Endogenous Acetylcholine Release and Labelled Acetylcholine Formation from [3H]Choline in the Myenteric Plexus of the Guinea-Pig Ileum

Abstract

The spontaneous release of acetylcholine (ACh) from the guinea-pig myenteric plexus – longitudinal muscle preparation superfused at a constant rate in the presence of physostigmine was 10 nmol∙g⁻¹∙h⁻¹. This release was decreased to one-third by tetrodotoxin or by MnCl₂ and increased 2.5 times by 0.1 Hz and 20 times by 16 Hz stimulation. The formation of [³H] ACh from [³H]choline increased from 3 to 33 nmol∙g⁻¹∙h⁻¹ when the concentration of [³H]choline was increased from 1 μM to 50 μM. The rate of [³H] ACh formation was not affected by tetrodotoxin, MnCl₂, or physostigmine in the absence of stimulation. It was increased by 50% by 0.1 Hz and by 100% by 16 Hz stimulation during the first 9 min of exposure to [³H]choline but not subsequently. The myenteric plexus – longitudinal muscle preparation contains 200 nmol/g choline. Results suggest that the apparent small [³H]ACh formation from low concentrations of [³H]choline is due to the dilution of [³H]choline by endogenous choline. The major part of [³H]ACh formation appears to be due to the intracellular turnover of ACh while the evoked release of [³H]ACh appears to originate from a small pool.