AJP - Gastrointestinal and Liver Physiology, Vol 246, Issue 1 16-G25, Copyright © 1984 by American Physiological Society
Evidence for a direct action of acetylcholine on the gastric oxyntic cell of the amphibian
M. C. Ruiz and F. Michelangeli
The possibility of a direct action of acetylcholine (ACh) on the oxyntic
cell not mediated by histamine release was studied in resting isolated
gastric mucosae. H+ secretion and histamine release, and their
relationship, were studied under ACh stimulation and other conditions. ACh
released histamine from mucosal stores and stimulated H+ secretion.
H2-receptor blocker cimetidine largely inhibited ACh-induced stimulation of
H+ transport but not histamine release. Atropine inhibited the cholinergic
response in both histamine release and H+ secretion. The dose-response
curves to ACh showed nonparallel increases in H+ secretion and histamine
release. When dose-response curves to exogenous and endogenous histamine
(released by ACh) were compared, it was found that ACh increased Vmax for
endogenous histamine. Comparison of the effects of ACh with other
experimental conditions such as tetragastrin treatment, K+ depolarization,
or Na+-free nutrient solution showed that the amount of histamine released
by ACh was insufficient to explain the observed rates of secretion.
Stimulation by ACh was faster, greater, and more transitory compared with
that by histamine. In mucosae maximally stimulated by histamine, ACh
induced a further increase in H+ secretion. In the presence of cimetidine,
potentiation between ACh and dibutyryl cAMP or isobutyl-methylxanthine was
found. The results are consistent with ACh having a dual action on oxyntic
and histamine-releasing cells. The action of ACh on the oxyntic cell would
potentiate the effect of released histamine. It is suggested that ACh and
released histamine act on the oxyntic cell through different second
messengers, resulting in the potentiated response.
