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Am J Physiol Gastrointest Liver Physiol 278: G18-G23, 2000;
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Vol. 278, Issue 1, G18-G23, January 2000

Motilin receptors in the human antrum

Paul Miller1, André Roy2, Serge St-Pierre3, Michel Dagenais2, Réal Lapointe2, and Pierre Poitras1

1 Gastrointestinal Unit and 2 Department of Surgery, Centre Hospitalier de l'Université de Montréal; and 3 Department of Chemistry, Université du Québec à Montréal, Montreal, Quebec, Canada H2X 3J4


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Motilin is an intestinal peptide that stimulates contraction of gut smooth muscle. The motilin receptor has not been cloned yet, but motilin-receptor agonists appear to be potent prokinetic agents for the treatment of dysmotility disorders. The aim of this study was to determine neural or muscular localization of motilin receptors in human upper gastrointestinal tract and to investigate their pharmacological characteristics. The binding of 125I-labeled motilin to tissue membranes prepared from human stomach and duodenum was studied; rabbit tissues were used for comparison. Solutions enriched in neural synaptosomes or in smooth muscle plasma membranes were obtained. Various motilin analogs were used to displace the motilin radioligand from the various tissue membranes. The highest concentration of human motilin receptors was found in the antrum, predominantly in the neural preparation. Human motilin receptors were sensitive to the NH2-terminal portion of the motilin molecule, but comparison with rabbit showed that both species had specific affinities for various motilin analogs [i.e., Mot-(1---9), Mot-(1---12), Mot-(1---12) (CH2NH)10-11, and erythromycin]. Motilin receptors obtained from synaptosomes or muscular plasma membranes of human antrum expressed different affinity for two motilin-receptor agonists, Mot-(1---12) and Mot-(1---12) (CH2NH)10-11, suggesting that they correspond to specific receptor subtypes. We conclude that human motilin receptors are located predominantly in nerves of the antral wall, are functionally (and probably structurally) different from those found in other species such as the rabbit, and express specific functional (and probably structural) characteristics dependent on their localization on antral nerves or muscles, suggesting the existence of specific receptor subtypes, potentially of significant physiological or pharmacological relevance.

gastrointestinal motility; regulatory peptides; receptor subtypes


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

MOTILIN IS A 22-amino acid peptide synthesized by endocrine cells of the duodenojejunal mucosa that stimulates the contraction of smooth muscles of the gastrointestinal tract (for review, see Ref. 23). Motilin appears as a circulating hormone controlling the interdigestive motility of the gastroduodenal tract through cyclic increases in circulating plasma motilin that regulate the phase III contraction of the migrating motor complex in the upper gut (4, 23, 24).

Motilin receptors were identified mostly in the upper part of the digestive tract. Motilin-induced contraction seems to involve both a direct action on the smooth muscle cell and/or a neural mediation through various neurotransmitters, such as acetylcholine. Motilin receptors can be activated by motilin and by motilin synthetic analogs constructed from the NH2-terminal portion of the native molecule (18, 25). Motilin receptors can also be activated by macrolide compounds derived from erythromycin (16, 22). Erythromycin and some derivatives, called motilides (named for their ability to act on motilin receptors), can indeed mimic the intestinal contraction induced by motilin. In fact, motilin receptor activation currently represents the most potent pharmacological stimulus for gastric contraction in humans. Since the discovery in 1990 by Janssens et al. (15) that erythromycin could rapidly empty the stomach of diabetics suffering from severe gastroparesis resistant to usual prokinetic therapy, motilin-receptor agonists are now considered the most potent gastrokinetic drugs and, although clinicians use erythromycin in various hypokinetic digestive disorders (for review, see Ref. 20), many pharmaceutical companies have worked to develop motilide compounds. These erythromycin derivatives, with a very high affinity for motilin receptors but devoid of antibiotic activity, are therefore capable of stimulating digestive motor activity; some of them have already been tested in clinical trials.

Characterization of the motilin receptor is very important, among other things, for the optimal design of motilides to be used for the treatment of patients with various clinical disorders due to dysmotility of the digestive tract. This study aims to determine the localization of motilin receptors on muscle or on neural cells of the human stomach and to explore the functional characteristics of the human motilin receptor.


    MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Tissue preparation: purification of enriched muscular and neuronal fractions from human and rabbit tissues. Human stomachs and duodena were obtained from organ donors. Rabbit antral smooth muscle tissue was obtained from female albino rabbits (2.5 kg) that were killed by intravenous pentobarbital injections. Smooth muscular tissue was dissected away from mucosa using a scalpel blade (antrum, corpus, and fundus) or with tweezers (duodenum) and washed in physiological saline solution. Human tissues were stored at -70°C.

Human and rabbit neural and muscular membranes were partially purified by a modification of the methods of Ahmad et al. (1, 2) used for purification of canine muscular and neural fractions. All purification steps were carried out at 4°C. Isolated smooth muscle tissues from antrum, corpus, fundus, and duodenum were minced and homogenized separately in 12 volumes of ice-cold homogenization buffer (50 mM Tris, 10 mM MgCl2, 1 mM EDTA, and 0.25 M sucrose, pH 8.0) in a Brinkmann polytron (PTA20 probe, setting 4, 3 × 8 s; Brinkmann, Lucerne, Switzerland). Homogenates were initially centrifuged at 1,500 g for 10 min, the pellet was discarded, and the supernatants (postnuclear supernatant, PNS) were immediately subjected to centrifugation at 170,000 g for 65 min. The supernatant containing the soluble protein fraction was discarded, and the pellet was resuspended manually in ice-cold homogenization buffer using a Potter apparatus. This suspension was centrifuged at 18,000 g for 10 min. The pellet [mitochondrial (Mit) fraction] and the supernatant [microsomal (Mic) fraction] were collected. The Mit fraction was resuspended manually in ice-cold homogenization buffer using a Potter apparatus.

The PNS, Mit, and Mic were assayed for saxitoxin binding to determine neural membrane content, for 5'nucleotidase (5'N) activity to determine smooth muscle membrane content, and for total membrane protein.

Binding assay. Porcine Mot-(1---22) was iodinated by the chloramine-T method and purified by HPLC. The mean radiospecific activity was ~1,000 cpm/fmol as determined by the RIA self-displacement method.

Binding of 125I-labeled motilin was performed on partially purified membrane extracts (Mit and Mic) in a total volume of 500 µl of 50 mM Tris · HCl (pH 8.0), 1 mM EDTA, 10 mM MgCl2, and 2% BSA at 30°C for 60 min. Membranes were harvested by aspiration onto presoaked filters (Whatman glass fiber filters type F, 10 % BSA in H2O, frozen at -20°C for 24 h) and washed with 25 ml of washing buffer (50 mM Tris · HCl, pH 8.0, 1 mM EDTA, and 10 mM MgCl2). The radioactivity was determined in a gamma counter. Specific binding of 125I-motilin was calculated from total and nonspecific binding determined in the absence and presence of 10-6 M porcine motilin. Nonspecific binding represented roughly 15-50% of total binding.

Maximal binding capacity in the human antral Mit fraction was determined in hot saturation experiments using 0.15-14.7 nM free concentrations of 125I-motilin. The maximal binding capacities of the human corpus, fundus, and duodenum were determined by cold saturation analysis of competition studies using INPLOT 4.0.

Competition binding studies were performed with a concentration of 2 nM 125I-motilin (human tissues) and 1 nM 125I-motilin (rabbit tissues). Competition binding experiments were performed in human and rabbit antral fractions to determine the receptor affinity for various NH2-terminal analogs of motilin, including Mot-(1---22), Mot-(1---19), Mot-(1---15), Mot-(1---12), Mot-(1---9), erythromycin, and the motilin antagonist Mot-(1---12) (CH2NH)10-11. The concentration of analog that displaced 50% of the labeled motilin (IC50) was determined. The IC50 is referred to in the text and graphs as the pIC50, indicating the negative logarithm of IC50.

All analogs of motilin, including porcine (or human) synthetic Mot-(1---22), Mot-(1---19), Mot-(1---15), Mot-(1---12), and Mot-(1---9) and the motilin analog Mot-(1---12) (CH2NH)10-11, used in this study were assembled in the laboratory of Dr. S. St-Pierre (Pte. Claire, PQ, Canada). Erythromycin lactobionate was purchased from Abbott Laboratories, (Montreal, PQ, Canada).

Data analysis. All binding data were analyzed for linear or nonlinear regression using INPLOT 4.0. Binding data among different fractions were compared by Student's t-test.


    RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

Distribution of motilin receptors in human intestinal tissues (antrum, corpus, fundus, and duodenum). Values for 5'N activity and [3H]saxitoxin binding in centrifugation-prepared human tissue fractions are shown in Table 1. In the Mic fraction, the concentration of 5'N increased five times over the basal levels found in the PNS, whereas the concentration of saxitoxin was only doubled. In the Mit fraction, 5'N increased five times, but a 13-fold increase in saxitoxin binding was obtained. We therefore considered that Mic and Mit fractions represented solutions enriched in muscle (Mic fraction) and in neural (Mit fraction) elements, respectively. The affinity for 125I-motilin and the binding capacity in these tissues are also shown in Table 1. Motilin binding could be detected in both nerve (Mit)- and muscle (Mic)- enriched solutions of the antrum but only in neural fractions of the corpus, fundus, and duodenum. The concentration of motilin receptor (251.9 ± 27.9 fmol/mg) in the antral synaptosomes exceeded by far the concentration detected in the other tissue preparations. Figure 1 graphically represents the relative distribution of motilin binding in correlation with 5'N activity and saxitoxin binding in the partially purified fractions of the human antrum.

                              
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  		Table 1.   Partial purification of muscular and neural elements



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  		Fig. 1.   Relative distribution of motilin (MOT) binding capacity in correlation with 5'nucleotidase (5'N) activity and saxitoxin (SAX) binding sites in centrifugation-prepared solutions of human antrum. PNS, supernatant; Mit, mitochondrial fraction; Mic, microsomal fraction.

A hot saturation analysis performed on human antral Mit fractions enriched in synaptosomes is shown in Fig. 2. The Scatchard analysis (Fig. 2, inset) reveals a single binding site with a dissociation constant value of 6.45 nM and a maximum binding capacity of 251.9 ± 27.9 fmol/mg. Competition displacement curves for human antral Mit fractions are shown in Fig. 3 for Mot-(1---22), Mot-(1---12), and Mot-(1---12) (CH2NH)10-11.


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  		Fig. 2.   From human antral mitochondrial fractions, hot saturation analysis and Scatchard analysis (inset), revealing a single binding site and a maximum motilin-binding capacity of 251.9 ± 27.9 fmol/mg.



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  		Fig. 3.   Competition displacement curves for motilin [Mot-(1---22)], motilin fragment Mot-(1---12), and motilin antagonist Mot-(1---12) (CH2NH)10-11 in mitochondrial fractions prepared from human antrum.

Analysis of motilin binding in human and rabbit antral nerve fractions. Partially purified Mit fractions from human and rabbit antrum were analyzed for receptor affinity and specificity of binding to various motilin fragments. Affinity (pIC50) was similar in the human and rabbit Mit fractions for Mot-(1---22) (9.00 ± 0.02 vs. 9.00 ± 0.03; not significant), Mot-(1---19) (9.20 ± 0.02 vs. 9.01 ± 0.08; not significant), and Mot-(1---15) (8.74 ± 0.06 vs. 8.86 ± 0.05, not significant), but it was significantly lower in human Mit fractions compared with rabbit Mit fractions for Mot-(1---12) (7.37 ± 0.03 vs. 8.28 ± 0.22; P = 0.0002) and Mot-(1---9) (3.49 ± 0.03 vs. 5.55 ± 0.12; P < 0.0001; Fig. 4). Erythromycin (6.09 ± 0.03 vs. 6.52 ± 0.04; P < 0.0001) and the motilin antagonist Mot-(1---12) (CH2NH)10-11 (6.05 ± 0.05 vs. 7.67 ± 0.12; P < 0.0001) also showed lower affinity in human than in rabbit tissues.


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  		Fig. 4.   Comparison of tissue affinity from rabbit or from human antrum for Mot-(1---22), Mot-(1---19), Mot-(1---12), Mot-(1---9), erythromycin (ERYTH), and Mot-(1---12) (CH2NH)10-11. * P < 0.001.

Analysis of motilin binding in human antral nerve and muscle fractions. To make sure that the data were absolutely comparable, antral Mic and Mit fractions from the same human specimens were tested in the same assay with the same analogs in a paired study. The pIC50 of Mot-(1---22) was similar in both prepared fractions (9.00 ± 0.02 and 9.02 ± 0.04, respectively; not significant, n = 6), but significantly lower in Mic (muscle-enriched solution) compared with Mit (neural synaptosome-enriched solution) fractions for Mot-(1---12) (6.36 ± 0.13 vs. 7.63 ± 0.08; P = 0.0007, n = 3) and for the antagonist Mot-(1---12) (CH2NH)10-11 (5.47 ± 0.06 vs. 6.07 ± 0.04; P < 0.0001, n = 4; Fig. 5).


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  		Fig. 5.   Binding affinity of synaptosome-enriched mitochondrial fractions and of plasma membrane microsomal fractions for Mot-(1---22), Mot-(1---12), and Mot-(1---12) (CH2NH)10-11. * P < 0.001.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

This study shows that 1) motilin receptors are in maximum concentration in the antrum of the stomach, 2) they are present on antral muscles, but they can be found predominantly in the neural structures of the antral wall, 3) the human motilin receptor recognizes the NH2-terminal structure of the motilin molecule, as previously demonstrated in the rabbit, 4) human and rabbit motilin receptors are functionally (and probably structurally) heterogeneous, and 5) motilin receptors in neural or in muscular membranes of the human antrum express different affinities for various motilin-receptor agonists and antagonists, therefore suggesting the existence of a heterogeneity of specific receptor subtypes.

Our finding that motilin receptors are predominantly located in the antrum are in agreement with all data published on this topic in humans and in other species. Peeters et al. (21) previously reported high binding capacity of the radiolabeled ligand by the human antrum and proximal duodenum, with a rapidly decreasing gradient along the more distal small intestine. In our experimental protocol, we did not differentiate the proximal or distal portions of the human duodenum, and this could possibly explain the low concentration of motilin receptors we found in the human duodenum. Motilin receptors have also been detected in the colon (5, 9) and outside of the digestive tract, such as in the central nervous system (8) or in vascular arterial vessels (14). These receptors are still of unknown physiological significance and were not considered in our study.

Although the specific localization of motilin receptors on nerves or muscles has been strongly debated in the past, this study reveals significant information about this topic. First, data obtained on smooth muscle strips of rabbit or human intestines tested in vitro (30) clearly showed that the contractile action of motilin could be obtained through the stimulation of a muscle receptor because the biological effect was always persistent in the presence of neurological inhibitors, including tetrodotoxin, a blocker of axonal conductance. Further studies with isolated muscle cells contracting in response to motilin or with histochemical localization of 125I-motilin binding to muscular coats of the intestinal wall brought additional support to the existence of a motilin receptor located on intestinal muscle cells (11). However, studies in dog, in vivo as well as in vitro, clearly indicated that, in this species, motilin-induced contraction was mediated through neural mechanisms, mostly through muscarinic influence (13, 29) and possibly by vagal mechanisms (10, 12). The concept of a motilin receptor present on intestinal nerves was then clearly recognized in human as in rabbit. In in vitro rabbit studies (27), purification of neural synaptosomes or muscular plasma membranes from antrum and duodenum (as we did in this study with human antral tissue) revealed data that suggested the presence of motilin receptors located predominantly on nerves in the rabbit antrum but on muscles in the rabbit duodenum. In studies on the ex vivo rabbit stomach (17), the motor effect of motilin was inhibited by blocking muscarinic transmission with atropine. In a very elegant study on the contraction of the rabbit stomach in vitro, Parkman et al. (19) showed that the effect of the motilin-receptor agonist erythromycin was obtained through two distinct receptors; one neural receptor was responsible for a chronotropic effect, and the other one, located on the muscle, had an inotropic action. Similar conclusions were obtained in humans in which we have shown that the phase III contractions induced in the antrum by intravenous motilin administration were blocked by the muscarinic antagonist atropine (3). More recently, Coulie et al. (6) published studies in conscious volunteers in which the phase III contractions induced by erythromycin during the fasting state could be blocked by atropine, which left intact the direct antral muscular stimulation obtained by using higher doses of erythromycin after a meal. The current binding study with human tissue is the first analysis of motilin receptors from human antral nerves.

Characterization of the motilin receptor has been obtained up to now in the rabbit, in which binding studies could be coupled with functional experiments. It has been well documented that the bioactive portion of human motilin relied on the NH2-terminal amino acid sequence of the molecule when tested on strips of rabbit duodenum in vitro (25). Binding studies on rabbit antral membranes confirmed the affinity of the rabbit receptor for the NH2-terminal motilin analogs (18). The current results confirm that the human receptor also expresses affinity for the NH2-terminal molecular sequence. However, the data obtained with the NH2-terminal motilin fragments also suggest heterogeneity between human and rabbit receptors. Indeed, as shown in Fig. 4, the receptor affinity of human or rabbit receptors for Mot-(1---22) and for motilin fragments Mot-(1---19) and Mot-(1---15) was similar, but it was different for shorter peptides such as Mot-(1---12) and Mot-(1---9). Moreover, the receptor affinity for the motilin antagonist Mot-(1---12) (CH2NH)10-11 (28), as well as for the motilin agonist erythromycin, was also different in the two species. These specific functional characteristics suggest structural heterogeneity for human and rabbit motilin receptors. This type of species-specific heterogeneity in the structure of motilin receptors has already been suspected because experimental reports have previously demonstrated significant differences in motilin analog activity in different species. For example, [Phe3,Leu13]Mot-(1---22) is a motilin-receptor antagonist in rabbit and in humans but is an important agonist when tested in the chicken (7). Therefore, species heterogeneity of motilin receptors should probably be considered in the pharmaceutical development of motilin-receptor agonists.

The most interesting observation in our study probably concerns the concept of receptor subtypes in the same species and more particularly in the same organ. We first suspected this condition in the dog, where in vivo data (29) revealed the existence of a neural receptor equally and highly sensitive to human and canine motilins (22-amino acid peptides with structural variations in positions 7, 8, 12, 13, and 14) but in which in vitro data (26) suggested the presence of a muscle receptor sensitive exclusively to the structure of canine motilin. More recently, in binding studies on rabbit tissues (27), we observed that the receptor affinity toward various motilin analogs was different for receptors found in a synaptosome-enriched solution from the antrum than for receptors present in a plasma membrane-enriched solution from the duodenum. It was not possible, in that experimental series, to determine if the documented difference was simply due to the different organs selected (stomach vs. duodenum) or to the specific organelles prepared (nerve vs. muscle). In the current study, we were able to prepare synaptosome- and plasma membrane-enriched solutions from the same antrum and to compare both tissue preparations in the same assay. The affinity for Mot-(1---22) was similar in both fractions, but the affinity for Mot-(1---12), as for motilin antagonist Mot-(1---12) (CH2NH)10-11, was clearly different, suggesting therefore that the motilin receptors on neural and muscular tissues were different. The presence of motilin receptor subtypes, muscular and neural, was also well supported in functional studies realized in rabbits as well as in humans. Parkman et al. (19), in the rabbit in vitro, demonstrated clearly that erythromycin could act via two different mechanisms: an inotropic effect was obtained via motilin receptors localized on muscles, and a chronotropic effect was seen at lower doses and was due to motilin receptors localized on cholinergic nerves. In humans, Coulie et al. (6) showed that the antral phase III contractions induced by low doses of erythromycin was inhibited by atropine, whereas the antral postprandial motor stimulation seen with high doses of erythromycin was independent of a muscarinic mediation and therefore could probably be due to a direct muscular effect of erythromycin. Both functional studies in humans (6) or in rabbits (19) suggested that the neural receptor was more sensitive than the muscle receptor to stimulation by erythromycin.

Erythromycin has already been shown to be a strong prokinetic agent and is currently being used by many clinicians for the management of some patients with gastroparesis or other dysmotility disorders (20). Motilin-receptor agonists derived from erythromycin, called motilides, have been developed, and some of them have already been tested in clinical trials. Further characterization of motilin receptors would eventually help to design and develop more specific and potent receptor agonists to be used in clinical practice to stimulate gastrointestinal motor activity.


    FOOTNOTES

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Address for reprint requests and other correspondence: P. Poitras, Centre Hospitalier de l'Université de Montréal, Saint-Luc, 1058, rue St-Denis, Montréal, Québec, Canada H2X 3J4 (E-mail: pierre.poitras{at}sympatico.ca).

Received 20 May 1999; accepted in final form 28 August 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES

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3.   Boivin, M., L. R. Pinelo, S. St-Pierre, and P. Poitras. Neural mediation of the motilin motor effect on the human antrum. Am. J. Physiol. Gastrointest. Liver Physiol. 272: G71-G76, 1997[Abstract/Free Full Text].

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30.   Strunz, U., W. Domschke, P. Mitznegg, S. Domschke, E. Schubert, E. Wunsch, E. Jaeger, and L. Demling. Analysis of the motor effects of 13-norleucine motilin on the rabbit, guinea pig, rat, and human alimentary tract in vitro. Gastroenterology 68: 1485-1491, 1975[ISI][Medline].

Am J Physiol Gastroint Liver Physiol 278(1):G18-G23
0193-1857/00 $5.00 Copyright © 2000 the American Physiological Society


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