Iberiotoxin

Iberiotoxin-sensitive and -insensitive BK currents in Purkinje neuron somata.[Pubmed:23446695]

J Neurophysiol. 2013 May;109(10):2528-41.

Purkinje cells have specialized intrinsic ionic conductances that generate high-frequency action potentials. Disruptions of their Ca or Ca-activated K (KCa) currents correlate with altered firing patterns in vitro and impaired motor behavior in vivo. To examine the properties of somatic KCa currents, we recorded voltage-clamped KCa currents in Purkinje cell bodies isolated from postnatal day 17-21 mouse cerebellum. Currents were evoked by endogenous Ca influx with approximately physiological Ca buffering. Purkinje somata expressed voltage-activated, Cd-sensitive KCa currents with Iberiotoxin (IBTX)-sensitive (>100 nS) and IBTX-insensitive (>75 nS) components. IBTX-sensitive currents activated and partially inactivated within milliseconds. Rapid, incomplete macroscopic inactivation was also evident during 50- or 100-Hz trains of 1-ms depolarizations. In contrast, IBTX-insensitive currents activated more slowly and did not inactivate. These currents were insensitive to the small- and intermediate-conductance KCa channel blockers apamin, scyllatoxin, UCL1684, bicuculline methiodide, and TRAM-34, but were largely blocked by 1 mM tetraethylammonium. The underlying channels had single-channel conductances of approximately 150 pS, suggesting that the currents are carried by IBTX-resistant (beta4-containing) large-conductance KCa (BK) channels. IBTX-insensitive currents were nevertheless increased by small-conductance KCa channel agonists EBIO, chlorzoxazone, and CyPPA. During trains of brief depolarizations, IBTX-insensitive currents flowed during interstep intervals, and the accumulation of interstep outward current was enhanced by EBIO. In current clamp, EBIO slowed spiking, especially during depolarizing current injections. The two components of BK current in Purkinje somata likely contribute differently to spike repolarization and firing rate. Moreover, augmentation of BK current may partially underlie the action of EBIO and chlorzoxazone to alleviate disrupted Purkinje cell firing associated with genetic ataxias.

Magnesium increases iberiotoxin-sensitive large conductance calcium activated potassium currents on the basilar artery smooth muscle cells in rabbits.[Pubmed:22196856]

Neurol Res. 2012 Jan;34(1):11-6.

OBJECTIVES: Magnesium has been known for treating vasospasm following subarachnoid hemorrhage. However, its action mechanism in cerebral vascular relaxation is not clear. Potassium channels play a pivotal role in the relaxation of smooth muscle cells. To investigate their role in magnesium-induced relaxation of basilar smooth muscle cells, we examined the effect of magnesium on potassium channels using the patch clamp technique on acutely isolated smooth muscle cells from rabbit basilar artery. METHOD: Fresh smooth muscle cells were isolated from the basilar artery by enzyme treatment. To identify which potassium channels are involved in the magnesium-induced currents, we used the potassium channel blockers tetraethylammonium (TEA), glibenclamide, apamin and Iberiotoxin (IBX). RESULTS: Magnesium (5 mM) increased the step pulse-induced outward K+ currents by 46% over control level (P < 0.01). The outward K+ current was decreased to 22% (P < 0.01) by TEA (10 mM), a non-specific K+ channel blocker, and to 60% of control level (P < 0.01) by IBX (0.1 muM,), a large-conductance Ca2+-activated K+ (BK) channel blocker, but was not inhibited by apamin (1 nM), a small-conductance Ca2+ -activated potassium (SK) channel blocker, or glibenclamide (3 mM), an adenosine triphosphate (ATP)-sensitive K+) channel blocker. Caffeine (3 mM) enhanced outward K+ currents. Magnesium-induced increase of outward K+ currents persisted in the presence of apamin. However, magnesium failed to increase the outward K+ currents in the presence of IBX. DISCUSSION: These results demonstrate that BK channels are functionally expressed in rabbit basilar smooth muscle cells and suggest that BK channels may play a pivotal role in magnesium-induced relaxation.

Current understanding of iberiotoxin-resistant BK channels in the nervous system.[Pubmed:25346692]

Front Physiol. 2014 Oct 9;5:382.

While most large-conductance, calcium-, and voltage-activated potassium channels (BK or Maxi-K type) are blocked by the scorpion venom Iberiotoxin, the so-called "type II" subtype has the property of toxin resistance. This property is uniquely mediated by channel assembly with one member of the BK accessory beta subunit family, the neuron-enriched beta4 subunit. This review will focus on current understanding of Iberiotoxin-resistant, beta4-containing BK channel properties and their function in the CNS. Studies have shown that beta4 dramatically promotes BK channel opening by shifting voltage sensor activation to more negative voltage ranges, but also slows activation to timescales that theoretically preclude BK ability to shape action potentials (APs). In addition, beta4 membrane trafficking is regulated through an endoplasmic retention signal and palmitoylation. More recently, the challenge has been to understand the functional role of the Iberiotoxin-resistant BK subtype utilizing computational modeling of neurons and neurophysiological approaches. Utilizing Iberiotoxin-resistance as a footprint for these channels, they have been identified in dentate gyrus granule neurons and in purkinje neurons of the cerebellum. In these neurons, the role of these channels is largely consistent with slow-gated channels that reduce excitability either through an interspike conductance, such as in purkinje neurons, or by replacing fast-gating BK channels that otherwise facilitate high frequency AP firing, such as in dentate gyrus neurons. They are also observed in presynaptic mossy fiber terminals of the dentate gyrus and posterior pituitary terminals. More recent studies suggest that beta4 subunits may also be expressed in some neurons lacking Iberiotoxin-resistant BK channels, such as in CA3 hippocampus neurons. Ongoing research using novel, specific blockers and agonists of BK/beta4, and beta4 knockout mice, will continue to move the field forward in understanding the function of these channels.

Effects of charybdotoxin and iberiotoxin on the spontaneous motility and tonus of different guinea pig smooth muscle tissues.[Pubmed:1717682]

J Pharmacol Exp Ther. 1991 Oct;259(1):439-43.

Charybdotoxin (ChTX) and Iberiotoxin (IbTX), two potent peptidyl blockers of the high conductance Ca(2+)-activated K+ channel (PK,Ca) were used to probe the role of this channel in regulating the contractility of various smooth muscles isolated from the guinea pig. Of the spontaneously contracting tissues that have been investigated, bladder and taenia coli are affected by ChTX, whereas portal vein and uterus are relatively insensitive. In the former two tissues, ChTX (10-100 nM) produces a concentration-dependent increase in contractility, with bladder being most sensitive to action of the toxin. ChTX also causes a contracture of quiescent aortic rings, although not affecting indomethacin-treated trachea. In order to demonstrate that the effects of ChTX are due specifically to blockage of PK,Ca, rather than to inhibition of some other K+ channel, two other inhibitors of PK,Ca were monitored. IbTX (10-100 nM), a selective inhibitor of PK,Ca, and tetraethylammonium ion, which blocks PK,Ca at low (0.1-3 mM) concentrations, both increase the myogenic activity of bladder, but not portal vein. In addition, IbTX causes a sustained contracture of aorta. Taken together, these data indicate that the increased contractility of certain guinea pig smooth muscles produced by ChTX, IbTX and tetraethylammonium ion is the result of selective inhibition of PK,Ca. It is suggested that in spontaneously active bladder and taenia coli, PK,Ca provides a repolarization pathway after tissue depolarization, whereas in quiescent aorta, PK,Ca maintains cellular resting potential. In contrast, in indomethacin-treated trachealis muscle, ChTX-sensitive K+ channel pathways are not involved in controlling resting tension.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and characterization of a unique, potent, peptidyl probe for the high conductance calcium-activated potassium channel from venom of the scorpion Buthus tamulus.[Pubmed:1694175]

J Biol Chem. 1990 Jul 5;265(19):11083-90.

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