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

High-conductance potassium channels of the SLO family

High-conductance, 'big' potassium (BK) channels encoded by the Slo gene family are among the largest and most complex of the extended family of potassium channels. The family of SLO channels apparently evolved from voltage-dependent potassium channels, but acquired a large conserved carboxyl extension, which allows channel gating to be altered in response to the direct sensing of several different intracellular ions, and by other second-messenger systems, such as those activated following neurotransmitter binding to G-protein-coupled receptors (GPCRs). This versatility has been exploited to serve many cellular roles, both within and outside the nervous system.

Verruculogen associated with Aspergillus fumigatus hyphae and conidia modifies the electrophysiological properties of human nasal epithelial cells

Background

The role of Aspergillus fumigatus mycotoxins in the colonization of the respiratory tract by conidia has not been studied extensively, even though patients at risk from invasive aspergillosis frequently exhibit respiratory epithelium damage. In a previous study, we found that filtrates of A. fumigatus cultures can specifically alter the electrophysiological properties of human nasal epithelial cells (HNEC) compared to those of non pathogenic moulds.

Results

We fractionated the organic phase of filtrate from 3-day old A. fumigatus cultures using high-performance liquid chromatography. The different fractions were tested for their ability to modify the electrophysiological properties of HNEC in an in vitro primary culture model.

The fraction collected between 20 and 30 min mimicked the effects of the whole filtrate, i.e. decrease of transepithelial resistance and increase of potential differences, and contained secondary metabolites such as helvolic acid, fumagillin, and verruculogen. Only verruculogen (10^-8 M) had effects similar to the whole filtrate. We verified that verruculogen was produced by a collection of 67 human, animal, plant and environmental A. fumigatus isolates. Using MS-MS analysis, we found that verruculogen was associated with both mycelium and conidia extracts.

Conclusion

Verruculogen is a secondary metabolite that modifies the electrophysiological properties of HNEC. The role of these modifications in the colonization and invasion of the respiratory epithelium by A. fumigatus on first contact with the epithelium remains to be determined.

Characterisation of large-conductance calcium-activated potassium channels (BK(Ca)) in human NT2-N cells

Large-conductance calcium-activated potassium (BK(Ca)) channels were studied in inside-out patches of human NTERA2 neuronal cells (NT2-N). In symmetrical (140 mM) K(+) the channel mean conductance was 265 pS, the current reversing at approximately 0 mV. It was selective (P(K)/P(Na)=20:1) and blocked by internal paxilline and TEA. The open probability-voltage relationship for BK(Ca) was fitted with a Boltzmann function, the V((1/2)) being 76.3 mV, 33.6 mV and -14.1 mV at 0.1 muM, 3.3 muM and 10 muM [Ca(2+)](i), respectively. The relationship between open probability and [Ca(2+)](i) was fitted by the Hill equation (Hill coefficient 2.7, half maximal activation at 2.0 muM [Ca(2+)](i)). Open and closed dwell time histograms were fitted by the sum of two and three voltage-dependent exponentials, respectively. Increasing [Ca(2+)](i) produced both an increase in the longer open time constant and a decrease in the longest closed time constant, so increasing mean open time. "Intracellular" ATP evoked a concentration-dependent increase in NT2-N BK(Ca) activity. At +40 mV half-maximum activation occurred at an [ATP](i) of 3 mM (30 nM [Ca(2+)](i)). ADP and GTP were less potent, and AMP-PNP was inactive. This is the first characterisation of a potassium channel in NT2-N cells showing that it is similar to the BK(Ca) channel of other preparations.

Contribution of epoxyeicosatrienoic acids to the hypoxia-induced activation of Ca2+-activated K+ channel current in cultured rat hippocampal astrocytes

Brief hypoxia differentially regulates the activities of Ca(2+)-activated K(+) channels (K(Ca)) in a variety of cell types. We investigated the effects of hypoxia (<2% O(2)) on K(Ca) channel currents and on the activities of cytochrome P450 2C11 epoxygenase (CYP epoxygenase) in cultured rat hippocampal astrocytes. Exposure of astrocytes to hypoxia enhanced macroscopic outward K(Ca) current, increased the open state probability (NPo) of 71 pS and 161 pS single-channel K(Ca) currents in cell-attached patches, but failed to increase the NPo of both the 71 pS and 161 pS K(Ca) channel currents recorded from excised inside-out patches. The hypoxia-induced enhancement of macroscopic K(Ca) current was attenuated by pretreatment with tetraethylammonium (TEA, 1 mM) or during recording using low-Ca(2+) external bath solution. Exposure of astrocytes to hypoxia was associated with generation of superoxide as detected by staining of cells with the intracellular superoxide detection probe hydroethidine (HE), attenuation of the hypoxia-induced activation of unitary K(Ca) channel currents by superoxide dismutation with tempol, and as quantitated by high-pressure liquid chromatography/fluorescence assay using HE as a probe. In cultured astrocytes in which endogenous CYP epoxygenase activity has been inhibited with either miconazole or N-methylsulfonyl-6-(2-propargyloxyphenyl) hexanamide (MSPPOH) hypoxia failed to increase the NPo of both the 71 pS and 161 pS K(Ca) currents and generation of superoxide. Hypoxia increased the level of P450 epoxygenase protein and production of epoxyeicosatrienoic acids (EETs) from cultured astrocytes, as determined by immunohistochemical staining and LC/MS analysis, respectively. Exogenous 11,12-EET increased the NPo of both the 71 pS and 161 pS K(Ca) single-channel currents only in cell-attached but not in excised inside-out patches of cultured astrocytes. These findings indicate that hypoxia enhances the activities of two types of unitary K(Ca) currents in astrocytes by a mechanism that appears to involve CYP epoxygenase-dependent generation of superoxide and increased production or release of EETs.

Neuregulin-1 isoform induces mitogenesis, KCa and Ca2+ currents in PC12 cells. A comparison with sciatic nerve conditioned medium

Neuregulin-1 (NRG-1) is an active component found in sciatic nerve conditioned medium (CM). NRG-1 is a growth and differentiation factor shown to have an effect on neuritogenesis and survival of neural cells. PC12 cells chronically treated with NRG-1 (beta1 isoform) show an increase in proliferation under low-serum condition (2.5% fetal bovine serum and 1.25% horse serum) and serum deprivation, without visible morphological changes. NRG-1 and CM treatments of PC12 cells induced an increase of voltage-activated Ca2+ currents and large-conductance calcium-activated K+ currents (KCa). AG825, a specific inhibitor for erbB2 receptor, abolishes KCa current, though Ca2+ currents were not inhibited. These results showed that NRG-1 is capable of inducing functional changes but is not sufficient on its own to have an effect on cell morphology.

MaxiK Blockade Selectively Inhibits the Lipopolysaccharide-Induced IκB-α/NF-κB Signaling Pathway in Macrophages

Macrophages have a pivotal function in innate immunity against bacterial infections. They are present in all body compartments and able to detect invading microorganisms with high sensitivity. LPS (endotoxin) of Gram-negative bacteria is among the most potent stimuli for macrophages and initiates a wide panel of cellular activation responses. The release of mediators such as TNF-alpha and ILs is essential for the initiation of a proinflammatory antibacterial response. Here, we show that blockade of the large-conductance Ca2+ -activated potassium channel MaxiK (BK) inhibited cytokine production from LPS-stimulated macrophages at the transcriptional level. This inhibitory effect of channel blockade was specific to stimulation with LPS and affected neither stimulation of macrophages with the cytokine TNF-alpha nor LPS-induced activation of cells that do not express MaxiK. Investigation of the upstream intracellular signaling pathways induced by LPS revealed that the blockade of MaxiK selectively inhibited signaling pathways leading to the activation of the transcription factor NF-kappaB and the MAPK p38, whereas activation of ERK was unaffected. We present data supporting that proximal regulation of the inhibitory factor IkappaB-alpha is critically involved in the observed inhibition of NF-kappaB translocation. Using alveolar macrophages from rats, we could show that the necessity of MaxiK function in activation of NF-kappaB and subsequent cytokine production is not restricted to in vitro-generated monocyte-derived macrophages but also can be observed in primary cells. Thus, MaxiK appears to be a central molecule in the NF-kappaB-dependent inflammatory response of macrophages to bacterial LPS.

Synthesis of a biotin derivative of iberiotoxin: binding interactions with streptavidin and the BK Ca2+-activated K+ channel expressed in a human cell line

Iberiotoxin (IbTx) is a scorpion venom peptide that inhibits BK Ca2+-activated K+ channels with high affinity and specificity. Automated solid-phase synthesis was used to prepare a biotin-labeled derivative (IbTx-LC-biotin) of IbTx by substitution of Asp19 of the native 37-residue peptide with N--(D-biotin-6-amidocaproate)-L-lysine. Both IbTx-LC-biotin and its complex with streptavidin (StrAv) block single BK channels from rat skeletal muscle with nanomolar affinity, indicating that the biotin-labeled residue, either alone or in complex with StrAv, does not obstruct the toxin binding interaction with the BK channel. IbTx-LC-biotin exhibits high affinity (KD = 26 nM) and a slow dissociation rate (koff = 5.4 x 10(-4) s(-1)) in a macroscopic blocking assay of whole-cell current of the cloned human BK channel. Titration of IbTx-LC-biotin with StrAv monitored by high performance size exclusion chromatography is consistent with a stoichiometry of two binding sites for IbTx-LC-biotin per StrAv tetramer, indicating that steric interference hinders simultaneous binding of two toxin molecules on each of the two biotin-binding faces of StrAv. In combination with fluorescent conjugates of StrAv or anti-biotin antibody, IbTx-LC-biotin was used to image the surface distribution of BK channels on a transfected cell line. Fluorescence microscopy revealed a patch-like surface distribution of BK channel protein. The results support the feasibility of using IbTx-LC-biotin and similar biotin-tagged K+ channel toxins for diverse applications in cellular neurobiology. .

A role for voltage-gated, but not Ca2+-activated, K+ channels in regulating spontaneous contractile activity in myometrium from virgin and pregnant rats

The roles of voltage-gated (K(V)) and large conductance Ca2+-activated K+ (BK(Ca)) channels in regulating basal contractility in myometrial smooth muscle are unresolved. The aim of this study was to determine the effects of inhibition of these channels on spontaneous rhythmic contraction in myometrial strips from four groups of rats: nonpregnant and during early (day 7), mid- (day 14), and late (day 21) pregnancy. BK(Ca) channels were inhibited using iberiotoxin (1-100 nM), paxilline (1-10 microM) or penitrem A (1-500, or 3000 nM); K(V) channels were inhibited using tetraethylammonium (TEA; 1-10 mM) and/or 4-aminopyridine (4-AP; 1-5 mM). Contractility was assessed as mean integral tension (MIT). Time/vehicle controls were also performed. None of the selective BK(Ca) channel inhibitors significantly affected contractility in myometrial strips from either nonpregnant or pregnant animals. 4-AP caused concentration-dependent increases in MIT in myometrium in all four groups. TEA (5 and 10 mM) significantly increased MIT in myometrium from nonpregnant, and mid- and late pregnant rats, but not in myometrium from early pregnant rats. TEA and 4-AP still caused an increase in MIT following treatment with 3000 nM penitrem A or a combination of propranolol, phentolamine, atropine (all 1 microM) and capsaicin (10 microM) in myometrial strips from nonpregnant rats. These results indicate that whereas BK(Ca) channels play little or no part in controlling basal rhythmicity in rat myometrium, K(V) channels appear to play a crucial role in this regard, especially during mid- and late pregnancy.

Dendritic calcium spikes are tunable triggers of cannabinoid release and short-term synaptic plasticity in cerebellar Purkinje neurons

Understanding the relationship between dendritic excitability and synaptic plasticity is vital for determining how dendrites regulate the input-output function of the neuron. Dendritic calcium spikes have been associated with the induction of long-term changes in synaptic efficacy. Here we use direct recordings from cerebellar Purkinje cell dendrites to show that synaptically activated local dendritic calcium spikes are potent triggers of cannabinoid release, producing a profound and short-term reduction in synaptic efficacy at parallel fiber synapses. Enhancing dendritic excitability by modulating dendritic large-conductance calcium-activated potassium (BK) channels improves the spread of dendritic calcium spikes and enhances cannabinoid release at the expense of spatial specificity. Our findings reveal that dendritic calcium spikes provide a local and tunable coincidence detection mechanism that readjusts synaptic gain when synchronous activity reaches a threshold, and they reveal a tight link between the regulation of dendritic excitability and the induction of synaptic plasticity.

BKCa Channels Activating at Resting Potential without Calcium in LNCaP Prostate Cancer Cells

Large-conductance Ca2+-dependent K+ (BK(Ca)) channels are activated by intracellular Ca2+ and membrane depolarization in an allosteric manner. We investigated the pharmacological and biophysical characteristics of a BK(Ca)-type K+ channel in androgen-dependent LNCaP (lymph node carcinoma of the prostate) cells with novel functional properties, here termed BK(L). K+ selectivity, high conductance, activation by Mg2+ or NS1619, and inhibition by paxilline and penitrem A largely resembled the properties of recombinant BK(Ca) channels. However, unlike conventional BK(Ca) channels, BK(L) channels activated in the absence of free cytosolic Ca2+ at physiological membrane potentials; the half-maximal activation voltage was shifted by about -100 mV compared with BK(Ca) channels. Half-maximal Ca2+-dependent activation was observed at 0.4 microM: for BK(L) (at -20 mV) and at 4.1 microM: for BK(Ca) channels (at +50 mV). Heterologous expression of hSlo1 in LNCaP cells increased the BK(L) conductance. Expression of hSlo-beta1 in LNCaP cells shifted voltage-dependent activation to values between that of BK(L) and BK(Ca) channels and reduced the slope of the P (open) (open probability)-voltage curve. We propose that LNCaP cells harbor a so far unknown type of BK(Ca) subunit, which is responsible for the BK(L) phenotype in a dominant manner. BK(L)-like channels are also expressed in the human breast cancer cell line T47D. In addition, functional expression of BK(L) in LNCaP cells is regulated by serum-derived factors, however not by androgens.

Revealing Complex Traits with Small Molecules and Naturally Recombinant Yeast Strains

Here we demonstrate that natural variants of the yeast Saccharomyces cerevisiae are a model system for the systematic study of complex traits, specifically the response to small molecules. As a complement to artificial knockout collections of S. cerevisiae widely used to study individual gene function, we used 314- and 1932-member libraries of mutant strains generated by meiotic recombination to study the cumulative, quantitative effects of natural mutations on phenotypes induced by 23 small-molecule perturbagens (SMPs). This approach reveals synthetic lethality between SMPs, and genetic mapping studies confirm the involvement of multiple quantitative trait loci in the response to two SMPs that affect respiratory processes. The systematic combination of natural variants of yeast and small molecules that modulate evolutionarily conserved cellular processes can enable a better understanding of the general features of complex traits.

Membrane-delimited inhibition of maxi-K channel activity by the intermediate conductance Ca2+-activated K channel

The complexity of mammalian physiology requires a diverse array of ion channel proteins. This diversity extends even to a single family of channels. For example, the family of Ca²⁺-activated K channels contains three structural subfamilies characterized by small, intermediate, and large single channel conductances. Many cells and tissues, including neurons, vascular smooth muscle, endothelial cells, macrophages, and salivary glands express more than a single class of these channels, raising questions about their specific physiological roles. We demonstrate here a novel interaction between two types of Ca²⁺-activated K channels: maxi-K channels, encoded by the K_Ca1.1 gene, and IK1 channels (K_Ca3.1). In both native parotid acinar cells and in a heterologous expression system, activation of IK1 channels inhibits maxi-K activity. This interaction was independent of the mode of activation of the IK1 channels: direct application of Ca²⁺, muscarinic receptor stimulation, or by direct chemical activation of the IK1 channels. The IK1-induced inhibition of maxi-K activity occurred in small, cell-free membrane patches and was due to a reduction in the maxi-K channel open probability and not to a change in the single channel current level. These data suggest that IK1 channels inhibit maxi-K channel activity via a direct, membrane-delimited interaction between the channel proteins. A quantitative analysis indicates that each maxi-K channel may be surrounded by four IK1 channels and will be inhibited if any one of these IK1 channels opens. This novel, regulated inhibition of maxi-K channels by activation of IK1 adds to the complexity of the properties of these Ca²⁺-activated K channels and likely contributes to the diversity of their functional roles.

Cocaine elicits action potential bursts in a central snail neuron: The role of delayed rectifying K+ current

The effects of cocaine were studied in an identifiable RP4 neuron of the African snail, Achatina fulica Ferussac, using the two-electrode voltage-clamp method. The RP4 neuron generated spontaneous action potentials and bath application of cocaine (0.3-1 mM) reversibly elicited action potential bursts of the central RP4 neuron in a concentration-dependent manner. The action potential bursts were not blocked when neurons were immersed in high-Mg(2+)solution, Ca(2+)-free solution, nor after continuous perfusion with atropine, d-tubocurarine, propranolol, prazosin, haloperidol, or sulpiride. Similarly, the action potential bursts were not abolished by pretreatment with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride, (9S,10S,12R)-2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid hexyl ester or anisomycin. Injection of hyperpolarizing current at an intensity of greater than 2 nA effectively suppressed the cocaine-elicited action potential bursts and no postsynaptic potentials were observed under these conditions. These results suggest that the generation of action potential bursts elicited by cocaine was not due to (1) the synaptic effects of neurotransmitters, (2) the cholinergic, adrenergic or dopaminergic receptors of the excitable membrane, or (3) the cAMP second messengers and new protein synthesis of the RP4 neuron. Notably, the induction of action potential bursts was blocked by pretreatment with 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione. Voltage-clamp studies conducted on the RP4 neuron revealed that cocaine at 0.3 mM decreased (1) the Ca(2+) current, (2) the delayed rectifying K(+) current, (3) the fast-inactivating K(+) current and (4) the Ca(2+)-activated K(+) current, but had no remarkable effects on the Na(+) current. Perfusion with Ca(2+)-free solution, which may abolish the Ca(2+) current and Ca(2+)-activated K(+) current, did not cause any bursts of action potentials in control RP4 neurons. Application of 4-aminopyridine, an inhibitor of fast-inactivating K(+) current, and paxilline, an inhibitor of Ca(2+)-activated K(+) current, failed to elicit action potential bursts, whereas tetraethylammonium chloride, a blocker of Ca(2+)-activated K(+) current and delayed rectifying K(+) current, and tacrine, an inhibitor of delayed rectifying K(+) current, successfully elicited action potential bursts. Further, while 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione did not affect the delayed rectifying K(+) current of the RP4 neuron, 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl)amino]hexyl]-1H-pyrrole-2,5-dione decreased the inhibitory effect of cocaine on the delayed rectifying K(+) current. It is concluded that cocaine elicits action potential bursts in the central snail RP4 neuron and that the effect is closely related to the inhibitory effects on the delayed rectifying K(+) current.

Large-conductance, calcium-activated potassium channels: structural and functional implications

The large-conductance, calcium-activated potassium channels (BK, also termed BK(Ca), Slo, or MaxiK) distributed in both excitable and non-excitable cells are involved in many cellular functions such as action potential repolarization; neuronal excitability; neurotransmitter release; hormone secretion; tuning of cochlear hair cells; innate immunity; and modulation of the tone of vascular, airway, uterine, gastrointestinal, and urinary bladder smooth muscle tissues. Because of their high conductance, activation of BK channels has a strong effect on membrane potential. BK channels differ from all other potassium (K(+)) channels due to their high sensitivity to both intracellular calcium (Ca(2+)) concentrations and voltage. These features make BK channels ideal negative feedback regulators in many cell types by decreasing voltage-dependent Ca(2+) entry through membrane potential hyperpolarization. The current review aims to give a comprehensive understanding of the structure and molecular biology of BK channels and their relevance to various pathophysiological conditions. The review will also focus on the therapeutic potential and pharmacology of the various BK channel activators and blockers.

A-type potassium current in retinal arteriolar smooth muscle cells

PURPOSE

By their control of membrane potential and intracellular free Ca(2+) ([Ca(2+)](i)), K(+) currents are pivotal in the regulation of arterial smooth muscle tone. The goal of the present study was to identify and characterize the A-type K(+) current in retinal microvascular smooth muscle (MVSM) and to examine its role in modulating membrane potential and cellular contractility.

METHODS

Whole-cell perforated patch-clamp recordings were made from MVSM cells within intact isolated arteriolar segments. Before patch-clamping, retinal arterioles were anchored in the physiological recording bath and perfused with an enzyme cocktail to remove surface basal lamina and to uncouple electrically the endothelial cells from the overlying MVSM cells.

RESULTS

K(+) currents were activated by depolarizing steps from -80 to +100 mV in 20-mV increments. A dominant, noninactivating current was elicited by depolarization to potentials positive of -50 mV. Inhibition of this current by 100 nM of the Ca(2+)-activated K(+) channel blocker, Penitrem A, revealed a rapidly inactivating K(+) current that resembled an A-type current. The A-type current was insensitive to tetraethylammonium (TEA) at 1 mM, but was partially suppressed by higher concentrations (10 mM). 4-Aminopyridine (10 mM; 4-AP) completely blocked the A-type current. The 4-AP-sensitive transient current was activated at a potential of -60 mV with peak current densities averaging 29.7 +/- 5.68 pA/pF at +60 mV. The voltage of half-inactivation was -28.3 +/- 1.9 mV, and the time constant for recovery from inactivation at +60 mV was 118.7 +/- 7.9 ms. Under current-clamp conditions 4-AP depolarized the membrane potential by approximately 3 to 4 mV and triggered small contractions and relaxations of individual MVSM cells within the walls of the arterioles.

CONCLUSIONS

A-type current is the major voltage-dependent K(+) current in retinal MVSM and appears to play a physiological role in suppressing cell excitability and contractility.

N-methyl-D-aspartate receptors and large conductance calcium-sensitive potassium channels inhibit the release of opioid peptides that induce mu-opioid receptor internalization in the rat spinal cord

Endogenous opioids in the spinal cord play an important role in nociception, but the mechanisms that control their release are poorly understood. To simultaneously detect all opioids able to activate the mu-opioid receptor, we measured mu-opioid receptor internalization in rat spinal cord slices stimulated electrically or chemically to evoke opioid release. Electrical stimulation of the dorsal horn in the presence of peptidase inhibitors produced mu-opioid receptor internalization in half of the mu-opioid receptor neurons. This internalization was rapidly abolished by N-methyl-D-aspartate (IC50=2 microM), and N-methyl-D-aspartate antagonists prevented this effect. mu-Opioid receptor internalization evoked by high K+ or veratridine was also inhibited by N-methyl-D-aspartate receptor activation. N-methyl-D-aspartate did not affect mu-opioid receptor internalization induced by exogenous endomorphins, confirming that the effect of N-methyl-D-aspartate was on opioid release. We hypothesized that this inhibition was mediated by large conductance Ca2+-sensitive K+ channels BK(Ca2+). Indeed, inhibition by N-methyl-D-aspartate was prevented by tetraethylammonium and by the selective BK(Ca2+) blockers paxilline, penitrem A and verruculogen. Paxilline did not increase mu-opioid receptor internalization in the absence of N-methyl-D-aspartate, indicating that it does not produce an increase in opioid release unrelated to the inhibition by N-methyl-d-aspartate. The BK(Ca2+) involved appears to be a subtype with slow association kinetics for iberiotoxin, which was effective only with long incubations. The BK(Ca2+) opener NS-1619 also inhibited the evoked mu-opioid receptor internalization, and iberiotoxin prevented this effect. We concluded that Ca2+ influx through N-methyl-D-aspartate receptors causes the opening of BK(Ca2+) and hyperpolarization in opioid-containing dorsal horn neurons, resulting in the inhibition of opioid release. Since mu-opioid receptors in the dorsal horn mediate analgesia, inhibition of spinal opioid release could contribute to the hyperalgesic actions of spinal N-methyl-D-aspartate receptors.

Patulin produced by an Aspergillus clavatus isolated from feed containing malting residues associated with a lethal neurotoxicosis in cattle

A severe neurotoxicosis, comprising tremors, ataxia, paresis, recumbency and death, occurred simultaneously among several herds of beef cattle in the region of Flanders (Belgium). After a first multi-toxin screening of some suspected diet elements, verruculogen was detected in a sample of a common feed ingredient. However, when the first animal necropsies revealed serious nervous lesions, including neuronal degeneration of the central nervous system and axonal degeneration in the peripheral nervous system, further investigations focused on fungal isolation. As expected from the pathological lesions, Aspergillus clavatus was found to be the dominant fungal species in a sample of compacted fodder, containing malting residues, consumed by all the affected herds. The isolated fungus appeared to produce patulin in culture medium. Traces of patulin were also detected in the fodder. These findings and their possible role in the intoxication are discussed.

The fungal neurotoxin lolitrem B inhibits the function of human large conductance calcium-activated potassium channels

The effects of the mycotoxin lolitrem B on the function of hSlo large conductance calcium-activated potassium channels expressed in HEK293 cells have been investigated using inside-out membrane patches. Lolitrem B potently inhibited hSlo potassium currents activated by depolarising voltage pulses in the presence of 10 microM free calcium. At a concentration of 100 nM, lolitrem B rapidly and completely inhibited outward potassium currents. The concentration that produced half-maximal inhibition was 3.7 nM, indicating a high apparent affinity for hSlo channels. This is the first time that a molecular site of action has been identified for a compound of the lolitrem structural class of indole diterpene and identifies a novel BK channel blocker.

Potassium channels in the peripheral microcirculation

Vascular smooth muscle (VSM) cells, endothelial cells (EC), and pericytes that form the walls of vessels in the microcirculation express a diverse array of ion channels that play an important role in the function of these cells and the microcirculation in both health and disease. This brief review focuses on the K+ channels expressed in smooth muscle and endothelial cells in arterioles. Microvascular VSM cells express at least four different classes of K+ channels, including inward-rectifier K+ channels (Kin), ATP-sensitive K+ channels (KATP), voltage-gated K+ channels (Kv), and large conductance Ca2+-activated K+ channels (BKCa). VSM KIR participate in dilation induced by elevated extracellular K+ and may also be activated by C-type natriuretic peptide, a putative endothelium-derived hyperpolarizing factor (EDHF). Vasodilators acting through cAMP or cGMP signaling pathways in VSM may open KATP, Kv, and BKCa, causing membrane hyperpolarization and vasodilation. VSMBKc. may also be activated by epoxides of arachidonic acid (EETs) identified as EDHF in some systems. Conversely, vasoconstrictors may close KATP, Kv, and BKCa through protein kinase C, Rho-kinase, or c-Src pathways and contribute to VSM depolarization and vasoconstriction. At the same time Kv and BKCa act in a negative feedback manner to limit depolarization and prevent vasospasm. Microvascular EC express at least 5 classes of K+ channels, including small (sKCa) and intermediate(IKCa) conductance Ca2+-activated K+ channels, Kin, KATP, and Kv. Both sK and IK are opened by endothelium-dependent vasodilators that increase EC intracellular Ca2+ to cause membrane hyper-polarization that may be conducted through myoendothelial gap junctions to hyperpolarize and relax arteriolar VSM. KIR may serve to amplify sKCa- and IKCa-induced hyperpolarization and allow active transmission of hyperpolarization along EC through gap junctions. EC KIR channels may also be opened by elevated extracellular K+ and participate in K+-induced vasodilation. EC KATP channels may be activated by vasodilators as in VSM. Kv channels may provide a negative feedback mechanism to limit depolarization in some endothelial cells.

Kv3 voltage-gated potassium channels regulate neurotransmitter release from mouse motor nerve terminals

Voltage-gated potassium (Kv) channels are critical to regulation of neurotransmitter release throughout the nervous system but the roles and identity of the subtypes involved remain unclear. Here we show that Kv3 channels regulate transmitter release at the mouse neuromuscular junction (NMJ). Light- and electron-microscopic immunohistochemistry revealed Kv3.3 and Kv3.4 subunits within all motor nerve terminals of muscles examined [transversus abdominus, lumbrical and flexor digitorum brevis (FDB)]. To determine the roles of these Kv3 subunits, intracellular recordings were made of end-plate potentials (EPPs) in FDB muscle fibres evoked by electrical stimulation of tibial nerve. Tetraethylammonium (TEA) applied at low concentrations (0.05-0.5 mM), which blocks only a few known potassium channels including Kv3 channels, did not affect muscle fibre resting potential but significantly increased the amplitude of all EPPs tested. Significantly, this effect of TEA was still observed in the presence of the large-conductance calcium-activated potassium channel blockers iberiotoxin (25-150 nM) and Penitrem A (100 nM), suggesting a selective action on Kv3 subunits. Consistent with this, 15-microM 4-aminopyridine, which blocks Kv3 but not large-conductance calcium-activated potassium channels, enhanced evoked EPP amplitude. Unexpectedly, blood-depressing substance-I, a toxin selective for Kv3.4 subunits, had no effect at 0.05-1 microM. The combined presynaptic localization of Kv3 subunits and pharmacological enhancement of EPP amplitude indicate that Kv3 channels regulate neurotransmitter release from presynaptic terminals at the NMJ.

Electrophysiological and pharmacological characterization of K+-currents in muscle fibres isolated from the ventral sucker ofFasciola hepatica

Fibres isolated from the ventral sucker ofFasciola hepaticawere identified as muscle on the basis of their contractility, and their actin and myosin staining. They were voltage-clamped at a holding potential of −40 mV and depolarization-activated outward currents were characterized both electrophysiologically and pharmacologically. Activation was well fitted by a Boltzmann equation with a half-maximal potential of +9 mV and a slope factor of −14·3 mV, and the kinetics of activation and deactivation were voltage-sensitive. Tail current analysis showed that the reversal potential was shifted by +46±3 mV when EKwas increased by 52 mV, confirming that this was a K+-current with electrophysiological characteristics similar to delayed rectifier and Ca2+-activated K+-currents in other tissues. The peak current at +60 mV was inhibited by 76±6% by tetrapentylammonium chloride (1 mM) and by 84±7% by Ba2+(3 mM), but was completely resistant to block by tetraethylammonium (30 mM), 3,4-diaminopyridine (100 μM) and 4-aminopyridine (10 mM). Penitrem A, a blocker of high-conductance Ca2+-activated K+-channels reduced the current at +60 mV by 23±5%. When the effects of Ca2+-channel blocking agents were tested, the peak outward current at +60 mV was reduced by 71±7% by verapamil (30 μM) and by 59±4% by nimodipine (30 μM). Superfusion with BAPTA-AM (50 μM), which is hydrolysed intracellularly to release the Ca2+-buffer BAPTA, also decreased the current by 44±16%. We conclude that voltage-and Ca2+-sensitive K+-channels are expressed in this tissue, but that their pharmacology differs considerably from equivalent channels in other phyla.

Development of Transient Outward Currents Coupled With Ca2+-Induced Ca2+Release Mediates Oscillatory Membrane Potential in Ascidian Muscle Cells

Isolated ascidian Halocynthia roretzi blastomeres of the muscle lineage exhibit muscle cell-like excitability on differentiation despite the arrest of cell cleavage early in development. This characteristic provides a unique opportunity to track changes in ion channel expression during muscle cell differentiation. Here, we show that the intrinsic membrane property of ascidian cleavage-arrested muscle-type cells becomes oscillatory by expressing transient outward currents ( Ito) activated by Ca2+-induced Ca2+release (CICR) in a maturation-dependent manner. In current-clamp mode, most day 4 (72 h after fertilization) cleavage-arrested muscle cells exhibited an oscillatory membrane potential of –20 mV at 15 Hz, whereas most day 3 (48 h after fertilization) cells exhibited a spiking pattern. In voltage-clamp mode, the day 4 cells exhibited prominent transient outward currents that were not present in day 3 cells. Itowas abolished by the application of 10 mM caffeine, implying that CICR was involved in Itoactivation. Itowas based on K+efflux and sensitive to tetraethylammonium and some Ca2+-activated K+channel inhibitors. We found a 60-pS single channel conductance that was activated by local Ca2+release in ascidian muscle cell. Voltage-clamp recording with an oscillatory waveform as a command pulse showed that CICR-activated K+currents were activated during the falling phase of the membrane potential oscillation. These results suggest that developmental expression of CICR-activated K+current plays a role in the maturation of larval locomotion by modifying the intrinsic membrane excitability of muscle cells.

Opening of Ca2+-activated K+ channels triggers early and delayed preconditioning against I/R injury independent of NOS in mice

Opening of Ca2+-activated K+ (KCa) channels has been shown to confer early cardioprotection. It is unknown whether the opening of these channels also induces delayed cardioprotection. In addition, we determined the involvement of nitric oxide synthases (NOSs), which have been implicated in cardioprotection induced by opening of mitochondrial ATP-sensitive K+ (KATP) channels. Adult male ICR mice were pretreated with the KCa-channel opener NS-1619 either 10 min or 24 h before 30 min of global ischemia and 60 min of reperfusion (I/R) in Langendorff mode. Infusion of NS-1619 (10 microM) for 10 min before I/R led to smaller infarct sizes as compared with the vehicle (DMSO)-treated group (P <0.05). This infarct-limiting effect of NS-1619 was associated with improvement in ventricular functional recovery after I/R. The NS-1619-induced protection was abolished by coadministration with the KCa-channel blocker paxilline (1 microM). Similarly, pretreatment with NS-1619 (1 mg/kg ip) induced delayed protection 24 h later (P <0.05). Interestingly, the NS-1619-induced late protection was not blocked by the NOS inhibitor Nomega-nitro-L-arginine methyl ester (15 mg/kg ip). Unlike diazoxide (the opener of mitochondrial KATP channels), NS-1619 did not increase the expression of inducible or endothelial NOS. Western blot analysis demonstrated the existence of alpha- and beta-subunits of KCa channels in mouse heart tissue. We conclude that opening of KCa channels leads to both early and delayed preconditioning effects through a mechanism that is independent of nitric oxide.

Calcium-activated potassium channels: multiple contributions to neuronal function

Calcium-activated potassium channels are a large family of potassium channels that are found throughout the central nervous system and in many other cell types. These channels are activated by rises in cytosolic calcium largely in response to calcium influx via voltage-gated calcium channels that open during action potentials. Activation of these potassium channels is involved in the control of a number of physiological processes from the firing properties of neurons to the control of transmitter release. These channels form the target for modulation for a range of neurotransmitters and have been implicated in the pathogenesis of neurological and psychiatric disorders. Here the authors summarize the varieties of calcium-activated potassium channels present in central neurons and their defining molecular and biophysical properties.

BK potassium channels control transmitter release at CA3-CA3 synapses in the rat hippocampus

Large conductance calcium- and voltage-activated potassium channels (BK channels) activate in response to calcium influx during action potentials and contribute to the spike repolarization and fast afterhyperpolarization. BK channels targeted to active zones in presynaptic nerve terminals have been shown to limit calcium entry and transmitter release by reducing the duration of the presynaptic spike at neurosecretory nerve terminals and at the frog neuromuscular junction. However, their functional role in central synapses is still uncertain. In the hippocampus, BK channels have been proposed to act as an 'emergency brake' that would control transmitter release only under conditions of excessive depolarization and accumulation of intracellular calcium. Here we demonstrate that in the CA3 region of hippocampal slice cultures, under basal experimental conditions, the selective BK channel blockers paxilline (10 microM) and iberiotoxin (100 nM) increase the frequency, but not the amplitude, of spontaneously occurring action potential-dependent EPSCs. These drugs did not affect miniature currents recorded in the presence of tetrodotoxin, suggesting that their action was dependent on action potential firing. Moreover, in double patch-clamp recordings from monosynaptically interconnected CA3 pyramidal neurones, blockade of BK channels enhanced the probability of transmitter release, as revealed by the increase in success rate, EPSC amplitude and the concomitant decrease in paired-pulse ratio in response to pairs of presynaptic action potentials delivered at a frequency of 0.05 Hz. BK channel blockers also enhanced the appearance of delayed responses, particularly following the second action potential in the paired-pulse protocol. These results are consistent with the hypothesis that BK channels are powerful modulators of transmitter release and synaptic efficacy in central neurones.

Genotoxicity assessment of five tremorgenic mycotoxins (fumitremorgen B, paxilline, penitrem A, verruculogen, and verrucosidin) produced by molds isolated from fermented meats

A number of toxinogenic fungal species, particularly producers of tremorgenic mycotoxins, have been isolated from traditional fermented meats. Tremorgenic mycotoxins are a group of fungal metabolites known to act on the central nervous system, causing sustained tremors, convulsions, and death in animals. However, the mode of action of these mycotoxins has not been elucidated in detail, and their genotoxic capacity has hardly been investigated. Because genotoxicity is one of the most prominent toxicological end points in food safety testing, we assessed the genotoxicity of five tremorgenic mycotoxins (fumitremorgen B, paxilline, penitrem A, verrucosidin, and verruculogen) associated with molds found in fermented meats. The mycotoxins were tested in two short-term in vitro assays with the use of different genotoxic end points in different phylogenetic systems (the Ames Salmonella/mammalian-microsome assay and the single-cell gel electrophoresis assay of human lymphocytes). According to the results obtained in this study, all of the investigated mycotoxins except penitrem A exhibited a certain degree of genotoxicity. Verrucosidin appeared to have the highest toxic potential, testing positive in both assays. Verruculogen tested positive in the Salmonella/mammalian-microsome assay, and paxilline and fumitremorgen B caused DNA damage in human lymphocytes. The use of fungal starter cultures to avoid tremorgen contamination in fermented meats is recommended.

Molecular analysis of two cytochrome P450 monooxygenase genes required for paxilline biosynthesis in Penicillium paxilli, and effects of paxilline intermediates on mammalian maxi-K ion channels

The gene cluster required for paxilline biosynthesis in Penicillium paxilli contains two cytochrome P450 monooxygenase genes, paxP and paxQ. The primary sequences of both proteins are very similar to those of proposed cytochrome P450 monooxygenases from other filamentous fungi, and contain several conserved motifs, including that for a haem-binding site. Alignment of these sequences with mammalian and bacterial P450 enzymes of known 3-D structure predicts that there is also considerable conservation at the level of secondary structure. Deletion of paxP and paxQ results in mutant strains that accumulate paspaline and 13-desoxypaxilline, respectively. These results confirm that paxP and paxQ are essential for paxilline biosynthesis and that paspaline and 13-desoxypaxilline are the most likely substrates for the corresponding enzymes. Chemical complementation of paxilline biosynthesis in paxG (geranygeranyl diphosphate synthase) and paxP, but not paxQ, mutants by the external addition of 13-desoxypaxilline confirms that PaxG and PaxP precede PaxQ, and are functionally part of the same biosynthetic pathway. A pathway for the biosynthesis of paxilline is proposed on the basis of these and earlier results. Electrophysiological experiments demonstrated that 13-desoxypaxilline is a weak inhibitor of mammalian maxi-K channels (Ki=730 nM) compared to paxilline (Ki=30 nM), indicating that the C-13 OH group of paxilline is crucial for the biological activity of this tremorgenic mycotoxin. Paspaline is essentially inactive as a channel blocker, causing only slight inhibition at concentrations up to 1 microM.

A natural product ligand of the oxysterol receptor, liver X receptor

Natural products have been identified as ligands for a number of members of the nuclear hormone receptor (NHR) superfamily. Often these natural products are used as dietary supplements to treat myriad ailments ranging from perimenopausal hot flashes to hypercholesterolemia and reduced cognitive function. Examples of some natural product ligands for NHRs include genestein (estrogen receptors NR3A1 and NR3A2), guggulsterone (farnesoid X receptor NR1H4), and St. John's wort (pregnane X receptor, NR1I2). In this study, we identified the first nonoxysterol natural product that functions as a ligand for the liver X receptor (LXRalpha and LXRbeta; NR1H3, NR1H2), a NHR that acts as the receptor for oxysterols and plays a key role in regulation of cholesterol metabolism and transport as well as glucose metabolism. We show that paxilline, a fungal metabolite, is an efficacious agonist of both LXRalpha and LXRbeta in biochemical and in vitro cell-based assays. Paxilline binds directly to both receptors and is an activator of LXR-dependent transcription in cell-based reporter assays. We also demonstrate that paxilline binding to the receptors results in efficient activation of transcription of two physiological LXR target genes, ABCA1 and SREBP. The discovery of paxilline, the first reported nonoxysterol natural product ligand of the LXRs, may provide insight into the mechanism of ligand recognition by these receptors and reaffirms the utility of examining natural product libraries for identifying novel NHR ligands.

Voltage-independent KCNQ4 currents induced by (+/-)BMS-204352

The compound BMS-204352 has been targeted for use against acute ischemic stroke, due to its activation of the large-conductance Ca2+-activated K-channel (BK). We have previously described that the racemate (+/-)BMS-204352 reversibly modulates KCNQ4 voltage dependency. Here we show that (+/-)BMS-204352 also induces a voltage-independent KCNQ4 current. The channels were stably expressed in human embryonic kidney cells (HEK293), and investigated by use of the whole-cell mode of the patch-clamp technique. (+/-)BMS-204352 was applied extracellularly (10 microM) in order to precipitate the robust appearance of the voltage-independent current. The voltage-independent KCNQ4 currents were recorded as instantaneous increases in currents upon hyperpolarizing or depolarizing voltage steps elicited from holding potentials of -90 or -110 mV. The voltage-independent current reversed at the equilibrium potential for potassium ( E(K)), hence was carried by a K+ conductance, and was blocked by the selective KCNQ channel blockers XE991 and linopirdine. Similar results were obtained with KCNQ4 channels transiently transfected into Chinese hamster ovary cells (CHO). When (+/-)BMS-204352 was applied to stably expressed BK channels, only the voltage dependency was modulated. Retigabine, the classic activator of KCNQ channels, did not induce voltage-independent currents. Our data indicate that KCNQ4 channels may conduct voltage-dependent and voltage-independent currents in the presence of (+/-)BMS-204352.

Ionic mechanisms mediating oscillatory membrane potentials in wide-field retinal amacrine cells

Particular types of amacrine cells of the vertebrate retina show oscillatory membrane potentials (OMPs) in response to light stimulation. Historically it has been thought the oscillations arose as a result of circuit properties. In a previous study we found that in some amacrine cells, the ability to oscillate was an intrinsic property of the cell. Here we characterized the ionic mechanisms responsible for the oscillations in wide-field amacrine cells (WFACs) in an effort to better understand the functional properties of the cell. The OMPs were found to be calcium (Ca2+) dependent; blocking voltage-gated Ca2+ channels eliminated the oscillations, whereas elevating extracellular Ca2+ enhanced them. Strong intracellular Ca2+ buffering (10 mM EGTA or bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid) eliminated any attenuation in the OMPs as well as a Ca2+-dependent inactivation of the voltage-gated Ca2+ channels. Pharmacological and immunohistochemical characterization revealed that WFACs express L- and N-type voltage-sensitive Ca2+ channels. Block of the L-type channels eliminated the OMPs, but omega-conotoxin GVIA did not, suggesting a different function for the N-type channels. The L-type channels in WFACs are functionally coupled to a set of calcium-dependent potassium (K(Ca)) channels to mediate OMPs. The initiation of OMPs depended on penitrem-A-sensitive (BK) K(Ca) channels, whereas their duration is under apamin-sensitive (SK) K(Ca) channel control. The Ca2+ current is essential to evoke the OMPs and triggering the K(Ca) currents, which here act as resonant currents, enhances the resonance as an amplifying current, influences the filtering characteristics of the cell membrane, and attenuates the OMPs via CDI of the L-type Ca2+ channel.

BK channel openers inhibit migration of human glioma cells

Large-conductance Ca(2+)-activated K(+) channels (BK channels) are highly expressed in human glioma cells. However, less is known about their biological function in these cells. We used the patch-clamp technique to investigate activation properties of BK channels and time-lapse microscopy to evaluate the role of BK channel activation in migration of 1321N1 human glioma cells. In whole cells, internal perfusion with a solution containing 500 nM free Ca(2+) and external application of the BK channel opener phloretin (100 micro M) shifted the activation threshold of BK channel currents toward more negative voltages of about -30 mV, which is close to the resting potential of the cells. The concentration of intracellular Ca(2+) in fura-2-loaded 1321N1 cells was measured to be 235+/-19 nM and was increased to 472+/-25 nM after treatment with phloretin. Phloretin and another BK channel opener NS1619 (100 micro M) reduced the migration velocity by about 50%. A similar reduction was observed following muscarinic stimulation of glioma cells with acetylcholine (100 micro M). The effects of phloretin, NS1619 and acetylcholine on cell migration were completely abolished by co-application of the specific BK channel blockers paxilline (5 micro M) and iberiotoxin (100 nM). The phloretin-induced increase in intracellular Ca(2+) was unaffected by the removal of extracellular Ca(2+) and co-application of paxilline. These findings indicate that glioma cell migration was inhibited through BK channel activation, independent of intracellular Ca(2+).

Distinct contributions of small and large conductance Ca2+-activated K+ channels to rat Purkinje neuron function

The cerebellum is important for many aspects of behaviour, from posture maintenance and goal-oriented reaching movements to timing tasks and certain forms of learning. In every case, information flowing through the cerebellum passes through Purkinje neurons, which receive input from the two primary cerebellar afferents and generate continuous streams of action potentials that constitute the sole output from the cerebellar cortex to the deep nuclei. The tonic firing behaviour observed in Purkinje neurons in vivo is maintained in brain slices even when synaptic inputs are blocked, suggesting that Purkinje neuron activity relies to a significant extent on intrinsic conductances. Previous research has suggested that the interplay between Ca2+ currents and Ca2+-activated K+ channels (KCa channels) is important for Purkinje cell activity, but how many different KCa channel types are present and what each channel type contributes to cell behaviour remains unclear. In order to better understand the ionic mechanisms that control the behaviour of these neurons, we investigated the effects of different Ca2+ channel and KCa channel antagonists on Purkinje neurons in acute slices of rat cerebellum. Our data show that Ca2+ entering through P-type voltage-gated Ca2+ channels activates both small-conductance (SK) and large-conductance (BK) KCa channels. SK channels play a role in setting the intrinsic firing frequency, while BK channels regulate action potential shape and may contribute to the unique climbing fibre response.

Tremorgenic mycotoxins increase gastric smooth muscle activity of sheep reticulum and rumen in vitro

Reticulum and rumen strips (consisting of both muscle layers and the myenteric plexus) were superfused with Tyrode Ringer and their contractions recorded isometrically. The strips were subjected to exogenous acetylcholine and electrical field stimulation (EFS) resulting in contractions that could be blocked by atropine. Responses to the tremorgenic mycotoxin penitrem A and others thought to be involved in ryegrass staggers, paxilline and lolitrem B (10(-10)-10(-6)M), were compared with those of control vehicle (0.1% DMSO). The tremorgens were without effect on quiescent preparations. Penitrem A and paxilline enhanced spontaneously active preparations and the amplitude of contractions in response to EFS. Responses to paxilline had a shorter latency than to penitrem A. Responses of spontaneously active preparations were resistant to atropine. Penitrem A, but not paxilline, increased responses to exogenous acetylcholine. Lolitrem B (10(-6)M) increased responses to EFS, but many responses were equivocal, possibly due to the lower solubility of lolitrem B in aqueous solutions compared to the other tremorgens. The results show that these mycotoxins have peripheral excitatory effects on the reticulorumen and it is suggested that such activity in vivo may reflexly affect centrally derived cyclical contractions.

Stimulatory effects of chlorzoxazone, a centrally acting muscle relaxant, on large conductance calcium-activated potassium channels in pituitary GH3 cells

Chlorzoxazone, a centrally acting muscle relaxant, has been used as a marker for hepatic CYP2E1 activity. However, little is known about the mechanism of chlorzoxazone actions on ion currents in neurons or neuroendocrine cells. We thus investigated its effects on ion currents in GH(3) lactotrophs. Chlorzoxazone reversibly increased Ca(2+)-activated K(+) current (I(K(Ca))) in a concentration-dependent manner with an EC(50) value of 30 microM. The chlorzoxazone-stimulated I(K(Ca)) was inhibited by iberitoxin (200 nM) or clotrimazole (10 microM), but not by glibenclamide (10 microM) or apamin (200 nM). Chlorzoxazone (30 microM) suppressed voltage-dependent L-type Ca(2+) current. In the inside-out configuration, chlorzoxazone applied to the intracellular side of the patch did not modify single-channel conductance of large conductance Ca(2+)-activated K(+) (BK(Ca)) channels, but did increase channel activity by increasing mean open time and decreasing mean closed time. Chlorzoxazone also caused a left shift in the activation curve of BK(Ca) channels. However, Ca(2+)-sensitivity of these channels was unaffected by chlorzoxazone. 1-Ethyl-2-benzimidazolinone (30 microM), 2-amino-5-chlorobenzoxazole (30 microM) or chlormezanone (30 microM) enhanced BK(Ca) channel activity, while 6-hydroxychlorzoxazone (30 microM) slightly increased it; however, chlorphenesin carbamate (30 microM) had no effect on it. Under the current-clamp condition, chlorzoxazone (10 microM) reduced the firing rate of action potentials. In neuroblastoma IMR-32 cells, chlorzoxazone (30 microM) also stimulated BK(Ca) channel activity. The stimulatory effects of chlorzoxazone on these channels may be responsible for the underlying mechanism of chlorzoxazone actions on neurons and neuroendocrine cells.

Behavior of nonselective cation channels and large-conductance Ca2+-activated K+ channels induced by dynamic changes in membrane stretch in cultured smooth muscle cells of human coronary artery

INTRODUCTION

The effects of membrane stretch on ion channels were investigated in cultured smooth muscle cells of human coronary artery.

METHODS AND RESULTS

In the cell-attached configuration, membrane stretch with negative pressure induced two types of stretch-activated (SA) ion channels: a nonselective cation channel and a large-conductance Ca2+-activated K+ (BK(Ca)) channel. The single-channel conductances of SA cation and BK(Ca) channels were 26 and 203 pS, respectively. To elucidate the mechanism of activation of these SA channels and to minimize mechanical disruption, a sinusoidal change in pipette pressure was applied to the on-cell membrane patch. During dynamic changes in pipette pressure, increases in SA cation channel activity was found to coincide with increases in BK(Ca) channel activity. In the continued presence of cyclic stretch, the activity of SA cation channels gradually diminished. However, after termination of cyclic stretch, BK(Ca) channel activity was greatly enhanced, but the activity of SA cation channels disappeared.

CONCLUSION

This study is the first to demonstrate that the behavior of SA cation and BK(Ca) channels in coronary smooth muscle cells is differentially susceptible to dynamic changes in membrane tension.

Prolonged activation of Ca2+-activated K+ current contributes to the long-lasting refractory period of Aplysia bag cell neurons

Stimulation of the bag cell neurons of Aplysia activates several biochemical pathways, including protein kinase C (PKC), and alters their excitability for many hours. After an approximately 30 min afterdischarge, these neurons enter an approximately 18 hr inhibited state during which additional stimulation fails to evoke discharges. In vivo, this refractory period limits the frequency of reproductive behaviors associated with egg laying. We have now examined the role of Ca2+-activated K+ (BK) currents in the refractory period. Outward currents gated by both intracellular Ca2+ and depolarization, with pharmacological characteristics of BK currents, were recorded in isolated bag cell neurons. These currents were enhanced by the BK channel activators phloretin and 1,3-dihydro-1-[2-hydroxy-5-(trifluoro-methyl)phenyl]-5-trifluoromethyl-2H-benzimidazol-2-one and inhibited by the BK blocker paxilline. The BK component of K+ current was enhanced by 12-O-tetradecanoyl-phorbol-13-acetate, an activator of PKC, and this effect was blocked by sphinganine and PKC(19-36), inhibitors of PKC in bag cell neurons. To test whether the BK current is altered during the refractory period, intact clusters were stimulated to afterdischarge, and neurons were isolated after the clusters had entered the refractory period. Compared with unstimulated cells, current density was almost doubled in refractory neurons. This increase in current was inhibited by preincubating clusters in sphinganine. Treatment of refractory clusters with paxilline significantly restored the ability of stimulation to evoke afterdischarges. Conversely, application of phloretin to previously unstimulated clusters inhibited the onset of afterdischarges. These results indicate that a prolonged increase in BK channel activity contributes to the prolonged refractory period of the bag cell neurons.

Characterization of norepinephrine-evoked inward currents in interstitial cells isolated from the rabbit urethra

Freshly dispersed interstitial cells from the rabbit urethra were studied by using the perforated-patch technique. When cells were voltage clamped at -60 mV and exposed to 10 microM norepinephrine (NE) at 80-s intervals, either large single inward currents or a series of oscillatory inward currents of diminishing amplitude were evoked. These currents were blocked by either phentolamine (1 microM) or prazosin (1 microM), suggesting that the effects of NE were mediated via alpha(1)-adrenoceptors. NE-evoked currents were depressed by the blockers of Ca(2+)-activated Cl(-) currents, niflumic acid (10 microM), and 9-anthracenecarboxylic acid (9-AC, 1 mM). The reversal potential of the above currents changed in a predictable manner when the Cl(-) equilibrium potential was altered, again suggesting that they were due to activation of a Cl(-) conductance. NE-evoked currents were decreased by 10 microM cyclopiazonic acid, suggesting that they were dependent on store-released Ca(2+). Inhibition of NE-evoked currents by the phospholipase C inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (100 microM) suggested that NE releases Ca(2+) via an inositol 1,4,5-trisphosphate (IP(3))-dependent mechanism. These results support the idea that stimulation of alpha(1)-adrenoceptors releases Ca(2+) from an IP(3)-sensitive store, which in turn activates Ca(2+)-activated Cl(-) current in freshly dispersed interstitial cells of the rabbit urethra. This elevates slow wave frequency in these cells and may underlie the mechanism responsible for increased urethral tone during nerve stimulation.

Delayed rectifier potassium channels contribute to the depressed pulmonary artery contractility in pneumonia

We investigated the role of K(+) channels in the attenuated pulmonary artery (PA) contractility characteristic of acute Pseudomonas pneumonia. Contractility of PA rings from the lungs of control or pneumonia rats was assessed in vitro by obtaining cumulative concentration-response curves to the contractile agonists KCl, phenylephrine, or PGF(2 alpha) on PA rings before and after treatment with K(+) channel blockers. In rings from pneumonia rats, paxilline (10 microM), tetraethylammonium (2 mM) (blockers of large-conductance Ca(2+)-activated K(+) channels), and glybenclamide (ATP-sensitive K(+) channel blocker, 80 microM) had no significant effect on the attenuated contractile responses to KCl, phenylephrine, and PGF(2 alpha). However, 4-aminopyridine (2 mM), a blocker of voltage-gated K(+) channels (delayed rectifier K(+) channel) reversed this depressed contractility. Therefore, large-conductance Ca(2+)-activated K(+) and ATP-sensitive K(+) channels do not contribute to the attenuated PA contractility observed in this model of acute pneumonia. In contrast, 4-aminopyridine enhances contraction in PA rings from pneumonia lungs, consistent with involvement of a voltage-gated K(+) channel in the depressed PA contractility in acute pneumonia. Unraveling the precise mechanism of attenuated contractility in pneumonia could lead to innovative therapies for the pulmonary vascular abnormalities associated with this disease.

BK channel activity determines the extent of cell degeneration after oxygen and glucose deprivation: a study in organotypical hippocampal slice cultures

BK channels are voltage- and calcium-dependent potassium channels whose activation tends to reduce cellular excitability. In hippocampal pyramidal cells, BK channels repolarize somatic action potentials, and recent immunogold and electrophysiological analyses have revealed a presynaptic pool of BK channels that can regulate glutamate release. Agents that modulate BK channel activity would therefore be expected to affect cell excitability and neurotransmitter release also under pathological conditions. We have investigated the role of BK potassium channels in a model of ischemia-induced nerve cell degeneration. Organotypical slice cultures of rat hippocampus were exposed to oxygen and glucose deprivation (OGD), and cell death was assessed by the fluorescent dye propidium iodide. OGD induced cell death in the CA1 region and to a lesser extent in CA3. Treatment with the BK channel blockers, paxilline and iberiotoxin, during and after OGD induced increased cell death in CA1 and CA3. Both BK channel blockers also sensitized the relatively resistant granule cells in fascia dentata to OGD. The effect of paxilline and iberiotoxin was evident from 3 h after OGD, indicating a role of BK channels early in the post-ischemic phase or during OGD itself. The BK channel opener, NS1619, turned out to be gliotoxic, and this effect was not counteracted by paxilline and iberiotoxin. Our data show that blockade of BK channels aggravates OGD-induced cell damage and suggest that BK channels act as a kind of 'emergency brake' during and/or after ischemia. Accordingly, the BK channel is a potential molecular target for neuroprotective therapy in stroke.

Dual and opposing roles of presynaptic Ca2+ influx for spontaneous GABA release from rat medial preoptic nerve terminals

Calcium influx into the presynaptic nerve terminal is well established as a trigger signal for transmitter release by exocytosis. By studying dissociated preoptic neurons with functional adhering nerve terminals, we here show that presynaptic Ca2+ influx plays dual and opposing roles in the control of spontaneous transmitter release. Thus, application of various Ca2+ channel blockers paradoxically increased the frequency of spontaneous (miniature) inhibitory GABA-mediated postsynaptic currents (mIPSCs). Similar effects on mIPSC frequency were recorded upon washout of Cd2+ or EGTA from the external solution. The results are explained by a model with parallel Ca2+ influx through channels coupled to the exocytotic machinery and through channels coupled to Ca2+-activated K+ channels at a distance from the release site.

Asian scorpion BmK venom induces plasma extravasation and thermal hyperalgesia in the rat

In the present study, the effects of scorpion Buthus martensi Karsch (BmK) venom on plasma extravasation and paw withdrawal latency (PWL) to radiant heat have been investigated in rats. BmK venom (20-200 microg/kg) by subcutaneous injection under the surface of the rat hindpaw causes dose-dependant increased plasma extravasation that could be partially inhibited by intraperitoneal (i.p.) injected morphine (6 mg/kg). Peak plasma extravasation was reached at 10 min and persisted for 60 min at a dose of 200 microg/kg. BmK venom induced cutaneous hyperalgesia as indicated by decreased PWL to radiant heat in the ipsilateral paw following subcutaneous injection of 20 microg/kg BmK venom without effect on PWL of the contralateral hindpaw. Meanwhile, it was found that i.p. morphine injection could inhibit this decreased ipsilateral PWL. The results thus suggest that BmK venom could induce peripheral inflammation in rat by subcutaneous injection, and may prove a valuable animal model for investigating the pathophysiology of a number of inflammatory diseases and identifying potential anti-inflammatory and analgesic drugs.

Channels underlying neuronal calcium-activated potassium currents

In many cell types rises in cytosolic calcium, either due to influx from the extracellular space, or by release from an intracellular store activates calcium dependent potassium currents on the plasmalemma. In neurons, these currents are largely activated following calcium influx via voltage gated calcium channels active during the action potentials. Three types of these currents are known: I(c), I(AHP) and I(sAHP). These currents can be distinguished by clear differences in their pharmacology and kinetics. Activation of these potassium currents modulates action potential time course and the repetitive firing properties of neurons. Single channel studies have identified two types of calcium-activated potassium channel which can also be separated on biophysical and pharmacological grounds and have been named BK and SK channels. It is now clear that BK channels underlie I(c) whereas SK channels underlie I(AHP). The identity of the channels underlying I(sAHP) are not known. In this review, we discuss the properties of the different types of calcium-activated potassium channels and the relationship between these channels and the macroscopic currents present in neurons.

Mycotoxins

The relevance of many mycotoxins to small animal health is difficult to assess, because available information has not been collated and reviewed. Only aflatoxins, penitrem A, and roquefortine have been confirmed in natural mycotoxicoses in pets. Effects of tricothecene mycotoxins, patulin, and penicillic acid on dogs and cats have only been studied experimentally and have not been confirmed in natural outbreaks. Although they make up only a small portion of the cases presented to veterinarians, mycotoxicoses often require special effort. Establishing an accurate diagnosis is crucial to minimize exposure and provide adequate treatment. In most cases, clinical examination, clinical pathologic testing, and analytical chemistry analysis of suspect feed are necessary to reach a diagnosis.

Presynaptic Ca2+-activated K+ channels in glutamatergic hippocampal terminals and their role in spike repolarization and regulation of transmitter release

Large-conductance Ca(2+)-activated K(+) channels (BK, also called Maxi-K or Slo channels) are widespread in the vertebrate nervous system, but their functional roles in synaptic transmission in the mammalian brain are largely unknown. By combining electrophysiology and immunogold cytochemistry, we demonstrate the existence of functional BK channels in presynaptic terminals in the hippocampus and compare their functional roles in somata and terminals of CA3 pyramidal cells. Double-labeling immunogold analysis with BK channel and glutamate receptor antibodies indicated that BK channels are targeted to the presynaptic membrane facing the synaptic cleft in terminals of Schaffer collaterals in stratum radiatum. Whole-cell, intracellular, and field-potential recordings from CA1 pyramidal cells showed that the presynaptic BK channels are activated by calcium influx and can contribute to repolarization of the presynaptic action potential (AP) and negative feedback control of Ca(2+) influx and transmitter release. This was observed in the presence of 4-aminopyridine (4-AP, 40-100 microm), which broadened the presynaptic compound action potential. In contrast, the presynaptic BK channels did not contribute significantly to regulation of action potentials or transmitter release under basal experimental conditions, i.e., without 4-AP, even at high stimulation frequencies. This is unlike the situation in the parent cell bodies (CA3 pyramidal cells), where BK channels contribute strongly to action potential repolarization. These results indicate that the functional role of BK channels depends on their subcellular localization.

Ceramide inhibits the inwardly rectifying potassium current in GH(3) lactotrophs

The effects of ceramide on ion currents in rat pituitary GH(3) cells were investigated. Hyperpolarization-elicited K(+) currents present in GH(3) cells were studied to determine the effect of ceramide and other related compounds on the inwardly rectifying K(+) current (I(K(IR))). Ceramide (C(2)-ceramide) suppressed the amplitude of I(K(IR)) in a concentration-dependent manner, with an IC(50) value of 5 microM. Ceramide caused a rightward shift in the midpoint for the activation curve of I(K(IR)). Pretreatment with PD-98059 (30 microM) or U-0126 (30 microM) did not prevent ceramide-mediated inhibition of I(K(IR)). However, the magnitude of ceramide-induced inhibition of I(K(IR)) was attenuated in GH(3) cells preincubated with dithiothreitol (10 microM). TNF alpha (100 ng/g) also suppressed I(K(IR)). In the inside-out configuration, application of ceramide (30 microM) to the bath slightly suppressed the activity of large conductance Ca(2+)-activated K(+) channels. Under the current clamp mode, ceramide (10 microM) increased the firing of action potentials. Cells that exhibited an irregular firing pattern were converted to those displaying a regular firing pattern after application of ceramide (10 microM). Ceramide also suppressed I(K(IR)) in neuroblastoma IMR-32 cells. Therefore, ceramide can produce a depressant effect on I(K(IR)). The blockade of this current by ceramide may affect cell function.

Paradoxical role of large-conductance calcium-activated K+ (BK) channels in controlling action potential-driven Ca2+ entry in anterior pituitary cells

Activation of high-conductance Ca(2+)-activated K(+) (BK) channels normally limits action potential duration and the associated voltage-gated Ca(2+) entry by facilitating membrane repolarization. Here we report that BK channel activation in rat pituitary somatotrophs prolongs membrane depolarization, leading to the generation of plateau-bursting activity and facilitated Ca(2+) entry. Such a paradoxical role of BK channels is determined by their rapid activation by domain Ca(2+), which truncates the action potential amplitude and thereby limits the participation of delayed rectifying K(+) channels during membrane repolarization. Conversely, pituitary gonadotrophs express relatively few BK channels and fire single spikes with a low capacity to promote Ca(2+) entry, whereas an elevation in BK current expression in a gonadotroph model system leads to the generation of plateau-bursting activity and high-amplitude Ca(2+) transients.

Role of IP(3) in modulation of spontaneous activity in pacemaker cells of rabbit urethra

Isolated interstitial ("pacemaker") cells from rabbit urethra were examined using the perforated-patch technique. Under voltage clamp at -60 mV, these cells fired large spontaneous transient inward currents (STICs), averaging -860 pA and >1 s in duration, which could account for urethral pacemaker activity. Spontaneous transient outward currents (STOCs) were also observed and fell into two categories, "fast" (<100 ms in duration) and "slow" (>1 s in duration). The latter were coupled to STICs, suggesting that they shared the same mechanism, while the former occurred independently at faster rates. All of these currents were abolished by cyclopiazonic acid, caffeine, or ryanodine, suggesting that they were activated by Ca(2+) release. When D-myo-inositol 1,4,5-trisphosphate (IP(3))-sensitive stores were blocked with 2-aminoethoxydiphenyl borate, the STICs and slow STOCs were abolished, but the fast STOCs remained. In contrast, the fast STOCs were more nifedipine sensitive than the STICs or the slow STOCs. These results suggest that while fast STOCs are mediated by a mechanism similar to STOCs in smooth muscle, STICs and slow STOCs are driven by IP(3). These results support the hypothesis that pacemaker activity in the urethra is driven by the IP(3)-sensitive store.