Kinetic Studies on the Release of [³H]-Acetylcholine from Guinea Pig Myenteric Plexus: Difference in the Effects of Morphine and Reduced Calcium Influx

Abstract

The guinea pig ileum and later the myenteric plexus-longitudinal muscle preparation derived from it, have been used extensively as convenient in vitro models for demonstrating the specific effects of opiates on neural function (8). Opiates reduce the contractions produced by low frequency stimulation, decrease the resting release of acetylcholine (ACh) and the release produced by low frequency stimulation but not by high frequency stimulation (11). The mechanism whereby opiates depress the release of ACh from the myenteric plexus is still not understood. One of the problems in investigating the action of opiates in this preparation is the impossibility of simultaneously measuring the effect of opiates on ACh release and on mechanical activity; cholinesterise inhibitors, which have to be used for measuring ACh release, cause a steady contraction of the preparation which precludes the detection of mechanical activity induced by stimulation. To overcome this problem, the method developed for measuring labelled ACh release from brain slices in the absence of Cholinesterase inhibitors (13) was adapted for the myenteric plexus-longitudinal muscle preparation. After labelling part of the ACh stores with [³H]-choline (Ch), the spontaneous release of radioactivity measured in the presence of physostigmine consists of a mixture of labelled ACh, Ch and phosphorylcholine.