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

1. Vinblastine induces reversible changes of the spontaneous release of acetylcholine (ACh) at the frog neuromuscular junction as characterized by the appearance of giant potentials. These large potentials occur soon after soaking the muscle in vinblastine and are not consequent to a large increase in the frequency of spontaneous release. Their number, relative to the total number of spontaneous potentials, increases with the duration of soaking. 2. Large potentials appear even in the presence of tetrodotoxin or in low Ca2+-high Mg2+ Ringer solution. 3. Following vinblastine treatment,the amplitude histogram of spontaneous potentials recorded from a number of fibres display an evident periodicity with peaks occurring regularly at simple multiples of the model amplitude of the unitary potentials. It is suggested that giant potentials are produced by the release of preformed pluriquantal packets of ACh. 4. Comparison of the amplitude distribution of spontaneous potentials and end-plates potentials show that only an insignificant number of large quanta are released by nerve stimuli. The absolute frequency of giant potentials does not markedly change when spontaneous discharge is accelerated by hypertonic solution. 5. The mechanism by which vinblastine induces the appearance of giant potentials is discussed.