Chemical messengers: a view from the gut.

The peptides usually called gastrointestinal hormones belong to a broader group of regulatory substances distributed in many parts of the body and delivered to their targets not only by the blood but also by neural and paracrine paths. The neural, endocrine, and paracrine cells as a group might be called "regulator cells" and the chemical messengers they produce might be called "regulins." Twenty peptides have been isolated from the alimentary tract and pancreas: 12 have been sequenced, 4 have been partially sequenced, and 4 more have been identified only by immunoreactivity. Gastrin, gastric inhibitory peptide, glucagon, insulin, and secretin can be regarded as established hormones that are released into the blood by identified stimuli and produce identified physiological responses. The evidence for the hormonal status of cholecystokinin, pancreatic polypeptide, and motilin is incomplete but suggestive. The possible physiological roles of the other 12 peptides remain to be determined. If specific antagonists of these peptides can be found, they will greatly assist in elucidating the peptides' physiological roles.