 125I-ChTX is a useful probe for elucidating the physiological role of this type of K+ channel.

K+ efflux pathways and neurotransmitter release associated to hippocampal ischemia: effects of glucose and of K+ channel blockers

Ischemia of hippocampal slices leads to 86Rb+ efflux and to amino acid neurotransmitter release. This 86Rb+ efflux which corresponds to the massive K+ efflux from neuronal cells observed in ischemic animals is inhibited by glucose (IC50 = 1.7 mM). Glucose also inhibits the ischemia induced liberation of GABA and aspartate. 86Rb+ efflux is insensitive to any type of known blockers for ATP-sensitive, Ca2(+)-sensitive and voltage-sensitive K+ channels.

K+ channel and 5-hydroxytryptamine1A autoreceptor interactions in the rat dorsal raphe nucleus: an in vitro electrophysiological study

Extracellular recordings were made from serotonergic neurons of the rat dorsal raphe nucleus in a slice preparation. In the presence of phenylephrine (3 microM) to restore the pacemaker activity of otherwise silent serotonergic neurons, superfusion with the 5-hydroxytryptamine1A agonist ipsapirone depressed the firing of these neurons with an IC50 of approximately 50 nM. Complete inhibition was achieved with 100-300 nM of the drug. Concomitant superfusion with the 5-hydroxytryptamine1A antagonists spiperone (100 nM) or propranolol (10 microM) markedly reduced the inhibitory effect of ipsapirone (100 nM). Superfusion with K+ channel blockers such as apamin (50-100 nM), charybdotoxin (100 nM) or Ba2+ (1 mM) did not induce any changes in the electrical activity of serotonergic neurons. However, 4-aminopyridine (0.1-1 mM) disrupted the regularity of their discharge without affecting the mean firing rate. The ipsapirone-induced inhibition was unchanged by apamin and charybdotoxin, but was markedly reduced by Ba2+ and 4-aminopyridine. Thus the IC50 of ipsapirone was shifted to approximately 150 nM in the presence of 1 mM of 4-aminopyridine. These results indicate that, in serotonergic neurons within the dorsal raphe nucleus, the K+ channel opened through the stimulation of 5-hydroxytryptamine1A autoreceptors is 4-aminopyridine-sensitive.

A Ca2(+)-activated K+ current in ras-transformed fibroblasts is absent from nontransformed cells

Biochemical similarities between ras proteins and the GTP-binding proteins and correlation of ras-induced cell transformation with altered transmembrane cation fluxes indicate that ras proteins may act to modulate ion channel activity. To test this idea, whole cell, tight-seal, patch-clamp recording was used to compare macroscopic currents of ras-transformed fibroblasts with currents of their nontransformed counterparts. A prominent calcium-activated, voltage-independent potassium current was observed in 83-100% of cells from three separate fibroblast lines transformed by two different oncogenic ras alleles, whereas the same current was present at much smaller amplitudes in only 0-15% of nontransformed cells. The calcium-activated potassium current is blocked by charybdotoxin and by concentrations of tetraethylammonium above 1 mM, but it is insensitive to apamin. Both normal and ras-transformed cells have another calcium-activated current that is not potassium selective, and, consistent with other studies, normal cells display a voltage-activated calcium conductance. These results suggest that the mechanisms by which ras triggers or maintains cell transformation may involve alterations in the number or activity of certain ion channels, in particular, a type of calcium-activated potassium channel.

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.

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.

Pharmacology of the Ca2(+)-dependent K+ channel in corn protoplasts

We investigated the sensitivity of the Ca2(+)-dependent K+ current, IK(Ca), present in corn protoplasts, to different K+ channel blockers. IK(Ca) was inhibited by external Cs+ (10 mM), Ba2+ (10 mM), and quinine (0.5 mM): reagents which block many types of outward-rectifying K+ channels. In contrast 4-aminopyridine (5 mM), an inhibitor of delayed rectifier or inactivating K+ currents, had no effect. Neither of the peptide toxins, apamin or charybdotoxin, specific for Ca2(+)-dependent K+ channels in animal cells, inhibited currents when used in the nanomolar concentration range. However, higher levels of charybdotoxin (10 microM) caused marked reduction of IK(Ca).

Synthesis and structural characterization of charybdotoxin, a potent peptidyl inhibitor of the high conductance Ca2(+)-activated K+ channel

Charybdotoxin (ChTX), a potent inhibitor of the high conductance Ca2(+)-activated K+ channel (PK,Ca) is a highly basic peptide isolated from venom of the scorpion Leiurus quinquestriatus hebraeus, whose primary structure has been determined (Gimenez-Gallego, G., Navia, M. A., Reuben, J. P., Katz, G. M., Kaczorowski, G. J., and Garcia, M. L. (1988) Proc. Natl. Acad. Sci. U. S. A. 85, 3329-3333). The synthesis of this peptide using continuous flow solid phase fluorenylmethyloxycarbonyl-pentafluorophenyl ester methodology has now been achieved. The 1-37-amino acid hexasulfhydryl peptide oxidizes readily to give the tricyclic disulfide structure in good yield. This folded synthetic material is identical to native toxin based on three criteria: co-migration with ChTX on reversed phase high performance liquid chromatography (HPLC); competitive inhibition of 125I-labeled monoiodotyrosine charybdotoxin binding to bovine aortic sarcolemmal membrane vesicles with a Ki (10 pM) identical to that of native toxin; blockade of PK,Ca activity in excised outside-out patches from bovine aortic smooth muscle with the potency and inhibitory properties characteristic of ChTX (i.e. appearance of silent periods interdispersed with normal bursts of channel activity in single channel recordings). Selective enzymatic digestion of native or synthetic ChTX by simultaneous exposure to chymotrypsin and trypsin yields identical reversed phase HPLC profiles. Analysis of the sequence and amino acid composition of the resulting fragments defines a disulfide bond arrangement (Cys7-Cys28, Cys13-Cys33, Cys17-Cys35) which differs from that previously suggested. This configuration predicts a highly folded tertiary structure for ChTX which, together with observations from electrophysiological and binding experiments, suggests a possible mechanism by which ChTX interacts with PK,Ca to block channel function.

Mechanisms of potassium channel block in rat alveolar epithelial cells

Block of inactivating delayed rectifier K+ currents was studied in rat alveolar epithelial cells in primary culture using the whole-cell configuration of the gigohm-seal voltage-clamp technique. Charybdotoxin was the only blocker studied which did not alter K+ current kinetics; it produced a simple block (K1 approximately 1 nM) which appeared to be independent of voltage or channel state (open, closed or inactivated). Tetraethylammonium slowed inactivation of K+ currents, consistent with the notion that blocked channels cannot inactivate. Verapamil and methoxyverapamil produced time-, voltage- and concentration-dependent "inactivation" or block of open channels during depolarizing pulses, with negligible block of closed channels at negative holding potentials. Capsaicin, chlorpromazine, phencyclidine, quinidine and tetrahydroaminoacridine both increased the rate of inactivation and decreased the peak K+ current. These characteristics suggest that both open and closed channels can be blocked, but that open channels are blocked preferentially. Nifedipine, like most other blockers, increased the rate of K+ current decay, but, unlike other blockers, resulted in two distinct kinetic components of current decay under some conditions. Because nifedipine is uncharged, the voltage and time-dependence of its block cannot be ascribed to a traditional ionic blockade mechanism. Mechanisms of K+ channel block are compared with block of Ca++ channels by calcium "antagonists" and block of Na+ channels by local anesthetics. Interactions between gating kinetics and K+ channel blockade seem to be the rule rather than the exception.

Selective inhibition of relaxation of guinea-pig trachea by charybdotoxin, a potent Ca(++)-activated K+ channel inhibitor

Airway smooth muscle plasma membranes are rich in K+ channels of various types. Charybdotoxin (ChTX) is a potent blocker of the high-conductance Ca(++)-activated K+ channel in smooth muscle and produces a concentration-dependent contraction of guinea pig trachea. In the present study, pharmacologic experiments were performed on carbachol-contracted (0.34 microM) guinea-pig trachea contracted further with ChTX in order to determine if Ca(++)-activated K+ channels play a role in the responses to cAMP-dependent and cAMP-independent bronchodilators. Relaxation concentration response curves to the beta-agonists, isoproterenol and salbutamol; the phosphodiesterase inhibitor, aminophylline; the cAMP mimic, N6-2'-O-adenosine 3':5'-cyclic monophosphate the guanylate cyclase activator, sodium nitroprusside; and the K+ channel agonists, BRL-34915 and pinacidil, were obtained in the absence and presence of ChTX. The concentration response curves to isoproterenol and salbutamol were shifted to the right (approximately 27-fold and greater than 40-fold, respectively) by 180 nM ChTX, whereas concentration response curves to N6-2'-O-adenosine 3':5'-cyclic monophosphate and aminophylline were affected significantly less (shifted approximately 7.5-fold). Concentration response curves to the cGMP-dependent relaxant sodium nitroprusside were also altered by ChTX (17-fold rightward shift at 180 nM). In the presence of 60 nM ChTX, the concentration response curves to the above relaxants were shifted only 3- to 5-fold. In contrast, ChTX (60 and 180 nM) failed to produce a significant rightward shift in the concentration response curves to BRL-34915 or pinacidil. Relaxation to BRL-34915 was however, blocked by glybenclamide, suggesting differences in the mechanism of relaxation. Contraction of tissues with depolarizing concentrations of KCl (20-80 mM) inhibited responses to all bronchodilators. These results suggest that hyperpolarization of tracheal smooth muscle as a result of opening various types of K+ channels can lead to relaxation of carbachol-contracted tracheal smooth muscle.

Alternative Shaker transcripts express either rapidly inactivating or noninactivating K+ channels

Two members of the Shaker K+ channel family designated ShA2 and ShD2 were characterized in the Xenopus oocyte expression system. The predicted amino acid sequences of ShA2 and ShD2 differ only in the amino terminus, which is located intracellularly according to the present topological model of K+ channels. The differing amino termini have profound effects on the electrophysiological and pharmacological properties of the K+ channel. Most markedly, the nature of the amino terminus determines whether the K+ channel mediates rapidly inactivating or noninactivating K+ currents. It also affects the 4-aminopyridine, tetraethylammonium, and charybdotoxin sensitivities of the K+ channels. These results suggest that the amino terminus of Shaker proteins affects K+ channel structures on both sides of the membrane.

Calcium-activated potassium channels in basolateral membranes of colon epithelial cells; reconstitution and functional properties

Using differential sedimentation, isopycnic and Ficoll-400 barrier centrifugation, basolateral membrane vesicles of surface and crypt cells of the rabbit distal colon were enriched 34- and 9-fold, respectively. 86Rb(+)-uptake into these vesicles, driven by an electrical potential difference, was stimulated by submicromolar Ca2+ activities and inhibited by Ba2+. These findings indicate the presence of Ca2(+)-activated K+ channels. The K+ channels in surface and crypt cell membranes differed with respect to inhibition by the bee venom apamin, the scorpion venom charybdotoxin and tetraethylammonium and exhibited a different pH dependence. Fusion of basolateral membrane vesicles with planar phospholipid bilayers revealed the presence of high-conductance Ba2(+)-sensitive K+ channels which were activated by micromolar Ca2+ and inhibited by crude scorpion venom and trifluoperazine. These K+ channels may be involved in the coupling of apical and basolateral membrane conductances during Na+ absorption and Cl- secretion, but they may also play a role in cell volume regulation.

Ca2(+)-activated K+ channels from rabbit kidney medullary thick ascending limb cells expressed in Xenopus oocytes

Ca2(+)-activated K+ channels are present in muscle, nerve, pancreas, macrophages, and renal cells. They are important in such diverse functions as neurotransmitter release, muscle excitability, pancreatic secretion, and cell volume regulation. Although much is known about the biophysics of Ca2(+)-activated K+ channels, the molecular structure, cDNA and amino acid sequences are unknown. We injected size-fractionated mRNA isolated from cultured rabbit kidney medullary thick ascending limb cells in Xenopus oocytes and observed newly expressed K+ currents using two-microelectrode voltage-clamp technique. The expressed K+ currents are Ca2+ dependent and inhibited by charybdotoxin, a specific blocker of Ca2(+)-activated K+ channels. Amplitudes of the current ranged from 30 nA to more than 1 microA at a membrane potential of +30 mV. Reversal potential of the current suggested a K(+)-selective channel. The peak activity of Ca2(+)-activated K+ channels were observed in fractions corresponding to a message RNA with size of approximately 4.5 kilobases.

Characterization of high affinity binding sites for charybdotoxin in synaptic plasma membranes from rat brain. Evidence for a direct association with an inactivating, voltage-dependent, potassium channel

Charybdotoxin (ChTX), a potent peptidyl inhibitor of several types of K+ channels, binds to sites in vascular smooth muscle sarcolemma (Vázquez, J., Feigenbaum, P., Katz, G. M., King, V. F., Reuben, J. P., Roy-Contancin, L., Slaughter, R. S., Kaczorowski, G. J., and Garcia, M. L. (1989) J. Biol. Chem. 265, 20902-20909) which are functionally associated with a high conductance Ca2(+)-activated K+ channel (PK,Ca). 125I-ChTX also binds specifically and reversibly to a single class of sites in plasma membranes prepared from rat brain synaptosomes. These sites exhibit a Kd of 25-30 pM, as measured by either equilibrium or kinetic binding protocols and display a maximum density of about 0.3-0.5 pmol/mg of protein. Competition studies with native ChTX yield a Ki of 8 pM for the noniodinated toxin. The highest density of ChTX sites exists in vesicle fractions of plasma membrane origin. Binding of 125I-ChTX is modulated by metal ions that interact with K+ channels: Ba2+, Ca2+, and Cs+ cause inhibition of ChTX binding; Na+ and K+ stimulate binding at low concentration before producing complete inhibition as their concentration is increased. Stimulation of binding is due to an allosteric interaction that decreases Kd whereas inhibition results from an ionic strength effect. Tetraethylammonium ion has no effect on binding, but tetrabutylammonium ion blocks binding with a Ki of 2.5 mM. Different toxins (i.e. alpha-dendrotoxin, noxiustoxin) that inhibit an inactivating, voltage-dependent K+ channel (PK,V) block 125I-ChTX binding in brain. In marked contrast, iberiotoxin, a selective inhibitor of PK,Ca, has no effect on ChTX binding in this preparation. Inhibition of ChTX binding by alpha-dendrotoxin and noxiustoxin results from an allosteric interaction between separate binding sites for these agents and the ChTX receptor. Taken together, these results suggest that the ChTX sites present in brain are associated with PK,V rather than with PK,Ca. Therefore, 125I-ChTX is a useful probe for elucidating the biochemical properties of a number of different types of K+ channels.

Rabbit distal colon epithelium: III. Ca2(+)-activated K+ channels in basolateral plasma membrane vesicles of surface and crypt cells

In the mammalian distal colon, the surface epithelium is responsible for electrolyte absorption, while the crypts are the site of secretion. This study examines the properties of electrical potential-driven 86Rb+ fluxes through K+ channels in basolateral membrane vesicles of surface and crypt cells of the rabbit distal colon epithelium. We show that Ba2(+)-sensitive, Ca2(+)-activated K+ channels are present in both surface and crypt cell derived vesicles with half-maximal activation at 5 x 10(-7) M free Ca2+. This suggests an important role of cytoplasmic Ca2+ in the regulation of the bidirectional ion fluxes in the colon epithelium. The properties of K+ channels in the surface cell membrane fraction differ from those of the channels in the crypt cell derived membranes. The peptide toxin apamin inhibits Ca2(+)-activated K+ channels exclusively in surface cell vesicles, while charybdotoxin inhibits predominantly in the crypt cell membrane fraction. Titrations with H+ and tetraethylammonium show that both high- and low-sensitive 86Rb+ flux components are present in surface cell vesicles, while the high-sensitive component is absent in the crypt cell membrane fraction. The Ba2(+)-sensitive, Ca2(+)-activated K+ channels can be solubilized in CHAPS and reconstituted into phospholipid vesicles. This is an essential step for further characterization of channel properties and for identification of the channel proteins in purification procedures.

The role of the divergent amino and carboxyl domains on the inactivation properties of potassium channels derived from the Shaker gene of Drosophila

Several products generated from the Drosophila Shaker gene by alternative splicing predict a group of similar proteins with an identical central and variable amino and carboxyl domains. We have constructed 9 Sh cDNAs combining 3 different 5' domains with 3 different 3' domains. RNA transcribed from 6 of these cDNAs induce K+ currents in Xenopus oocytes. All currents share similar properties of voltage dependence, potassium selectivity, and block by 4-AP, TEA, and charybdotoxin. These properties presumably result from a channel core formed by the identical central region of the proteins. The currents differ in macroscopic inactivation kinetics. Five RNAs induced K+ currents which inactivate, each at distinct rates, during short depolarizations. The sixth RNA induces a current that essentially does not inactivate unless depolarized for many seconds. This raises the possibility that Sh may encode nontransient as well as transient K+ currents. Analysis of currents produced by the various combinations suggests that the divergent amino domains influence the stability of a first, nonabsorbing, inactivated state. This results in striking differences in the probability of channel reopening, as observed in single-channel recordings, of those channels with identical carboxyl but different amino domains. Furthermore, based on macroscopic analysis of the currents, we suggest that the primary role of the carboxyl domains is to influence the relative stability between the first and a second inactivated state. The second inactivated state is essentially absorbing, and recovery from this state is very slow. The observed differences in the rates of recovery from inactivation of channels containing different carboxyl domains reflect differences in the rates at which they enter this second inactivated state.

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.

A simple assay for agonist-regulated Cl and K conductances in salt-secreting epithelial cells

We developed a convenient flux assay that permits simultaneous measurement of Cl and K conductance pathways in Cl-secreting epithelial cells. Monolayers of the colonic tumor cell line T84 were preloaded with 125I and 86Rb, and isotope effluxes were monitored by a sample-replace procedure. The adenosine 3',5'-cyclic monophosphate (cAMP)-mediated agonists forskolin and prostaglandin E2 increased I efflux with little effect on Rb efflux, whereas the Ca-mediated agonists ionomycin, A23187, and carbachol increased both I and Rb effluxes. Simultaneous determinations of I and Cl or Rb and K effluxes indicated that I and Rb provide good measures of the effluxes of Cl and K, respectively. Forskolin- and ionomycin-stimulated I effluxes were inhibited by the Cl-channel blockers diphenylamine-2-dicarboxylate (DPC), 5-nitro-2-(3-phenylpropyl-amino)benzoic acid (NPPB), and 2-[cyclopentyl-6,7-dichloro-2,3-dihydro-2-methyl-1-oxo-1H- inden-5-yl)oxy]acetic acid (IAA-94) and by high external K. The Rb efflux evoked by ionomycin was inhibited by the K-channel blockers Ba and charybdotoxin. These findings suggest that I and Rb effluxes provide qualitative estimates of agonist-stimulated Cl and K conductance pathways. Thus this method can provide a simple and relatively inexpensive screening assay for Cl and K conductances in cultured cells to assess the effects of agonist, blockers, or genetic manipulations.

Evidence for two calcium-dependent potassium conductances in lizard motor nerve terminals

Action potentials and afterpotentials were recorded with a microelectrode inserted into lizard motor axons within a few millimeters of their motor terminals. In the presence of 1 mM 4-aminopyridine (4-AP), the duration of the action potential recorded near motor terminals was Ca-sensitive: repolarization was more rapid when bath [Ca] was elevated, and became slower when bath [Ca] was removed or when 0.1-1 mM Mn was added. Repolarization was also slowed following addition of 3-10 nM charybdotoxin or 100-300 microM tetraethylammonium (TEA) to the bath, and following intra-axonal injection of the Ca buffer BAPTA. These results, in agreement with published extracellular recordings, indicate that the motor nerve terminal membrane contains rapidly activating, Ca-activated K channels. When these (and other) K channels were blocked by 10 mM TEA, the action potential recorded near motor terminals was followed by Ca-dependent depolarizing afterpotentials, followed in turn by a slow hyperpolarizing afterpotential (h.a.p.) that lasted several seconds. This slow h.a.p. was also Ca-sensitive: it became larger with increasing bath [Ca] and was abolished by removal of bath [Ca] and by addition of 1 mM Mn. Intra-axonal injection of BAPTA reduced the amplitude of the slow h.a.p., and prolonged injections promoted repetitive discharge. The slow h.a.p. following single action potentials was observed in 100 microM ouabain and in K-free solutions and thus is pharmacologically distinct from the hyperpolarization that follows tetanic stimulation. The slow h.a.p. was selectively inhibited by 100 nM apamin, but persisted in 100 nM charybdotoxin. This afterpotential was enhanced by 0.1-1 mM 4-AP and by the dihydropyridine Bay K 8644 (0.1-1 microM). These results suggest that the slow h.a.p. in lizard motor nerve terminals is mediated by Ca-activated K channels that can be activated near the resting potential and are pharmacologically distinct from the Ca-activated K channels that contribute to action potential repolarization. The slow h.a.p. was enhanced by 0.1-1 mM caffeine and inhibited by 100 microM procaine, raising the possibility that this afterpotential may be activated not only by Ca entering via the plasma membrane, but also by Ca released from intra-terminal stores.

Solution synthesis of charybdotoxin (ChTX), a K+ channel blocker

Charybdotoxin, a 37 amino acid peptide which is a minor component of Leiurus quinquestriatus venom, was synthesized by the solution procedure applying our maximum protection strategy. After formation of the three disulfide bonds, for which a redox buffer was necessary, the final product was purified to homogeneity and found to have similar biological potency to that reported by others for the natural product. The disulfide bond configuration was found to be: Cys7-Cys28; Cys13-Cys33; Cys17-Cys35. Conformational analysis by 1H-NMR showed that the molecule exists as a very tightly folded structure, in which residues 1-7 and 24-37 form a triple-stranded beta-sheet, with a turn at positions 30-31. The region from 11-20 appears to adopt an alpha-helical conformation.

Purification and characterization of a unique, potent, peptidyl probe for the high conductance calcium-activated potassium channel from venom of the scorpion Buthus tamulus

An inhibitor of the high conductance, Ca2(+)-activated K+ channel (PK,Ca) has been purified to homogeneity from venom of the scorpion Buthus tamulus by a combination of ion exchange and reversed-phase chromatography. This peptide, which has been named iberiotoxin (IbTX), is one of two minor components of the crude venom which blocks PK,Ca. IbTX consists of a single 4.3-kDa polypeptide chain, as determined by polyacrylamide gel electrophoresis, analysis of amino acid composition, and Edman degradation. Its complete amino acid sequence has been defined. IbTX displays 68% sequence homology with charybdotoxin (ChTX), another scorpion-derived peptidyl inhibitor of PK,Ca, and, like this latter toxin, its amino terminus contains a pyroglutamic acid residue. However, IbTX possesses 4 more acidic and 1 less basic amino acid residue than does ChTX, making this toxin much less positively charged than the other peptide. In single channel recordings, IbTX reversibly blocks PK,Ca in excised membrane patches from bovine aortic smooth muscle. It acts exclusively at the outer face of the channel and functions with an IC50 of about 250 pM. Block of channel activity appears distinct from that of ChTX since IbTX decreases both the probability of channel opening as well as the channel mean open time. IbTX is a selective inhibitor of PK,Ca; it does not block other types of voltage-dependent ion channels, especially other types of K+ channels that are sensitive to inhibition by ChTX. IbTX is a partial inhibitor of 125I-ChTX binding in bovine aortic sarcolemmal membrane vesicles (Ki = 250 pM). The maximal extent of inhibition that occurs is modulated by K+, decreasing as K+ concentration is raised, but K+ does not affect the absolute inhibitory potency of IbTX. A Scatchard analysis indicates that IbTX functions as a noncompetitive inhibitor of ChTX binding. Taken together, these data suggest that IbTX interacts at a distinct site on the channel and modulates ChTX binding by an allosteric mechanism. Therefore, IbTX defines a new class of peptidyl inhibitor of PK,Ca with unique properties that make it useful for investigating the characteristics of this channel in target tissues.

Diffusion-controlled binding of a peptide neurotoxin to its K+ channel receptor

Single Ca2(+)-activated K+ channels were reconstituted into planar lipid bilayer membranes, and the effect of charybdotoxin, a pore-blocking peptide from scorpion venom, was studied. In particular, the effect of solution viscosity on the kinetics of block was assessed in order to test the idea that toxin binding is diffusion-controlled. This idea is supported by the strictly inverse relation between solution viscosity and the rate constants of both association and dissociation of peptide with the K+ channel mouth. However, at an ionic strength high enough to suppress local electrostatic potentials, the diffusion-controlled on-rate constant is surprisingly low, 10(5) M-1 s-1. These slow, viscosity-dependent kinetics may be understood if charybdotoxin can attain the bound state only from a rare set of encounters with the K+ channel.

Basolateral K+ channels in airway epithelia. I. Regulation by Ca2+ and block by charybdotoxin

In airway epithelia, adenosine 3',5'-cyclic monophosphate (cAMP) stimulates Cl- secretion by activating apical membrane Cl- channels and basolateral membrane K+ channels. Cl- channels are regulated by cAMP-dependent phosphorylation, whereas K+ channels are regulated by the cytosolic Ca2+ concentration, [Ca2+]c. Our recent observation that cAMP increases [Ca2+]c suggested that cAMP might indirectly regulate K+ channels by increasing [Ca2+]c. To study regulation of K+ channels we measured 86Rb efflux, single K+ channels in membrane patches, and [Ca2+]c with the fluorescent indicator fura-2. Isoproterenol and Ca2+ ionophore, A23187, transiently increased [Ca2+]c and transiently stimulated 86Rb efflux. Stimulation of 86Rb efflux resulted from release of intracellular Ca2+ stores. 86Rb efflux was blocked by Ba2+ or charybdotoxin, but not by tetraethylammonium. Charybdotoxin prevented all of the 86Rb efflux that was stimulated by A23187 or by forskolin. Charybdotoxin also blocked the low-conductance inwardly rectifying K+ channel (KCLIC) in membrane patches. These results indicate that the KCLIC channel is responsible for the Ca2(+)-dependent increase in K+ permeability in airway epithelial cells. They also indicate that cAMP-induced release of intracellular Ca2+ is sufficient to activate K+ channels.

Basolateral K+ channels in airway epithelia. II. Role in Cl- secretion and evidence for two types of K+ channel

We previously described a Ca2(+)-activated K+ channel (KCLIC) in airway epithelial cells [J. D. McCann, J. Matsuda, M. Garcia, G. Kaczorowski, and M. J. Welsh. Am. J. Physiol 258 (Lung Cell. Mol. Physiol. 2): L334-L342, 1990]. To determine whether the KCLIC channel is a basolateral membrane channel and to understand its role in Cl- secretion, we studied airway epithelial cells grown on permeable supports. When cells were stimulated with A23187, charybdotoxin (ChTX) inhibited Cl- secretion and 86Rb efflux at the same concentrations, indicating that the KCLIC channel is required for Ca2(+)-stimulated Cl- secretion. We also investigated the function of K+ channels in adenosine 3',5'-cyclic monophosphate-stimulated secretion. Addition of isoproterenol caused a biphasic increase in Cl- secretion; the time course of the transient component correlated with the time course of the isoproterenol-induced increase in Ca2+ concentration [( Ca2+]c). ChTX inhibited the transient component, but not the prolonged component of secretion; Ba2+ inhibited the sustained component. These results suggest that when cells are grown on permeable supports isoproterenol-induced secretion depends on activation of two types of K+ channel: the KCLIC channel that is stimulated initially and a ChTX-insensitive K+ channel that is stimulated during sustained secretion. This conclusion was supported by measurement of 86Rb efflux from cell monolayers.

Bradykinin stimulates airway epithelial Cl- secretion via two second messenger pathways

In canine airway epithelial cells, bradykinin increases intracellular concentrations of D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], cytosolic calcium concentration ([Ca2+]c), and adenosine 3',5'-cyclic monophosphate (cAMP). To determine the role of these second messengers in bradykinin-stimulated Cl- secretion, we studied the secretory response to this peptide using canine tracheal monolayers mounted in Ussing chambers. Bradykinin stimulated Cl- secretion [measured as short-circuit current (Isc)] when added to submucosal or mucosal surfaces; however, secretory responses differed substantially. Submucosal addition of bradykinin induced a biphasic increase in secretion; mucosal addition induced a monophasic increase in secretion. Both responses were mediated by B2 receptors. We show that activation of bradykinin receptors can stimulate Cl- secretion in two ways. 1) Bradykinin added to either surface stimulates prostaglandin synthesis and release at the basolateral surface. This leads to activation of prostaglandin E2-sensitive receptors on the basolateral surface that are coupled to cAMP production and an increase in apical membrane Cl- conductance. 2) In addition, bradykinin added to the submucosal surface increases Ins(1,4,5)P3 and [Ca2+]c levels, which enhance basolateral K+ conductance and the electrical driving force for apical Cl- exit. Whereas secretion requires activation of apical Cl- channels, the data show that Cl- secretion can also be modulated by activation of basolateral K+ channels. These data indicate that bradykinin-induced transepithelial Cl- secretion is mediated by two independent, second messenger pathways. These results provide the first evidence for expression of both pathways in a polar fashion in an epithelial monolayer.

Tetraethylammonium blockade of apamin-sensitive and insensitive Ca2(+)-activated K+ channels in a pituitary cell line

1. The pharmacological sensitivities and physiological contributions of two types of Ca2(+)-activated K+ channels (BK and SK) in GH3 cells were examined by the outside-out, whole-cell and cell-attached modes of the patch-clamp technique. 2. BK channels (250-300 pS in symmetrical 150 mM-K+) in outside-out patches were blocked by external tetraethylammonium (TEA) and by 50 nM-charybdotoxin (CTX), but were not blocked by apamin. 3. SK channels (9-14 pS in symmetrical 150 mM-K+) in outside-out patches were blocked by external TEA and by apamin, but were not blocked by 50 nM-CTX. 4. The dissociation constant (Kd) for TEA block of SK channels (3.1 +/- 0.37 mM) was 12-fold greater than the Kd for the BK channels (260 +/- 21 microM). The TEA blockade of both channels was not strongly voltage dependent: for both channels the TEA binding site sensed less than 20% of the membrane electric field. 5. Application of blockers of the BK channels (1 mM-TEA and 50 nM-CTX) to whole cells under current clamp prolonged action potential duration; whereas application of apamin, a selective blocker of the SK channel, inhibited a slowly decaying after-hyperpolarization and had little effect on action potential duration. Apamin also increased the firing rate in 30% of the spontaneously pacing cells. 6. It is suggested that BK channels contribute to action potential repolarization: whereas SK channels contribute to the regulation of action potential firing rate.

Ion channel expression by white matter glia: the O-2A glial progenitor cell

We describe electrophysiological properties of the O-2A glial progenitor cell in a new serum-free culture system. O-2A progenitors have many properties characteristic of neurons: they have glutamate-activated ion channels, express the neuronal form of the sodium channel, fire single regenerative potentials, and synthesize the neurotransmitter GABA by an alternative synthetic pathway. Nearly identical properties were observed in acutely isolated O-2A progenitors, indicating that this phenotype is not an artifact of culture. The O-2A did not express a simple subset of channel types found in its descendant cells, the type-2 astrocyte and oligodendrocyte, studied in the same culture system. During development, these electrophysiological properties may contribute to O-2A function in vivo.

Effect of lowered extracellular pH on Ca2(+)-dependent K+ currents in type I cells from the neonatal rat carotid body

1. The whole-cell configuration of the patch-clamp technique was used to record K+ currents from type I cells enzymatically dispersed from the neonatal rat carotid body. The current-voltage (I-V) relationship for the K+ currents showed a prominent, outward shoulder at test potentials of between +10 and +30 mV. 2. The shoulder of the I-V curve could be enhanced by raising extracellular Ca2+ concentration or by bath application of 5 microM-Bay K 8644. It could also be suppressed by bath application of 100 microM-Cd2+ or 5 microM-methoxyverapamil (D600), indicating that a large component of the K+ current in these cells was activated by an influx of Ca2+ through its own channels during cell depolarization. 3. Potassium currents were also reversibly suppressed by 8 nM-charybdotoxin but unaffected by 100 nM-apamin, suggesting that the Ca2(+)-dependent K+ current was carried through large or intermediate conductance Ca2(+)-activated K+ channels. 4. Lowering the pH of the bathing medium from 7.40 to 7.00 reversibly reduced the K+ current amplitudes, and suppressed the shoulder normally seen in the I-V relationship. This effect was enhanced in the presence of 5 microM-Bay K 8644 and abolished in the presence of 5 microM-D600. 5. It is concluded that the Ca2(+)-dependent K+ channels of type I carotid body cells are selectively suppressed by extracellular acidity. Possible mechanisms underlying this effect, and its role in excitation of the carotid body are discussed.

Calcium-independent cell volume regulation in human lymphocytes. Inhibition by charybdotoxin

The properties of the K+ pathway underlying regulatory volume decrease (RVD) in human blood lymphocytes were investigated. Evidence is presented for the existence of three types of K+ conductance in these cells. Ionomycin, a Ca2+ ionophore, induced a K(+)-dependent hyperpolarization, indicating the presence of Ca2(+)-activated K+ channels, which were blocked by charybdotoxin (CTX). CTX also induced a depolarization of the resting membrane potential, even at subphysiological cytosolic [Ca2+]([Ca2+]i), which suggests the existence of a second CTX-sensitive, but Ca2(+)-independent conductance. A CTX-resistant K+ conductance was also detected. RVD in blood lymphocytes was partially (approximately 75%) blocked by CTX. However, volume regulation was not accompanied by detectable changes in [Ca2+]i, nor was it prevented by removal of extracellular Ca2+ and depletion or buffering of intracellular Ca2+. These observations suggest that K+ loss during RVD is mediated by Ca2(+)-independent, CTX-sensitive channels or that Ca2(+)-dependent channels can be activated by cell swelling at normal or subnormal [Ca2+]i. The former interpretation is supported by findings in rat thymic lymphocytes. These cells also displayed a CTX-sensitive Ca2(+)-dependent hyperpolarization. However, CTX did not significantly alter the resting potential, suggesting the absence of functional Ca2(+)-independent, toxin-sensitive channels. Volume regulation in thymic lymphocytes was less efficient than in human blood cells. In contrast to blood lymphocytes, RVD in thymocytes was not affected by CTX. These observations indicate that, though present in lymphocytes, Ca2(+)-activated K+ channels do not play an important role in volume regulation. Instead, RVD seems to be mediated by Ca2(+)-independent K+ channels. We propose that two types of channels, one CTX sensitive and the other CTX insensitive, mediate RVD in human blood lymphocytes, whereas only the latter type is involved in rat thymocytes.

Characterization of high affinity binding sites for charybdotoxin in sarcolemmal membranes from bovine aortic smooth muscle. Evidence for a direct association with the high conductance calcium-activated potassium channel

Charybdotoxin (ChTX), a peptidyl inhibitor of the high conductance Ca2+-activated K+ channel (PK,Ca), has been radiolabeled to high specific activity with 125I, and resulting derivatives have been well separated. The monoiodotyrosine adduct blocks PK,Ca in vascular smooth muscle with slightly reduced potency compared with the native peptide under defined experimental conditions. [125I]ChTX, representing this derivative, binds specifically and reversibly to a single class of sites in sarcolemmal membrane vesicles prepared from bovine aortic smooth muscle. These sites display a Kd of 100 pM for the iodinated toxin, as determined by either equilibrium or kinetic binding analyses. Binding site density is about 500 fmol/mg of protein in isolated membranes. The addition of low digitonin concentrations to disrupt the vesicle permeability barrier increases the maximum receptor concentration to 1.5 pmol/mg of protein, correlating with the observations that ChTX binds only at the external pore of PK,Ca and that the membrane preparation is of mixed polarity. Competition studies with ChTX yield a Ki of about 20 pM for native toxin. Binding of [125I]ChTX is modulated by ionic strength as well as by metal ions that are known to interact with PK,Ca. Moreover, tetraethylammonium ion, which blocks PK,Ca with moderately high affinity when applied at the external membrane surface, inhibits [125I]ChTX binding in an apparently competitive fashion with a Ki similar to that found for channel inhibition. In marked contrast, agents that do not inhibit PK,Ca in smooth muscle (e.g. tetrabutylammonium ion, other toxins homologous with ChTX, and pharmacological agents that modulate the activity of dissimilar ion channels) have no effect on [125I]ChTX binding in this tissue. Taken together, these results suggest that the binding sites for ChTX which are present in vascular smooth muscle are directly associated with PK,Ca, thus identifying [125I]ChTX as a useful probe for elucidating the biochemical properties of these channels.

Charybdotoxin is a new member of the K+ channel toxin family that includes dendrotoxin I and mast cell degranulating peptide

A polypeptide was identified in the venom of the scorpion Leiurus quinquestriatus hebraeus by its potency to inhibit the high-affinity binding of the radiolabeled snake venom toxin dendrotoxin I (125I-DTX1) to its receptor site. It has been purified, and its properties investigated by different techniques were found to be similar to those of MCD and DTXI, two polypeptide toxins active on a voltage-dependent K+ channel. However, its amino acid sequence was determined, and it was shown that this toxin is in fact charybdotoxin (ChTX), a toxin classically used as a specific tool to block one class of Ca2+-activated K+ channels. ChTX, DTXI, and MCD are potent convulsants and are highly toxic when injected intracerebroventricularly in mice. Their toxicities correlate well with their affinities for their receptors in rat brain. These three structurally different toxins release [3H]GABA from preloaded synaptosomes, the efficiency order being DTXI greater than ChTX greater than MCD. Both binding and cross-linking experiments of ChTX to rat brain membranes and to the purified MCD/DTXI binding protein have shown that the alpha-subunit (Mr = 76K-78K) of the MCD/DTXI-sensitive K+ channel protein also contains the ChTX binding sites. Binding sites for DTXI, MCD, and ChTX are in negative allosteric interaction. Our results show that charybdotoxin belongs to the family of toxins which already includes the dendrotoxins and MCD, which are blockers of voltage-sensitive K+ channels. ChTX is clearly not selective for Ca2+-activated K+ channel.

Charybdotoxin inhibits proliferation and interleukin 2 production in human peripheral blood lymphocytes

We demonstrate that blockade of the lymphocyte voltage-gated K+ channel by charybdotoxin (CTX) inhibits lymphocyte mitogenesis. Charybdotoxin blocks conductance with a Ki of 0.3 nM and inhibits mitogen- and antigen-stimulated proliferation with a Ki of 0.5 nM. As opposed to the other blockers of the lymphocyte K+ channel, the inhibition of mitogenesis by CTX can be overcome selectively by exogenous recombinant interleukin 2 (IL-2); endogenous levels of IL-2 in the culture supernatants of stimulated cells are decreased by the presence of CTX. Our results suggest that the voltage-gated K+ channel is either directly or indirectly involved in IL-2 synthesis and/or secretion.

Properties of ShB A-type potassium channels expressed in Shaker mutant Drosophila by germline transformation

We have used P element-mediated germline transformation to express ShB channels in Shaker mutant Drosophila and have examined their properties by patch-clamp of embryonic myotubes. The transformed ShB cDNA was placed under the transcriptional control of a heat shock promoter (hsp70). Northern blots revealed that transformed DNA is efficiently transcribed in response to heat shock. Cultured myotubes from the transformants produced large A-type potassium currents in response to heat shock. Although qualitatively similar to native Shaker A-currents in wild-type myotubes, transformant A-current inactivates more rapidly and recovers from inactivation more rapidly, similar to ShB channels expressed in Xenopus oocytes. Unlike the channels in oocytes, however, the transformant A-current is insensitive to 50 nM charybdotoxin.

Functional modification of a Ca2+-activated K+ channel by trimethyloxonium

Single Ca2+-activated K+ channels from rat skeletal muscle plasma membranes were studied in neutral phospholipid bilayers. Channels were chemically modified by briefly exposing the external side to the carboxyl group modifying reagent trimethyloxonium (TMO). TMO modification, in a "multi-hit" fashion, reduces the single-channel conductance without affecting ion selectivity. Modification also shifts the voltage activation curve toward more depolarized voltages and reduces the affinity of the channel blocker charybdotoxin (CTX). CTX, bound to the channel during the TMO exposure, prevents the TMO-induced reduction of the single-channel conductance. These data suggest that the high-conductance Ca2+-activated K+ channel has carboxyl groups on its external surface. These groups influence ion conduction, gating, and the binding of CTX.

Expression of Drosophila Shaker potassium channels in mammalian cells infected with recombinant vaccinia virus

A recombinant vaccinia virus containing a Drosophila potassium channel (Shaker H4) cDNA was constructed by homologous recombination between wild-type vaccinia virus DNA and a transfer plasmid. The new virus was used to infect four types of mammalian cells in culture. Electrophysiological recording 24-72 hr after infection revealed the expression of voltage-gated transient potassium channels in all four cell types. The properties of the induced currents were identical to those previously observed following injection of the Shaker H4 transcript into oocytes. Vaccinia promises to be an effective vehicle for the heterologous expression of transmembrane ion channels in a variety of cell types.

Mutant potassium channels with altered binding of charybdotoxin, a pore-blocking peptide inhibitor

The inhibition by charybdotoxin of A-type potassium channels expressed in Xenopus oocytes was studied for several splicing variants of the Drosophila Shaker gene and for several site-directed mutants of this channel. Charybdotoxin blocking affinity is lowered by a factor of 3.5 upon replacing glutamate-422 with glutamine, and by a factor of about 12 upon substituting lysine in this position. Replacement of glutamate-422 by aspartate had no effect on toxin affinity. Thus, the glutamate residue at position 422 of this potassium channel is near or in the externally facing mouth of the potassium conduction pathway, and the positively charged toxin is electrostatically focused toward its blocking site by the negative potential set up by glutamate-422.

Dendrotoxin and charybdotoxin increase the cytosolic concentration of free Ca2+ in cerebrocortical synaptosomes: an effect not shared by apamin

Nanomolar concentrations of charybdotoxin or dendrotoxin increase the cytoplasmic free Ca2+ concentration in isolated central nerve terminals. The effects of the two toxins, normally considered to be blockers of K+ channels controlled by voltage in a Ca2+-sensitive or -insensitive manner, respectively, show only marginal additivity. Apamin, and inhibitor of low conductance Ca2+-activated K+ channels, was without effect in either the absence or presence of dendrotoxin. The effect of charybdotoxin on polarized, isolated central nerve terminals seems to be mediated largely by a block of K+ channels sensitive to dendrotoxin. Apparently, these voltage-operated K+ channels make a more significant contribution to maintaining the polarized potential of synaptosomes than do those activated by Ca2+.

Interactions between dendrotoxin, a blocker of voltage-dependent potassium channels, and charybdotoxin, a blocker of calcium-activated potassium channels, at binding sites on neuronal membranes

Dendrotoxin I (DpI) from black mamba venom (Dendroaspis polylepis) has high affinity binding sites on rat brain synaptic membranes. Native DpI displaced [125I]-DpI binding with a Ki of 1 x 10(-10) M, and over 90% of specific binding was displaceable. Charybdotoxin isolated from the Israeli scorpion venom (Leiurus quinquestriatus hebraeus), also displaced [125I]-DpI binding, with a Ki of approximately 3 x 10(-9) M, although the displacement curve was shallower than with native DpI. Both toxins are thought to be high affinity blockers of specific K+ currents. Charybdotoxin selectively blocks some types of Ca2+-activated K+ channels, whereas dendrotoxins only block certain voltage-dependent K+ channels. The interaction between the two types of toxin at the DpI binding site is unexpected and may suggest the presence of related binding sites on different K+ channel proteins.

Ca2+ induces charybdotoxin-sensitive membrane potential changes in rat lymphocytes

There is disagreement regarding the existence of Ca2+-activated K+ channels in lymphocytes. Depolarization, hyperpolarization, or little change in membrane potential (Em) has been reported following elevation of free cytosolic Ca2+ concentration ([Ca2+]i). Patch-clamping studies have demonstrated inhibition of voltage-gated K+ channels, but Ca2+-activated K+ channels have not been detected. We used charybdotoxin (CTX), a potent inhibitor of Ca2+-activated K+ channels, to assess their presence in rat thymic lymphocytes. Fluorescent probes were used to measure Em and [Ca2+]i in cell suspensions treated with ionomycin. At basal [Ca2+]i, CTX had no effect on Em, suggesting that Ca2+-activated K+ channels do not contribute importantly to the resting potential. Elevation of [Ca2+]i in the submicromolar range induced a hyperpolarization that was dependent on the outward K+ gradient. The shape and duration of the Em change closely followed the elevation of [Ca2+]i. This hyperpolarization was inhibited by nanomolar concentrations of CTX. When [Ca2+]i approached or exceeded 1 microM, a biphasic Em change was recorded. A transient, CTX-sensitive hyperpolarization was followed by a sustained depolarization. The latter was greatly reduced when external Na+ was omitted. The data suggest that thymic lymphocytes possess Ca2+-sensitive K+ channels, which are activated by moderate increases in [Ca2+]i, resulting in hyperpolarization. At higher [Ca2+]i, the effect of K+ channels on Em is superseded by opening of nonselective cation channels, producing depolarization. Variations in the level of [Ca2+]i attained in earlier studies can explain existing discrepancies.

Analysis of the blocking activity of charybdotoxin homologs and iodinated derivatives against Ca2+-activated K+ channels

Two charybdotoxin peptides were purified from venom of the Israeli scorpion, Leiurus quinquestriatus hebraeus. Microsequencing of the most abundant toxin, ChTX-Lq1, revealed identity with the 37-residue peptide previously sequenced by Gimenez-Gallego et al. [Gimenez-Gallego, G., et al., Proc. Natl. Acad. Sci. USA 85:3329-3333 (1988)]. Sequence data on the minor peptide, ChTX-Lq2, showed substantial homology to ChTX-Lq1 with differences observed at eight positions. These two charybdotoxin sequences, along with that of noxiustoxin, define a distinct family of scorpion peptide toxins with activity against K+ channels. Both charybdotoxin homologs inhibited Ca2+-dependent K+ efflux from human erythrocytes with similar potency, K0.5 approximately 40 nM. In planar bilayer assays of single K(Ca) channels from rat muscle, ChTX-Lq1 and ChTX-Lq2 blocked with intrinsic Kd's of 1.3 and 43 nM, respectively, in the presence of 50 mM external KCl. A new application of dwell-time histogram analysis of single-channel blocking events was used to characterize the kinetic homogeneity of toxin samples and the blocking kinetics of ChTX derivatives. The lower blocking affinity of ChTX-Lq2 was the combined result of a faster dissociation rate and a slower association rate as compared to ChTX-Lq1. The blocking activity of two mono-iodinated derivatives of ChTX-Lq1 was also analyzed. Blocked dwell-time histograms of the iodinated peptides were characterized by predominately brief (0.2-2 sec) blocking events in comparison to the native toxin (20 sec). Histogram analysis revealed that mono-iodination of ChTX-Lq1 impairs blocking activity by adverse effects on both dissociation and association rate constants. Frequency density histograms of single channel blocking events provide a sensitive assay of toxin purity suitable for quantitating structure-activity relationships of charybdotoxin derivatives.

Charybdotoxin blocks both Ca-activated K channels and Ca-independent voltage-gated K channels in rat brain synaptosomes

Charybdotoxin (ChTX), a 4.3 kDa polypeptide toxin from the venom of the scorpion Leiurus quinquestriatus, blocks both a Ca-activated K channel (IC50 approximately 15 nM) and a Ca-independent voltage-gated K channel (IC50 approximately 40 nM) in rat brain synaptosomes. These results indicate that in this preparation ChTX is not specific for the Ca-activated K channel and suggest that there may be structural homology among the toxin-binding sites on various types of K channels.

Charybdotoxin blocks dendrotoxin-sensitive voltage-activated K+ channels

Charybdotoxin, a short scorpion venom neurotoxin, which was thought to be specific for the blockade of Ca2+-activated K+ channels also blocks a class of voltage-sensitive K+ channels that are known to be the target of other peptide neurotoxins from snake and bee venoms such as dendrotoxin and MCD peptide. Charybdotoxin also inhibits 125I-dendrotoxin and 125I-MCD peptide binding to their receptors. All these effects are observed with an IC50 of about 30 nM.