Neuromuscular effects of some potassium channel blocking toxins from the venom of the scorpion Leiurus quinquestriatus hebreus

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.