Substance P and neurotensin in the control of gastrointestinal function.

Our experimental data show that neurotensin or a metabolite satisfies the rigid physiological criteria that should be met before a substance is classified as a true hormone with endocrine function. The gastrointestinal actions are produced by very small concentrations; the release into the blood is produced by a physiological stimulus (ingestion of food, especially fat); and the plasma concentrations of NTLI following the ingestion of food are comparable to those obtained by giving exogenous neurotensin at doses which produce effects in the gastrointestinal tract.