Blockade of Neuromuscular Transmission by Enzymatically Active and Inactive β -Bungarotoxin

β -Bungarotoxins have been shown to be presynaptic blockers of neuromuscular transmission. This paper reports experiments using the most positively charged β -bungarotoxin that elutes from a CM-Sephadex C-25 column. The toxin is known to be a single polypeptide with a molecular weight of approximately 11,000 and has phospholipase A2activity. The application of the enzymatically active toxin to the frog sciatic nerve-sartorius muscle preparation results in an initial decrease in the average endplate potential amplitude followed by a temporary rebound in endplate potential amplitude, and finally a complete inhibition of endplate potentials. Similarly, miniature endplate potential frequency is initially reduced upon toxin application but then increases dramatically. After the phospholipase A2of the toxin is inactivated, treatment with the toxin results in only the initial decrease in transmitter release. These results suggest that this β -bungarotoxin acts in two functionally separate steps: (i) by binding to a specific presynaptic site possibly associated with calcium entry, and (ii) by perturbing the presynaptic membrane by its enzyme action, which results in an increase and then a failure in transmitter release.