A further study of the phospholipase‐independent action of beta‐bungarotoxin at frog end‐plates.

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Abstract
The effect of .beta.-bungarotoxin (.beta.-BuTx) at the frog neuromuscular junction was investigated further to distinguish more clearly between phospholipase-independent and phospholipase-dependent action on transmitter release. Inhibition of the enzymatic activity, by substitution of Sr for Ca, allowed determination of the dose-response curve of the early rapid decrease in transmitter release caused by the toxin. In the presence of Sr ions there was, however, still .apprx. 7% residual enzymatic activity, and electrophysiological evidence of it was seen in room-temperature experiments at high concentrations of .beta.-BuTx. This residual enzymatic activity could be suppressed by lowering the temperature to 5.degree. C. In normal Ca-Ringer solution .beta.-BuTx produced the typical triphasic effect on the amplitude of end-plate potentials (e.p.p.). Lowering the temperature markedly delayed and then diminished the secondary transient increase. There was comparatively little temperature influence on the 1st rapid decrease in e.p.p. amplitude. Enzymatic assays confirmed the temperature dependence of the toxin''s phospholipase activity on model phospholipid substrates. The kinetics of the phospholipase-independent action of .beta.-BuTx were examined in Sr-Ringer and compared to Ca-Ringer solution, as well as in Ca-Ringer at different temperatures. Both the time to onset of inhibition and the time to 50% inhibition of the e.p.p. during the 1st phase of toxin action, are temperature-dependent and briefer in Ca than in Sr-Ringer solution. It is suggested that Ca is more effective than Sr in promoting this phospholipase-independent interaction of .beta.-BuTx with the nerve terminal membrane.