Heterogeneity of motilin-immunoreactive cells in the duodenum and pyloric region of several avian species

Potentiation of motilin-induced contraction by nitric oxide synthase inhibition in the isolated chicken gastrointestinal tract

The present experiments were designed to determine whether or not endogenous nitric oxide (NO) modifies the contractile response to chicken motilin (ch-MT) in the gastrointestinal (GI) tract (proventriculus and small intestine) of the chicken. ch-MT (1 nmol L(-1)-1 micromol L(-1)) caused contractions of longitudinal muscle strips of the proventriculus through both myogenic and neurogenic (mostly cholinergic) mechanisms. On the other hand, ch-MT (0.1 nmol L(-1)-100 nmol L(-1)) contracted the small intestine (duodenum, jejunum and ileum) only through a myogenic mechanism. L-Nitroarginine methylester (L-NAME) potentiated, and L-arginine inhibited, the ch-MT- induced contraction without affecting the responsiveness of acetylcholine (ACh) or 5-hydroxytryptamine in the proventriculus. Electrical field stimulation (EFS)- and 1,1-dimethyl-4-phenylpiperazinium (DMPP)- induced contractions were also potentiated by L-NAME. The potentiation by L-NAME was prevented by L-arginine but not by D-arginine. However, in the presence of atropine or tetrodotoxin, neither L-NAME nor L-arginine modified the responses to ch-MT and DMPP. In contrast to the proventriculus, L-NAME and L-arginine were both ineffective in modifying the ch-MT-induced contraction in the small intestine. These results indicate that NO synthase inhibition potentiates the contractile response of ch-MT, EFS and DMPP in the chicken proventriculus through reduction of endogenous NO-mediated presynaptic inhibition on neural ACh release. However, NOS inhibition did not modify the myogenic (direct) action of ch-MT in gastric and intestinal smooth muscles of the chicken.

Immunohistochemical survey of the gut endocrine cells in the common tree shrew (Tupaia belangeri)

Regional distribution and relative frequency of endocrine cells in the gastrointestinal tract of the common tree shrew (Tupaia belangeri) were studied immunohistochemically. Ten types of immunoreactive endocrine cells were localized in the gastric mucosa, i.e., chromogranin-, serotonin-, gastrin-, somatostatin-, bovine pancreatic polypeptide (BPP)-, enteroglucagon-, pancreatic glucagon-, peptide tyrosine tyrosine (PYY)-, motilin-, and substance P (SP)-immunoreactive (IR) cells. In the intestine, 13 types of immunoreactive cells were observed, i.e., chromogranin-, serotonin-, somatostatin-, gastrin-, BPP-, enteroglucagon-, PYY-, secretin-, cholecystokinin (CCK)-, gastric inhibitory peptide (GIP)-, motilin-, neurotensin-, and SP-IR cells. The regional distribution and relative frequency of the cell types varied along the gastrointestinal tract. Basically, the types, distribution, and relative frequency of the gut endocrine cells were similar to those reported in other mammalian species. However, some characteristic findings were noted in the present study: (1) the considerably large number of gastrin-IR cells in the pyloric region; (2) numerous serotonin-IR cells in the stomach; (3) appreciable number of BPP-IR cells in the transitional region of the stomach; and (4) wide distribution of PYY- and motilin-IR cells in the gut.

Functional characterization of neural and smooth muscle motilin receptors in the chicken proventriculus and ileum

To characterize the motilin receptors present in the chicken, the effects of chicken motilin (Phe-Val-Pro-Phe-Phe-Thr-Gln-Ser-Asp-Ile-Gln-Lys-Met-Gln-Glu-Lys-Glu-Arg -Asn-Lys-Gly-Gln), Leu13 porcine motilin, canine motilin and three erythromycin derivatives (EMA, EM523, GM611) on the contractility of the chicken gastrointestinal (GI) smooth muscles were investigated in vitro and compared with those in the rabbit duodenum. In the proventriculus longitudinal and circular muscle layers, chicken motilin (3 nM-1 microM) caused an atropine- and a tetrodotoxin-sensitive contraction (EC50 = 39-49 nM), and potentiated the EFS-induced contraction without affecting the responsiveness of acetylcholine. EM523 and GM611 (3-100 microM) contracted the proventriculus longitudinal muscle, and the maximum amplitudes of contraction were about 60% of that induced by chicken motilin. Chicken motilin (0.1 nM-100 nM) also caused contraction of the ileum (EC50 = 7 nM) through direct action on the smooth muscle cells. On the other hand, erythromycin derivatives showed only a weak contractile efficacy (about 20% of the maximum response of chicken motilin) even at high concentrations (10-100 microM). The rank order of potency in the ileum was chicken motilin > canine motilin > or = Leu13 porcine motilin > > GM611 > or = EM523 > or = EMA. GM109 slightly inhibited the ideal contractions induced by Leu13 porcine motilin at 100 microM (pA2 = 3.86). In the rabbit duodenum, chicken motilin was a full agonist with the same intrinsic activity as Leu13 porcine motilin, canine motilin and the erythromycin derivatives. However, the rank order of potency (Leu13 porcine motilin > or = canine motilin > chicken motilin > GM611 > or = EM523 > EMA) was different from that in the chicken ileum. In conclusion, chicken motilin causes an excitatory response in the chicken GI tract through activation of neural (proventriculus) and smooth muscle motilin receptors (ileum). The motilin receptor present in the ileum is different from that demonstrated in the rabbit intestine, because of a different rank order of motilin peptides in producing the contraction, low contracting activity of erythromycin derivatives and low antagonistic efficacy of GM109. Different pharmacological characteristics of the mechanical response induced by motilin peptides and erythromycin derivatives between the proventriculus and the ileum are discussed.

Isolation, sequence, and bioactivity of chicken motilin

Motilin was isolated from acid extracts of the small intestine of chickens by a combination of gel filtration chromatography, ion-exchange, and reverse-phase HPLC. The purification was monitored using a radioreceptor assay. The sequence of chicken motilin is FVPFFTQSDIQKMQEK-ERNKGQ. Although the six residues differing from porcine motilin (4, 7-10, and 12) are mostly in the pharmacophore of porcine motilin, the affinity of chicken motilin and of the (1-14) fragment of chicken motilin for the motilin receptor of rabbit antral smooth muscle is not much reduced (pKds of 8.90 and 8.45), compared with the affinity of [Nle13]porcine motilin (pKd 9.12). With smooth muscle tissue of the chicken, however, receptors could not be demonstrated with binding studies. In the tissue bath chicken motilin induced a dose-dependent tonic contraction, which was most pronounced with muscle strips prepared from chicken jejunum. This response was blocked by the Ca2+ antagonist verapamil, but atropine, TTX, L-NNA, guanethidine, prazosin, and yohimbine had no effect. The pEC50 for chicken motilin in the chicken jejunum was 7.41. Motilins from other species had lower potencies, and [Phe3, Leu13]porcine motilin, an antagonist in the rabbit, was an agonist in the chicken. The motilin agonists erythromycin A and EM-523 were almost without effect. Tested against rabbit duodenum, chicken motilin had a smaller potency than mammalian motilins. Thus, chicken motilin and the chicken motilin receptor differ from their mammalian counterparts.

Excitatory action of [Leu13]motilin on the gastrointestinal smooth muscle isolated from the chicken

The effects of a porcine motilin analogue, [Leu13]motilin (LMT) on the smooth muscle preparations isolated from the chicken gastrointestinal (GI) tract were investigated in vitro. In the proventriculus, LMT (100 nM to 30 microM) caused an atropine-sensitive contraction and enhanced the electrical field stimulation (EFS)- or 1,1-dimethyl-4-phenyl-piperazinium (DMPP)-induced contraction without affecting the response to acetylcholine (ACh). LMT also caused a concentration-dependent contraction of the intestinal tract (duodenum, jejunum, ileum, and colon). The responsiveness to LMT was strongest in the jejunum and weakest in the colon. The responses to LMT in the intestinal segments were not affected by tetrodotoxin, atropine, hexamethonium, pyrilamine, spantide, and 5-hydroxyltryptamine-induced desensitzation, but significantly decreased by verapamil or removal of external Ca2+. LMT did not enhance the EFS- or DMPP-induced contraction in the ileum. Canine motilin also contracted the intestinal segments in a similar concentration range to LMT with an equal potency, but erythromycin A (EMA) and N-ethyl-N-demethyl-8,9-anhydroerythromycin A, 6-9-hemiketal (EM523) showed only a weak contractile activity even at high concentration (up to 100 microM), indicating that motilin receptors in the chicken intestine were somewhat different from those of mammals. In conclusion, LMT produces an excitatory response in the chicken GI tract with a different sensitivity from region to region. The mechanisms of the action were different between the proventriculus and small intestine; that is, LMT contracts the small intestine through the direct action on the smooth muscle cells, but this peptide acts on the enteric cholinergic neurones and stimulates ACh release, and thus regulates autonomic neuroeffector transmission in the proventriculus.