Apamin as a selective blocker of the calcium-dependent potassium channel in neuroblastoma cells: voltage-clamp and biochemical characterization of the toxin receptor

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

The recent development of techniques for recording currents through single ionic channels has led to the identification of a K+-specific channel that is activated by cytoplasmic Ca2+. The channel has complex properties, being activated by depolarizing voltages and having a voltage-sensitivity that is modulated by cytoplasmic Ca2+ levels. The conduction behaviour of the channel is also unusual, its high ionic selectivity being displayed simultaneously with a very high unitary conductance. Very little is known about the biochemistry of this channel, largely due to the lack of a suitable ligand for use as a biochemical probe for the channel. We describe here a protein inhibitor of single Ca2+-activated K+ channels of mammalian skeletal muscle. This inhibitor, a minor component of the venom of the Israeli scorpion, Leiurus quinquestriatus, reversibly blocks the large Ca2+-activated K+ channel in a simple biomolecular reaction. We have partially purified the active component, a basic protein of relative molecular mass (Mr) approximately 7,000.

Effects of apamin, quinine and neuromuscular blockers on calcium-activated potassium channels in guinea-pig hepatocytes

The bee venom peptide, apamin, has been radiolabelled with 125I, the monoiodinated derivative purified, and its binding to intact guinea-pig liver cells studied. At 37 degrees C 125I-monoiodoapamin associated with, and dissociated from, guinea-pig hepatocytes remarkably rapidly. The association and dissociation rate constants were 1.4 X 10(8) M-1 s-1 and 0.035 s-1 respectively. Equilibrium binding studies demonstrated a saturable binding component compatible with 1:1 binding to a single class of site and having an equilibrium dissociation constant (KL) of 390 pM. The maximal binding capacity was 1.1 fmol mg-1 dry wt. of tissue. Unlabelled apamin displaced bound 125I-monoiodoapamin with a KI of 380 pM, which is consistent with the concentration of apamin required to inhibit Ca2+-activated K+ permeability (PK(Ca) ) in these cells. Inhibitable binding of 125I-monoiodoapamin to rat hepatocytes was much less than to guinea-pig hepatocytes and could not be reliably quantified. Neither was there any discernible inhibitable binding to human erythrocytes. This is in keeping with the reported lack of apamin-sensitive Ca2+-activated K+ channels in these cell types. Various agents were tested for their ability to inhibit monoiodoapamin binding to, and Ca2+-mediated K+ efflux from, guinea-pig hepatocytes. All compounds tested which inhibited binding also blocked K+ efflux at similar concentrations. TEA and quinine affected hepatocytes only at high concentration (KI = 5.8 and 0.51 mM respectively). 9-aminoacridine, quinacrine and chloroquine were slightly more effective (KI = 70-180 microM). By far the most active compounds (apart from apamin) were the neuromuscular blocking agents; tubocurarine, pancuronium and atracurium (KI = 7.5, 6.8 and 4.5 microM respectively). Gallamine was slightly less effective (KI = 14 microM) and decamethonium and hexamethonium much less so (KI = 620 and 760 microM respectively). 3,4-diaminopyridine, alpha-bungarotoxin and tetrodotoxin were among several compounds which showed little or no affinity for apamin binding sites or inhibition of K+ efflux in guinea-pig hepatocytes. The saturable binding of 125I-monoiodoapamin to guinea-pig hepatocytes corresponds to about 1700 sites per cell. Assuming, tentatively, that binding sites correspond to channels the rate of K+ loss observed following agonist action can readily be explained if these channels have unitary conductances in the range reported for PK(Ca) in other tissues.

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.

Effects of apamin on the outward potassium current of isolated frog skeletal muscle fibres

The effect of apamin, a polypeptidic toxin from bee venom which is a specific blocker of certain Ca2+-dependent K+ channels, has been tested (50-100nM) on voltage clamped single skeletal muscle fibres of the frog. The results have shown the existence of an inhibitory effect of the toxin on the slow outward K+ current which suggests the existence of a Ca2+-sensitive component of the slow K+ permeability in the plasma membrane of the frog muscle fibre.

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.

Characterization, solubilization, affinity labeling and purification of the cardiac Na+ channel using Tityus toxin gamma

Saturable, high-affinity binding of iodinated toxin gamma from Tityus serrulatus scorpion venom (TiTx gamma) to Na+ channel receptor was identified in sarcolemma membrane of chick heart. A binding capacity of 450-600 fmol/mg of protein was found similar to that of tetrodotoxin-binding component. The enrichment of these membrane-bound toxin binding sites follows that of other sarcolemma markers. Kinetic data and displacement of 125I-TiTx gamma from its binding sites by unlabeled TiTx gamma gave an equilibrium dissociation constant (Kd) of 1-3 pM. The gating component and the selectivity filter of the voltage-sensitive Na+ channel, identified as binding sites of TiTx gamma and of tetrodotoxin respectively, have been efficiently solubilized with Nonidet P-40. Purification was achieved by ion-exchange chromatography on DEAE-Sephadex A-25, affinity chromatography on wheat-germ-agglutinin-Sepharose and sucrose density gradient centrifugation. An enrichment of 1400-fold from the original detergent extract was measured for both toxin binding sites (1120-1230 pmol/mg of protein). Sodium dodecyl sulfate gel electrophoresis reveals a single large polypeptide component of Mr230000-270000. The purified material exhibits an apparent sedimentation coefficient of 8.8S. Covalent cross-linking of 125I-TiTx gamma to its membrane-embedded cardiac receptor shows that the cross-linked material, solubilized and purified by the same procedure comprises a single polypeptide chain of the same Mr of 230000-270000. Furthermore, as seen for Electrophorus electricus electroplax and rat brain, the tetrodotoxin-binding component and the TiTx gamma-binding component are carried by the same polypeptide chain. The functional Na+ channel might be an oligomer of this subunit of Mr23000-270000.

High-affinity neurotensin binding sites in differentiated neuroblastoma N1E115 cells

This paper describes the interaction of neurotensin with mouse neuroblastoma N1E115 cells. Neurotensin binding sites are undetectable in nondifferentiated neuroblastoma cells. They appear during cell differentiation in the presence of a low serum concentration and dimethyl sulfoxide, and reach a maximal level after 50-60 h of incubation under these conditions. The binding of monoiodo[Trp11]neurotensin to homogenates of differentiated N1E115 cells is specific, saturable, and reversible. The interaction is characterized by a dissociation constant of 150 pM and a maximal binding capacity of 9 fmol/mg of protein at 0 degrees C, pH 7.5. These binding parameters, as well as the specificity toward a series of neurotensin analogues, are similar for neurotensin receptors in N1E115 cells and for the high-affinity binding sites that had been previously characterized in rat brain synaptic membranes by means of the same radiolabeled ligand. The presence of high-affinity binding sites for neurotensin in the neuroblastoma N1E115 provides a useful model to study the cellular responses that are generated by the association of neurotensin to its receptor in electrically excitable cells.

Differentiation of receptor sites for [3H]nitrendipine in chick hearts and physiological relation to the slow Ca2+ channel and to excitation-contraction coupling

The properties of interaction of the Ca2+ channel antagonist [3H]nitrendipine have been investigated in chick hearts at various stages of in ovo and post-natal development and in cultured cells. The dissociation constant of the [3H]nitrendipine-receptor complex is between 0.4 nM and 0.5 nM for intact ventricle and cultured cells. [3H]Nitrendipine binding is antagonized by nitrendipine analogs. The order of efficacy of the different dihydropyridine molecules is nitrendipine greater than nimodipine greater than nifedipine greater than nisoldipine with Kd values ranging from 0.5 to 4 nM. Inhibition of [3H]nitrendipine binding by other antiarrhythmic molecules like amiodarone, F13004 and bepridil was observed. Half-maximum inhibitions (K0.5) were found for verapamil and D600 at concentrations between 0.23 and 0.26 microM. The potency of organic Ca2+ blockers to depress by 50% the maximum amplitude of spontaneous beating of heart cells is closely related to K0.5 values obtained from [3H]nitrendipine binding experiments. Electrophysiological results indicate that the slow channel is insensitive to nitrendipine at the younger stage of development (3-day-old) whereas, in adult like cells, nitrendipine (50 nM) abolished both slow action potential due to the slow Ca2+ channel and contraction. The maximum binding capacity for [3H]nitrendipine is found to increase during development of the embryonic heart from 40 fmol/mg protein at day 3 to 100 fmol/mg protein at day 14, to stay relatively stable until day 18. Then the number of sites increases rapidly to reach a second plateau at 210 fmol/mg protein on day 4 after hatching. Treatment with 6-hydroxydopamine results in 35% increase in [3H]nitrendipine binding, whereas reserpine treatment is without effect. Developmental properties of nitrendipine-sensitive Ca2+ channels have been compared with those of tetrodotoxin-sensitive Na+ channels and muscarinic receptors. These results indicate that nitrendipine receptors exist at the early stage of development (3-day-old-hearts) but that they do not correspond to functional slow Ca2+ channels, that in ovo development corresponds both to an increase of the number of [3H]nitrendipine receptors and to the transformation of silent Ca2+ channels into functional Ca2+ channels, and that there is a regulation of the level nitrendipine-sensitive Ca2+ channels by innervation.

A micro-radioimmunoassay for apamin

A radioimmunoassay is described which allows detection of quantities of apamin between 5 and 1200 fmoles (10-2400 pg) in 50 microliter aliquots. The assay requires a few milliliters of apamin antiserum diluted 10,000-20,000 times and 125I apamin at a specific radioactivity of 2000 Ci/mmole. This assay is specific for apamin.

Resistance to apamin of the Ca2+-activated K+ permeability in pancreatic B-cells

The bee venom neurotoxin apamin failed to affect 86Rb outflow and insulin release from rat pancreatic islets stimulated by D-glucose or the Ca2+-ionophore A23187. Apamin, in contrast to quinine or A23187, also failed to affect bioelectrical activity in mouse islet cells. These findings suggest that, like in erythrocytes, and at variance with the situation found in smooth muscle, liver or neuroblastoma cells, the Ca2+-activated K+ permeability in the pancreatic B-cell is resistant to apamin.

High affinity binding of [125I]monoiodoapamin to isolated guinea-pig hepatocytes

The bee venom neurotoxin apamin has been labelled with 125I and its binding to isolated guinea-pig hepatocytes measured under physiological conditions. A single saturable component of [125I]monoiodoapamin binding with a Kd of 350 pM and Bmax of 0.99 fmol/mg dry wt was identified. Native apamin displaced labelled apamin with a Kd of 376 pM which is broadly in keeping with the concentrations found to inhibit K loss from guinea-pig hepatocytes. These observations, together with the binding found in other tissues, suggest that specific binding of labelled apamin is particularly associated with apamin-sensitive, Ca-activated K-channels.