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 ra diolabelled dendrotoxin to K+ channel proteins on synaptosomal membran es and that facilitated acetylcholine release in chick biventer cervic is nerve-muscle preparations. In addition to the previously characteri zed charybdotoxin, three toxins were isolated: 14-2, 15-1 and 18-2. To xin 14-2 has a blocked N-terminus and because of low quantities, it ha s not been sequenced; 15-1 is a newly sequenced toxin of 36 residues w ith some overall homology to charybdotoxin and noxiustoxin; 18-2 is id entical to charybdotoxin-2. The apparent K-i against dendrotoxin bindi ng 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 mu M) had a presynaptic facilitatory eff ect on neuromuscular preparations. Toxin 15-1 augmented responses to d irect muscle stimulation, probably because it blocked Ca2+-activated K + currents in muscle fibres. Toxin 18-2 (charybdotoxin-2) had a potent presynaptic facilitatory action, with less effect on direct muscle st imulation. This contrasts with the relatively weak neuromuscular effec ts of the highly homologous charybdotoxin. On a Ca2+-activated K+ curr ent in mouse motor nerve endings, charybdotoxin and toxin 18-2 produce d maximal block at around 100 nM, whereas 15-1 was inactive at 300 nM. Charybdotoxin can increase quantal content, but this is more likely t o result from block of voltage-dependent K+ channels than Ca2+-activat ed channels: the increase in transmitter release occurred in condition s in which little I-KCa would be present; higher concentration of char ybdotoxin and longer exposure times were required to increase transmit ter release than those needed to block I-KCa, and the facilitatory eff ects of charybdotoxin and toxin 18-2 correlated more with their effect s on dendrotoxin binding than on block of I-KCa.