title: Neuromuscular effects of some potassium channel blocking toxins from the venom of the scorpion Leiurus quinquestriatus hebreus.
abstract: The scorpion venom Leiurus quinquestriatus hebreus was fractionated by chromatography in order to isolate toxins that affected binding of radiolabelled dendrotoxin to K+ channel proteins on synaptosomal membranes and that facilitated acetylcholine release in chick biventer cervicis nerve-muscle preparations. In addition to the previously characterized charybdotoxin, three toxins were isolated: 14-2, 15-1 and 18-2. Toxin 14-2 has a blocked N-terminus and because of low quantities, it has not been sequenced; 15-1 is a newly sequenced toxin of 36 residues with some overall homology to charybdotoxin and noxiustoxin; 18-2 is identical to charybdotoxin-2. The apparent Ki against dendrotoxin binding were: charybdotoxin, 3.8 nM; 14-2, 150 nM; 15-1, 50 nM; and 18-2, 0.25 nM. Toxin 14-2 (75 nM-1.5 microM) had a presynaptic facilitatory effect on neuromuscular preparations. Toxin 15-1 augmented responses to direct muscle stimulation, probably because it blocked Ca(2+)-activated K+ currents in muscle fibres. Toxin 18-2 (charybdotoxin-2) had a potent presynaptic facilitatory action, with less effect on direct muscle stimulation. This contrasts with the relatively weak neuromuscular effects of the highly homologous charybdotoxin. On a Ca(2+)-activated K+ current in mouse motor nerve endings, charybdotoxin and toxin 18-2 produced maximal block at around 100 nM, whereas 15-1 was inactive at 300 nM. Charybdotoxin can increase quantal content, but this is more likely to result from block of voltage-dependent K+ channels than Ca(2+)-activated channels: the increase in transmitter release occurred in conditions in which little IKCa would be present; higher concentration of charybdotoxin and longer exposure times were required to increase transmitter release than those needed to block IKCa, and the facilitatory effects of charybdotoxin and toxin 18-2 correlated more with their effects on dendrotoxin binding than on block of IKCa.
authors.affiliation: Department of Physiology and Pharmacology, University of Strathclyde, Glasgow, U.K.
authors.forename: D L
authors.initials: DL
authors.lastname: Marshall
authors.forename: H
authors.initials: H
authors.lastname: Vatanpour
authors.forename: A L
authors.initials: AL
authors.lastname: Harvey
authors.forename: P
authors.initials: P
authors.lastname: Boyot
authors.forename: S
authors.initials: S
authors.lastname: Pinkasfeld
authors.forename: Y
authors.initials: Y
authors.lastname: Doljansky
authors.forename: F
authors.initials: F
authors.lastname: Bouet
authors.forename: A
authors.initials: A
authors.lastname: Ménez
mesh_terms: D000109:Acetylcholine / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D001667:Binding, Competitive; D018999:Charybdotoxin; D005591:Chemical Fractionation; D002645:Chickens; D002851:Chromatography, High Pressure Liquid; D002852:Chromatography, Ion Exchange; D004305:Dose-Response Relationship, Drug; D004546:Elapid Venoms / Q000302:isolation & purification / Q000378:metabolism / Q000633:toxicity; D066298:In Vitro Techniques; D008297:Male; D051379:Mice; D008969:Molecular Sequence Data; D009469:Neuromuscular Junction / Q000187:drug effects*; D009498:Neurotoxins / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism / Q000633:toxicity*; D010791:Phrenic Nerve / Q000378:metabolism; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000378:metabolism; D011869:Radioligand Assay; D012604:Scorpion Venoms / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism / Q000633:toxicity*; D017386:Sequence Homology, Amino Acid; D013329:Structure-Activity Relationship; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism
chemical_list: D004546:Elapid Venoms; D009498:Neurotoxins; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D012604:Scorpion Venoms; D018999:Charybdotoxin; C020273:dendrotoxin; D000109:Acetylcholine