The increase in spontaneous transmitter release produced by beta-bungarotoxin and its modification by inorganic ions.
The excitatory phase of the biphasic action on transmitter release of the neurotoxin, beta-bungarotoxin (beta-BuTX; 0.5 microgram ml-1), was studied on miniature end-plate potentials (MEPPs) at frog sciatic nerve-sartorius muscle junctions. The most common type of excitatory response was characterized by a continuous increase in MEPP frequency that reached a plateau; a less common form was characterized by irregular episodic bursts of firing. There was a direct relationship between toxin activity and [K+]O (2.5-10.0 mM) with virtually no effect of the toxin at normal [K]O+ at the concentration of toxin used. In the absence of Mg++, there was a paradoxical inverse relationship between toxin activity and [Ca++]O (0.5-4.0 mM) at higher [K+]O. However, in the presence of 1.0 mM Mg++ the increased MEPP frequency produced by beta-BuTX was independent of [Ca++]O. The action of beta-BuTX was very sensitive to blockade by Mg++. Toxin activity was demonstrated in a Sr++-containing, Ca++-free solution, but not in a Mg++-containing, Ca++-free solution. It is probable that beta-BuTX causes a slight depolarization of the nerve terminals by a mechanism not sensitive to blockade by tetrodotoxin and that the ability of beta-BuTX to depolarize the terminals can account for the enhancement of the response by raising [K+]O and the depression of the response by Mg++. Alternatively, beta-BuTX could be producing its effects by some, as yet undefined, direct action on the release process.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001001:Anura; D002038:Bungarotoxins / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology; D002412:Cations / Q000494:pharmacology*; D004347:Drug Interactions; D066298:In Vitro Techniques; D008274:Magnesium / Q000494:pharmacology; D009045:Motor Endplate / Q000187:drug effects; D018377:Neurotransmitter Agents / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D011894:Rana pipiens; D013268:Stimulation, Chemical
D002038:Bungarotoxins; D002412:Cations; D018377:Neurotransmitter Agents; D008274:Magnesium; D011188:Potassium; D002118:Calcium