SPECIFICITY OF IONIC FIXATION WITH SILICOTUNGSTIC ACID FOR CYTOCHEMICAL LOCALIZATION AND IDENTIFICATION OF ACETYLCHOLINE IN SYNAPTIC VESICLES

Abstract

Silicotungstic acid (STA), as well as phosphomolybdic acid (PMA) and phosphotungstic acid (PTA), are known to exist as a large molecular complex of heteropolyanion in aqueous solution, and to be a potent precipitating agent of quaternary ammonium compounds, like acetylcholine (ACh) and choline (ionic fixation). In most cytochemical detection of acetylcholine in synaptic vesicles, STA is preferentially employed, because of the stability of the heteropolyanion structure in a wide pH range. The specificity of the ionic fixation for ACh was examined by a model experiment in vitro. Among various transmitter substances, ACh can be precipitated in the lowest concentration by STA. After the treatment of nervous tissue by STA, a bulk of ACh (more than 1/3) was recovered from the STA-precipitate. STA applied to nerve fibers in the sinus venosus of frog heart formed a punctiform precipitate in clear vesicles, but not in large cored vesicles in varicosities. It is suggested that electron-dense punctiform precipitates in synaptic vesicles in the present study and in previous cytochemical observations on neuromuscular junction represent ACh-silicotungstate precipitates.