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

Lanthanum (La3+, 0.1-2mM) was applied to frog cutaneous pectoris muscles at 20-25 degrees C, or at 3-5 degrees C, and the mean amplitude and rate of occurrence, mean value of r, of the miniature endplate potentials (mepps) were measured as functions of time at single neuromuscular junctions. Some muscles were fixed at 3-5 degrees C and their nerve terminals examined in the electron microscope. When 1 or 2 mM La3+ was applied at room temperature, mean value of r rose to peak values of 0.8-3.4 x 10(3) and then declined to less than 100/s after 30-60 min. When the results are corrected for the dispersion in mepp amplitudes, we estimate that approximately 1.8 x 10(6) mepps occurred in this time. If 0.1 mM La3+ was applied, or if 1 mM La3+ was removed when mean value of r was near its peak, then mean value of r remained high for at least 1 h and approximately 4 x 10(6) mepps occurred. All these mepp counts exceed the 0.7 x 10(6) quanta stored in resting nerve terminals. When 1 or 2 mM La3+ was applied at 3-5 degrees C, mean value of 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, mean value of r declined approximately 50% over the next hour; approximately 0.7 x 10(6) 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 degrees C. Millimolar concentrations of La3+ reduced the mepp amplitude by 70-80% at both temperatures and abolished almost completely the depolarization produced by bath applied acetylcholine or carbachol. These effects were reversible.