Abstract
Motilin, produced in endocrine cells in the mucosa of the upper intestine, is an important regulator of gastrointestinal (GI) motility and mediates the phase III of interdigestive migrating motor complex (MMC) in the stomach of humans, dogs and house musk shrews through the specific motilin receptor (MLN-R). Motilin-induced MMC contributes to the maintenance of normal GI functions and transmits a hunger signal from the stomach to the brain. Motilin has been identified in various mammals, but the physiological roles of motilin in regulating GI motility in these mammals are well not understood due to inconsistencies between studies conducted on different species using a range of experimental conditions. Motilin orthologs have been identified in non-mammalian vertebrates, and the sequence of avian motilin is relatively close to that of mammals, but reptile, amphibian and fish motilins show distinctive different sequences. The MLN-R has also been identified in mammals and non-mammalian vertebrates, and can be divided into two main groups: mammal/bird/reptile/amphibian clade and fish clade. Almost 50 years have passed since discovery of motilin, here we reviewed the structure, distribution, receptor and the GI motility regulatory function of motilin in vertebrates from fish to mammals.
Highlights
Motilin was identified in the 1970s from the mucosa of the porcine upper intestine as a stimulant of gastric motility [1,2,3]
Motilin initiated phase III-like activity in the stomach in a vagus nerve-independent manner, and a ghrelin receptor antagonist or an MLN-R antagonist decreased the phase III activity of the gastric MMC. These results indicate that motilin is involved in the induction of phase III of gastric MMC as in humans, dogs and that ghrelin is involved in initiation of phase II and subsequently enhances motilinmediated phase III contractions [173]
Contraction in the proventriculus was decreased by tetrodotoxin or atropine, being different from the response in the small intestine, suggesting that motilin acts on MLN-Rs located on enteric cholinergic nerves, which is consistent with the results in humans and rabbits [58, 145]
Summary
Motilin was identified in the 1970s from the mucosa of the porcine upper intestine as a stimulant of gastric motility [1,2,3]. We found that turtle and alligator motilins cause contraction of the rabbit duodenum, but the affinity and amplitude of turtle motilin are considerably low compared with those of alligator, chicken, and human motilins (Figure 2A) This indicates the significance of the N-terminal sequence for GI-stimulating activity of motilin in mammals. We tried to examine the contractile activity of newt motilin in isolated rabbit duodenum and chicken ileum and found that newt motilin induced a small contraction in the rabbit duodenum but no response in the chicken ileum (Figure 2), while newt motilin showed a high responsiveness in the newt stomach (Figure 2) These results suggest that binding affinity of amphibian motilin to mammalian and avian MLN-Rs is very low due to the critical sequence differences, and amphibian motilin has an ability to bind MLN-R and to cause GI contraction of amphibians itself. It may be that the molecules retain vestiges of the process of evolving into land animals
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