Physical studies of the interactions of acetylcholine chloride with membrane constituents

The binding of acetylcholine to pure lipids, and lipids, proteins and lipoproteins extracted from synaptic membranes, was investigated by monolayer and n.m.r. techniques. No specific binding of acetylcholine could be detected at the concentration used, although its muscarinic and nicotinic antagonists [atropine and (+)-tubocurarine respectively] could be shown to interact with the membrane components. It is concluded that the binding of the nicotinic and muscarinic antagonists of acetylcholine is not necessarily indicative of the existence of a specific acetylcholine receptor. Measurements of the displacement of 45Ca2+ from monolayers of phosphatidylserine by acetylcholine and the variation of electrophoretic mobility of phosphatidylserine particles with concentration of acetylcholine indicated that in these systems acetylcholine was acting as a counterion at the negatively charged lipid interface. But studies of the salting-in and salting-out of negatively charged lipid aggregates showed that acetylcholine and other quaternary ammonium compounds did not here behave simply as counterions. Electrostrictively hydrated cations such as Na+ and K+ were found to salt out, whereas hydrophobically hydrated cations such as acetylcholine salted in such aggregates. The possible role of the hydration of acetylcholine in synaptic transmission is discussed.