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

Maxi-K+ channel in single isolated cochlear efferent nerve terminals

Patch clamp recordings were obtained from isolated cochlear efferent nerve terminals. Channel activity was found in 85% of membrane patches, was present in on-terminal and excised patches and was characterized to originate from a maxi-K+ channel. An average of 2.0 +/- 0.1 (N = 33) maxi-K+ channels were found per active patch. In symmetrical solutions, the current-voltage relationship was linear and the single-channel conductance was 221 +/- 5 pS (N = 22). The open probability of the maxi-K+ channel increased with depolarization of the membrane potential and with an increasing free Ca2+ concentration on the cytosolic side. The open probability was insensitive to changes in the free Ca2+ concentration on the extracellular side. TEA (20 mM) and charybdotoxin (10(-7) M) decreased the open probability to nearly zero from the extracellular side but had no effect from the cytosolic side. The high incidence with which this channel was found suggests that the maxi-K+ channel is physiologically relevant which might include protection against overstimulation of the efferent synapse.

Synthetic charybdotoxin-iberiotoxin chimeric peptides define toxin binding sites on calcium-activated and voltage-dependent potassium channels

Charybdotoxin (ChTX) and iberiotoxin (IbTX) are highly charged peptidyl toxins which exhibit 68% sequence identity and share a similar three-dimensional structure. Despite these structural similarities, IbTX and ChTX differ in their selectivity for two types of potassium channels; large conductance calcium-activated potassium (maxi-K) channels and slowly inactivating voltage-gated (Kv1.3) potassium channels. ChTX blocks with high affinity both maxi-K and Kv1.3 channels, while IbTX blocks the maxi-K but not the voltage-gated channel. To identify regions of the toxins which impart this this selectivity, we have constructed by solid-phase synthesis two chimeric toxins, ChTX1-19IbTX20-37 (Ch-IbTX) and IbTX1-19ChTX20-37 (Ib-ChTX), as well as a truncated peptide, ChTX7-37. These peptides were assayed for their ability to inhibit [125I]ChTX binding in sarcolemmal vesicles from smooth muscle (maxi-K binding) and [125I]ChTX binding to plasma membranes from brain (Kv1.3 binding). The ability of the peptides to block the maxi-K channel was determined from recordings of single maxi-K channels incorporated into planar lipid bilayers. Block of Kv1.3 was determined from recordings of whole cell currents in Xenopus oocytes injected with mRNA encoding the cloned Kv1.3 channel. Both chimeric toxins inhibited [125I]ChTX binding to sarcolemmal membranes from smooth muscle, and they both blocked the maxi-K channel in planar lipid bilayers. In contrast, [125I]ChTX binding in brain and Kv1.3 currents expressed in oocytes were inhibited only by the chimera Ib-ChTX.(ABSTRACT TRUNCATED AT 250 WORDS)

Different effects of the K+ channel blockers 4-aminopyridine and charybdotoxin on sensory nerves in guinea-pig lung

In the isolated guinea-pig bronchus, the potassium channel blocking agent 4-aminopyridine (10(-4) M) caused a contraction which was abolished by capsaicin tachyphylaxis, suggesting involvement of sensory neuropeptides. Charybdotoxin (10(-8), 5 x 10(-8) M), which is a potent blocker of the high-conductance Ca(2+)-activated K+ channel in smooth muscle, caused slowly developing and long lasting bronchoconstriction, which was resistant to capsaicin tachyphylaxis. Neither 4-aminopyridine (10(-3), 10(-4) M) nor charybdotoxin (10(-8), 5 x 10(-8) M) had any significant effect on the bronchoconstriction induced by electrical field stimulation. Furthermore, charybdotoxin had no significant influence on the inhibitory effect of the alpha 2-adrenoceptor agonist SKF 35886 (5 x 10(-7) M) on the bronchoconstriction induced by electrical field stimulation. In the isolated perfused guinea-pig lung, 4-aminopyridine (3 x 10(-5) -10(-3) M) caused bronchoconstriction and enhanced both basal and (at 3 x 10(-5) M) vagal nerve stimulation-evoked calcitonin gene-related peptide outflow from pulmonary sensory nerves. In conclusion, 4-aminopyridine stimulated capsaicin-sensitive sensory neurons and enhanced the sensory activation induced by vagal nerve stimulation in guinea-pig lung. Charybdotoxin, on the other hand, caused bronchial contraction independently of capsaicin-sensitive nerves.

Characterization of large-conductance, calcium-activated potassium channels from human myometrium

OBJECTIVES

The purpose of our study was to detect and characterize potassium channels in the plasma membrane of smooth muscle cells from human myometrium.

STUDY DESIGN

Plasma membrane vesicles were incorporated into lipid bilayers to record single potassium channel activity.

RESULTS

We predominantly found a "maxi" calcium-activated potassium channel (261 picosiemens). This channel was calcium (micromoles per liter range) and voltage sensitive, highly selective for K+ over Na+ and Cs+, and was sensitive to external tetraethylammonium (dissociation constant approximately 220 mumol/L) and charybdotoxin (dissociation constant approximately 23 nmol/L). External apamin and 4-aminopyridine had no effect on this channel. Another type of potassium channel that was less frequently observed was also identified. It had a smaller conductance (142 picosiemens) and it seemed to be calcium independent (up to 50 nmol/L).

CONCLUSION

Human myometrium possesses abundant "maxi" calcium-activated potassium channels. This channel shares common characteristics with other "maxi" calcium-activated potassium channels, including calcium and voltage gating, high conductance and selectivity, and channel pharmacologic profile.

Comparison of the distribution of binding sites for the potassium channel ligands [125I]apamin, [125I]charybdotoxin and [125I]iodoglyburide in the rat brain

Potassium channels represent a diverse and promising target for drug development. Pharmacological subtypes of K channels have begun to emerge based on the development of both organic molecules and peptide toxins which possess subtype selectivity. In order to evaluate the neuroanatomical distribution of these subtypes we have utilized the ligands [125I]apamin, [125I]charybdotoxin and [125I]iodoglyburide in an autoradiographic study of rat brain. In the rat brain, these ligands have selectivity for the low conductance Ca(2+)-activated, voltage-gated K channels and ATP-sensitive K channels respectively. The distribution of binding sites for these three ligands were distinctly different. [125I]Apamin binding was highest in various thalamic and hippocampal structures, while only low to moderate levels of [125I]charybdotoxin binding were seen in these regions. In contrast, very high levels of [125I]charbydotoxin were seen in white matter regions such as the lateral olfactory tract and fasciculus retroflexus. High levels of [125I]charybdotoxin binding were also seen in gray matter-containing regions such as the zona incerta, medial geniculate and superior colliculus, where low to moderate [125I]apamin binding was found. [125I]Iodoglyburide presented a more uniform binding with the highest levels in the globus pallidus, islands of Calleja, anteroventral nucleus of the thalamus and zonas reticulata of the substantia nigra. These results indicate that subtypes of K channels have very different distributions in the brain. As such, the results imply differing CNS actions for potential modulators of K channel subtypes.

Drugs acting on the intracellular signalling system

Cellular response to extracellular messages is a basic process to maintain and to support cell life. Several signalling molecules important as sites of therapeutic drug action are involved in the response. Recent studies on life sciences have elucidated molecular properties of intracellular signalling factors and mechanisms of cascading. Novel drugs acting on signalling molecules and possessing new sites and mechanisms of action have been found. This article summarizes the properties (subtypes, structures, functions) of signalling factors (receptors, ion channels, GTP binding proteins, second messenger-generating enzymes, second messenger-metabolizing enzymes, second messengers protein kinases, protein phosphatases) and lists in Tables A-H drugs that act on signalling molecules and which should find clinical use.

Potassium channels in airway smooth muscle: a tale of two channels

Potassium channels are an important determinant of smooth muscle excitability and force generation. Two potassium channels have been fully described in airway smooth muscle: large conductance, calcium-activated potassium channels and voltage-dependent delayed rectifier channels. This article will review the biophysics and pharmacology of these channels and discuss what is currently known with respect to their regulation and physiological significance.

Characterization and partial purification from pheochromocytoma cells of an endogenous equivalent of scyllatoxin, a scorpion toxin which blocks small conductance Ca(2+)-activated K+ channels

This work describes the partial purification of a heat-stable peptide which has the same properties as the scorpion toxin, scyllatoxin, a specific blocker of one class of Ca(2+)-activated K+ channels: (i) it competes with [125I]apamin for binding to the same site, (ii) like apamin and scyllatoxin, it blocks the after-potential hyperpolarization in skeletal muscle cells in culture, (iii) like apamin and scyllatoxin, it contracts guinea-pig taenia coli relaxed by epinephrine, (iv) it cross-reacts with antibodies raised against scyllatoxin but not with antibodies raised against apamin.

Potassium and calcium currents activated by foetal calf serum in Balb-c 3T3 fibroblasts

In quiescent Balb-c mouse 3T3 fibroblasts, the application of whole or dialyzed 10% foetal calf serum elicits a biphasic electrical response, consisting of a transient outward current, flowing through Ca(2+)-activated K+ channels, followed by an inward one, lasting up to 15 min. On the basis of experiments with ion substitutions and blockers, the inward current can be attributed to the opening of cationic channels permeable to Na+ and Ca2+ ions. This current could mediate the calcium influx involved in the sustained elevation of [Ca2+]i that has been observed in many preparations in response to mitogen stimulation and that is involved in triggering cell proliferation.

Substrate influences rat osteoclast morphology and expression of potassium conductances

1. We studied the electrophysiological properties of freshly isolated rat osteoclasts using the whole-cell configuration of the patch-clamp technique. Membrane currents were recorded from cells plated on three substates: dentine, type I collagen and glass. 2. Based on their morphology, we defined two categories of osteoclasts. 'Rounded' osteoclasts were dome-shaped and lacked lamellipodia. 'Spread' osteoclasts were flattened and had lamellipodia. The proportion of 'rounded' osteoclasts was significantly greater when cells were plated on dentine or type I collagen than when cells were plated on glass. 3. 'Spread' osteoclasts expressed an inwardly rectifying K+ conductance regardless of the substrate on which they were plated. 4. 'Rounded' osteoclasts, on all substrates, expressed a transient, outwardly rectifying conductance that was selective for K+ based on: reversal of deactivation tail currents at -74 mV; a 60 mV shift in tail current reversal potential for 10-fold change in [K+]o; and blockade of outward current by extracellular 4-aminopyridine, charybdotoxin, and intracellular Cs+. The outward K+ current had an activation threshold of approximately -50 mV, with half-activation at -29 mV. The current also exhibited voltage-dependent inactivation, with half-inactivation at approximately -40 mV. 5. Outward K+ current in 'rounded' osteoclasts was reduced when extracellular Ca2+ was removed and upon addition of Ni2+, but was unaffected by Cd2+ or nifedipine. 6. 'Rounded' osteoclasts had large whole-cell capacitance for their apparent surface area. Capacitance was positively correlated with K+ conductance. The additional surface membrane we detected through capacitance measurements may be the 'ruffled border' of actively resorbing osteoclasts. 7. We conclude that substrate influences the expression of osteoclast phenotype, as defined by morphology and K+ conductances. 'Rounded' osteoclasts express an outwardly rectifying K+ conductance, with no apparent inwardly rectifying K+ conductance. In contrast, 'spread' osteoclasts exhibit an inwardly rectifying K+ conductance with no outwardly rectifying K+ conductance. The 'spread' phenotype may represent a motile phase, while the 'rounded' phenotype may represent a resorptive phase of osteoclastic activity.

A novel large-conductance Ca(2+)-activated potassium channel and current in nerve terminals of the rat neurohypophysis

1. Nerve terminals of the rat posterior pituitary were acutely dissociated and identified using a combination of morphological and immunohistochemical techniques. Terminal membrane currents were studied using the 'whole-cell' patch clamp technique and channels were studied using inside-out and outside-out patches. 2. In physiological solutions, but with 7 mM 4-aminopyridine (4-AP), depolarizing voltage clamp steps from different holding potentials (-90 or -50 mV) elicited a fast, inward current followed by a slow, sustained, outward current. This outward current did not appear to show any steady-state inactivation. 3. The threshold for activation of the outward current was -30 mV and the current-voltage relation was 'bell-shaped'. The amplitude increased with increasingly depolarized potential steps. The outward current reversal potential was measured using tail current analysis and was consistent with that of a potassium current. 4. The sustained potassium current was determined to be dependent on the concentration of intracellular calcium. Extracellular Cd2+ (80 microM), a calcium channel blocker, also reversibly abolished the outward current. 5. The current was delayed in onset and was sustained over the length of a 150 ms-duration depolarizing pulse. The outward current reached a peak plateau and then decayed slowly. The decay was fitted by a single exponential with a time constant of 9.0 +/- 2.2 s. The decay constants did not show a dependence on voltage but rather on intracellular Ca2+. The time course of recovery from this decay was complex with full recovery taking > 190 s. 6. 4-AP (7 mM), dendrotoxin (100 nM), apamin (40-80 nM), and charybdotoxin (10-100 nM) had no effect on the sustained outward current. In contrast Ba2+ (200 microM) and tetraethylammonium inhibited the current, the latter in a dose-dependent manner (apparent concentration giving 50% of maximal inhibition (IC50) = 0.51 mM). 7. The neurohypophysial terminal outward current recorded here corresponds most closely to a Ca(2+)-activated K+ current (IK(Ca)) and not to a delayed rectifier or IA-like current. It also has properties different from that of the Ca(2+)-dependent outward current described in the magnocellular neuronal cell bodies of the hypothalamus. 8. A large conductance channel is often observed in isolated rat neurohypophysial nerve terminals. The channel had a unit conductance of 231 pS in symmetrical 150 mM K+.(ABSTRACT TRUNCATED AT 400 WORDS)

Dendrotoxin-like effects of noxiustoxin

Noxiustoxin from the Mexican scorpion (Centruroides noxius Hoffmann) is known to block neuronal K+ channels. Noxiustoxin facilitated acetylcholine release in chick biventer cervicis nerve-muscle preparations, but not in mouse phrenic nerve-hemidiaphragm preparations. Noxiustoxin displaced binding of a radiolabelled dendrotoxin from synaptosomal membranes from rat brain, with a Ki of 10(-10) M. It is concluded that noxiustoxin shares some pharmacological properties with the K+ channel blocking dendrotoxins.

Isolation and physiological characterization of taicatoxin, a complex toxin with specific effects on calcium channels

Taicatoxin is a new complex oligomeric toxin that was isolated from the venom of the Australian taipan snake Oxyuranus scutellatus scutellatus. It is composed of three different molecular entities: an alpha-neurotoxin-like peptide of mol. wt 8000, a neurotoxic phospholipase of mol. wt of 16,000 and a serine protease inhibitor of mol. wt 7000, linked by non-covalent bonds, at an approximate stoichiometry of 1:1:4. The most active form of the complex was isolated by ion exchange chromatography through DE-Cellulose followed by two steps of CM-Cellulose chromatography at pH 4.7 and pH 6.0, respectively. At this stage the complex migrates as a single component in beta-alanine-acetate-urea gel electrophoresis and is very toxic to mice (1 or 2 micrograms of the complex protein kills a mouse of 20 g within 2 hr). It blocks the high threshold calcium channel current of excitable membranes in heart and does not affect the low threshold calcium channel current. The block occurs at a site that is accessible extracellularly but not intracellularly. The block is selective for calcium channels, reversible, does not affect single channel conductance but only changes channel gating, and is voltage dependent with higher affinity for inactivated channels. The phospholipase activity of the complex toxin can be separated by affinity-chromatography using a phospholipid analog (PC-Sepharose). The resulting complex contains only alpha-neurotoxin and protease inhibitor and is still capable of blocking calcium channels, although with less potency than the native oligomeric form. Sephadex G-50 gel filtration chromatography in the presence of high salt (1M NaCl) at alkaline pH (8.2), separates the alpha-neurotoxin-like peptide from the protease inhibitor, but at this stage the resulting peptides lose physiological activity towards the calcium channels. The amino acid sequence of the protease inhibitor was determined by automatic Edman degradation. The alpha-neurotoxin-like peptide and two isosubunits displaying phospholipase activity were sequenced at the N-terminal part of the molecule.

Excitatory and neurotoxic actions of platelet-activating factor on rat myenteric neurons in cell culture

At micromolar concentrations, PAF causes intense excitation and elevation of intracellular Ca in a subset of myenteric neurons. When applied for more than about 10 seconds, these concentrations of PAF kill a subset of myenteric neurons. The excitation and elevation of Ca levels are accompanied by increased membrane conductance and enhanced synaptic activity. The effects of brief applications are reversible, but responses to subsequent applications of PAF are substantially reduced. If such responses can be elicited in enteric neurons by the concentrations of PAF that are generated in vivo, they would account for the potent ability of PAF to evoke neurally-mediated secretory responses in GI tissues.

Charybdotoxin, dendrotoxin and mast cell degranulating peptide block the voltage-activated K+ current of fibroblast cells stably transfected with NGK1 (Kv1.2) K+ channel complementary DNA

The blocking actions of the K+ channel toxins charybdotoxin, dendrotoxin and mast cell degranulating peptide were studied in B82 mouse fibroblast cells transformed to express NGK1 (Kv1.2) K+ channels. All three toxins were potent blockers of the K+ current in these cells, with KD values of 1.7, 2.8 and 185 nM, respectively. The toxin block exhibited a weak voltage-dependence with the degree of inhibition decreasing at positive membrane potentials. For charybdotoxin and dendrotoxin, reducing [K+]i did not increase the fractional block, demonstrating that the relief of block at positive membrane potentials is not due to displacement of the toxin molecules by outward flow of K+ ions. A voltage-jump protocol was used to determine the rates of binding and unbinding of dendrotoxin and mast cell degranulating peptide; binding of charybdotoxin was too rapid to be quantitatively evaluated in this manner. The binding rates (dendrotoxin, approximately 5 x 10(7)/M per s; mast cell degranulating peptide, approximately 0.8 x 10(7)/M per s) were largely voltage-independent, suggesting that association of the toxin molecules with the channel is diffusion limited. The rates of unbinding (dendrotoxin, approximately 0.3/s; mast cell degranulating peptide, approximately 3/s at +60 mV) of both toxins increased e-fold per approximately 40 mV change in membrane potential, thus accounting for the voltage-dependence of the equilibrium block. Internal perfusion with the three toxins failed to affect the K+ current (in contrast to internal tetraethylammonium which strongly blocked the current), indicating that the toxins exert their blocking action by binding to extracellular sites.

Resting membrane potential and potassium currents in cultured parasympathetic neurones from rat intracardiac ganglia

1. Whole-cell K+ currents contributing to the resting membrane potential and repolarization of the action potential were studied in voltage-clamped parasympathetic neurones dissociated from neonatal rat intracardiac ganglia and maintained in tissue culture. 2. Rat intracardiac neurones had a mean resting membrane potential of -52 mV and mean input resistance of 850 M omega. The current-voltage relationship recorded during slow voltage ramps indicated the presence of both leakage and voltage-dependent currents. The contribution of Na+, K+ and Cl- to the resting membrane potential was examined and relative ionic permeabilities PNa/PK = 0.12 and PCl/PK < 0.001 were calculated using the Goldman-Hodgkin-Katz voltage equation. Bath application of the potassium channel blockers, tetraethylammonium ions (TEA; 1 mM) or Ba2+ (1 mM) depolarized the neurone by approximately 10 mV. Inhibition of the Na(+)-K+ pump by exposure to K(+)-free medium or by the addition of 0.1 mM ouabain to the bath solution depolarized the neurone by 3-5 mV. 3. In most neurones, depolarizing current pulses (0.5-1 s duration) elicited a single action potential of 85-100 mV, followed by an after-hyperpolarization of 200-500 ms. In 10-15% of the neurones, sustained current injection produced repetitive firing at maximal frequency of 5-8 Hz. 4. Tetrodotoxin (TTX; 300 nM) reduced, but failed to abolish, the action potential. The magnitude and duration of the TTX-insensitive action potential increased with the extracellular Ca2+ concentration, and was inhibited by bath application of 0.1 mM Cd2+. The repolarization rate of the TTX-insensitive action potential was reduced, and after-hyperpolarization was replaced by after-depolarization upon substitution of internal K+ by Cs+. The after-hyperpolarization of the action potential was reduced by bath application of Cd2+ (0.1 mM) and abolished by the addition of Cd2+ and TEA (10 mM). 5. Depolarization-activated outward K+ currents were isolated by adding 300 nM TTX and 0.1 mM Cd2+ to the external solution. The outward currents evoked by step depolarizations increased to a steady-state plateau which was maintained for > 5 s. The instantaneous current-voltage relationship, examined under varying external K+ concentrations, was linear, and the reversal (zero current) potential shifted in accordance with that predicted by the Nernst equation for a K(+)-selective electrode. The shift in reversal potential of the tail currents as a function of the extracellular K+ concentration gave a relative permeability, PNa/PK = 0.02 for the delayed outward K+ channel(s).(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of charybdotoxin and leiurotoxin I on potassium currents in bullfrog sympathetic ganglion and hippocampal neurons

The effects of charybdotoxin and leiurotoxin I were examined on several classes of K+ currents in bullfrog sympathetic ganglion and hippocampal CA1 pyramidal neurons. Highly purified preparations of charybdotoxin selectively blocked a large voltage- and Ca(2+)-dependent K+ current (IC) responsible for action potential repolarization (IC50 = 6 nM) while leiurotoxin I selectively blocked a small Ca(2+)-dependent K+ conductance (IAHP) responsible for the slow afterhyperpolarization following an action potential (IC50 = 7.5 nM) in bullfrog sympathetic ganglion neurons. Neither of the toxins had significant effects on other K+ currents (M-current [IM], A-current [IA] and the delayed rectifier [IK]) present in these cells. Leiurotoxin I at a concentration of 20 nM had no detectable effect on currents in hippocampal CA1 pyramidal neurons. This lack of effect on IAHP in central neurons suggests that the channels underlying slow AHPs in those neurons are pharmacologically distinct from analogous channels in peripheral neurons.

Direct vascular actions of hydrochlorothiazide and indapamide in isolated small vessels

The mechanism by which thiazides lower peripheral resistance is unresolved. The aim of this study was to investigate the mechanisms of the acute vasodilator action of hydrochlorothiazide and the 'thiazide-like' diuretic indapamide on human, guinea pig and rat small arteries. Vessels were mounted on a myograph and the relaxation profile of both drugs and interactions with K+ channels and the vascular eicosanoid system were studied. Neither drug had any relaxant effect in rat mesenteric vessels and indapamide did not relax human arteries. Hydrochlorothiazide in both human and guinea pig vessels produced significantly more relaxation of noradrenaline than K(+)-constricted vessels (P less than 0.001). Relaxation to hydrochlorothiazide was reduced in the presence of charybdotoxin. Maximal-induced hydrochlorothiazide relaxation was reduced by 64% in human arteries (P less than 0.001) and by 91% in guinea pig vessels (P less than 0.001). Incubation with glibenclamide and indomethacin had no effect on the relaxant activity of hydrochlorothiazide and indapamide. Indapamide-induced relaxation was unaffected in the presence of charybdotoxin. These results show marked differences in the acute vasodilator action of hydrochlorothiazide and indapamide. There appears to be involvement of Ca(2+)-activated K+ channels in the acute vasorelaxant activity of hydrochlorothiazide.

Determination of the three-dimensional structure of iberiotoxin in solution by 1H nuclear magnetic resonance spectroscopy

The solution structure of chemically synthesized iberiotoxin, a scorpion toxin that blocks Ca(2+)-activated K+ channels, has been determined using 2D 1H NMR spectroscopy. Analysis of the NOEs, coupling constants, and HN-DN exchange rates indicates the structure consists of an antiparallel beta-sheet from residues 25 to 36, with a type 1 turn at residues 30-31, and a helix from residues 13 to 21. The carboxyl-terminal residues form a short, and distorted, third strand of the sheet. The NMR data are consistent with disulfide bonds from residues 7 to 28, 13 to 33, and 17 to 35. The disulfide bridging presents the same profile as in other scorpion toxins, where a Cys-X-Cys sequence in a strand of sheet forms two disulfide bonds to a Cys-X-X-X-Cys sequence in a helix. Three-dimensional structures were generated using the torsion angle space program PEGASUS. The best ten structures had an average rmsd over all pairwise comparisons of 1.49 A. The average rmsd to a calculated average structure is 1.0 A. The resulting structures appear very similar to those of charybdotoxin, a related scorpion toxin.

Role of K+ channels in the modulation of cholinergic neural responses in guinea-pig and human airways

1. Several agonists modulate cholinergic neurotransmission in airways raising the possibility that there may be a common inhibitory mechanism, such as the activation of a common K+ channel in the nerve ending. To test this hypothesis, we examined whether blockers of K+ channels are able to depress the prejunctional inhibitory modulation of cholinergic contractile responses by various agonists in guinea-pig and human airways in vitro. 2. Electrical field stimulation (40 V, 0.5 ms) was applied to guinea-pig (0.5 Hz) or human (1 Hz) tracheal strips every 4 min to elicit cholinergic neural responses. The effects of the K+ channel blockers, charybdotoxin (ChTX, 10 nM), apamin (100 nM) and glibenclamide (1 microM), on the prejunctional inhibition of cholinergic contraction evoked by neuropeptide Y (NPY, 100 nM), an alpha 2-agonist, clonidine (10 nM), a mu-opioid agonist, [D-Ala2, NMePhe4, Gly-ol5]-enkephalin (DAMGO, 100 nM), and a KATP channel opener, lemakalim (300 nM) were tested in guinea-pigs. In human tissues, the effect of ChTX (10 nM) on the mu-opioid (DAMGO, 300 nM)-induced inhibition of cholinergic nerves was examined. 3. In guinea-pigs, ChTX (10 nM) significantly reversed the prejunctional inhibition of cholinergic contraction by NPY (84.2 +/- 16.2%), clonidine (71.9 +/- 22.4%), DAMGO (67.3 +/- 13.1%) and lemakalim (20.9 +/- 9.4%) (n = 5, P < 0.05, respectively), while apamin (100 nM) had no effect. Glibenclamide (10 microM) reduced only the lemakalim-induced inhibitory modulation. ChTX (10 nM) itself potentiated cholinergic contraction (24.6 +/- 9.4%, n = 5, P < 0.05) without affecting exogenously applied acetylcholine dose-response curves. Pretreatment with ChTX (10 nM) significantly reduced the inhibitory modulation of cholinergic nerves by NPY, clonidine and DAMGO, but not by lemakalim. 4. In human tissues, ChTX significantly reduced DAMGO-induced prejunctional inhibition of cholinergic contraction (13.6 +/- 8.5% with and 46.5 +/- 5.5% without ChTX, respectively; n = 5, P < 0.05). 5. These results may support a hypothesis that the activation of ChTX-sensitive K+ channels is involved in the inhibitory modulation of cholinergic neuro-transmission by agonists acting on presynaptic receptors in guinea-pig and human airways.

Tetrandrine blocks a slow, large-conductance, Ca(2+)-activated potassium channel besides inhibiting a non-inactivating Ca2+ current in isolated nerve terminals of the rat neurohypophysis

The effects of tetrandrine, a bis-benzyl-isoquinoline alkaloid, on voltage-gated Ca2+ currents (ICa) and on Ca(2+)-activated K+ current (IK(Ca)) and channels in isolated nerve terminals of the rat neurohypophysis were investigated using patch-clamp techniques. The non-inactivating component of ICa was inhibited by external tetrandrine in a voltage- and dose-dependent manner, with an IC50 = 10.1 microM. IK(Ca) was elicited by depolarizations when approximately 10 microM Ca2+ was present on the cytoplasmic side. Only externally applied tetrandrine, at 1 microM, decreased the amplitude of IK(Ca), whereas the fast inward Na+ current and transient outward K+ current were not affected. Tetrandrine, applied to the extracellular side of outside-out patches excised from the nerve terminals, induced frequent and short closures of single type II, maxi-Ca(2+)-activated K+ channels. Tetrandrine decreased the channel-open probability, within bursts, with an IC50 = 0.21 microM. Kinetic analysis of the channel activity showed that the open-time constant decreased linearly with increasing tetrandrine concentrations (0.01-3 microM), giving an association rate constant of 8.8 x 10(8) M-1 s-1, whereas the arithmetic mean closed time did not change, giving a dissociation rate constant of 136.6 s-1. These results show that tetrandrine is a high-affinity blocker of the type II, maxi-Ca(2+)-activated K+ channel of the rat neurohypophysial terminals.

Mapping function to structure in a channel-blocking peptide: electrostatic mutants of charybdotoxin

Electrostatic interactions between charybdotoxin (CTX), a specific peptide pore blocker of K+ channels, and a Ca(2+)-activated K+ channel were investigated with a genetically manipulable recombinant CTX. Point mutations at certain charged residues showed only small effects on the binding affinity of the toxin molecule: Lys11, Glu12, Arg19, His21, Lys31, and Lys32. Replacement by Gln at Arg25, Lys27, or Lys34 strongly decreased the affinity of the toxin. These affinity changes were mainly due to large increases of toxin dissociation rates without much effect on association rates, as if close-range interactions between the toxin and its receptor site of the channel were disrupted. We also found that the neutralization of Lys27 to Gln removed the toxin's characteristic voltage dependence in dissociation rate. Mutation and functional mapping of charged residues revealed a molecular surface of CTX which makes direct contact with the extracellular mouth of the K+ channel.

Block of potassium outward currents in the crayfish stretch receptor neurons by 4-aminopyridine, tetraethylammonium chloride and some other chemical substances

The effects of 4-aminopyridine (4-AP) and tetraethylammonium (TEA) on the outward potassium currents in the rapidly and slowly adapting stretch receptor neurons (SRNs) of the crayfish (Pacifastacus leniusculus) were studied using a two micro-electrode voltage-clamp technique. The leakage current was not affected by either 4-AP or TEA. External 4-AP blocked the peak outward current in a dose-dependent manner (1:1 stoichiometry) with an apparent dissociation constant (Kd) of 2.3 +/- 0.2 mM (mean +/- SEM) in the slowly and 1.4 +/- 0.2 mM in the rapidly adapting SRN, the block being voltage dependent. External application of TEA resulted in a block of the steady state current enhancing the transient characteristics of the current response. The block appeared to deviate from a 1:1 stoichiometry and the apparent Kd for TEA was 9.6 +/- 3.4 mM with a cooperativity factor n = 0.43 +/- 0.03 in the slowly adapting SRN and 34.5 +/- 9.2 mM and 0.37 +/- 0.03 respectively in the rapidly adapting SRN. Low Ca2+, apamin and charybdotoxin, which are known to block Ca(2+)-dependent K-currents, had no effects on the outward current as was also the case with catechol. It is concluded that the different effects of TEA and 4-AP on the outward current in the two types of SRNs can be explained by the presence of at least two, probably heteromultimeric, channel populations having similar sensitivity to 4-AP but different sensitivity to TEA. One channel has a high affinity (Kd = 0.8-1.6 mM) for TEA and the other a low affinity (Kd = 173-213 mM) for TEA. The low-affinity channel seems to dominate in the slowly adapting SRN while both channels are equally common in the rapidly adapting SRN. Further, the present results do not support the existence of a macroscopic Ca(2+)-dependent K+ current in the SRNs.

An after-depolarization following action potentials and its modulation by substance P in rat sympathetic neurons

Sympathetic neurons in rat coeliac-superior mesenteric ganglia (C-SMG) displayed an after-depolarization (ADP) following action potentials and after-hyperpolarization. The ADP had amplitudes of 3-10 mV and lasted for 2-6 s, during which membrane resistance and excitability of C-SMG neurons increased. The reversal potential of ADP was dependent on external K+ concentrations. The ADP was suppressed in a solution containing Cd2+ or zero Ca2+. The ADP thus appears to be produced by inhibition of certain K+ channels via a Ca(2+)-dependent process. Substances P (SP) depolarized ganglion cells and increased the ADP, leading to a long-lasting increase in membrane excitability of rat C-SMG neurons.

Ca2+ release from isolated sarcoplasmic reticulum of guinea-pig psoas muscle induced by K(+)-channel blockers

A Ca(2+)-sensitive electrode was used to measure the Ca2+ concentration of the medium containing the heavy fraction of the fragmented sarcoplasmic reticulum (SR) prepared from guinea-pig psoas muscle. Among K(+)-channel blockers tested, 4-aminopyridine (4-AP), tetraethylammonium (TEA) and charybdotoxin elicited Ca2+ release from the SR, but apamin and glibenclamide did not. These results suggest that a reduction of SR K+ conductance leads to Ca2+ release from the SR.

Interaction of charybdotoxin with permeant ions inside the pore of a K+ channel

Charybdotoxin (CTX) blocks high conductance Ca(2+)-activated K+ channels by binding to a receptor site in the externally facing "mouth." Toxin bound to the channel can be destabilized from its site by K+ entering the channel from the opposite, internal, solution. By analyzing point mutants of CTX expressed in E. coli, assayed with single Ca(2+)-activated K+ channels reconstituted into planar lipid bilayers, we show that a single positively charged residue of the peptide, Lys-27, wholly mediates this interaction of K+ with CTX. If position 27 carries a positively charged residue, internal K+ accelerates the dissociation rate of CTX in a voltage-dependent manner; however, if a neutral Asn or Gln is substituted at this position, the dissociation rate is completely insensitive to either internal K+ or applied voltage. Position 27 is unique in this respect; charge-neutral substitutions made at other positions fail to eliminate the K+ destabilization phenomenon. The results argue that CTX bound to the channel positions Lys-27 physically close to a K(+)-specific binding site on the external end of the conduction pathway and that a K+ ion occupying this site destabilizes CTX via direct electrostatic repulsion with the epsilon-amino group of Lys-27.

A peptide derived from the Shaker B K+ channel produces short and long blocks of reconstituted Ca(2+)-dependent K+ channels

A 20 amino acid synthetic peptide, corresponding to the amino-terminal region of the Shaker B (ShB) K+ channel and responsible for its fast inactivation, can block large conductance Ca(2+)-dependent K+ channels from rat brain and muscle. The ShB inactivation peptide produces two kinetically distinct blocking events in these channels. At lower concentrations, it produces short blocks, and at higher concentrations long-lived blocks also appear. The L7E mutant peptide produces only infrequent short blocks (no long-lived blocks) at a much higher concentration. Internal tetraethylammonium competes with the peptide for the short block, which is also relieved by K+ influx. These results suggest that the peptide induces the short block by binding within the pore of Ca(2+)-dependent K+ channels. The long block is not affected by increased K+ influx, indicating that the binding site mediating this block may be different from that involved in the short block. The short block of Ca(2+)-dependent K+ channels and the inactivation of Shaker exhibit similar characteristics with respect to blocking affinity and open pore blockade. This suggests a conserved binding region for the peptide in the pore regions of these very different classes of K+ channel.

Mechanism of iberiotoxin block of the large-conductance calcium-activated potassium channel from bovine aortic smooth muscle

The interaction of iberiotoxin (IbTX) with the large-conductance calcium-activated potassium (maxi-K) channel was examined by measuring single-channel currents from maxi-K channels incorporated into planar lipid bilayers. Addition of nanomolar concentrations of IbTX to the external side of the channel produced long nonconducting silent periods, which were interrupted by periods of normal channel activity. The distributions of durations of blocked and unblocked periods were both described by single exponentials. The mean duration of the unblocked periods decreased in proportion with the external concentration of IbTX, while the mean duration of the blocked periods was not affected. These results suggest that IbTX blocks the maxi-K channel through a simple bimolecular binding reaction where the silent periods represent times when a single toxin molecule is bound to the channel. In symmetric solutions of 150 mM KCl, with a membrane potential of 40 mV, the mean duration of the blocked periods produced by IbTX was 840 s, and the association rate was 1.3 x 10(6) M-1 s-1, yielding an equilibrium dissociation constant of about 1 nM. Raising the internal potassium concentration increased the dissociation rate constant of IbTX in a manner which was well described by a saturable binding function for potassium. External tetraethylammonium ion increased the average duration of the unblocked periods without affecting the blocked periods, suggesting that tetraethylammonium and IbTX compete for the same site near the conductance pathway of the channel. Increasing the external concentration of monovalent cations from 25 to 300 mM with either potassium or sodium decreased the rate of binding of IbTX to the channel by approximately 24-fold, with little effect on the rate of toxin dissociation.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca(2+)-activated K+ channels from an insulin-secreting cell line incorporated into planar lipid bilayers

This study evaluates the use of the planar lipid bilayer as a functional assay of Ca(2+)-activated K+ channel activity for use in purification of the channel protein. Ca(2+)-activated K+ channels from the plasma membrane of an insulin-secreting hamster Beta-cell line (HIT T15) were incorporated into planar lipid bilayers. The single channel conductance was 233 picoSiemens (pS) in symmetrical 140 mmol/l KCl and the channel was strongly K(+)-selective (PCl/PK = 0.046; PNa/PK = 0.027). Channels incorporated into the bilayer with two orientations. In 65% of cases, the probability of the channel being open was increased by raising calcium on the cis side of the bilayer (to which the membrane vesicles were added) or by making the cis side potential more positive. At a membrane potential of + 20 mV, which is close to the peak of the Beta-cell action potential, channel activity was half-maximal at a Ca2+ concentration of about 15 mumol/l. Charybdotoxin greatly reduced the probability of the channel being open when added to the side opposite to that at which Ca2+ activated the channel. These results resemble those found for Ca(2+)-activated K+ channels in native Beta cell membranes and indicate that the channel properties are not significantly altered by incorporation in a planar lipid bilayer.

Induction of a Ca(2+)-activated K+ channel in human erythrocytes by mechanical stress

Mechanical deformation of normal ATP-replete human erythrocytes increased their permeability to Ca2+ sufficiently to turn on the Ca(2+)-activated K+ channel (the Gardos channel). When Ca2+ is absent, mechanical deformation of normal erythrocytes induces an equivalent increase the permeability of both Na+ and K+, In the presence of 0.1 to 1 mM Ca2+, a further increase in the K+ efflux rate was seen. There was no increase in Na+ flux above that induced by deformation itself. The involvement of the Ca(2+)-activated H channel was verified by showing the specific inhibitors of the channel, quinine and charybdotoxin, prevent the Ca(2+)-induced increase in K+ efflux. These results are consistent with a model of sickle cell dehydration proposed by Bookchin et al. ((1987) Prog. Clin. Biol. Res. 240, 193-200). The estimated rate of Ca2+ entry under these conditions (37 degrees C, 1000 dyne/cm2, and laminar shear) was about 1 mmol/loc per h.

Autoradiographic localization of [125I]charybdotoxin binding sites in rat brain

Charybdotoxin, a 37 amino acid peptide isolated from scorpion venom, is a potent inhibitor of potassium channel function. [125I]charybdotoxin was originally believed to be a selective ligand for the Ca(2+)-sensitive channel in many tissues, but it appears to bind only to a voltage-sensitive potassium channel in brain. We found high densities of [125I]charybdotoxin binding in lateral olfactory tract, interpeduncular nucleus and a variety of mesencephalic nuclei. Moderate levels were found in the cerebral cortex, medial thalamus, hypothalamus and selected thalamic nuclei. These results indicate that [125I]charybdotoxin identifies a potassium channel or channels with a unique distribution in the brain.

Transformation of renal tubule epithelial cells by simian virus-40 is associated with emergence of Ca(2+)-insensitive K+ channels and altered mitogenic sensitivity to K+ channel blockers

We compared the pattern of K+ channels and the mitogenic sensitivity to K+ channel blocking agents in primary cultures of rabbit proximal tubule cells (PC.RC) (Ronco et al., 1990) and two derived SV-40-transformed cell lines exhibiting specific functions of proximal (RC.SV1) and more distal (RC.SV2) tubule cells (Vandewalle et al., 1989). First, K+ channel equipment surveyed by the patch-clamp technique was modified after SV-40 transformation in both cell lines; although a high conductance Ca(2+)-activated K+ channel [K+200 (Ca2+)] remained the most frequently recorded K+ channel, the transformed state was characterized by emergence of three Ca(2+)-insensitive K+ channels (150, 50, and 30 pS), virtually absent from primary culture, contrasting with reduced frequency of two Ca(2+)-sensitive K+ channels (80 and 40 pS). Second, quinine (Q), tetraethylammonium ion (TEA) and charybdotoxin (CTX), at concentrations not affecting cell viability, all decreased 3H-TdR incorporation and cell growth in PC.RC cultures, but only TEA had similar effects in transformed cells. The latter were further characterized by paradoxical effects of Q that induced a marked increase in thymidine incorporation. Q also exerted contrasting effects on channel activity: it inhibited the [K+200 (Ca2+)] when the channel was highly active, with a Ki (0.2 mM) similar to that measured for 3H-TdR incorporation in PC.RC cells (0.3 mM), but increased the mean current through poorly active channels. TEA blocked all K+ channels with conductance greater than or equal to 50 pS, including the [K+200 (Ca2+)], in a range of concentrations that substantially affected cell proliferation. The unique effect of TEA on SV-40-transformed cells might be related to broad inhibition of K+ channels.

Effects of cromakalim on the contraction and the membrane potential of the circular smooth muscle of guinea-pig stomach

1. The effects of cromakalim on mechanical and electrical activities of the circular smooth muscles of guinea-pig stomach antrum were observed. 2. Cromakalim (greater than 1 x 10(-7) M) decreased the amplitude of spontaneous rhythmic contractions and also the acetylcholine-enhanced spontaneous contractions. Cromakalim was less effective against the 25.9 mM and 35.9 mM K(+)-induced tonic contractions. 3. Glibenclamide (1 x 10(-6) M) itself caused no detectable change in the spontaneous contractions, those potentiated by acetylcholine or tonic contractions induced by high K+ solutions, but attenuated the actions of cromakalim. On the other hand, charybdotoxin (3 x 10(-8) M) increased the amplitude of spontaneous contractions but failed to affect the actions of cromakalim. 4. Cromakalim (greater than 1 x 10(-6) M) decreased the amplitude and duration of slow waves, and hyperpolarized the membrane. These actions of cromakalim were completely antagonized by 1 x 10(-6) M glibenclamide, whereas part of the effects of cromakalim on mechanical activity was resistant to glibenclamide. 5. The results suggest that the inhibition by cromakalim of the electrical activity and the hyperpolarization, which may be associated with the opening of glibenclamide-sensitive K+ channel, are responsible for its inhibitory action on circular smooth muscle of guinea-pig stomach. Further, some effects independent of glibenclamide-sensitive K+ channel may also be responsible for the mechanical effect.

Comparison of the actions of acetylcholine and BRL 38227 in the guinea-pig coronary artery

1. The contractile and electrical responses to acetylcholine (ACh) in isolated segments of guinea-pig and rabbit coronary arteries were compared to those of the putative adenosine 5'-triphosphate (ATP)-dependent K+ channel opener, BRL 38227. 2. Both ACh and BRL 38227 produced concentration-dependent relaxation of vessel segments contracted with the H1-receptor agonist, 2-(2-aminoethyl)pyridine. 3. An IC90 of either vasodilator also produced 17-20 mV of hyperpolarization of the guinea-pig coronary artery. 4. Glibenclamide (1-35 microM) depolarized the guinea-pig coronary artery by 8-12 mV and antagonized BRL 38227- but not ACh-induced relaxation and hyperpolarization. 5. In the guinea-pig coronary artery, the K+ channel blockers phencyclidine (PCP, 100 microM), tetraethylammonium (TEA, 10 mM) and scorpion venom (8.7 micrograms ml-1) all significantly reduced ACh-induced relaxation and hyperpolarization whereas only PCP was an effective antagonist of both relaxation and hyperpolarization with BRL 38227. 6. Similar effects of glibenclamide and scorpion venom on ACh- and BRL 38227-induced relaxation were observed in the rabbit coronary artery. 7. Apamin (3.5 microM) was without effect on either the ACh- or BRL 38227-induced relaxation in the guinea-pig coronary artery. 8. In conclusion, the actions of BRL 38227 in coronary artery are compatible with its proposed effects on ATP-dependent K+ channels. In contrast, the results with ACh suggest that some step between the initial binding of ACh to endothelial muscarinic receptors and the final relaxation of the smooth muscle depends upon the opening of Ca(2+)-activated K+ channels.

Potassium currents of rat basilar artery smooth muscle cells

Primary isolates of smooth muscle cells from the basilar artery of the rat were studied using whole-cell and single-channel patch-clamp techniques. Two distinct potassium currents were characterized. With low intracellular calcium, depolarization above 0 mV elicited an outward current of a few hundred pA (at +120 mV) with sigmoidal onset and little inactivation during 1.25 s steps. This current was reduced by bath application of 1 mM procaine or 1 mM strychnine, but not by 500 nM charybdotoxin. These are characteristics of the delayed rectifier potassium current in other preparations. With higher intracellular calcium, depolarization above 0 mV elicited a non-inactivating potassium current of several nA (at +120 mV). This current persisted in the presence of 1 mM procaine or strychnine but was reduced by bath application of 100 nM charybdotoxin. In whole-cell recordings in which intracellular calcium was unbuffered with EGTA, spontaneous transient outward currents were manifest and displayed voltage dependence and tail currents similar to the calcium-dependent current. The spontaneous transient current and the calcium-dependent current had similar sensitivity to charybdotoxin. Cell-free membrane patches contained one or more channels of 220 pS (in solutions symmetrical with respect to potassium) with similar voltage and calcium dependence. These are characteristics of the large conductance calcium-activated potassium current in other preparations.

Regulation of arterial tone by activation of calcium-dependent potassium channels

Blood pressure and tissue perfusion are controlled in part by the level of intrinsic (myogenic) vascular tone. However, many of the molecular determinants of this response are unknown. Evidence is now presented that the degree of myogenic tone is regulated in part by the activation of large-conductance calcium-activated potassium channels in arterial smooth muscle. Tetraethylammonium ion (TEA+) and charybdotoxin (CTX), at concentrations that block calcium-activated potassium channels in smooth muscle cells isolated from cerebral arteries, depolarized and constricted pressurized cerebral arteries with myogenic tone. Both TEA+ and CTX had little effect on arteries when intracellular calcium was reduced by lowering intravascular pressure or by blocking calcium channels. Elevation of intravascular pressure through membrane depolarization and an increase in intracellular calcium may activate calcium-activated potassium channels. Thus, these channels may serve as a negative feedback pathway to control the degree of membrane depolarization and vasoconstriction.

Mapping hydrophobic residues of the interaction surface of charybdotoxin

Images

A small-conductance charybdotoxin-sensitive, apamin-resistant Ca(2+)-activated K+ channel in aortic smooth muscle cells (A7r5 line and primary culture)

A small conductance K+ channel was identified in smooth muscle cells of the rat aortic cell line A7r5 and also in rat aortic smooth muscle cells in primary culture, using conventional single-channel recording techniques. The single-channel conductance shows no rectification, either in the range -70 to +40 mV under asymmetrical conditions (9.1 pS), or in the range -100 to +50 mV in symmetrical 150 mM K+ (37 pS). Channel activity is reversibly inhibited by extracellular application of charybdotoxin, with a concentration of 8 nM producing half-maximal inhibition. It is unaffected by apamin or scyllatoxin. Channel activity depends on the presence of free Ca2+ on the cytosolic face of the membrane, with an activation zone between 0.1 and 1 microM. This small-conductance, charybdotoxin-sensitive, Ca(2+)-regulated K+ channel is activated by vasoconstrictors such as vasopressin and endothelin.

Toxin pharmacology of the ATP-induced hyperpolarization in Madin-Darby canine kidney cells

The effects of Leiurus quinquestriatus hebraeus (LQH) venom, mamba venom, Buthus tamulus (BT) venom, purified apamin and synthetic charybdotoxin on the membrane hyperpolarization induced by extracellular ATP were examined in Madin-Darby canine kidney cells. For this we used a membrane potential probe (bisoxonol) to determine the potential variations. The relation between bisoxonal fluorescence and membrane potential was established by treating Madin-Darby canine kidney cells suspended in solutions containing various external sodium concentrations with gramicidin. Extracellular ATP induced a rapid hyperpolarization that was blocked by LQH venom and synthetic charybdotoxin. BT venom also blocked the response but at a much higher concentration than that of LQH. Mamba venom (Dendroaspis polylepis) and apamin did not modify the ATP-induced hyperpolarization. We concluded that the ATP induced hyperpolarization was due to the augmentation of the potassium conductance probably through Ca(2+)-activated K+ channels sensitive to charybdotoxin but not to mamba venom. The interaction previously described between charybdotoxin and dendrotoxin (the main toxin of mamba venom) was not observed in our case.

Calcium-activated potassium channels mediate prejunctional inhibition of peripheral sensory nerves

Activation of several receptors, including mu-opioid, alpha 2-adrenergic, and neuropeptide Y receptors, inhibits excitatory nonadrenergic noncholinergic (NANC) neural responses in airways, which were mediated by the release of peptides from capsaicin-sensitive sensory nerves. This raises the possibility of a common inhibitory mechanism, which may be related to an increase in K+ conductance in sensory nerves. To examine this hypothesis, we have studied whether K(+)-channel blockers inhibit the effects of neuromodulators of sensory nerves in guinea pig bronchi by using selective K(+)-channel blockers. Charybdotoxin (ChTX; 10 nM), which blocks large conductance Ca(2+)-activated K(+)-channel function, completely blocked and reversed the inhibitory effects of a mu-opioid agonist, neuropeptide Y, and an alpha 2-adrenoceptor agonist on excitatory NANC responses. Neither inhibitors of ATP-sensitive K+ channels (BRL 31660 or glibenclamide, both at 10 microM) nor an inhibitor of small conductance Ca(2+)-activated K+ channels (apamin; 0.1 microM) were effective. This suggests that ChTX-sensitive K(+)-channel activation may be a common mechanism for the prejunctional modulation of sensory nerves in airways. This may have important implications for the control of neurogenic inflammation.

Ca(2+)-activated K+ channels in human leukemic T cells

Using the patch-clamp technique, we have identified two types of Ca(2+)- activated K+ (K(Ca)) channels in the human leukemic T cell line. Jurkat. Substances that elevate the intracellular Ca2+ concentration ([Ca2+]i), such as ionomycin or the mitogenic lectin phytohemagglutinin (PHA), as well as whole-cell dialysis with pipette solutions containing elevated [Ca2+]i, activate a voltage-independent K+ conductance. Unlike the voltage-gated (type n) K+ channels in these cells, the majority of K(Ca) channels are insensitive to block by charybdotoxin (CTX) or 4- aminopyridine (4-AP), but are highly sensitive to block by apamin (Kd less than 1 nM). Channel activity is strongly dependent on [Ca2+]i, suggesting that multiple Ca2+ binding sites may be involved in channel opening. The Ca2+ concentration at which half of the channels are activated is 400 nM. These channels show little voltage dependence over a potential range of -100 to 0 mV and have a unitary conductance of 4-7 pS in symmetrical 170 mM K+. In the presence of 10 nM apamin, a less prevalent type of K(Ca) channel with a unitary conductance of 40-60 pS can be observed. These larger-conductance channels are sensitive to block by CTX. Pharmacological blockade of K(Ca) channels and voltage- gated type n channels inhibits oscillatory Ca2+ signaling triggered by PHA. These results suggest that K(Ca) channels play a supporting role during T cell activation by sustaining dynamic patterns of Ca2+ signaling.

Subtypes of substantia nigra dopaminergic neurons revealed by apamin: Autoradiographic and electrophysiological studies

In the intact animal, some substantia nigra dopaminergic neurons exhibit regular, and some exhibit burst firing patterns. In the in vitro slice preparation, however, all dopaminergic neurons exhibit a nonburst firing pattern. Burst firing patterns are thought to be regulated, in part, by a small conductance calcium-activated potassium channel (SK channels). To test whether SK channels reside within the midbrain dopaminergic cell regions of the mouse, receptor autoradiographic experiments were conducted with the SK channel antagonist, 125I-apamin. To determine whether SK channels play a role in burst firing pattern generation in substantia nigra dopaminergic neurons, changes in firing patterns of these cells were examined in the in vitro slice preparation following apamin superfusion (1-1000 nM). It was demonstrated that a) specific binding of radiolabeled apamin was found within the dopaminergic cell regions of the substantia nigra pars compacta, and ventral tegmental area (2.7-4.7 fmol/mg tissue); b) the firing patterns of less than half of the dopaminergic neurons were changed from a regular pattern to that of a burster with concentrations as low as 1 nM, but the firing patterns of many neurons were not changed by the drug; and c) blockade of the SK channel did not interfere with the inhibitory effects of dopamine on dopaminergic neuronal impulse flow, indicating that the known hyperpolarizing effects mediated by this dopamine receptor are not importantly mediated via the SK channel.

Intracellular Mg2+ inhibits the IP3-activated IK(Ca) in NG108-15 cells. [Why intracellular citrate can be useful for recording IK(Ca)]

Receptor-mediated formation of inositol 1,4,5-trisphosphate (IP3) can induce an outward Ca(2+)-activated K+ current [IK(Ca)] in some neural cells. We have investigated IK(Ca) activated by intracellular injections of IP3 in whole-cell patch-clamped neuroblastoma x glioma hybrid cells. The current could only be recorded reliably using citrate as the anion in the pipette, but not using acetate, aspartate, chloride, fluoride, gluconate or methylsulphate. This could be attributed to buffering of intracellular Mg2+ by citrate. Theoretical calculations suggested free [Mg2+] of 1.0 and 0.07 mM respectively in the acetate- and citrate-based recording solutions. Further, IP3-activated IK(Ca) could be recorded when the free Mg2+ level in the acetate, chloride or methylsulphate solutions was lowered to the range (0.05 mM) calculated for the citrate solution. Thus, raised [Mg2+] blocks IK(Ca). This appeared to be due to inhibition of the response to released Ca2+, since high [Mg2+] also blocked the response to intracellular injections of Ca2+ ions. Mean Mg2+ levels in intact neuroblastoma x glioma hybrid cells measured by Mag-Indo-1/AM fluorescence were estimated to be less than 0.14 mM. We therefore conclude that IP3-induced IK(Ca) is expressed under normal conditions, but may be subject to regulation by intracellular Mg2+.