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At least 19 recordsLinked to original sources
PubMed1986

Properties of the apamin-sensitive Ca2+-activated K+ channel in PC12 pheochromocytoma cells which hyper-produce the apamin receptor.

Undifferentiated PC12 cell produce high levels of apamin receptors (measured with 125I-apamin) after 7 days in culture. These levels are at least 50 times higher than those found in other cellular types which are also known to have apamin receptors and apamin-sensitive Ca2+-activated K+ channels in their membranes. Treatment of undifferentiated PC12 cells with nerve growth factor maintains these cells in a state having a low level (10 times less after 7 days of culture) of apamin receptors. Ca2+ injection into PC12 cells with the calcium ionophore A23187 has been used to monitor the activity of the Ca2+-activated K+ channel following 86Rb+ efflux. A large component of this Ca2+-activated 86Rb+ efflux is inhibited by apamin. Half-maximum inhibition by apamin of both 86Rb+ efflux and 125I-apamin binding was observed at 240 pM apamin. Another component of 86Rb+ efflux is due to another type of Ca2+-activated K+ channel which is resistant to apamin and sensitive to tetraethylammonium. The Ca2+ channel activator Bay K8644 also triggers an apamin-sensitive Ca2+-dependent 86Rb+ efflux. Bay K8644 has been used to analyze the internal Ca2+ concentration dependence of the apamin-sensitive channel activity. Under normal conditions, the internal Ca2+ concentration is 109 +/- 17 nM, and the apamin-sensitive channel is not activated. The channel is fully activated at an internal Ca2+ concentration of 320 +/- 20 nM.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1979

Solid phase synthesis of 13-lysine-apamin, 14-lysine-apamin and the corresponding guanidinated derivatives.

In order to study the importance of arginine residues 13 and 14 in apamin, the bee venom neurotoxin, four analogues, [Lys13]-apamin, [Lys14]-apamin, [Har4, Har13]-apamin and [Har4, har14]-apamin were synthesized and tested with respect to their neurotoxicity. The two lysine-apamins were prepared by the solid phase method on benzhydrylamine resins. Before oxidation to disulphides, the (S-Acm)4-peptides were isolated and characterized. Portions of the purified lysin peptides were converted to homoarginine analogues by guanidination. The four apamin analogues were lethal, but the lethal doses differed significantly. The results demonstrate that the arginine residue at position 14 is more important for the high toxicity than is the one at position 13. The circular dichroism (CD) spectrum of [Lys13]-apamin was identical with that of apamin itself, whereas the spectrum of [Lys14]-apamin showed certain deviations.

Amino Acids↗
PubMed1985

The all-or-none role of innervation in expression of apamin receptor and of apamin-sensitive Ca2+-activated K+ channel in mammalian skeletal muscle.

The long-lasting after-hyperpolarization(s) (AHP) that follows the action potential in rat myotubes differentiated in culture is due to Ca2+-activated K+ channels. These channels have the property to be specifically blocked by the bee venom toxin apamin at low concentrations. Apamin has been used in this work to analyze, by electrophysiological and biochemical techniques, the role of innervation in expression of these important channels. The main results are as follows: (i) Long-lasting AHP that follows the action potential in rat myotubes in culture disappears when myotubes are cocultured with nerve cells from the spinal cord under the conditions of in vitro innervation. (ii) Extensor digitorum longus muscles from adult rats have action potentials that are not followed by AHP but AHP are systematically recorded after muscle denervation and they are blocked by apamin. (iii) Specific 125I-labeled apamin binding is undetectable in innervated muscle fibers but it becomes detectable 2-4 days after muscle denervation to be maximal 10 days after denervation. (iv) Apamin receptors detected with 125I-labeled apamin are present at fetal stages with biochemical characteristics identical to those found in myotubes in culture. The receptor number decreases as maturation proceeds and 125I-labeled apamin receptors completely disappear after the first week of postnatal life, in parallel with the disappearance of multi-innervation. All these results taken together strongly suggest an all-or-none effect of innervation on the expression of apamin-sensitive Ca2+-activated K+ channels.

Animals↗
PubMed1977

Concept of internal structural controls for evaluation of inactive synthetic peptide analogs: synthesis of [Orn13,14]apamin and its guanidination to an apamin derivative with full neurotoxic activity.

The importance of arginine residues 13 and 14 in the bee venom neurotoxin, apamin, was teste by the synthesis of replacement analogs. [13,14-di-Ndelta-trifluoroacetylornithine]Apamin was synthesized by the solid phase method on a benzhydrylamine resin. It was deprotected to [13,14-diornithine]apamin, which was then guanidinated to produce the 4-homoarginine-13,14-diarginine analog, [Har4]apamin. Neither the trifluoroacetylornithine analog nor the ornithine analog produced any detectable symptoms when injected intravenously into mice. However, the synthetic [Har4]apamin exhibited the full neurotoxic activity of native apamin and of [Har4]apamin derived from the natural toxin. This provided an internal structural control for the correctness of the primary structure of the inactive synthetic analogs and strengthened the conclusion that one, or both, of the arginine residues plays an important role in the action of apamin.

Animals↗
PubMed1995

Recombinant and chemical derivatives of apamin. Implication of post-transcriptional C-terminal amidation of apamin in biological activity.

The use of the colicin A lysis protein to direct the extracellular release of a fusion protein from Escherichia coli was investigated as an approach for the preparation of recombinant animal toxins. Apamin, a bee venom neurotoxin, was used as the model toxin. It is reticulated by two disulfide bridges and interacts with small conductance Ca(2+)-activated K+ channels. Substantial amounts of free recombinant apamin were obtained by CNBr cleavage of the fusion protein [col-(1-171)-apa] and HPLC purification. It was recognized by conformation-dependent monoclonal antibodies with a K0.5 value close to that for natural apamin, indicating that folding was correct. In toxicity and binding experiments, the recombinant apamin displayed low activity. The recombinant and natural molecules differed by the amidation of the C-terminal histidine residue. Previous structure/activity relationship studies do not implicate this C-terminal residue in activity but the role of its amidation was not investigated. An apamin analog with a non-amidated C-terminal residue was then chemically synthesized. The biological properties of both recombinant and chemical molecules were determined. Amidation of the C-terminal alpha-carboxyl of apamin appears to be essential for full expression of its biological activity.

Amides↗
PubMed1978

Solid phase synthesis of apamin, the principal neurotoxin in bee venom. Isolation and characterization of acetamidomethyl apamin.

The synthesis of apamin, the principal neurotoxin in bee venom, has been accomplished by the solid phase method on a benzhydrylamine resin, 2-Phenylisopropyloxycarbonyl amino acids were used throughout the synthesis except for the C-terminal histidine. Improved yields in the coupling steps in the N-terminal part of the molecule were obtained by coupling each amino acid both in dichloromethane and dimethylformamide. The use of acetamidomethyl as an S-protecting group for cysteine made it possible to isolate and purify the linear peptide. The deblocked and oxidized peptide was fractionated by ion-exchange chromatography (Bio-Rex 70) to obtain a highly purified apamin with full biological activity and with the same physical and chemical properties as the natural peptide. Circular dichroism (CD) spectra of the synthetic and natural apamin were identical.

Amino Acid Sequence↗
PubMed1985

The apamin-sensitive Ca2+-dependent K+ channel molecular properties, differentiation and endogenous ligands in mammalian brain.

Apamin is a bee venom neurotoxin of 10 amino acids containing two disulphide bridges. Current-clamp and voltage-clamp experiments have shown that apamin externally applied blocks specifically at low concentration (0.1 microM) the Ca2+-dependent slow K+ conductance which mediates the long-lasting after-hyperpolarization in neuroblastoma cells and rat muscle cells in culture. The apamin-sensitive Ca2+-dependent slow K+ conductance is voltage-dependent and tetraethylamonium-insensitive. It is distinct from the high conductance Ca+-dependent K+ channel revealed by patch-clamp experiments. Biochemical characterization of the apamin receptor in rat striated muscle, neuroblastoma cells, rat synaptosomes, smooth muscles and hepatocytes was carried out with the use of a radiolabelled monoiodo-apamin derivative (125I-apamin) of high specific radioactivity (2000 Ci/mmol). The dissociation constant of the apamin-receptor complex is between 15 and 60 pM for all tissue preparations. The density of binding sites is very low: between 1 and 40 fmol/mg of protein. Radiation-inactivation analysis indicates a molecular mass for the apamin receptor of 250 000 Da whereas affinity labelling with 125I-apamin results in covalent labelling of a single polypeptide chain with a molecular mass of about 30 000 Da. Autoradiography of 125I-apamin binding sites reveals the presence of Ca2+-activated K+ channels in many regions of the brain. There is an all-or-none control of the expression of the apamin-sensitive Ca2+-dependent K+ channel by innervation in mammalian skeletal muscle. There exists an endogenous equivalent of apamin in rat brain.

Action Potentials↗
PubMed1989

Structural analysis of the interaction of apamin with Ia and its recognition by Ad- or Ab-restricted mouse T cells.

Apamin is a single-chain, disulfide-bonded, 18-amino acid peptide that elicits mouse T cell responses when presented by cells expressing syngeneic Ad or Ab class II MHC molecules. We previously showed that both the unfolding of this peptide by APC and the integrity of its N terminus segment were required for efficient apamin T cell recognition. To seek further information on the sites through which this peptide interacts with Ia and/or TCR, we used a panel of Ad- or Ab-restricted, apamin-specific THC to probe the antigenicity of a series of synthetic apamin analogs. These included peptides either truncated at the N terminus, or substituted by Ala at position 2, 4, 6, 7, 8, or 10. Analysis of THC responses to apamin analogs and use of the latter in competition assays for peptide presentation revealed the following: 1) optimal apamin T cell recognition critically involved Lys4, Ala5, Pro6, Glu7, and Leu10. The role of these residues in either "Ia or TCR binding regions" was found to depend upon the restricting Ia molecules at play. Thus, Lys4, Glu7, and Leu10 were TCR-binding residues in both Ad- and Ab-apamin complexes, whereas Lys4 participated in apamin/Ab but not, or to a marginal extent, in apamin/Ad interaction. Furthermore, Pro6 was associated either with an Ia contact region or a TCR interaction site when apamin was presented by Ab or Ad molecules, respectively. Unfolded apamin and the unrelated chicken OVA323-339 peptide were found to bind to the same, or closely related site(s) of Ad, as shown by their ability to compete reciprocally for recognition by appropriate Ad-restricted THC. Four distinct TCR V beta genes (V beta 2, V beta 4, V beta 6, and V beta 8) were found to be used in our panel of 16 apamin-specific THC. These data indicate that apamin interacts with Ad or TCR through a motif resembling other beta-sheeted, Ad-binding sequences; however, based on the spacing of the critical residues (i.e., 4, 7, and 10), the possibility exists that apamin processing permits the folding of this sequence into an alpha-helix.

Amino Acid Sequence↗
PubMed1985

Existence of a Ca2+-dependent K+ channel in synaptic membrane and postsynaptic density fractions isolated from canine cerebral cortex and cerebellum, as determined by apamin binding.

Apamin, a 18-amino acid neurotoxin isolated from bee venom, is a specific blocker of one class of the Ca2+-dependent K+ channels. The monoiodo derivative of the toxin with high specific radioactivity (1600 Ci/mmol) has been used to study its binding to synaptic membrane (SM) and postsynaptic density (PSD) fractions isolated from cerebral cortex (CTX) and cerebellum (CL) of canine brains. The Bmax (30.2 fmol/mg protein) for CTX-PSD is about twice that for CTX-SM (17.3 fmol/mg protein), suggesting a concentration of the apamin receptor protein in CTX-PSD over CTX-SM fractions. The lower value of Bmax for CL-PSD (12.3 fmol/mg protein), and the higher Kd value (51 pM) than for CTX-SM (33 pM), CTX-PSD (24 pM), and CL-SM (39 pM), may reflect the disruptive effect of Triton X-100 on these thin structures. The values of Bmax and Kd for CTX-SM are similar to those (22.0 fmol/mg protein and 33 pM) for rat CTX-SM. Both Ca2+ and Na+ inhibit apamin binding to CTX-PSD with K0.5 values of 14 and 31 mM, respectively, while the optimum concentration of KCl for activation is 5 mM. All these values are similar to those found for rat synaptosomes. Covalent labeling of the apamin binding protein, using the non-cleavable cross-linker, disuccinimidyl suberate, reveals an apamin binding polypeptide of 27 kdaltons under reducing and denaturing conditions in both the CTX-SM and CTX-PSD preparations, similar to that (28 kdaltons) reported for rat CTX-SM fractions. Prior phosphorylation of isolated CTX-PSD had no effect on apamin binding, nor did apamin binding influence subsequent phosphorylation of CTX-PSD. Calmodulin, an intrinsic PSD protein, may not play a role in apamin binding to PSD, since addition of calmodulin, or removal of the calmodulin by EGTA treatment, resulted in no change in the binding capacity of the PSD. The apamin binding protein seems to be bound quite firmly in the CTX-PSD fraction since treatments with 0.5% deoxycholate, 1% N-lauroyl sarcosinate, 4 M guanidine-HCl, pH 7.0, 0.5 M KCl and 1.0 M KCl, could only remove the apamin-receptor complexes from CTX-PSD by 40, 55, 52, 12 and 15%, respectively. These results contrast with the findings that the two detergents mentioned solubilize 80-93% of the receptor from synaptosomal or synaptic membrane fractions, indicating that a good deal of the receptor in these fractions is membrane-bound and not connected to the PSD.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

Molecular properties of the apamin-binding component of the Ca2+-dependent K+ channel. Radiation-inactivation, affinity labelling and solubilization.

Radiation-inactivation was used to assess the functional size of the apamin-binding component of the Ca2+-dependent K+ channel. The amount of specific binding of 125I-apamin to receptors in synaptic membranes of rat cortex decayed exponentially with increasing doses of ionizing radiation and target size analysis was consistent with a relative molecular mass of 250 000 +/- 20 000 for the 125I-apamin receptor. Analysis on sodium dodecyl sulfate gels following covalent cross-linking of 125I-apamin to its receptor in a synaptosomal membrane preparation from rat cortex revealed a single labelled polypeptide chain of Mr = 33 000 +/- 2000 in the presence of protease inhibitors. Our results suggest that the Ca2+-dependent K+ channel from rat cortex is an oligomeric structure of Mr = 250 000 +/- 20 000 containing an apamin-binding subunit of Mr = 33 000 +/- 2000. The apamin-binding component of the Ca2+-dependent K+ channel from rat synaptosomes was solubilized using detergents such as sodium cholate or 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate. Phospholipids did not increase the stability of the apamin-binding component during the solubilization. Binding of apamin to its solubilized receptor is reversible and saturable. The dissociation constant of the apamin-receptor complex is 40-150 pM, the rates constants of association and dissociation being 3.2 X 10(6) M-1s-1 and 1.4 X 10(-4)s-1 respectively. These binding characteristics are similar to those found for the membrane-bound apamin receptor.

Affinity Labels↗
PubMed1983

The effect of apamin on non-adrenergic, non-cholinergic vasodilator mechanisms in the intestines of the cat.

The effects of apamin, a polypeptide isolated from bee venom, on different vasodilator mechanisms in the small and large intestines were studied in atropinized cats. In the large intestine vasodilatation in response to pelvic nerve stimulation was either abolished or markedly diminished by I.A. apamin. However, neither the contraction of colonic muscle which occurred under these conditions nor sympathetic vasoconstriction was significantly influenced by apamin, suggesting that the effect of the peptide was not a non-specific effect on nerves or vascular smooth muscle. In the small intestine it was observed that the nervous vasodilatation induced by transmural electrical field stimulation or mechanical mucosal stimulation was either diminished or abolished by apamin. Intestinal vasodilatation, caused by close I.A. infusions of 5-hydroxytryptamine (5-HT), was abolished by apamin. After giving apamin 5-HT infusions induced a vasoconstriction in five out of six experiments. Vasodilatation induced by vasoactive intestinal polypeptide (VIP) was not significantly affected by apamin. In a series of in vitro experiments on rat portal vein, dose-response curves of several putative intestinal neurotransmitters were determined in the presence and absence of apamin. The following substances were tested: VIP, substance P, bradykinin, 5-HT, ATP and adenosine. Apamin had no effect on the dose-response curves of any of these compounds. The results are discussed in relation to the possibility that apamin may act by blocking the release of a putative peptidergic transmitter from nerve terminals.

Adenosine Triphosphate↗
PubMed1984

Apamin: a specific toxin to study a class of Ca2+-dependent K+ channels.

Apamin is a bee venom neurotoxin of 18 amino-acids containing two disulfide bridges. Current clamp and voltage clamp experiments have shown that externally applied apamin blocks specifically at low concentration (0.1 microM) the Ca2+-dependent slow K+ conductance which mediates the long-lasting after-hyperpolarization in neuroblastoma cells and rat muscle cells in culture. The apamin-sensitive Ca2+-dependent slow K+ conductance is voltage-dependent and tetraethylammonium (TEA) insensitive. It is distinct from the high conductance Ca2+-dependent K+ channel revealed by patch clamp experiments. Biochemical characterization of the apamin receptor in rat striated muscle, neuroblastoma cells, rat synaptosomes, smooth muscles and hepatocytes was carried out with the use of a radiolabelled monoiodo-apamin derivative (125I-apamin) of high specific radioactivity (2 000 Ci/mmol). The dissociation constant of the apamin-receptor complex is between 15 and 60 pM for all tissue preparations. The density of binding sites is very low; it varied between 1 and 40 fmol/mg of protein. Radiation inactivation analysis indicates a molecular weight for the apamin receptor of 250 000 daltons whereas affinity labelling with 125I-apamin results in covalent labelling of a single polypeptide chain with a molecular weight of about 30 000 daltons. We conclude that the apamin-sensitive Ca2+-dependent K+ channel is probably a large oligomeric structure containing one subunit of 30 000 daltons.

Animals↗
PubMed1995

Density of apamin-sensitive Ca(2+)-dependent K+ channels in bovine chromaffin cells: relevance to secretion.

Three objectives were defined when planning this study: (i) to identify binding sites for [125I]-apamin in intact bovine adrenal medulla chromaffin cells and to estimate their density and selectivity; (ii) to determine whether apamin modified the release of catecholamines evoked by brief pulses of dimethylphenylpiperazinium (DMPP, 1 or 5 microM for 10 sec), histamine (10 microM for 10 sec) or high K+ (20, 35 or 70 mM for 10 sec) applied to superfused cells; and (iii) to test whether apamin affected the profiles of the changes in cytosolic Ca2+ concentrations [Ca2+]i obtained in suspensions of cells loaded with fura-2 and stimulated with DMPP or histamine. At equilibrium, increasing concentrations of [125I]-apamin gave a saturation curve whose Scatchard transformation produced a Kd of 132 pM and a Bmax of 0.72 fmol/10(6) cells. Quinine, tetraethylammonium, charybdotoxin or glibenclamide (blockers of various subtypes of K+ channels) did not inhibit [125I]apamin binding. Binding was blocked by apamin and by d-tubocurarine, two blockers of small-conductance Ca(2+)-activated K+ channels (SK channels). The number of binding sites for [125I]apamin amounted to approx. 900 per single chromaffin cell, 0.72 sites per micron 2 surface area. Apamin (1 microM) enhanced the secretory response to histamine (10 microM), DMPP (1 or 5 microM) and high K+ (20 or 35 mM) by 2-3-fold. The response to 70 mM K+, however, was unaffected. Apamin also enhanced the peak [Ca2+]i increase produced by DMPP or histamine by approx. 30%. Overall, these results strongly support the hypothesis that under physiological conditions, SK channels control some of the electrical activity of chromaffin cells and indirectly, the opening of voltage-dependent Ca2+ channels, the access of Ca2+ to the secretory machinery and the rate of catecholamine release to the circulation from the intact adrenal gland.

Adrenal Medulla↗
PubMed1984

The presence in pig brain of an endogenous equivalent of apamin, the bee venom peptide that specifically blocks Ca2+-dependent K+ channels.

An apamin-like factor has been isolated from pig brain after extraction of the tissue and purification on sulfopropyl-Sephadex C-25 and on reversed-phase high pressure liquid chromatography. The apamin-like factor has the following properties: (i) it prevents 125I-labeled apamin binding to its specific receptor site present on rat brain synaptosomes, (ii) it is active in the radioimmunoassay for apamin (i.e., it prevents 125I-labeled apamin precipitation by anti-apamin antibodies), (iii) it induces contraction of guinea pig intestinal smooth muscle previously relaxed with epinephrine, and (iv) it blocks Ca2+-dependent K+ channels responsible for the long-lasting afterpotential hyperpolarization following the action potential in rat skeletal muscle cells in culture. All these properties are those of apamin itself. The apamin-like factor is a peptide that, like apamin, is destroyed by trypsin and unaffected by chymotrypsin. These results suggest the presence in mammalian brain of a potent Ca2+-dependent K+-channel modulator.

Action Potentials↗
PubMed1986

Blockade of Ca-activated K conductance by apamin in rat sympathetic neurones.

Effects of apamin on rat sympathetic neurones were investigated by means of intracellular and extracellular recording. Apamin (50 nM) significantly shortened the after-hyperpolarization (AH) following the spike evoked by current injection and slightly decreased its peak amplitude without affecting the time course of the spike. The AH following the synaptically-evoked spike was also blocked by apamin. This effect was dose- and time-dependent (ID50 estimated by extracellular recording approximately 15 nM, 20 min after application) and poorly reversible. Transmission of a single volley was not affected by 50 nM apamin. Though a long depolarizing current caused one or two spikes in the cell, greater repetitive firing was observed in the presence of apamin. Spontaneous repetitive firing, however, was not observed except for anodal-break spikes. Resting potential and input membrane resistance were essentially unchanged by apamin. The maximum rate of rise of the Ca spike was not decreased by 50 nM apamin but the duration of the spike was lengthened by 60%. The AH following the Ca spike was also blocked by apamin. These results suggest that apamin suppressed the slow AH without any inhibition of the Ca flux into the cell and is useful as a blocker of GK(Ca) in the rat sympathetic neurone.

Action Potentials↗
PubMed1988

Intracellular injection of apamin reduces a slow potassium current mediating afterhyperpolarizations and IPSPs in neocortical neurons of cats.

Electrophysiologic effects of intracellularly injected apamin, a Ca2+-dependent K+ channel blocker, were investigated in neurons of the motor cortex of awake cats. Single-electrode voltage clamp techniques were used to measure changes in membrane currents including those that were synaptically activated. All changes occurred within 2-4 min after pressure injection of apamin with partial recovery observed within 8-15 min. Apamin selectively abolished an outward current that mediated a slow afterhyperpolarization (AHP) following intracellular depolarizing current pulses and action potentials without influencing the time course of the action potentials or an associated fast AHP component. In addition apamin increased the number and frequency of spike discharges evoked by the depolarizing current pulses and produced a small increase in the rate of background firing activity. The baseline resting potential and input resistance were essentially unchanged by apamin. Apamin also diminished a late, slowly decaying component of inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) elicited by stimulation of the ventrolateral thalamus or the pyramidal tract. The apamin-induced changes were concomitant with a decrease of the decay time constant of both IPSPs and IPSCs and a positive shift in their reversal potential. The results suggest that the late, slowly decaying component of these inhibitory postsynaptic responses is generated by an apamin-sensitive Ca2+-dependent K+ conductance which is also responsible for the slow AHP.

Action Potentials↗
PubMed1991

Effect of apamin, a toxin that inhibits Ca(2+)-dependent K+ channels, on learning and memory processes.

Apamin, a neurotoxin extracted from bee venom, specifically binds to a particular class of Ca(2+)-activated K+ channels which are involved in the slow afterhyperpolarization (S-AHP) that follows action potentials in many excitable cells. We tested in mice the effects of apamin on learning and memory processes. The results showed that pre-training injection of apamin accelerated the acquisition of a bar-pressing response but also increased the bar-pressing rates of the animals. This latter result suggests that apamin accelerated acquisition because it increased behavioral activity in general and the number of bar-presses in particular. Post-training apamin injection retroactively and non-contingently facilitated memory processes taking place shortly after training in a bar-pressing task. The lack of an effect of the delayed apamin injection showed that apamin did not act proactively on memory retrieval processes. These results suggest that apamin-sensitive KCa channels may contribute to memory processes.

Animals↗