Molybdic and tungstic heteropolyanions for "ionic fixation" of acetylcholine in cholinergic motor nerve terminals.

The treatment of neuromuscular junctions with phosphomolybdic acid (PMA) and silicotungstic acid (STA) heteropolyanions permits the visualization of electron dense precipitates in the synaptic vesicles of the cholinergic motor nerve terminals. At the light microscopic level, the uncolored molybdenum salt is visualized after reduction to molybdenum blue. The blue coloration is confined to the nerve terminals. Since PMA and STA are known as strong precipitating agents of quaternary ammonium compounds (cations) it is supposed that they have insolubilized in situ the acetylcholine (Ach) of the synaptic vesicles by means of a rapid ionic interaction. Furthermore, in spite of the strong acidity of PMA and STA solutions, the ultrastructure of the treated tissue is not significantly altered but on the contrary seems to be well preserved. The ionic insolubilization of Ach, added to the good preservation of the ultrastructure prompted us to use the term "ionic fixation".