A study of the concentrations of substance P and neurotensin in the gastrointestinal tract of various mammals.

Immunoreactive substance P and neurotensin in extracts of the digestive tract of man, cat, guinea-pig, pig, rabbit, and rat were measured by radioimmunoassay using antisera directed against the C-terminal portions of the two peptides. In all species except the cat, the concentrations of substance P were highest in the small intestine, intermediate in the large intestine and lowest in the stomach and oesophagus: the digestive tract of the cat displayed a rather even distribution of substance P. As observed in the ileum of guinea-pig, rabbit, and rat, the external muscle layer including the myenteric plexus contained 2-5 times higher concentrations of substance P than the whole ileal wall, whereas the substance P concentrations in the mucosa were only about one sixth of those in the whole wall. High performance liquid chromatography of extracts of human, feline and rabbit ileum showed that all the immunoreactive substance P eluted at the positions of substance P and substance P sulfoxide. The distribution of immunoreactive neurotensin along the digestive system of all six species was very similar. The highest concentrations of neurotensin were measured in the distal part of the small intestine, whereas the large intestine, stomach and oesophagus contained only low concentrations of neurotensin relative to the concentrations in the ileum. As examined in the ileum of guinea-pig, rabbit, and rat, the mucosa exhibited 2.5-4 times higher concentrations of neurotensin than the whole ileal wall, while the concentrations of neutrotensin in the external muscle layer including the myenteric plexus were only 4-20% of those in the whole wall. High performance liquid chromatography of the immunoreactive neurotensin extracted from the cat ileum yielded a single peak corresponding to neurotensin while the immunoreactive neurotensin extracted from the ileum of man and rabbit was eluted in two peaks, 55 and 72% of the recovered immunoreactivity, respectively, corresponding to neurotensin. These findings are in line with the proposed roles of substance P in the neural, and neurotensin in the endocrine, control and maintenance of gastrointestinal motility.