Effect of low calcium and of oxotremorine on the kinetics of the evoked release of [3H]acetylcholine from the guinea-pig myenteric plexus; comparison with morphine.

Abstract

1. Guinea-pig myenteric plexus-longitudinal muscle preparations were superfused at a constant rate and their mechanical activities were monitored. 2. Acetylcholine stores were labelled with [3H] choline during 0.1 Hz supramaximal stimulation and the rate of the release of [3H]ACh evoked by the same supramaximal stimulation was followed in the presence of hemicholinium-3 but without cholinesterase inhibition. 3. The evoked release of [3H]ACh had an initial fast and a late slow component. Low concentration (1.26 mM) of Ca2+ reduced the rate but not the releasable pool size of [3H]ACh, while 50 nM oxotremorine depressed the evoked release of [3H]ACh. Both low Ca2+ and oxotremorine caused a contraction of the preparation. 4. In the presence of 0.5 mM MnCl2 which suppressed contractions, evoked release proceeded at a single rate. The rate of release, but not the size of the releasable pool, was reduced in low Ca2+ and by oxotremorine. The effect of oxotremorine but not that of low Ca2+ was antagonized by atropine. 5. Morphine (1 μM) in the presence of MnCl2 reduced the releasable pool size but not the rate of evoked release. Naloxone overcame the effect of morphine. 6. The results suggest that low Ca2+ and oxotremorine reduced the amount of ACh released per action potential because of a decreased influx, of Ca2+. In contrast, morphine reduced the evoked release of [3H]ACh by decreasing the number of neurones excited by field-stimulation but did not interfere with the release of ACh directly.