Effect of lanthanum ions on function and structure of frog neuromuscular junctions

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Abstract
1. Electrophysiological and electron-microscopic studies were made of the effect of lanthanum ions on frog neuromuscular junctions. 2. In the presence of 1 mM La$^{2+}$, nerve impulses continued to invade the nerve terminals but ceased to release transmitter. 3. Lanthanum caused a rapid and large increase in the frequency of miniature end-plate potentials; presumably because La activates the mechanism of transmitter release without the usual prerequisite of presynaptic membrane depolarization. At 4 degrees C, La caused a 10 000-fold, or even larger increase in the rate of leakage of transmitter quanta. Such high rate of transmitter release was not accompanied by obvious changes in electron-microscopic structure of the nerve terminals. 4. With continued La-treatment, the frequency of miniature end-plate potentials subsides slowly until they are no longer detectable at most end-plates. During this period the number of synaptic vesicles is reduced until practically all the endings become completely depleted of synaptic vesicles. In contrast, coated vesicles and membrane-bound tubes and cysternae become more numerous.

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