A re-examination of the effects of lanthanum on the frog neuromuscular junction

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Abstract
Lanthanum (La3+, 0.1–2 mM) was applied to frog cutaneous pectoris muscles at 20–25°C, or at 3–5°C, and the mean amplitude and rate of occurrence, <r>, of the miniature endplate potentials (mepps) were measured as functions of time at single neuromuscular junctions. Some muscles were fixed at 3–5°C and their nerve terminals examined in the electron microscope. When 1 or 2 mM La3+ was applied at room temperature, <r> rose to peak values of 0.8–3.4×103/s and then declined to 6 mepps occurred in this time. If 0.1 mM La3+ was applied, or if 1 mM La3+ was removed when <r> was near its peak, then <r> remained high for at least 1 h and ∼4×106 mepps occurred. All these mepp counts exceed the 0.7×106 quanta stored in resting nerve terminals [4]. When 1 or 2 mM La3+ was applied at 3–5°C, <r> rose to peak values of 50–700/s and then fell to 20–200/s after 20–30 min. If the La3+ was then removed, <r> declined ∼50% over the next hour; ∼0.7×106 mepps occurred at the junctions treated with 1 mM La3+, and their terminals still contained about 69% of their vesicles. Thus, vesicles can be recycled at 3–5°C. Millimolar concentrations of La3+ reduced the mepp amplitude by70–80% at both temperatures and abolished almost completely the depolarization produced by bath applied acetylcholine or carbachol. These effects were reversible.