Vagal stimulation increases gastrin secretion. This is mediated by
|A||GRP or gastrin-releasing peptide|
The vagal endings on the gastrin-producing G cells secrete GRP and not acetylcholine; the GRP mediates the gastrin release from the G cells. GRP has 27 amino acids; out of this, the 10 amino acids at the carboxyl terminal are almost same as bombesin (a peptide found in amphibians).
Substance P is found in cells and neurons of the GIT; it increases the motility of the small intestine.
(Both GRP and substance P may enter the circulation)
Apart from the pancreas, glucagon is secreted by the GIT; this may be partly responsible for the hyperglycemia that occurs after pancreatectomy.