Action of vinblastine on the spontaneous release of acetylcholine at the frog neuromuscular junction.

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Abstract
Vinblastine induced reversible changes of the spontaneous release of acetylcholine (ACh) at the frog [Rana esculenta and R. temporaria] neuromuscular junction as characterized by the appearance of giant potentials. These large potentials occurred soon after soaking the muscle in vinblastine and were not consequent to a large increase in the frequency of spontaneous release. Their number, relative to the total number of spontaneous potentials, increased with the duration of soaking. Large potentials appeared even in the presence of tetrodotoxin or in low Ca2+-high Mg2+ Ringer solution. Following vinblastine treatment, the amplitude histogram of spontaneous potentials recorded from a number of fibers displayed an evident periodicity with peaks occurring regularly at simple multiples of the model amplitude of the unitary potentials. Giant potentials may be produced by the release of preformed pluriquantal packets of ACh. Comparison of the amplitude distribution of spontaneous potentials and end-plates potentials showed that only an insignificant number of large quanta were released by nerve stimuli. The absolute frequency of giant potentials did not markedly change when spontaneous discharge was accelerated by hypertonic solution. The mechanism by which vinblastine induces the appearance of giant potentials was discussed.