Effect of potassium channel blockers on relaxations to a nitric oxide donor and to nonadrenergic nerve stimulation in guinea pig trachea

Nonadrenergic, noncholinergic (NANC) relaxations were elicited by field stimulation (1-16 Hz, 1 msec, 12 V for 15 sec) of guinea pig trachea desensitized with capsaicin (3 microM); pretreated with atropine (1 microM), propranolol (1 microM), indomethacin (3 microM) and alpha-chymotrypsin (2 U/ml) and contracted with 3 microM histamine. The nitric oxide (NO) synthase inhibitor L-nitro-N-arginine (L-NNA) significantly inhibited these responses, which is indicative of NO involvement. The ability of the large conductance Ca(++)-activated K+ channel antagonists iberiotoxin (IbTx) and charybdotoxin (ChTx) and the small conductance Ca(++)-activated K+ channel antagonist apamin to modify relaxations to NANC nerve stimulation and to the NO donor 3-morpholinosydnonimine-N-ethylcarbamide (SIN-1) was studied. Both IbTx (100 nM) and ChTx (100 nM) were found to inhibit the L-NNA-sensitive relaxations elicited by field stimulation and to inhibit the relaxations to SIN-1. In contrast, apamin did not inhibit the relaxations to either field stimulation or SIN-1. These results suggest that in the guinea pig trachea, responses to endogenous or exogenously added NO are at least in part mediated by the large conductance Ca(++)-activated K+ channel.

Evidence that Ca(2+)-activated K+ channels participate in the regulation of pituitary prolactin secretion

We evaluated the role of Ca(2+)-activated K+ channels in the regulation of prolactin (PRL) secretion with a perifusion system using acutely dispersed rat anterior pituitary cells. Apamin, which blocks Ca(2+)-activated K+ channels, induced PRL secretion in a dose-dependent fashion between 1 and 300 nM (r = 0.99, P < 0.01). Charybdotoxin, another Ca(2+)-activated K+ channel-blocker, also induced PRL secretion at 20 nM concentration. These were not non-specific toxic effects, since stimulation of PRL secretion by 10 nM thyrotropin-releasing hormone (TRH) was not different before and after applying the channel-blockers. Both 10 microM dopamine and 2 microM nifedipine significantly, but incompletely, depressed PRL secretion induced by 100 nM apamin; 10 microM dopamine completely blocked PRL secretion induced by 20 nM charybdotoxin. Our data indicate that Ca(2+)-activated K+ channels may play an important role in the regulation of PRL secretion.

Renal transepithelial phosphate secretion: luminal membrane voltage and Ca2+ dependence

The role of apical membrane electrical potential, the possibility of K+ channel involvement, and the role of extracellular Ca2+ in transepithelial P(i) secretion were examined in primary monolayer cultures of flounder renal proximal tubule cells in Ussing chambers. Exposure to 200 nM thapsigargin (TG) significantly increased net P(i) secretion. In TG-stimulated tissues, substitution of 100 mM KCl for 100 mM NaCl in the luminal medium depolarized the apical membrane potential from -64 +/- 2.8 to -26 +/- 3.9 mV and strongly inhibited net P(i) secretion. In 32P(i)-preloaded tissues, cell-to-lumen exit of 32P(i) was significantly decreased to approximately 50% of control by high luminal K+ while cell-to-peritubular bath movement was unchanged. Addition of BaCl2 (2 mM) or charybdotoxin (20 nM) to the luminal surface significantly reduced TG-stimulated net P(i) secretion. The elevation of bath Ca2+ from 2 to 5 mM significantly increased secretory flux and decreased reabsorptive flux. The effect of TG on net P(i) secretion was reduced by the Ca2+ channel blocker verapamil (VE, 100 microM) to 65% of control and by calmodulin antagonist W-7 (20 microM) to 35% of control but it was not blocked by the protein kinase inhibitor H-7 (100 microM). VE also significantly inhibited the P(i) secretion induced by acidification of the peritubular bathing medium. The data indicate that transepithelial P(i) secretion induced by TG is significantly influenced by apical membrane electrical polarity, which may be regulated in part by Ca(2+)-activated K+ channels.

Different electrical responses to vasoactive agonists in morphologically distinct smooth muscle cell types

Vascular smooth muscle cells (SMCs) in the blood vessel wall are frequently heterogeneous in nature, differing in their gross morphology, size, and shape, subcellular organelles, cytoskeleton, and contractile protein composition. In adult rat arterial vessels, two populations of SMCs have been shown to predominate: elongated bipolar cells, representing the majority of cells, and epithelial-like SMCs. We examined the ionic responses of these two types of SMCs, isolated by multiple subculture, to vasoactive stimuli. Elevations in intracellular Na+ and Ca2+ were measured with SBFI and fura 2, respectively, and changes in membrane potential were measured using the potential-sensitive fluorescent probe bis-oxonol. The resting membrane potential of the elongated bipolar cells was less negative than that of the epithelial-like SMCs. Exposure of the elongated SMCs to endothelin 1, alpha-thrombin, or arginine vasopressin induced elevations in [Ca2+]i and [Na+]i and membrane depolarization. Depolarization occurred because of entry of both Na+ and Ca2+, and pharmacological blockade of Cl- or K+ channels did not attenuate the depolarization. In contrast, when [Ca2+]i was elevated by the same agonists in the epithelial-like SMCs there was a pronounced hyperpolarization that appeared to be the consequence of enhanced activity of charybdotoxin-sensitive Ca(2+)-activated K+ channels because it was abolished by charybdotoxin (20 nmol/L), partially attenuated by tetraethylammonium chloride (10 mmol/L), and unaffected by apamin (1 mumol/L), glibenclamide (1 mumol/L), or 4-aminopyridine (5 mmol/L). Chelation of [Ca2+]i also abolished the hyperpolarization; instead, a small depolarization was observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional and molecular evidence for Shaker-like K+ channels in rabbit renal papillary epithelial cell line

The rabbit papillary epithelial cell line GRB-PAP1 was used to determine the ion transport characteristics of a model of the distal nephron and terminal collecting duct. When grown on permeable supports, monolayers developed a significant electrical resistance and a benzamil-sensitive short-circuit current, indicating that they had the property of electrogenic Na+ transport. Using the whole cell patch-clamp technique, we found that the dominant current in these cells was a slowly inactivating, time- and voltage-dependent K+ current. This current was activated by voltages more positive than -30 mV. At +30 mV, the peak outward currents were > 300 pA. The magnitude of the outward currents and their reversal potentials depended strongly on the extracellular concentration of K+ and not on the extracellular concentration of Cl-. These currents were inhibited by either tetraethylammonium, 4-aminopyridine, charybdotoxin, or dendrotoxin. These characteristics, together with the kinetics of activation and inactivation, are the general characteristics of delayed rectifier channels seen in many muscle and neuronal cells. Because many of these types of channels share sequence homology with the Shaker family of channels cloned from Drosophila, we sought to identify a molecular correlate. Using reverse transcription followed by polymerase chain reaction to amplify Shaker-like sequences, we cloned and sequenced a single 881-bp fragment. The sequence shared identity with a recently reported rabbit Shaker channel that belongs to the subclass Kv 1.2. These data show that this renal papillary epithelial cell line, which has the capability of electrogenic Na+ transport, expresses functional delayed rectifier channels.

Charybdotoxin and iberiotoxin but not apamin abolish the slow after-hyperpolarization in myenteric plexus neurons

Myenteric neurons of guinea-pig ileum were studied with intracellular microelectrodes. The specific toxins charybdotoxin, iberiotoxin and apamin were used to characterize the prolonged after-hyperpolarizations of AH neurons in this preparation. Charybdotoxin and iberiotoxin blocked prolonged after-hyperpolarizations in 23 of 24 AH neurons, but apamin had no effect on 5 of 5 AH neurons. Abolition of the after-hyperpolarizations was accompanied by depolarization and increases in input resistances of those AH neurons affected, but the shapes of action potentials were unchanged. The excitability of the AH neurons was enhanced as shown by an increase in the number of action potentials evoked by a 500-ms depolarizing current pulse or by a train of 15-ms depolarizing current pulses (10Hz). The other class of myenteric neurons, S neurons, was also investigated. The 19 S neurons studied fired action potentials only at the start of a 500 ms depolarization, but the toxins had no effect on this behaviour or on their other properties. Intracellular injection of Neurobiotin into the neurons studied and subsequent immunohistochemical staining to localise the calcium-binding protein, calretinin, indicated that all major classes of S neurons were included in the sample. Thus, the prolonged after-hyperpolarizations in AH neurons may be due to opening of a large-conductance (BK) calcium-dependent potassium channel, but similar channels play little or no role in regulation of the excitability of S neurons.

Evidence for a K+ channel in bovine chromaffin granule membranes: single-channel properties and possible bioenergetic significance

A K+ channel was incorporated into voltage-clamped planar lipid bilayers from bovine chromaffin granules and resealed granule membranes ("ghosts"). It was not incorporated from plasma membrane-rich fractions from the adrenal medulla. The channel had a conductance of approximately 400 pS in symmetric 450 mM KCl, with the permeability sequence K+ > Rb+ > Cs+ > Na+ > Li+, and was insensitive to both Ca2+ and charybdotoxin. It exhibited complex gating kinetics, consistent with the presence of multiple open and closed states, and its gating was voltage-dependent. The channels appeared to incorporate into bilayers with the same orientation, and were blocked from one side (the side of vesicle addition) by 0.2-1 mM TEA+. The block was slightly voltage-dependent. Acidification of resealed granule membranes in response to external ATP (which activated the vacuolar-type ATPase) was significantly reduced in the presence of 1 mM intralumenal TEACl (with 9 mM KCl), and parallel measurements with the potential-sensitive dye Oxonol V showed that such vesicles tended to develop higher internal-positive membrane potentials than control vesicles containing only 10 mM KCl. 1 mM TEA+ had no effect on proton-pumping activity when applied externally, and did not directly affect either the proton-pumping or ATP hydrolytic activity of the partially-purified ATPase. These results suggest that chromaffin granule membranes contain a TEA(+)-sensitive K+ channel which may have a role in regulating the vesicle membrane potential.

Voltage-activated potassium currents of rabbit osteoclasts: effects of extracellular calcium

The effects of increased extracellular Ca2+ concentration ([Ca2+]e) were examined on a delayed-rectifier K+ current (IK) and an inward-rectifier K+ current (IK1) in rabbit osteoclasts. Elevation of [Ca2+]e from 1.8 to 18 mM shifted the half point for IK activation by +11.5 mV and the voltage dependence of inactivation by +9.7 mV and slowed the rate of IK activation and deactivation. These effects of elevated [Ca2+]e on IK are consistent with screening of cell surface negative charge. However, elevation of [Ca2+]e increased the voltage-dependent kinetics of IK inactivation at all potentials tested, inconsistent with that predicted by simple surface charge theory. This finding suggests an additional, regulatory role for [Ca2+]e in the gating of IK channels. Some osteoclasts had an IK1, which was decreased when [Ca2+]e was raised from 1.8 to 18 mM. The physiological function of both types of K+ currents remains to be determined, and it is not clear whether these currents are involved with the coupling of cytosolic [Ca2+] to [Ca2+]e.

Electrostatic distance geometry in a K+ channel vestibule

Many voltage-gated K+ channels carry in the external vestibule a receptor for charybdotoxin, a peptide channel blocker. We use point mutagenesis of both charybdotoxin and a Shaker K+ channel to isolate the electrostatic interaction energy between chosen pairs of residues, one on the channel and one on bound toxin. The results allow estimates of physical distances between such residue pairs and, in combination with the known structure of charybdotoxin, localize specific channel residues in three-dimensional space.

Vasodilating actions of cromakalim in resting and contracting states of carotid arteries from spontaneously hypertensive rats

To determine the properties of cromakalim-opened K+ channels in arterial smooth muscle of spontaneously hypertensive rats (SHR), the effects of cromakalim on tension and 86Rb efflux were compared in endothelium-denuded strips of carotid arteries from 13-week-old SHR and normotensive Wistar-Kyoto rats (WKY). The addition of cromakalim or of nifedipine to resting strips caused a relaxation only in SHR. When strips from both strains were contracted with 15.9 mM K+, the magnitude of the precontraction was greater in SHR than in WKY. Under these conditions, relaxant responses to lower concentrations of cromakalim were decreased and those to higher concentrations of cromakalim were increased in SHR. When strips from both strains were contracted with a different concentration of K+ to an equivalent magnitude (78% of the maximum) relaxant responses to cromakalim were greater in SHR than in WKY. When strips were contracted with 10(-7) M norepinephrine, the precontraction was similar between SHR and WKY, and relaxant responses to cromakalim were greater in SHR. In both strains, the relaxant responses to cromakalim were competitively antagonized by glibenclamide, a blocker of ATP-sensitive K+ (KATP) channels, with a pA2 value of approximately 7.3. Charybdotoxin (10(-7) M), a blocker of Ca(2+)-activated K+ (KCa) channels, did not inhibit the relaxant responses to cromakalim in both strains. Charybdotoxin alone elicited a contraction, which was greater in SHR than in WKY. In resting strips preloaded with 86Rb, the basal 86Rb efflux rate constant was higher in SHR than in WKY. The addition of cromakalim (10(-5) M) to the resting strips increased the 86Rb efflux rate constant in both strains to a similar peak value. The addition of nifedipine (10(-7) M) to the resting strips decreased the basal 86Rb efflux rate constant only in SHR, and concomitantly affected the action of cromakalim in SHR. The results suggest that (1) cromakalim caused arterial relaxation via the opening of KATP channels in both SHR and WKY, (2) although the relaxant effects of cromakalim tended to be greater in SHR than in WKY, the differences were rather small, depended on the precontraction tone and varied with the concentration of the vasoconstrictors, and (3) there was an increased basal Ca2+ influx and a high activation of KCa channels in the resting state of SHR arteries, and these changes might influence the effects of cromakalim.

Covalent attachment of charybdotoxin to the beta-subunit of the high conductance Ca(2+)-activated K+ channel. Identification of the site of incorporation and implications for channel topology

Purified high conductance Ca(2+)-activated K+ (maxi-K) channels from bovine tracheal smooth muscle have been covalently labeled employing monoiodotyrosine charybdotoxin ([125I]ChTX) and different bifunctional cross-linking reagents. [125I]ChTX was specifically incorporated into the beta-subunit, which was thereafter isolated by size exclusion high performance liquid chromatography. Proteolytic fragments of the [125I]ChTX-labeled beta-subunit were generated by digestion with various endoproteinases. Glu-C or Asp-N cleavage yielded a glycosylated [125I]ChTX-labeled fragment of 13-14 kDa. A site-directed antiserum raised against residues 62-75 of the cloned beta-subunit of the maxi-K channel specifically recognizes the beta-subunit in immunostaining experiments and was capable of immunoprecipitating these ChTX-labeled peptides. Lys-C cleavage resulted in two fragments of 16 and 28 kDa, respectively, which were both precipitated by anti-beta (62-75). However, only the 28-kDa fragment was recognized by anti-beta(118-132) and shown to carry double the amount of N-linked carbohydrates. Taken together, these data restrict the site of covalent incorporation of ChTX into the beta-subunit exclusively at Lys69, confirm the predicted topology of this subunit, and indicate that both canonical N-linked glycosylation sites are occupied with complex carbohydrates of 5-6 kDa each. We propose that an extracellularly located portion of the beta-subunit is located within 7.7 A of the ChTX receptor site and could even participate in the formation of this receptor by close apposition of its extracellular domain with structural elements provided by the alpha-subunit.

Functional role of charybdotoxin-sensitive K+ channels in the resting state of dog basilar artery

To determine the possible role of Ca(2+)-activated K+ (KCa) channels in the regulation of resting tone of cerebral arteries, the effect of agents which interact with these channels on tension and 86Rb efflux was examined in endothelium-denuded strips of dog basilar artery, and data were compared with findings in the mesenteric artery. The basilar artery maintained a myogenic tone; that is, the resting tone decreased when nifedipine was added. The addition of charybdotoxin, a blocker of large conductance KCa channels, caused a concentration-dependent contraction in the basilar artery but not in the mesenteric artery. In strips preloaded with 86Rb, the basal 86Rb efflux rate constant was significantly greater in the basilar artery than in the mesenteric artery. Nifedipine decreased the basal 86Rb efflux rate constant only in the basilar artery. The basal 45Ca influx in the resting state of the basilar artery was significantly increased when compared with the mesenteric artery and this increase in the basilar artery was reduced by nifedipine. The results suggest that the charybdotoxin-sensitive KCa channels regulate the myogenic tone in the resting state of the basilar artery. The activation of KCa channels in the basilar artery appears to be secondary to the increased transmembrane Ca2+ influx probably via the activation of L-type voltage-dependent Ca2+ channels in the resting state of this artery.

Use of cultured human neuroblastoma cells in rapid discovery of the voltage-gated potassium-channel blockers

Depolarization of human neuroblastoma cells by high concentrations of extracellular potassium ions, leads to the activation of the voltage-gated potassium channels. The activity of such potassium channels can be effectively and rapidly monitored by tracking the efflux of 86Rb from pre-loaded target cells in response to the depolarizing stimulus. The inclusion of compounds with unknown activity in the assay medium, can result in the identification of novel blockers of the voltage-gated potassium channels. Since this functional assay is performed in 96-well microtitre plates, it represents a rapid and high-volume primary screening method for the detection and identification of the voltage-gated potassium-channel blockers, which may have therapeutic utility in several indications including memory degeneration and cardiac arrhythmias.

High-level expression and functional reconstitution of Shaker K+ channels

Voltage-gated K+ channels were expressed in COS cells transiently transfected with a plasmid carrying a cDNA for an inactivation-removed Shaker K+ channel driven by an adenovirus promoter. Channel expression was followed by immunological detection, binding of radioactive charybdotoxin (CTX), and functional reconstitution into planar lipid bilayers. About 10(7) channels per transfected cell are expressed on the plasma membrane. The expressed channels are glycosylated and competent to bind CTX with the expected characteristics. Channels observed after insertion into planar lipid bilayers displayed the voltage-dependent gating, conduction, and ion selectivity behavior expected for this channel. Channels were solubilized in several detergents without loss of CTX binding activity. The results make plausible a systematic attack on the purification of milligram-level amounts of functional K+ channels from a heterologous expression system.

Nitric oxide and cGMP cause vasorelaxation by activation of a charybdotoxin-sensitive K channel by cGMP-dependent protein kinase

Nitric oxide (NO)-induced relaxation is associated with increased levels of cGMP in vascular smooth muscle cells. However, the mechanism by which cGMP causes relaxation is unknown. This study tested the hypothesis that activation of Ca-sensitive K (KCa) channels, mediated by a cGMP-dependent protein kinase, is responsible for the relaxation occurring in response to cGMP. In rat pulmonary artery rings, cGMP-dependent, but not cGMP-independent, relaxation was inhibited by tetraethylammonium, a classical K-channel blocker, and charybdotoxin, an inhibitor of KCa channels. Increasing extracellular K concentration also inhibited cGMP-dependent relaxation, without reducing vascular smooth muscle cGMP levels. In whole-cell patch-clamp experiments, NO and cGMP increased whole-cell K current by activating KCa channels. This effect was mimicked by intracellular administration of (Sp)-guanosine cyclic 3',5'-phosphorothioate, a preferential cGMP-dependent protein kinase activator. Okadaic acid, a phosphatase inhibitor, enhanced whole-cell K current, consistent with an important role for channel phosphorylation in the activation of NO-responsive KCa channels. Thus NO and cGMP relax vascular smooth muscle by a cGMP-dependent protein kinase-dependent activation of K channels. This suggests that the final common pathway shared by NO and the nitrovasodilators is cGMP-dependent K-channel activation.

Voltage-dependent ionic conductances in Chinese hamster ovary cells

Chinese hamster ovary (CHO) cells are becoming a widely used biological material. A number of studies report membrane ion conductance changes after transfection of channels and receptors, but there are few data available on the properties of membrane ion conductances of CHO cells before transfection. In this work we studied voltage-dependent ionic conductances in cultures of CHO native (CHO-K1) cells. Three types of voltage-dependent ionic conductances were identified: 1) a K+ conductance showing sensitivity to Ca2+ and a unit conductance of approximately 210 pS in symmetrical 150 mM K+ outside-out patches (this conductance, which did not inactivate during a 160-ms pulse, was inhibited by 30 nM charybdotoxin but not by 30 mM extracellular tetraethylammonium); 2) a rapidly activating and inactivating tetrodotoxin (TTX)-sensitive inward current, peaking at about -10 to 0 mV (this current showed characteristics similar in many respects to Na+ current recorded in neurons); and 3) another voltage-dependent inward current, which had slow inactivation, was TTX insensitive but was blocked by Co2+ (current was also carried by Ba2+, peaked at approximately 0 to +10 mV, was identified as a Ca2+ conductance, and was inhibited by dihydropyridines but not by 10 microM omega-conotoxin). Cell-attached patch recordings of single Ca2+ channel currents demonstrated a unitary conductance of approximately 20 pS.

Early effects of PRL on ion conductances in CHO cells expressing PRL receptor

Chinese hamster ovary (CHO-K1) cells were stably transfected with prolactin (PRL) receptor cDNA. These cells (CHO-E32) expressed the long form of functional PRL receptor. Using microfluorimetric and patch-clamp techniques, we have investigated the effects of PRL on intracellular Ca2+ concentration ([Ca2+]i) and membrane ion conductances. Exposure of CHO-E32 cells to 5 nM PRL resulted in an increase in [Ca2+]i. Two types of response were observed: 1) a stimulation of Ca2+ entry and 2) an intracellular Ca2+ mobilization. As PRL inhibited voltage-activated Ca2+ current, the PRL-induced Ca2+ increase does not involve voltage-activated Ca2+ channels. PRL also increased a charybdotoxin-sensitive Ca(2+)-dependent K+ conductance. Simultaneous measurements showed that PRL hyperpolarized the membrane potential before increasing intracellular Ca2+ levels. In voltage clamp, hyperpolarizing voltage steps were associated with increased Ca2+ concentrations, whereas depolarizing voltage steps decreased [Ca2+]i. Cell-free patch-clamp experiments showed that PRL directly stimulates K+ channel activity. Our results suggest the existence of a regulatory complex involving a protein kinase tightly associated with the Ca(2+)-activated K+ channels and that PRL stimulates these channels by means of the activation of protein kinase. The resulting hyperpolarization stimulates Ca2+ entry, probably through voltage-insensitive nonspecific channels.

Evidence in support of a role for Ca(2+)-activated K+ channels in the hamster sperm acrosome reaction

The sperm acrosome reaction (AR) is a crucial step for mammalian fertilization. This work describes experiments to test the effect of the cesium ion (Cs+) and charybdotoxin (ChTX) on the Ca2+ or Na+/K+ ionophores stimulated hamster sperm AR in vitro. Cs+ and ChTX, a polypeptide toxin from the venom of the scorpion Leirus quinquestriatus, are considered blockers of Ca(2+)-activated K+ channels in several somatic cells. Both agents inhibited the AR by 55-66%. The inhibition was completely reversed by the Na+/K+ ionophore nigericin, but not by the Ca2+ ionophore A-23187. Results give evidence in support of a role for Ca(2+)-activated K+ channels in K+ influx required for the occurrence of the hamster sperm acrosome reaction.

Injection of a K+ channel (Kv1.3) cRNA in fertilized eggs leads to functional expression in cultured myotomal muscle cells from Xenopus embryos

The synthetic cRNA encoding for the major T lymphocyte K+ channel (Kv1.3) was injected into Xenopus fertilized eggs. Somites from embryos of stage 20-22 (about 40 h post-fertilization at 19 degrees C) were dissociated and myotomal muscle cells were cultured in vitro for 2 days. The whole cell configuration of the tight seal patch-clamp technique was used to record K+ channel activity in cultured myocytes. These myocytes have two endogenous delayed-rectifiers (sustained and transient) and an inward-rectifier K+ currents, all of which are insensitive to the scorpion toxin charybdotoxin. Cultured myocytes dissociated from embryos injected with the Kv1.3 cRNA expressed the exogenous Kv1.3 channel. The Kv1.3 channel was identified by its physiological (a very low recovery from inactivation) and its pharmacological properties (a high sensitivity to charybdotoxin). This work demonstrates that Xenopus cultured myotomal muscle cells represent a very efficient and practical assay system for the functional expression of cloned ion channels.

Spontaneous transient inward currents and rhythmicity in canine and guinea-pig tracheal smooth muscle cells

Spontaneous transient inward currents (STICs) were recorded in canine and guinea-pig tracheal myocytes held at negative membrane potentials. STICs were Cl- selective since their reversal potential was dependent on the Cl- gradient and they were blocked by the Cl- channel blocker niflumic acid. STICs were insensitive to Cs+, charybdotoxin, and nifedipine. Ca(2+)-activated K+ currents often preceded STICs, suggesting that the STICs are Ca2+ dependent. In support of this suggestion, we found the Cl- currents were: (1) abolished by depleting intracellular Ca2+ stores using caffeine, acetylcholine, histamine, or substance P; (2) enhanced by increasing external concentrations of Ca2+; (3) evoked by voltage-dependent Ca2+ influx. The channels responsible for this Cl- current are of small unitary conductance (< 20 pS). Decay of the STICs was described by a single exponential with a time constant of 94 +/- 9 ms at -70 mV; the time constant increased considerably at more positive potentials. Using Ca(2+)-dependent Cl- currents and contractions as indices of internal levels of Ca2+, we found that isolated tracheal cells are capable of exhibiting rhythmic behaviour: bursts of currents and contractions with a periodicity of less than 0.1 Hz and which continued for more than 20 min. These rhythmic events were recorded at negative membrane potentials, suggesting that cyclical release of internally sequestered Ca2+ is responsible. We conclude that spontaneous release of Ca2+ from intracellular stores in tracheal muscle cells leads to transient currents in some cases accompanied by rhythmic contractions. Our studies provide evidence for a cellular mechanism that could underly myogenic oscillations of membrane potential in smooth muscle.

Involvement of apamin-sensitive K+ channels in antigen-induced spasm of guinea-pig isolated trachea

1. In order to examine whether K+ channels play a role in antigen-induced airway responses, the effect of K+ channel blockers on antigen-induced airway smooth muscle contraction and mediator release was examined in vitro in guinea-pigs actively sensitized with ovalbumin (OA). 2. Tracheal strips from sensitized animals were suspended in organ baths under a resting tension of 1 g and isometric tension was continuously measured. Cumulative concentration-response curves to OA (0.1-1000 ng ml-1) or histamine (10 nM-1 mM) were obtained in the presence and absence of K+ channel blockers. 3. OA (10, 100 or 1000 ng ml-1) was incubated with minced lung tissues from the same animals for 15 min in the presence and absence of K+ channel blockers, and released histamine and leukotriene C4 (LTC4) in the incubating medium were measured. 4. Apamin, a small conductance Ca(2+)-activated K+ channel (PK,Ca) blocker, (0.1, 0.3 and 1 microM) significantly inhibited OA-induced smooth muscle contraction, while charybdotoxin (ChTX, 10 nM), an intermediate and large conductance PK,Ca blocker, and iberiotoxin (IbTX, 3 nM), a large conductance PK,Ca blocker, were without effect. Apamin (0.3 microM) had no effect on exogenously administered histamine-induced airway smooth muscle contraction, suggesting that the inhibition of OA-induced contraction by apamin did not occur at the smooth muscle level. 5. The inhibition of OA-induced contraction by apamin (0.3 microM) was not significantly affected by pretreatment with a leukotriene antagonist, ONO-1078 (10 microM), but was abolished by pretreatment with a histamine H1-receptor blocker, pyrilamine (1 microM). 6. Apamin by itself (up to 0.1 MicroM) had no effect on spontaneous histamine release from minced lung tissues. Histamine release induced by low and intermediate concentrations of OA (10 and 100 ng ml-1)was significantly suppressed by apamin pretreatment (P<0.05 and P<0.001), whereas LTC4 release was not affected. ChTX (0.1 MicroM) and IbTX (10 nM) had no significant effect on either spontaneous or OA (100 ng ml-1)-induced histamine release.7. These results suggest that apamin partially but substantially inhibits antigen-induced smooth muscle contraction, presumably by inhibiting antigen-induced histamine release from airway mast cells through small conductance PKca closure.

Physiological role of Ca(2+)-activated and voltage-dependent K+ currents in rabbit coronary myocytes

The properties and function of Ca(2+)-activated K+ (KCa) and voltage-dependent K+ (IK) currents of rabbit coronary myocytes were studied under whole cell voltage-clamp conditions (22 degrees C). Inhibition of KCa by tetraethylammonium chloride (1-10 mM) or charybdotoxin (50-100 nM) suppressed noisy outward rectifying current elicited by 5-s voltage steps or ramp at potentials > 0 mV, reduced the hump of the biphasic ramp current-voltage relation, and shifted by less than +5 mV the potential at which no net steady-state current is recorded (Enet; index of resting membrane potential). Inhibition of steady-state inward Ca2+ currents [ICa(L)] by nifedipine (1 microM) displaced Enet by -11 mV. Analysis of steady-state voltage dependence of IK supported the existence of a "window" current between -50 and 0 mV. 4-Aminopyridine (2 mM) blocked a noninactivating component of IK evoked between -30 and -40 mV, abolished the hump current during ramps, and shifted Enet by more than +15 mV; hump current persisted during 2-min ramp depolarizations and peaked near the maximum overlap of the steady-state activation and inactivation curves of IK (about -22 mV). A threefold rise in extracellular Ca2+ concentration (1.8-5.4 mM) enhanced time-dependent outward K+ current (6.7-fold at +40 mV) and shifted Enet by -30 mV. It is concluded that, under steady-state conditions, IK and ICa(L) play a major role in regulating resting membrane potential at a physiological level of intracellular Ca2+ concentration, with a minor contribution from KCa. However, elevation of intracellular Ca2+ concentration enhances KCa and hyperpolarizes the myocyte to limit Ca2+ entry through ICa(L).

Nitric oxide-mediated inhibitory response of rat proximal colon: independence from changes in membrane potential

1. We studied the relation of nitric oxide-mediated relaxation of smooth muscle to changes in membrane potential of cells in the proximal colon of rats. 2. The resting membrane potential and electrical field stimulation (EFS)-induced junction potentials were recorded from the circular and longitudinal muscle cells. 3. Localized distension with a small balloon caused relaxation of the circular muscle on the anal side of the distended region (descending relaxation). Relaxation of the longitudinal muscle was also induced by EFS. 4. Inhibitory junction potentials (i.j.ps) were recorded from all circular muscle cells tested, but rarely from the longitudinal muscle cells. 5. The i.j.ps were recorded only in the presence of atropine but relaxations of both muscles were induced even in the absence of atropine. 6. Apamin (100 nM) completely abolished the i.j.ps recorded in both circular and longitudinal muscle cells, but had no significant effect on the relaxations of either. 7. In contrast to apamin, Ng nitro-L-arginine (10 microM) inhibited the relaxations of both muscles, but did not affect the i.j.ps. 8. Exogenously added nitric oxide (0.1-10 microM) induced relaxations of both muscles concentration-dependently, but did not affect the membrane potentials at these concentrations. 9. These data strongly suggest that nitric oxide-mediated relaxation of rat proximal colon is not associated with the i.j.ps of the cell membrane.

The charybdotoxin receptor of a Shaker K+ channel: peptide and channel residues mediating molecular recognition

Charybdotoxin (CTX) is a peptide of known structure that inhibits Shaker K+ channels by a pore-blocking mechanism. Point mutagenesis of all 30 solvent-exposed residues identified the part of the CTX molecular surface making contact with the receptor in the K+ channel. All close-contact residues are clustered in a well-defined interaction surface; the shape of this surface implies that the outer opening of the Shaker channel conduction pore abruptly widens to a 25 x 35 A plateau. A mutagenic scan of the S5-S6 linker sequence of the Shaker K+ channel identified those channel residues influencing CTX binding affinity. The Shaker residues making the strongest contribution to toxin binding are located close to the pore-lining sequence, and more distant residues on both sides of this region influence CTX binding weakly, probably by an electrostatic mechanism. Complementary mutagenesis of both CTX and Shaker suggests that Shaker-F425 contacts a specific area near T8 and T9 on the CTX molecular surface. This contact point constrains Shaker-F425 to be located at a 20 A radial distance from the pore axis and 10-15 A above the "floor" of the CTX receptor.

Pharmacological characterization of five cloned voltage-gated K+ channels, types Kv1.1, 1.2, 1.3, 1.5, and 3.1, stably expressed in mammalian cell lines

We have analyzed the biophysical and pharmacological properties of five cloned K+ (Kv) channels (Kv1.1, Kv1.2, Kv1.3, Kv1.5, and Kv3.1) stably expressed in mammalian cell lines. Kv1.1 is biophysically similar to a K+ channel in C6 glioma cells and astrocytes, Kv1.3 and Kv3.1 have electrophysiological properties identical to those of the types n and l K+ channels in T cells, respectively, and Kv1.5 closely resembles a rapidly activating delayed rectifier in the heart. Each of these native channels may be formed from the homomultimeric association of the corresponding Kv subunits, and pharmacological compounds that selectively modulate them may be useful for the treatment of neurological, immune, and cardiac disorders. The cell lines described in this report could be used to identify such drugs and we have therefore embarked on a pharmacological characterization of the five cloned channels. The compounds tested in this study include 4-aminopyridine, capsaicin, charybdotoxin, cromakalim, dendrotoxin, diltiazem, D-sotalol, flecainide, kaliotoxin, mast cell degranulating peptide, nifedipine, noxiustoxin, resiniferatoxin, and tetraethylammonium.

Responses of salivary acinar cells to intracellular alkalinization

Responses of rat submandibular acini to intracellular alkalinization were investigated. Intracellular alkalinization was induced by addition of NH4Cl or methyl amines, or by prepulse with Na butyrate. Only partial recovery occurred following Na butyrate prepulse or methylated amine addition, but full recovery was observed following addition of NH4Cl. The latter recovery was DIDS and dimethylamiloride-insensitive but was inhibited by bumetanide or high [K+] and stimulated in Na(+)-free buffer and by ouabain. Acetylcholine stimulated recovery from NH4Cl- or Na butyrate pre-pulse-induced alkalinization and reduced the extent of alkalinization induced by methylated amines. Acetylcholine-stimulated recovery from NH4Cl-induced alkalinization was mimicked by substance P or ionomycin and was partially Ca(2+)-dependent. This stimulated recovery was bumetanide-insensitive but was partially sensitive to charybdotoxin. Taken together, these data indicate that in unstimulated cells, recovery from alkalinization induced by NH4Cl occurs by bumetanide-sensitive transport of the NH4+ ion, that DIDS-inhibitable anion transport contributes little to this recovery, and that acetylcholine and other Ca(2+)-elevating agents accelerate recovery from NH4Cl-induced alkaline challenge by a mechanism insensitive to bumetanide, DIDS, ouabain, and dimethylamiloride but sensitive to extracellular Ca2+ and to charybdotoxin. Partial recovery from alkaline challenge can also occur in the absence of NH4+ ions, and acetylcholine also stimulates this mode of recovery. Together, these data suggest that these cells have little intrinsic ability to recover from intracellular alkalinization and that the NH4+ ion may be a surrogate for K+ in at least two ion transport pathways.

Secondary structure of noxiustoxin and charybdotoxin from hydropathy power spectra

The analysis of the hydropathy profile power spectra provides a basis for studies of pattern matching between the primary and secondary structure of peptides. The structural motif obtained with Noxiustoxin (NTX), the first K+ channel blocking peptide described, is composed of a N-terminal beta-strand, a central alpha-helix and a final beta-strand zone, probably forming a beta-sheet. These results were compared with those of Charybdotoxin (ChTX), a potent inhibitor of the high conductance Ca(2+)-activated K+ channel, which presents about 48% similarity with NTX in the amino acid sequence. Our prediction for ChTX secondary structure, which is known by 2D-NMR spectroscopy, yielded a Chou-Fasman quality index Q = 90%. The comparison between the two toxins has guided the interpretation of the data obtained.

Functional unit size of the charybdotoxin receptor in smooth muscle

Target inactivation analysis was used to determine the functional size of the charybdotoxin (ChTX) receptor in aortic and tracheal sarcolemmal membrane vesicles. This receptor has previously been shown to be an integral component of the high-conductance Ca2+-activated K+ (Maxi-K) channel in these smooth muscles. Exposure of either bovine aortic or bovine tracheal sarcolemma to high-energy irradiation results in disappearance of 125I-labeled ChTX binding activity as a monoexponential function of radiation dose; from these functions molecular masses of 88 +/- 10 kDa and 89 +/- 6 kDa, respectively, can be calculated. Similar results were obtained from radiation inactivation studies with the detergent-solubilized ChTX receptor from aortic sarcolemmal membranes. The effect of radiation on 125I-labeled ChTX binding is to decrease the number of functional ChTX receptors without affecting the affinity of receptors for the toxin, indicating that radiation is destroying, rather than altering, the binding site. The validity of the radiation inactivation technique in these membrane preparations is supported by data obtained in parallel experiments in which target sizes of the alpha 1 subunit of the L-type Ca2+ channel and 5'-nucleotidase were measured. The molecular masses determined for these entities are in excellent agreement with those expected from previous studies. The present data are discussed in terms of the recently determined subunit composition of the smooth muscle Maxi-K channel. In light of the target size, a single alpha beta subunit heterodimer complex could serve as the ChTX receptor.

Tremorgenic indole alkaloids potently inhibit smooth muscle high-conductance calcium-activated potassium channels

Tremorgenic indole alkaloids produce neurological disorders (e.g., staggers syndromes) in ruminants. The mode of action of these fungal mycotoxins is not understood but may be related to their known effects on neurotransmitter release. To determine whether these effects could be due to inhibition of K+ channels, the interaction of various indole diterpenes with high-conductance Ca(2+)-activated K+ (maxi-K) channels was examined. Paspalitrem A, paspalitrem C, aflatrem, penitrem A, and paspalinine inhibit binding of [125I]charybdotoxin (ChTX) to maxi-K channels in bovine aortic smooth muscle sarcolemmal membranes. In contrast, three structurally related compounds, paxilline, verruculogen, and paspalicine, enhanced toxin binding. As predicted from the binding studies, covalent incorporation of [125I]ChTX into the 31-kDa subunit of the maxi-K channel was blocked by compounds that inhibit [125I]ChTX binding and enhanced by compounds that stimulate [125I]ChTX binding. Modulation of [125I]ChTX binding was due to allosteric mechanisms. Despite their different effects on binding of [125I]ChTX to maxi-K channels, all compounds potently inhibited maxi-K channels in electrophysiological experiments. Other types of voltage-dependent or Ca(2+)-activated K+ channels examined were not affected. Chemical modifications of paxilline indicate a defined structure-activity relationship for channel inhibition. Paspalicine, a deshydroxy analog of paspalinine lacking tremorgenic activity, also potently blocked maxi-K channels. Taken together, these data suggest that indole diterpenes are the most potent nonpeptidyl inhibitors of maxi-K channels identified to date. Some of their pharmacological properties could be explained by inhibition of maxi-K channels, although tremorgenicity may be unrelated to channel block.

Role of calcium-activated potassium channels in the relaxation of tracheal smooth muscles by forskolin

1. The role of calcium-activated potassium (KCa) channels in bronchodilation produced by a direct adenylyl cyclase activator, forskolin, was investigated. The involvement of intracellular cyclic AMP (cAMP) in the process was also examined. 2. The isometric tension records from guinea-pig tracheal smooth muscles indicated that application of charybdotoxin (ChTX), a selective inhibitor of large conductance KCa channels, led to a suppression of the relaxant effect of forskolin in the precontracted tissue by carbachol (CCh). However, the inhibitory action by ChTX had a much greater effect on the relaxation caused by isoproterenol than by forskolin. 3. In contrast to the effect of ChTX, glybenclamide, a cromakalim-sensitive K+ channel inhibitor and apamin, a small conductance KCa channel blocker, had no effects on the bronchodilation produced by forskolin. 4. The effects of forskolin and nifedipine on tone produced by high K+ was compared. Concentration-inhibition curves in guinea-pig trachealis precontracted by 20 mmol/L K+ solution were similar for forskolin and nifedipine. Conversely, relaxation by forskolin was significantly diminished when tissues were contracted with 40 mmol/L K+ solution, whereas nifedipine relaxations were unaffected. 5. A single channel record from a cell-attached patch in a porcine tracheal myocyte demonstrated that forskolin stimulates reversibly KCa channels without affecting the unitary amplitude. 6. The results are consistent with forskolin-induced relaxation occurring at least in part through the opening of ChTX-sensitive KCa channels, by means of a cAMP-dependent channel modulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of calcium influx by ATP and store depletion in primary cultures of renal proximal cells

Cytoplasmic calcium changes and calcium influx evoked by adenosine triphosphate (ATP) were investigated in primary cultures of rabbit proximal convoluted tubule cells. Extracellular ATP (50 microM) induced a biphasic increase of [Ca2+]i measured with the calcium probe fura-2. In the early phase, the mobilization of intracellular pools resulted in a transient increase of [Ca2+]i from 106 +/- 11 nM (n = 36) to 1059 +/- 115% (n = 29) of the resting level within 10 s. In the presence of external calcium, [Ca2+]i then decreased within 3 min to a sustained level (398 +/- 38%, n = 8). Measurements of fura-2 quenching by external manganese revealed that this phase was the result of an increased Ca2+ uptake, blocked by lanthanum (10 microM) and verapamil (100 microM) but not by the nifedipin (25 microM). Internal calcium store depletion by ATP induced an increased calcium influx through lanthanum- and verapamil-sensitive, nifedipin-insensitive calcium channels, located on the apical membrane of the cells. As indicated by 86Rb+ efflux measurements, ATP activated a potassium efflux that was blocked by barium and Leiurus quinquestriatus hebraeus (LQH) venom (containing charybdotoxin) indicating the involvement of Ca(2+)-sensitive K+ channels. Moreover, in the presence of the LQH venom, the internal calcium stores were not replenished after being depleted by ATP. Our results indicate that an ATP-evoked hyperpolarization of the plasma membrane leads to increased Ca2+ influx, which facilitates the replenishment of the internal stores.

Deformation of swollen erythrocytes provides a model of sickling-induced leak pathways, including a novel bromide-sensitive component

Deoxygenation-induced red blood cell (RBC) sickling probably activates multiple cation leak pathways. In an attempt to model this, we examined the net passive K efflux ("K leak") from normal and sickle RBCs undergoing elliptical deformation in hypotonic media (200 mOsmol/L). This hypotonic deformation activates two deformation-dependent K leak pathways that are not detectable during the balanced leak (Kefflux = Nainflux) resulting from deformation of RBCs in isotonic medium. These are (1) a calcium-dependent leak component and (2) a novel leak pathway that is inhibited by substitution of bromide (but not sulfamate) for chloride, which converts the unbalanced K leak (Kefflux > Nainflux) of hypotonic deformation to a residual balanced leak. This dramatic effect of hypotonic deformation is reversible, is detected in both normal and sickle RBCs, and is inhibited significantly by 4,4'-diisothiocyano-2,2'-stilbene disulfonate. Remarkably, bromide also inhibits by 55% the K leak resulting from authentic deoxygenation-induced RBC sickling and, thereby, blunts the imbalance of accompanying monovalent cation leaks. The unique effect of bromide is not readily explainable on the basis of known behaviors of known ion leak/transport pathways. The mechanical threshold for triggering K leak during hypotonic deformation is at applied shear stress of 164 dyne/cm2, a value similar to the abnormal susceptibility we previously found for oxygenated sickle RBCs during isotonic deformation. These data suggest that membrane stretch accompanying hypotonic deformation activates the same multiple leak pathways that contribute to net K leak during authentic RBC sickling, including a previously unknown bromide-sensitive leak.

Atrial natriuretic peptide enhances activity of potassium conductance in adrenal glomerulosa cells

Aldosterone secretion from the adrenal glomerulosa (AG) cells is inhibited by atrial natriuretic peptide (ANP). Inasmuch as alterations in K+ conductance can modulate aldosterone secretion, the effect of ANP on intracellular K+ homeostasis was investigated. Intracellular K+ concentration ([K+]i) of AG cells was assessed by spectrofluorometry using the K(+)-sensitive dye, K(+)-binding benzofuran isophthalate. The resting value of [K+]i in AG cells was determined to be 120 +/- 1.2 mM (n = 37) in a HCO3-free, N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered medium. Exposure of AG cells to ANP led to a dose-dependent, transient decrease in [K+]i, from 21 +/- 3.2% (n = 7) at 100 pM to 31 +/- 2.3% at 1 microM (n = 7). In the continued presence of ANP, a rapid recovery to near basal values of [K+]i was attained within 90 s. Measurements of membrane voltage using the potential sensitive dye 1-3(-sulfonatopropyl)-4-[beta-(-(di-n-butylamino)-6-naphthyl)vinyl ]- pyridinium betaine documented an accompanying change in membrane potential. Pretreatment of AG cells with barium (0.5 mM), tetraethylammonium (0.1 mM), charybdotoxin (100 nM), or ethylene glycol-bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid (0.5 mM) blunted the ANP-induced decrease in [K+]i. ANP-(7-23), the ANP-C-receptor selective agonist, which does not elevate guanosine 3',5'-cyclic monophosphate (cGMP) did not alter [K+]i in contrast to cGMP (50 microM), which did. We conclude that ANP via the activation of the ANP A receptor alters K+ homeostasis through a Ca(2+)-activatable K(+)-conductive pathway likely to be the maxi-K channel.

Enhanced role of potassium channels in relaxations to acetylcholine in hypercholesterolemic rabbit carotid artery

The effect of hypercholesterolemia for 10 wk on endothelium-dependent relaxations to acetylcholine was studied in isolated rings of rabbit carotid artery and abdominal aorta contracted with phenylephrine or elevated potassium. In these arteries obtained from hypercholesterolemic rabbits, endothelium-dependent relaxations to acetylcholine were not significantly different from those of normal rabbits. In normal and hypercholesterolemic arteries, partial relaxation persisted in the presence of NG-nitro-L-arginine methyl ester (L-NAME), which blocked acetylcholine-induced increases in arterial guanosine 3',5'-cyclic monophosphate (cGMP). Combined treatment with L-NAME and the calcium-dependent potassium-channel inhibitor, charybdotoxin, blocked relaxations in both groups, suggesting that L-NAME-resistant relaxations are mediated by an endothelium-derived hyperpolarizing factor. Charybdotoxin alone or depolarizing potassium had no significant effect on normal carotid artery or normal and hypercholesterolemic abdominal aorta but significantly inhibited relaxations of the carotid artery from cholesterol-fed rabbits. The enhanced role of calcium-dependent potassium channels and the hyperpolarizing factor in relaxation of the hypercholesterolemic carotid artery suggested by these results was likely related to the fact that acetylcholine failed to stimulate cGMP only in that artery. These data suggest that endothelium-dependent relaxation in these rabbit arteries is mediated by nitric oxide-cGMP-dependent and -independent mechanisms. In hypercholesterolemia, the contribution of nitric oxide-cGMP in the carotid artery is reduced, but a hyperpolarizing factor and calcium-dependent potassium channels maintain normal acetylcholine-induced relaxation.

Fractal analysis of role of smooth muscle Ca2+ fluxes in genesis of chaotic arterial pressure oscillations

We have investigated the role of vascular smooth muscle Ca2+ fluxes in the genesis of chaotic pressure oscillations induced by histamine in isolated resistance arteries from the rabbit ear. The responses exhibited distinct "fast" and "slow" components, with periods of 5-20 s and 1-5 min, respectively, which could be dissociated pharmacologically. The fast subsystem involved ion movements at the cell membrane and was inhibited by both low (< 2 mM) and high (> 5 mM) extracellular Ca2+ concentration ([Ca2+]o) by verapamil (which inhibits voltage-dependent Ca2+ influx) and by charybdotoxin (ChTX) and apamin (which block Ca(2+)-activated K+ channels). In contrast, the slow subsystem was intracellular and was selectively attenuated by ryanodine, which inhibits Ca(2+)-induced Ca2+ release from sarcoplasmic reticulum. The effects of these interventions on the complexity of the responses were quantified by calculating their fractal dimension, a parameter that estimates the minimum number of independent variables contributing to an irregular time series. Its mean value was generally > 2 under control conditions but decreased to < 2 in a concentration-dependent fashion in the presence of verapamil, ChTX, apamin, or ryanodine and when [Ca2+]o was outside the range of 2-3 mM. Each intervention thus removed one dimension of complexity from the mechanisms generating the rhythmic activity. We conclude that the interaction of a fast membrane oscillator, which involves Ca2+ influx, Ca(2+)-activated K+ efflux, and therefore presumably changes in membrane potential, and a slow intracellular oscillator involving Ca2+ sequestration and release from stores is responsible for vascular chaos in our model. The coupling between these subsystems is likely to be mediated by cytosolic [Ca2+].

Potassium single-channel properties in normal and Rous sarcoma virus-transformed chicken embryo fibroblasts

Ion channels in normal and Rous sarcoma virus (RSV)-transformed chicken embryo fibroblasts (CEFs) were examined by using the patch-clamp technique. Three different types of ion channels were observed with single-channel conductances in symmetrical 140 mM KCl (with frequencies of occurrence in parentheses) of 186 pS (70%), 110 pS (10%), and 65 pS (20%), which are identical in normal and RSV-transformed CEFs. The total channel density in both cell types is about 0.13 per micron2. All three types of channels are highly selective for K+ ions, they are Ca(2+)- and voltage-dependent, and they can be completely blocked by external tetraethylammonium (10 mM) in both normal and RSV-transformed cells. Some channel properties, however, are different in normal and RSV-transformed CEFs. The K186 channel of normal CEFs is almost completely activated in the presence of about 1 nM free internal Ca2+ and is insensitive to charybdotoxin (100 nM). In contrast, the K186 channel of RSV-transformed CEFs has an EC50 value for activation by internal Ca2+ of about 100 nM and is highly sensitive to charybdotoxin (IC50 = 9 nM). In normal CEFs, the K186 channel activity starts at membrane potentials more positive than -50 mV and reaches a high open state probability of 0.94 at +50 mV. In RSV-transformed CEFs, the threshold of K186 channel activity is also -50 mV but the maximal open state probability is only 0.70 at +50 mV membrane potential. Averages of current traces of K186 channels show the typical features of the macroscopic K+ currents described previously for normal and RSV-transformed CEFs.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that prostaglandin E2 can block calcium-activated 86Rb efflux from rat brain synaptosomes via a protein kinase C-dependent mechanism

The effects of prostaglandin E2 (PGE2) on 86Rb efflux from rat brain synaptosomes were studied to explore its role in nerve ending potassium (K+) channel modulation. A selective dose-dependent inhibition of the calcium-activated charybdotoxin-sensitive component of efflux was found upon application of PGE2. No significant effect was seen on basal and voltage-dependent components over the concentration range of 10(-8) to 10(-5) M. The protein kinase C (PKC) inhibitors H-7 (10 microM) and staurosporine (100 nM), as well as prolonged preincubation (90 min) with 4 beta-phorbol 12,13-dibutyrate, which has been reported to down-regulate PKC, abolished the PGE2-induced inhibition, whereas HA1004 (10 microM) and Rp-3',5'-cyclic phosphorothioate (100 nM), which are relatively more selective for protein kinase A than PKC, did not. 4 beta-Phorbol 12,13-dibutyrate (100 nM), an activator of PKC, produced a similar inhibition of the Ca(2+)-dependent component of 86Rb efflux but also had no effect on the basal and voltage-dependent components. These data suggest that PGE2 can inhibit rat brain nerve ending calcium-activated 86Rb efflux, and this inhibition may involve PKC activation.

Differential modulation of large-conductance KCa channels by PKA in pregnant and nonpregnant myometrium

Uterine excitability depends on ion channel activity, the expression of which is regulated by sexual hormones. We show now that the action of protein kinase A (PKA) on large-conductance calcium-activated K+ (KCa) channel activity also depends on the hormonal status. PKA-dependent phosphorylation of reconstituted KCa channels from midpregnant rats usually stimulated channel activity; in contrast, KCa channels from nonpregnant rat and human myometrium were primarily inhibited by this mechanism. Both effects were reversible by phosphatase treatment. These results suggest that one important factor modulating uterine contractility during pregnancy or the regular cycle may be the differential response of KCa channels toward PKA-induced phosphorylation.

Effect of potassium channel blockade and alpha 2-adrenoceptor activation on the release of nitric oxide from non-adrenergic non-cholinergic nerves

1. Using a superfusion bioassay cascade, we studied the effect of K+ channel blockers and alpha 2-adrenoceptor agents on the release of a transferable factor, previously characterized as nitric oxide (NO) or a nitric oxide-related substance (NO-R), in response to non-adrenergic non-cholinergic (NANC) nerve stimulation in the canine ileocolonic junction (ICJ). 2. The non-selective K+ channel blockers, 4-aminopyridine (4-AP, 50 microM) and tetraethylammonium (TEA, 1 mM) and the more selective blocker of Ca(2+)-activated K+ channels, charybdotoxin (Leiurus quinquestriatus venom (LQV), 0.4 microgram ml-1), significantly enhanced the release of NO-R induced by low frequency stimulation (2-4 Hz). In the presence of 4-AP and TEA, the release of NO-R was nearly abolished by tetrodotoxin (2 microM), and by L-NG-nitroarginine (L-NOARG, 0.1 mM). Relaxations induced by direct injection of exogenous NO (5-50 pmol) or nitroglycerin (GTN, 10-30 pmol) onto the rabbit aortic detector ring were not affected. 3. The alpha 2-adrenoceptor agonist, UK-14,304 (0.3 microM) inhibited the release of NO-R induced by low (2-4 Hz), but not that induced by high (16 Hz), frequency stimulation. This inhibitory effect was completely reversed by the alpha 2-adrenoceptor antagonist, yohimbine (0.3 microM). Neither UK-14,304 nor yohimbine affected the relaxations induced by exogenous NO (5 pmol) or GTN (10 pmol) on the aortic detector ring.3+

Nitric oxide directly activates calcium-dependent potassium channels in vascular smooth muscle

Nitric oxide is the major endothelium-derived relaxing factor (EDRF), and it is thought to relax smooth muscle cells by stimulation of guanylate cyclase, accumulation of its product cyclic GMP, and cGMP-dependent modification of several intracellular processes, including activation of potassium channels through cGMP-dependent protein kinase. Here we present evidence that both exogenous nitric oxide and native EDRF can directly activate single Ca(2+)-dependent K+ channels (K+Ca) in cell-free membrane patches without requiring cGMP. Under conditions when guanylate cyclase was inhibited by methylene blue, considerable relaxation of rabbit aorta to nitric oxide persisted which was blocked by charybdotoxin, a specific inhibitor of K+Ca channels. These studies demonstrate a novel direct action of nitric oxide on K+Ca channels.

Potassium and calcium channel dependence of bursting in cultured neuronal networks

Increases in extracellular potassium concentrations reliably increase burst rates in cultured fetal murine spinal cord networks. This effect could be mimicked by either blocking voltage-gated potassium conductances or facilitating excitatory synaptic interactions, but not by blocking specific calcium-dependent potassium conductances or tonic depolarization. Spontaneous bursting in cultured networks is apparently dependent on potassium currents and intracellular calcium levels, but not on the pharmacologically characterized calcium-dependent potassium conductances.

NS 004--an activator of Ca(2+)-dependent K+ channels in cerebellar granule cells

The large-conductance Ca(2+)-dependent K+ channels or BK channels in cerebellar granule cells were studied by patch-clamp technique, and the effects on channel activity of the molecule NS 004 (1-(2-hydroxy-5-chlorophenyl)-5-trifluoromethyl-2- benzimidazolone) were investigated. The channels had a unit conductance of 187 pS, were blocked by charybdotoxin and activated by internal Ca2+. NS 004 (10-30 microM) significantly increased the single channel opening frequency as well as the mean open time. In whole-cell recordings the compound shifted the BK current-voltage relationship by up to 40 mV towards negative membrane potentials. NS 004 is an efficient BK channel opener, which may represent a novel approach to relaxation of neuronal cells expressing this type of K+ channel.

Synthesis and characterization of sapecin and sapecin B

Two insect defencins, sapecin and sapecin B, were chemically synthesized to confirm their structure and antibacterial activity and also to examine the possibility that these peptides bind to the same site on the large conductance calcium-activated potassium channel as charybdotoxin. Both synthetic peptides showed the same antibacterial activity as native sapecins, indicating that the synthetic peptides folded correctly in the chemical synthesis. Synthetic sapecins did not show an inhibitory effect on [125I]charybdotoxin binding to rat brain synaptic membranes, suggesting that sapecin B recognizes a different binding site from that of charybdotoxin despite the similar structural motif.

Inhibition of excitatory non-adrenergic non-cholinergic bronchoconstriction in guinea-pig airways in vitro by activation of an atypical 5-HT receptor

1. The effect of 5-hydroxytryptamine (5-HT) was studied on excitatory neurally mediated non-adrenergic non-cholinergic (NANC) contractions evoked by electrical field stimulation (EFS) in guinea-pig isolated bronchi. 2. 5-HT (0.1-100 microM) produced a concentration-dependent inhibition of the excitatory NANC response with 50.9 +/- 5.0% (n = 5, P < 0.01) inhibition at 100 microM. This inhibition was not significantly affected by the 5-HT2 antagonist, ketanserin (1 microM) when inhibitions (+/- ketanserin) at each concentration of 5-HT were compared by unpaired t tests; however, this concentration appeared to produce a leftward shift (approximately 10 fold) of the 5-HT concentration-inhibition curve. Ketanserin (1 microM) was effective in blocking bronchoconstriction evoked by activation of 5-HT2A receptors on airway smooth muscle. In the presence of ketanserin (1 microM) 5-HT (100 microM) evoked an inhibition of 57.4 +/- 5.9% (n = 5, P < 0.01) with an EC50 of 0.57 microM. 3. Inhibition evoked by 5-HT (0.1-100 microM) was unaffected by the alpha-adrenoceptor antagonist phentolamine (1 microM), the beta 2-adrenoceptor antagonist, ICI 118551 (0.1 microM), the 5-HT1A/B antagonist, cyanopindolol (1 microM) or the 5-HT3/4 antagonist, ICS 205-930 (1 microM). 4. Methiothepin (0.1 microM) produced an insurmountable inhibition of the effect of 5-HT (0.1-100 microM), reducing the maximum inhibition produced by 5-HT (100 microM) to 30.2 +/- 5.0% (n = 5, P < 0.001) and suggesting a non-competitive antagonism. Methiothepin inhibited the effect of 5-HT (10 microM) in a concentration-dependent manner with an IC50 of 81 nM. 5. Selective 5-HT receptor agonists were also tested on excitatory NANC responses. 5-Carboxamidotryptamine (5-CT, 0.1-100 MicroM) was the most potent, producing a concentration-dependent inhibition with an EC50 of 0.13 MicroM. Calculation of approximate IC25 values (concentration of the agonist required to give a 25% inhibition of the excitatory NANC response) gave a rank order of potency 5-CT > 5-HT> > 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) >alpha-methyl-5-hydroxytryptamine (alpha-Me-5HT). Sumatriptan, 5-methoxytryptamine (5-MeOT) and 2-methyl-5-hydroxytryptamine (2-Me-5HT) were essentially inactive with IC25> 100 MicroM.6. 5-HT (10 microM) did not significantly affect contractile responses to exogenously applied substance P(1 nM-10 Microm).7. The effect of 5-HT was unchanged after incubation with the nitric oxide (NO) synthase inhibitor L-NG-nitroarginine methyl ester (L-NAME, 100 Microm). However, pretreatment with charybdotoxin (ChTX,0.1-30 nM), a blocker of the large conductance Ca2+-activated K+channel (K+ca), produced a concentration-dependent inhibition of the effect of 5-HT (10 MicroM).8. 5-HT evokes a concentration-dependent inhibition of e-NANC bronchoconstriction in guinea-pig isolated bronchi but does not affect cumulative concentration-dependent contractile responses to substance P, suggesting that inhibition is via a prejunctional receptor. Effects of selective antagonists and agonists suggest that an atypical 5-HT receptor mediates this inhibition. The inhibitory effect of 5-HT does not involve the production of NO, but may involve the opening a ChTX-sensitive K+ca channel.These data suggest that an atypical 5-HT receptor inhibits the release of neuropeptides from sensory C fibres and may act as other inhibitory neuromodulators via the opening of a common K'channel.

Engineering a uniquely reactive thiol into a cysteine-rich peptide

Cysteine mutagenesis for the purpose of chemical labelling was applied to the K+ channel neurotoxin charybdotoxin, a 37-residue peptide with six functionally essential cysteines. An additional 'spinster cysteine' was introduced at a position far away in space from the toxin's known interaction surface where it contacts its K+ channel receptor. Despite the presence of the extra unpaired cysteine residue, the toxin still folds efficiently and may be labelled by fluorescent and radioactive reagents to give a functionally competent toxin.

Characterization of Ca(2+)-activated K+ channels in excised patches of human T lymphocytes

Ca(2+)-activated K+ [K(Ca)] channels were studied in excised patches of resting and activated human peripheral blood T lymphocytes. The K(Ca) channel had a single-channel conductance of 50 +/- 6 pS in symmetrical high-K+ solutions in the potential range of -100 to -10 mV and was inwardly rectifying at more depolarized potentials. The channel was sensitive to block by charybdotoxin (10 nM) and insensitive to apamin (3 nM). Half-maximum activation occurred at an internal free Ca2+ concentration of 360 +/- 110 nM. The concentration-effect curve had a slope factor of 0.83 +/- 0.12, suggesting a 1:1 interaction of Ca2+ ions with the channel. Ca2+ affects the open time probability of the K(Ca) channels, mainly by modulating the frequency of channel opening. The open probability did not show voltage dependence. The kinetics of the channel could be described assuming one open state and two closed states. The time constant of the exponential describing the open time distribution amounted to 2.8 +/- 1.2 ms, whereas the closed time distribution could be described with two exponentials with time constants of 0.2 +/- 0.05 ms and 8.0 +/- 2.1 ms, respectively. Resting T lymphocytes expressed a low number of channels but the density of channels increased dramatically during chronic phytohaemagglutinin stimulation.

Reconstitution of expressed KCa channels from Xenopus oocytes to lipid bilayers

Reconstitution of large conductance calcium-activated potassium (KCa) channels from native cell membranes into planar lipid bilayers provides a powerful method to study single channel properties, including ion conduction, pharmacology, and gating. Recently, KCa channels derived from the Drosophila Slowpoke (Slo) gene have been cloned and heterologously expressed in Xenopus oocytes. In this report, we describe the reconstitution of cloned and expressed Slo KCa channels from Xenopus oocyte membranes into lipid bilayers. The reconstituted channels demonstrate functional properties characteristic of native KCa channels. They possess a mean unitary conductance of approximately 260 pS in symmetrical potassium (250 mM), and they are voltage- and calcium-sensitive. At 50 microM Ca2+, their half-activation potential was near -20 mV; and their affinity for calcium is in the micromolar range. Reconstituted Slo KCa channels were insensitive to external charybdotoxin (40-500 nM) and sensitive to micromolar concentrations of external tetraethylammonium (KD = 158 microM, at 0 mV) and internal Ba2+ (KD = 76 microM, at 40 mV). In addition, they were blocked by internally applied "ball" inactivating peptide (KD = 480 microM, at 40 mV). These results demonstrate that cloned KCa channels expressed in Xenopus oocytes can be readily incorporated into lipid bilayers where detailed mechanistic studies can be performed under controlled internal and external experimental conditions.

Calcium-activated potassium channels in rat visceral sensory afferents

The purpose of the present study was to describe, at the single-channel level, the activity of a calcium-sensitive potassium channel in rat visceral-sensory neurons which has been suggested to be involved in sensory neuron excitability. Single-channel recordings in the inside-out configuration identified a 220 pS conductance calcium-activated potassium channel (KCa). From a -20 mV holding potential, increasing [Ca2+]i from 0.01 microM to 1.0 microM increased the open probability of this channel 92% (from 0.12 to 0.23). However, from a +20 mV holding potential, increasing [Ca2+]i from 0.01 to 1.0 microM increased the open probability by 326% (from 0.15 to 0.64). In addition, this large conductance KCa channel was blocked by TEA (1.0 microM) and charybdotoxin (40 microM) when applied to the external surface. These results are the first to characterize a large conductance KCa channel in the sensory afferent neurons of the rat nodose ganglia and should further expand the understanding of the ionic currents involved in the regulation of sensory afferent neuronal activity.

Calcium-sensitive potassium current in isolated canine coronary smooth muscle cells

This study characterizes K+ current in canine coronary artery and investigates its role in regulation of vascular smooth muscle tone during the resting and activated state. Isolated rings and whole-cell K+ current as well as single K+ channels were studied. Tetraethylammonium (< 3 mM) did not increase the resting tension in isolated rings; however, 0.3 mM tetraethylammonium increased tension in vessels that were precontracted by elevated [K+]o or 5-hydroxytryptamine (5-HT). The whole-cell K+ current showed voltage and Ca2+ dependency and sensitivity to tetraethylammonium (31 +/- 7, 72 +/- 2, and 83 +/- 4% depression by 1, 10, and 30 mM tetraethylammonium, respectively). A large-conductance (100 pS) K+ channel was identified in cell-attached patches with open-time distribution fitted with two exponentials. Calcium ionophore A23187 (10 microM) increased the probability of opening, mean open time, and amplitude of this channel in cell-attached patches, suggesting Ca2+ dependency. A23187 shifted the plot of unitary current as a function of pipette potential to the right, suggesting A23187-induced cell hyperpolarization. In inside-out patches, increase in cytoplasmic-side [Ca2+] from 10(-7) to 10(-6) M increased both the frequency of channel opening and duration of the open state, without changing its conductance. Tetraethylammonium (1 mM) on the cytoplasmic side caused a reversible decrease in the current amplitude. Charybdotoxin (100 nM) decreased the probability of opening and mean open time and increased mean closed time, while apamin (100 nM) did not significantly affect channel kinetics. In summary, this study demonstrates the existence and important functional role of a large-conductance, Ca(2+)-sensitive K+ channel in regulation of membrane potential and cell excitability, as well as some aspects of regulation and kinetics of this channel in canine coronary arterial cells.

Solution structure of a core peptide derived from scyllatoxin

An analysis of the sequences of scyllatoxin and charybdotoxin suggested that it would be possible to design a core peptide sequence which would still fold to give the beta-hairpin and helix seen in the toxins, but which would eliminate one disulfide and connecting residues. The core sequence was modeled, then synthesized and purified. The cysteines oxidize in air to give the same disulfide pairings as seen in the parent toxins as the major product. The three-dimensional structure of the core sequence peptide, termed Max, was determined using proton NMR spectroscopy and found to be identical in secondary structure to the toxins. However differences were found in the relative orientation of the beta-hairpin and helix. The use of this structural motif, found in many insect toxins, as a disulfide framework for exploring sequence/structure/activity relationships is discussed.