An improved isolated single electroplax preparation: I. Effect of compounds acting primarily at the synapses

Abstract

Acetylcholine and related structures have been tested as to their effects on the electrical activity of an improved isolated single electroplax preparation. As a measure for comparing the relative strength of the compounds, the concentration which produced 50% decrease in spike height within 20 min or less (usually from 5–15 min) was used. This concentration is referred to as the minimum active concentration. (a) Acetylcholine, decamethonium and d-tubocurarine were about 5–10 times as active as in previous preparations; no marked increase in sensitivity was observed with neostigmine and dimethylaminoethyl acetate. (b) Among the newly tested compounds succinylcholine proved to be twice as strong as acetylcholine, propionylcholine about 100 times weaker, and benzoylcholine several hundred fold weaker; the S-analogue of acetylcholine was about 3–5 times weaker. Dimethyl curare is only about half as strong as curare. (c) The decrease in spike height caused by the various quaternary compounds tested showed much better reversibility than in the previous preparation. (d) Curare in concentrations about 1000 times as high as those required to block the indirect response, abolished the propagated spike evoked by direct stimulation. In previous preparations no effect was obtained with curare outside the synaptic region even in higher concentrations than those applied in the present studies. (e) Following block of the indirect spike by curare, acetylcholine when applied in higher concentrations was able to overcome the block thys adding new evidence for the competitive action between acetylcholine and curare for the receptor. Dimethylaminoethyl acetate and the S-analogue of acetylcholine did not affect the direct spike in presence of curare in the concentrations tested. (f) Dimethylaminoethyl acetate acts much more strongly at pH 6 to 8, where it is present mostly in the cationic (protonated) form, than at pH 9, where it is present mostly in its uncharged form. The role of the Coulombic forces in the receptor protein thus appears similar to that previously observed with acetylcholinesterase. (g) The various changes of the response to acetylcholine and related structures after the removal of the adjacent membrane are a new demonstration that the effect of externally applied compounds on biological membranes depends to a great extent on the permeability barriers protecting the active membrane.