Gastrin- and cholecystokinin-like polypeptides have been found to release acetylcholine from the Auerbach plexus of longitudinal muscle strips of guinea pig ileum. The evidence is consistent with the view that contractions of intestine in response to these peptides are due to the acetylcholine released. Tetrodotoxin inhibited actions of peptides both on acetylcholine release and on mechanical responses of intestine; however, hexamethonium failed to affect their actions, indicating that their site of action is situated on non-nicotinic receptors of ganglionic cells. C-terminal octapeptide amide of cholecystokinin-pancreozymin and caerulein produced significant acetylcholine release in concentrations as low as 9 × 10−10 and 7.5 × 10−10 M, respectively. Human gastrin I was much less effective, its threshold concentration being 7 × 10−8 M. Both exogenous and endogenous noradrenaline reduced both the acetylcholine release and the contractions of longitudinal muscle produced by peptides. Noradrenaline acted at a presynaptic site and its action was mediated through α receptors. It is concluded that gastrointestinal hormones may play a role as hormonal factors in regulating gastrointestinal motility, and their effect is continuously controlled by the tonic activity of the sympathetic nervous system which reduces acetycholine release.
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