The beta-bungarotoxin-induced depolarization of the synaptosomal plasma membrane monitored by the efflux of 86Rb+ is potentiated by raising the albumin in the incubation, is Ca2+-dependent and is due neither to inhibition of the (Na+ + K+)-dependent ATPase nor to activation of the voltage-dependent Na+ channel. Occupancy of the beta-bungarotoxin-binding site by dendrotoxin inhibits partially the action of beta-bungarotoxin. The efflux of 86Rb+ is parallelled by a release of lactate dehydrogenase from the synaptosome, and the two processes are maximal with 2 nM-toxin. Digitonin induces a release of 86Rb+ and lactate dehydrogenase closely similar to that seen with beta-bungarotoxin. It is concluded that the toxicity of beta-bungarotoxin for mammalian nerve terminals can be largely accounted for by specific site-directed phospholipase A2-induced permeabilization of the plasma membrane.

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