FIGURE 8-8 Primary amino acid sequence of porcine motilin.
Motilin is believed to play a role in the stimulation of small intestinal motility during fasting. Its release from the endocrine EC cells of the duodenum is cyclical, occurring approximately every 2 hr. Significantly, most of motilin's actions do not coincide with dietary intake, but with fasting. It has been suggested that it enhances the movement of digested food particles in preparation for the next meal. The physiological role of motilin remains to be clarified.
Neurotensin is a 13-amino acid polypeptide (see Figure 8-9) found in the N cells of the ileum; it is also present extensively in nerve cells of the CNS. The secretion of neurotensin into circulation is effected by the ingestion of a meal. Neurotensin was named because of its potency to cause hypotension and vasodilation; in the gut, neurotensin mediates inhibition of gastric motility and stimulation of the secretion of PP and glucagon, but inhibits insulin release.
J. Pancreatic Polypeptide Family (PP)
The pancreatic polypeptide family comprises three 36-amino-acid peptides; they include pancreatic peptide (PP), peptide YY (PYY), and neuropeptide (NPY). Table 8-6 provides a comparison of the biological properties of this peptide family.
There is an emerging area of research linking leptin (see Chapter 7) and neuropeptide Y. Leptin is a hormone produced by adipocytes that may signal to the hypothalamus to inhibit the secretion of appetite-stimulating (orexigenic) NPY. High concentrations of NPY are known to stimulate food intake, while low concentrations have the opposite effect.
Somatostatin is a cyclic peptide with two naturally occurring forms: either a 28 (SS-28)-or a 14 (SS-14)-amino-acid peptide (see Figure 8-10). Somatostatin was
Neurotensin ELYENKPRRPY I L -COOH FIGURE 8-9 Amino acid sequence of neurotensin.
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