Vagus-dependent and vagus-independent mechanisms of action of the erythromycin derivative EM574 and motilin in dogs

Molecular characterization and distribution of motilin and motilin receptor in the Japanese medaka Oryzias latipes

Motilin (MLN) is a peptide hormone originally isolated from the mucosa of the porcine intestine. Its orthologs have been identified in various vertebrates. Although MLN regulates gastrointestinal motility in tetrapods from amphibians to mammals, recent studies indicate that MLN is not involved in the regulation of isolated intestinal motility in zebrafish, at least in vitro. To determine the unknown function of MLN in teleosts, we examined the expression of MLN and the MLN receptor (MLNR) at the cellular level in Japanese medaka (Oryzias latipes). Quantitative PCR revealed that mln mRNA was limitedly expressed in the gut, whereas mlnr mRNA was not detected in the gut but was expressed in the brain and kidney. By in situ hybridization and immunohistochemistry, mlnr mRNA was detected in the dopaminergic neurons of the area postrema in the brain and the noradrenaline-producing cells in the interrenal gland of the kidney. Furthermore, we observed efferent projections of mlnr-expressing dopaminergic neurons in the lobus vagi (XL) and nucleus motorius nervi vagi (NXm) of the medulla oblongata by establishing a transgenic medaka expressing the enhanced green fluorescence protein driven by the mlnr promoter. The expression of dopamine receptor mRNAs in the XL and cholinergic neurons in NXm was confirmed by in situ hybridization. These results indicate novel sites of MLN activity other than the gastrointestinal tract. MLN may exert central and peripheral actions through the regulation of catecholamine release in medaka.

Motilin Comparative Study: Structure, Distribution, Receptors, and Gastrointestinal Motility

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.

Regulation of Gastrointestinal Motility by Motilin and Ghrelin in Vertebrates

The energy balance of vertebrates is regulated by the difference in energy input and energy expenditure. Generally, most vertebrates obtain their energy from nutrients of foods through the gastrointestinal (GI) tract. Therefore, food intake and following food digestion, including motility of the GI tract, secretion and absorption, are crucial physiological events for energy homeostasis. GI motility changes depending on feeding, and GI motility is divided into fasting (interdigestive) and postprandial (digestive) contraction patterns. GI motility is controlled by contractility of smooth muscles of the GI tract, extrinsic and intrinsic neurons (motor and sensory) and some hormones. In mammals, ghrelin (GHRL) and motilin (MLN) stimulate appetite and GI motility and contribute to the regulation of energy homeostasis. GHRL and MLN are produced in the mucosal layer of the stomach and upper small intestine, respectively. GHRL is a multifunctional peptide and is involved in glucose metabolism, endocrine/exocrine functions and cardiovascular and reproductive functions, in addition to feeding and GI motility in mammals. On the other hand, the action of MLN is restricted and species such as rodentia, including mice and rats, lack MLN peptide and its receptor. From a phylogenetic point of view, GHRL and its receptor GHS-R1a have been identified in various vertebrates, and their structural features and various physiological functions have been revealed. On the other hand, MLN or MLN-like peptide (MLN-LP) and its receptors have been found only in some fish, birds and mammals. Here, we review the actions of GHRL and MLN with a focus on contractility of the GI tract of species from fish to mammals.

Importance of the enteric nervous system in the control of the migrating motility complex

The migrating motility complex (MMC), a cyclical phenomenon, represents rudimentary motility pattern in the gastrointestinal tract. The MMC is observed mostly in the stomach and gut of man and numerous animal species. It contains three or four phases, while its phase III is the most characteristic. The mechanisms controlling the pattern are unclear in part, although the neural control of the MMC seems crucial. The main goal of this article was to discuss the importance of intrinsic innervation of the gastrointestinal tract in MMC initiation, migration, and cessation to emphasize that various MMC-controlling mechanisms act through the enteric nervous system. Two main neural regions, central and peripheral, are able to initiate the MMC. However, central regulation of the MMC may require cooperation with the enteric nervous system. When central mechanisms are not active, the MMC can be initiated peripherally in any region of the small bowel. The enteric nervous system affects the MMC in response to the luminal stimuli which can contribute to the initiation and cessation of the cycle, and it may evoke irregular phasic contractions within the pattern. The hormonal regulators released from the endocrine cells may exert a modulatory effect upon the MMC mostly through the enteric nervous system. Their central action could also be considered. It can be concluded that the enteric nervous system is involved in the great majority of the MMC-controlling mechanisms.

The motilin receptor agonist erythromycin stimulates hunger and food intake through a cholinergic pathway

BACKGROUND

Motilin-induced phase III contractions have been identified as a hunger signal. These phase III contractions occur as part of the migrating motor complex (MMC), a contractility pattern of the gastrointestinal tract during fasting. The mechanism involved in this association between subjective hunger feelings and gastrointestinal motility during the MMC is largely unknown, however, as is its ability to stimulate food intake.

OBJECTIVES

We sought to 1) investigate the occurrence of hunger peaks and their relation to phase III contractions, 2) evaluate whether this relation was cholinergically driven, and 3) assess the ability of the motilin receptor agonist erythromycin to induce food intake.

DESIGN

An algorithm was developed to detect hunger peaks. The association with phase III contractions was studied in 14 healthy volunteers [50% men; mean ± SEM age: 25 ± 2 y; mean ± SEM body mass index (BMI; in kg/m(2)): 23 ± 1]. The impact of pharmacologically induced phase III contractions on the occurrence of hunger peaks and the involvement of a cholinergic pathway were assessed in 14 healthy volunteers (43% men; age: 29 ± 3 y; BMI: 23 ± 1). Last, the effect of erythromycin administration on food intake was examined in 15 healthy volunteers (40% men; age: 28 ± 3 y; BMI: 22 ± 1).

RESULTS

The occurrence of hunger peaks and their significant association with phase III contractions was confirmed (P < 0.0001). Pharmacologically induced phase III contractions were also significantly associated with hunger peaks (P < 0.05), and this association involved a cholinergic pathway. Administering erythromycin significantly stimulated food intake compared with placebo (53% ± 13% compared with 10% ± 5%; P < 0.05).

CONCLUSIONS

Motilin-induced phase III contractions induced hunger feelings through a cholinergic pathway. Moreover, erythromycin stimulated food intake, suggesting a physiologic role of motilin as an orexigenic signal from the gastrointestinal tract. This trial was registered at www.clinicaltrials.gov as NCT02633579.

Redefining the functional roles of the gastrointestinal migrating motor complex and motilin in small bacterial overgrowth and hunger signaling

During the fasting state the upper gastrointestinal tract exhibits a specific periodic migrating contraction pattern that is known as the migrating motor complex (MMC). Three different phases can be distinguished during the MMC. Phase III of the MMC is the most active of the three and can start either in the stomach or small intestine. Historically this pattern was designated to be the housekeeper of the gut since disturbances in the pattern were associated with small intestinal bacterial overgrowth; however, its role in the involvement of hunger sensations was already hinted in the beginning of the 20th century by both Cannon (Cannon W, Washburn A. Am J Physiol 29: 441-454, 1912) and Carlson (Carlson A. The Control of Hunger in Health and Disease. Chicago, IL: Univ. of Chicago Press, 1916). The discovery of motilin in 1973 shed more light on the control mechanisms of the MMC. Motilin plasma levels fluctuate together with the phases of the MMC and induce phase III contractions with a gastric onset. Recent research suggests that these motilin-induced phase III contractions signal hunger in healthy subjects and that this system is disturbed in morbidly obese patients. This minireview describes the functions of the MMC in the gut and its regulatory role in controlling hunger sensations.

Anti-emetic and emetic effects of erythromycin in Suncus murinus: role of vagal nerve activation, gastric motility stimulation and motilin receptors

Paradoxically, erythromycin is associated with nausea when used as an antibiotic but at lower doses erythromycin activates motilin receptors and is used to treat delayed gastric emptying and nausea. The aim of this study was to characterise pro- and anti-emetic activity of erythromycin and investigate mechanisms of action. Japanese House musk shrews (Suncus murinus) were used. Erythromycin was administered alone or prior to induction of emesis with abnormal motion or subcutaneous nicotine (10mg/kg). The effects of erythromycin and motilin on vagal nerve activity and on cholinergically mediated contractions of the stomach (evoked by electrical field stimulation) were studied in vitro. The results showed that erythromycin (1 and 5mg/kg) reduced vomiting caused by abnormal motion (e.g., from 10.3 ± 1.8 to 4.0 ± 1.1 emetic episodes at 5mg/kg) or by nicotine (from 9.5 ± 2.0 to 3.1 ± 2.0 at 5mg/kg), increasing latency of onset to emesis; lower or higher doses had no effects. When administered alone, erythromycin 100mg/kg induced vomiting in two of four animals, whereas lower doses did not. In vitro, motilin (1, 100 nM) increased gastric vagal afferent activity without affecting jejunal afferent mesenteric nerve activity. Cholinergically mediated contractions of the stomach (prevented by tetrodotoxin 1 μM or atropine 1 μM, facilitated by l-NAME 300 μM) were facilitated by motilin (1-100 nM) and erythromycin (10-30 μM). In conclusion, low doses of erythromycin have anti-emetic activity. Potential mechanisms of action include increased gastric motility (overcoming gastric stasis) and/ or modulation of vagal nerve pathways involved in emesis, demonstrated by first-time direct recording of vagal activation by motilin.

Motilin receptor neuropharmacology: revised understanding

Although motilin was identified >40 years ago as a gastrointestinal hormone capable of stimulating gastric emptying, the relatively recent availability of molecular tools and focus on its neuronal activities are now clarifying mechanisms of action. In rodents, only motilin receptor pseudogenes are identified. In human stomach, facilitation of enteric cholinergic activity is identified as the main mechanism by which gastric emptying is increased; some motilin agonists act in a prolonged manner, contrasting with motilin itself and with studies using recombinant receptors. As such, assays using recombinant receptors seem poor predictors of in vivo activity. High-throughput screening enabled selective motilin agonists to be identified, which together with enhanced understanding into neuromuscular actions of motilin, promises to deliver rational treatments of disorders with delayed gastric emptying.

Myenteric neural network activated by motilin in the stomach of Suncus murinus (house musk shrew)

BACKGROUND

It has been shown in human and canine studies that motilin, a gastroprokinetic hormone, induces gastric phase III contractions via the enteric nervous; however, the center of motilin action in the stomach has not been clearly revealed. In the present study, we investigated the neural pathway of motilin-induced gastric contraction by using Suncus murinus, a new animal model for motilin study.

METHODS

An isolated suncus stomach was used in vitro to determine the mechanism of motilin action through the myenteric plexus. Synthetic suncus motilin (10(-11) -10(-7) molL(-1) ) was added to an organ bath, and the spontaneous contraction response was expressed as a percent of ACh (10(-5) molL(-1) ) responses. Motilin-induced contractions were also studied by a pharmacological method using several receptor antagonists and enzyme inhibitor.

KEY RESULTS

Suncus motilin induced a concentration-dependent gastric contraction at concentrations from 10(-9) to 10(-7) molL(-1) . The responses to suncus motilin in the stomach were completely abolished by atropine and tetrodotoxin treatment and significantly suppressed by administration of hexamethonium, verapamil, phentolamine, yohimbine, ondansetron, and naloxone, whereas ritanserin, prazosin, timolol, and FK888 did not affect the action of motilin. Additionally, N-nitro l-arginine methylester slightly potentiated the contractions induced by motilin.

CONCLUSIONS & INFERENCES

The results indicate that motilin directly stimulates and modulates suncus gastric contraction through cholinergic, adrenergic, serotonergic, opioidergic, and NO neurons in the myenteric plexus.

The hungry stomach: physiology, disease, and drug development opportunities

During hunger, a series of high-amplitude contractions of the stomach and small intestine (phase III), which form part of a cycle of quiescence and contractions (known as the migrating motor complex, MMC), play a "housekeeping" role prior to the next meal, and may contribute toward the development of hunger. Several gastrointestinal (GI) hormones are associated with phase III MMC activity, but currently the most prominent is motilin, thought to at least partly mediate phase III contractions of the gastric MMC. Additional GI endocrine and neuronal systems play even more powerful roles in the development of hunger. In particular, the ghrelin-precursor gene is proving to have a complex physiology, giving rise to three different products: ghrelin itself, which is formed from a post-translational modification of des-acyl-ghrelin, and obestatin. The receptors acted on by des-acyl-ghrelin and by obestatin are currently unknown but both these peptides seem able to exert actions which oppose that of ghrelin, either indirectly or directly. An increased understanding of the actions of these peptides is helping to unravel a number of different eating disorders and providing opportunities for the discovery of new drugs to regulate dysfunctional gastric behaviors and appetite. To date, ghrelin and motilin receptor agonists and antagonists have been described. The most advanced are compounds which activate the ghrelin and motilin receptors which are being progressed for disorders associated with gastric hypomotility.

Physiological characteristics of gastric contractions and circadian gastric motility in the free-moving conscious house musk shrew (Suncus murinus)

Although many studies have demonstrated the physiological action of motilin on the migrating motor complex, the precise mechanisms remain obscure. To obtain new insights into the mechanisms, we focused on the house musk shrew ( Suncus murinus, suncus used as a laboratory name) as a small model animal for in vivo motilin study, and we studied the physiological characteristics of suncus gastrointestinal motility. Strain gauge transducers were implanted on the serosa of the gastric body and duodenum, and we recorded gastrointestinal contractions in the free-moving conscious suncus and also examined the effects of intravenous infusion of various agents on gastrointestinal motility. During the fasted state, the suncus stomach and duodenum showed clear migrating phase III contractions (intervals of 80–150 min) as found in humans and dogs. Motilin (bolus injection, 100–300 ng/kg; continuous infusion, 10–100 ng·kg−1·min−1) and erythromycin (80 μg·kg−1·min−1) induced gastric phase III contractions, and motilin injection also increased the gastric motility index in a dose-dependent manner ( P < 0.05, vs. saline). Pretreatment with atropine completely abolished the motilin-induced gastric phase III contractions. On the other hand, in the free-feeding condition, the suncus showed a relatively long fasting period in the light phase followed by spontaneous gastric phase III contractions. The results suggest that the suncus has almost the same gastrointestinal motility and motilin response as those found in humans and dogs, and we propose the suncus as a new small model animal for studying gastrointestinal motility and motilin in vivo.

Effects of oral mitemcinal (GM-611), erythromycin, EM-574 and cisapride on gastric emptying in conscious rhesus monkeys

We assessed and compared the effects of oral mitemcinal (an orally active, erythromycin-derived motilin-receptor agonist; Code name: GM-611), erythromycin, EM-574 and cisapride on gastric emptying in conscious Rhesus monkeys using the acetaminophen method. Mitemcinal and erythromycin induced significant, dose-dependent increases in indices of gastric emptying, but mitemcinal required a much lower dose for the same effect. Cisapride induced a bell-shaped dose response, and EM-574, a potent erythromycin derivative and originally developed as an enteric coated formulation, had little effect when it was given orally uncoated. Since our previous study showed that response to motilin is similar in Rhesus monkeys and humans, these results suggest that oral mitemcinal may be effective for the treatment of symptoms in human disorders related to delayed gastric emptying (e.g., functional dyspepsia and gastroparesis). Combined with the results of other studies, these results suggest that mitemcinal may be able to replace the withdrawn drug, cisapride, as the drug of choice for treating delayed gastric emptying.

Molecular identification and characterization of the dog motilin receptor

Motilin, a 22-amino acid peptide hormone secreted by endocrine cells of the intestinal mucosa, plays an important role in the regulation of gastrointestinal motility. The actions of motilin agonists have been extensively investigated in dogs due to physiological similarities between the dog and human alimentary tracts. The amino acid sequence of the dog motilin receptor, however, was previously unknown. We have cloned a cDNA from dog stomach corresponding to the motilin receptor. The deduced protein shared 71% and 72% sequence identity with the human and rabbit motilin receptors, respectively. Expression of the dog motilin receptor in CHO cells promoted the typical cellular responses to the agonists, motilin and erythromycin. The rank order of potency determined for these agonists was similar to that found for the human motilin receptor, with motilin being more potent than erythromycin. Immunohistochemistry of the dog stomach revealed that the motilin receptor was localized in neuronal cell bodies and fibers. This is the first study detailing the cloning, expression, and functional characterization of the dog motilin receptor. Determination of the full sequence and functional properties of the dog motilin receptor will provide useful information enabling us to interpret previous and future studies of motilin agonists in dogs.

Effects of mitemcinal (GM-611), an acid-resistant nonpeptide motilin receptor agonist, on the gastrointestinal contractile activity in conscious dogs

The effects of mitemcinal (GM-611) on the gastrointestinal contractile activity were investigated using chronically implanted force transducers in conscious dogs and were compared with the effects of porcine motilin (pMTL), EM-523 and EM-574. In the interdigestive state, intravenous and oral administration of mitemcinal, EM-523 and EM-574 induced the gastrointestinal contractile activity in a manner similar to pMTL. The contractile activity caused by mitemcinal was suppressed by continuous intravenous infusion of a motilin receptor antagonist. In the digestive state, intravenous and oral administration of mitemcinal, EM-523 and EM-574 also stimulated the gastrointestinal contractile activity. Mitemcinal, EM-523 and EM-574 given intravenously increased the gastric contractile activity in a similar dose range; however, mitemcinal was approximately 10 times more potent than EM-523 and EM-574 when administered orally in the digestive state. These results indicate that the mitemcinal-induced gastrointestinal contractile activity operates via motilin receptors and possesses a higher activity than EM-523 and EM-574 when administered orally in conscious dogs in the digestive state. Mitemcinal may therefore be useful in the treatment of several gastrointestinal disorders involving dysmotility, such as gastroparesis and functional dyspepsia, even when administered orally.

Evidence for the presence of motilin, ghrelin, and the motilin and ghrelin receptor in neurons of the myenteric plexus

Motilin, a 22-amino acid gastrointestinal peptide, and ghrelin, the natural ligand of the growth hormone secretagogue receptor, form a new group of structurally related peptides. Several lines of evidence suggest that motilin and ghrelin are involved in the control of gastrointestinal motility by the activation of receptors on enteric neurons. The aim of this study was to look for the existence of motilin, ghrelin, and their respective receptors in the myenteric plexus of the guinea pig. We used longitudinal muscle/myenteric plexus (LMMP) preparations and cultures of myenteric neurons of the guinea pig ileum, immunohistochemistry, and reverse transcriptase-polymerase chain reaction (RT-PCR). Most of the motilin-immunoreactive (IR; 72.8%) and motilin receptor-IR (68.9%) neurons were also positive for neuronal nitric oxide synthase (nNOS), 72.8% and 68.9%, few for choline acetyl transferase (ChAT), 11.4% and 11.9%, respectively. In contrast, ghrelin was mainly colocalized with ChAT (72.2%), and only 3.6% of ghrelin-positive cells showed nNOS-IR in the LMMP. Neither motilin nor the motilin receptor or ghrelin colocalized with calbindin. RT-PCR studies revealed motilin, ghrelin, and ghrelin receptor mRNA transcripts in LMMP preparations and in cultured myenteric neurons. In conclusion, this study, for the first time, provides direct evidence for the existence of motilin and ghrelin in myenteric neurons and suggests that both peptides may play a role in the activation of the enteric nervous system and hence in the regulation of gastrointestinal motility.

Octreotide enhances the accelerating effect of erythromycin on gastric emptying in healthy subjects

BACKGROUND

Erythromycin exhibits gastrokinetic properties through cholinergic pathways. Reports regarding the action of octreotide on gastric emptying are conflicting.

AIM

: To assess: (i) the hypothesis that serotonin receptors are involved in the accelerating effect of erythromycin on gastric emptying; and (ii) any modification of the gastrokinetic action of erythromycin induced by octreotide.

SUBJECTS AND METHODS

Gastric emptying of a standard meal was estimated in 20 healthy subjects by scintigraphy on three different occasions in a double-blind, placebo-controlled manner and in random order: (i) after placebo; (ii) after 200 mg of intravenous erythromycin; and (iii) after 200 mg of intravenous erythromycin following pre-treatment with either 4 mg of intravenous ondansetron (10 subjects) or 50 micro g octreotide.

RESULTS

Erythromycin significantly accelerated gastric emptying in all subjects by abolishing the lag phase. Pre-treatment with ondansetron abolished the accelerating effect of erythromycin by restoring the emptying times to placebo levels. Octreotide significantly enhanced the accelerating effect of erythromycin by reducing both the lag and post-lag phases of gastric emptying.

CONCLUSIONS

Serotonin receptors are involved in the accelerating effect of erythromycin on gastric emptying. This effect seems to be enhanced by pre-treatment with octreotide, possibly as a result of the modification of the gastrointestinal hormonal environment.

The effect of erythromycin on human esophageal motility is mediated by serotonin receptors

OBJECTIVE

Erythromycin exhibits prokinetic properties. The drug enhances esophageal and gastric motility by acting as a motilin agonist and promoting acetylocholine release. 5-HT3 receptors are involved in the spontaneously occurring migrating motor complex and the effect of erythromycin on antral motility in dogs. The aim of the study was to investigate the hypothesis that 5-HT3 receptors are also involved in the action of erythromycin on the human esophagus.

METHODS

A total of 18 healthy volunteers underwent standard esophageal manometry on three different occasions in a double-blind, placebo-controlled, randomized manner, as follows: 1) after placebo, 2) after 200 mg of erythromycin i.v., and 3) after 200 mg of i.v. erythromycin subsequent to pretreatment with either 4 mg of i. v. ondansetron (serotonin receptor antagonist) (10 subjects) or 12 microg/kg of i.v. atropine (8 subjects).

RESULTS

Erythromycin significantly increased a) the amplitude of peristalsis at 5 cm (from 87 +/- 19 mm Hg to 108 +/- 26 mm Hg; p = 0.0007), 10 cm (from 72 +/- 24 mm Hg to 81 +/- 26 mm Hg; p = 0.016), and 15 cm (from 47 +/- 15 mm Hg to 55 +/- 17 mm Hg; p = 0.014) proximal to LES, b) the duration of peristalsis at 5 cm (from 4.5 +/- 0.9 s to 5.7 +/- 1.2 s; p < 0.0001) and 10 cm (from 4.1 +/- 1 s to 4.9 +/- 1 s; p < 0.0001) proximal to the LES and c) the strength of peristalsis at 5 cm proximal to the LES (from 180 +/- 49 mm Hg x s to 276 +/- 100 mm Hg x s; p < 0.0001), and decreased the velocity of peristalsis at distal esophagus (from 4.1 +/- 1 cm/s to 3.8 +/- 0.9 cm/s; p = 0.03). In addition, erythromycin significantly increased the resting pressure of the LES (from 36 +/- 10 mm Hg to 44 +/- 12 mm Hg; p = 0.002). Pretreatment with ondansetron totally reversed all of the effects of erythromycin to the placebo state. Pretreatment with atropine not only prevented the effects of erythromycin, but it reduced the amplitude and strength of peristalsis at the distal esophagus at significantly lower levels than after placebo.

CONCLUSIONS

Erythromycin exerts its prokinetic action on the lower esophagus by stimulating cholinergic pathways. This action includes not only an increase in LES pressure, but significant increases in the amplitude and duration of esophageal peristalsis, as well. 5-HT3 receptors are also involved in this process.

The stimulation of antral motility by erythromycin is attenuated by hyperglycemia

OBJECTIVE

Diabetic gastroparesis is usually treated with prokinetic drugs, of which the most potent, when given intravenously during euglycemia, is erythromycin. Recent studies have demonstrated that the gastrokinetic effects of erythromycin are attenuated by hyperglycemia. The aim of this study was to determine whether the effects of erythromycin on antropyloroduodenal motility, including the organization of antral pressure waves, are modified by hyperglycemia.

METHODS

A total of eight healthy male volunteers (median age 24 yr) were studied on 2 days each in randomized order. A manometric assembly, incorporating six antral, two pyloric, and seven duodenal sideholes and a pyloric sleeve sensor, was positioned with the sleeve spanning the pylorus. The blood glucose concentration was stabilized at about 5 mmol/L (euglycemia) or 15 mmol/L (hyperglycemia). After 30 min (T = 0), an intraduodenal lipid infusion (1.5 kcal/min) was commenced and continued until the end of the study. At T = 20 minutes, erythromycin (200 mg) as the lactobionate was infused intravenously over 20 min, followed by 100 mg over the next 40 min.

RESULTS

Intravenous erythromycin increased the amplitude of antral waves during intraduodenal lipid infusion at both blood glucose concentrations (p < 0.01 for euglycemia and p < 0.05 for hyperglycemia). After erythromycin (T = 20 to T = 80), the frequency (p < 0.05) and amplitude (p < 0.01) of antral waves were less during hyperglycemia than euglycemia. Both propagated (p < 0.0005) and nonpropagated (p < 0.01) antral waves were decreased by hyperglycemia, but the suppression of propagated waves was greater (p < 0.05). Erythromycin reduced the frequency (p = 0.09) but increased the amplitude (p < 0.05) of phasic pyloric pressures, and decreased basal pyloric pressure (p < 0.0005). The frequency (p = 0.06) and amplitude (p < 0.05) of phasic pyloric waves during erythromycin infusion were slightly less during hyperglycemia than euglycemia, whereas there was no effect of the blood glucose concentration on basal pyloric pressure. Erythromycin increased the amplitude (p < 0.001) but not the frequency of duodenal waves; the frequency and amplitude of duodenal waves did not differ between the two blood glucose concentrations.

CONCLUSIONS

Hyperglycemia attenuates the stimulation of antral pressures and propagated antral sequences by erythromycin, but not the effects of erythromycin on pyloric or duodenal motility.

EM574, an erythromycin derivative, improves delayed gastric emptying of semi-solid meals in conscious dogs

The gastroprokinetic effects of de(N-methyl)-N-isopropyl-8, 9-anhydroerythromycin A 6,9-hemiacetal (EM574), a non-peptide motilin receptor agonist, were investigated in conscious dogs in a normal state and with experimentally-induced gastroparesis. Gastric emptying of semi-solid meals was assessed indirectly from acetaminophen absorption with simultaneous recording of gastric antral motility. In the normal state, post-prandial intraduodenal administration of EM574 (0.03 mg/kg) [corrected] stimulated antral motility and significantly enhanced gastric emptying as potently as did intravenous porcine motilin (0.003 mg/kg/h). Intraduodenal cisapride at 1 mg/kg denal cisapride at 1 mg/kg elicited antral contractions and tended to accelerate gastric emptying but at 3 mg/kg, gastric emptying was not enhanced despite a further increase in the motor index. In dogs with gastroparesis induced by intraduodenal oleic acid or intravenous dopamine, EM574 (0.03 mg/kg) increased antral motility and reversed the delayed gastric emptying completely. Cisapride (1 mg/kg) partially ameliorated the impaired emptying under these conditions. In atropinized dogs, no acceleration of gastric emptying by EM574 was observed. These results indicate that EM574 potently accelerates gastric emptying of caloric meals in dogs in a normal state and with experimentally-induced gastroparesis, and also suggest that the effect is mediated through stimulation of a cholinergic neural pathway.

Motilin receptors in the human antrum

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 (125)I-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 NH(2)-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) (CH(2)NH)(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) (CH(2)NH)(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.

Scintigraphic measurement of regional gastrointestinal transit in the dog

Scintigraphic techniques can measure sequentially gastric emptying, small bowel transit, and colonic transit in humans, and comparable methods for experimental studies in animals would be useful. We developed such a method in dogs and examined the effects of prokinetic drugs on regional transit. Two isotopes were given to fasting dogs. Polystyrene pellets labeled with 99mTc were mixed in a can of dog food and 111In- labeled pellets were given in a gelatin capsule coated with a pH-sensitive polymer, designed to dissolve in the distal bowel. Gamma camera images were obtained for up to 24 h. Prokinetic drugs were given by intravenous injection. Duplicate baseline studies showed good agreement in seven dogs. In a second group (n = 4), intra- and interanimal variabilities were established. Two novel prokinetic drugs (AU-116 and AU-130) accelerated small bowel and colonic transit. A simple noninvasive method for measuring whole gut transit in dogs was developed and validated. Two new prokinetics accelerated small bowel and colonic transit.

Erythromycin gastrokinetic activity is partially vagally mediated

Erythromycin overcomes postvagotomy gastroparesia in patients without a distal stomach and functional pylorus. We investigate the role of the vagus in gastric emptying increased by erythromycin, using a model that preserves the physiology of the distal stomach and pylorus. The effects of erythromycin lactobionate (10 mg/kg) on transpyloric flow pattern and pyloric resistance were evaluated during repetitive bilateral vagal cooling in anesthetized pigs. Vagal cooling during erythromycin infusion produced a marked decreased of pyloric outflow (23 +/- 1.1 vs 50 +/- 2.6 ml/min) related to a reduced stroke volume of the flow pulses (7.8 +/- 3.31 vs. 14.1 +/- 2.44 ml). The amplitude and frequency of gastric and duodenal pressure events were unchanged or slightly reduced during vagal cooling. The smaller stroke volume of flow pulse was the consequence of increased pyloric resistance (6.2 +/- 1.98 vs. 2.3 +/- 0.21 mmHg.ml-1.s), which is associated with changes in the temporal relationship between a pyloric pressure event and flow pulse. In conclusion, erythromycin activity on the pylorus requires the integrity of vagal pathways. Enhancement of gastric outflow by erythromycin is also modulated by the vagus, since pyloric resistance was able to overcome increased gastric motility.

EM574, an erythromycin derivative, is a motilin receptor agonist in the rabbit

This study was performed to examine whether an erythromycin derivative, de(N-methyl)-N-isopropyl-8,9-anhydroerythromycin A 6,9-hemiacetal (EM574) is a motilin receptor agonist in the rabbit gastrointestinal tract. EM574 and porcine motilin induced contractions in segments of isolated rabbit intestine with pEC50 values of 8.26 +/- 0.04 and 8.69 +/- 0.07, respectively, but not in rat or guinea pig preparations. The sensitivity and efficacy of the response to both compounds in rabbits decreased aborally and was insensitive to pretreatment with atropine or tetrodotoxin, but was markedly suppressed under Ca(2+)-free conditions. EM574 and porcine motilin specifically displaced [125I-Tyr23]canine motilin bound to gastric antral smooth muscle homogenates with plC50 values of 8.21 +/- 0.13 and 9.20 +/- 0.11, respectively. The pEC50 value for the contractile response and plC50 value for the receptor binding for motilin, EM574, erythromycin A and three other derivatives correlated well (r = 0.94, P < 0.01). Tissue section autoradiography in the antrum revealed that specific labeled motilin binding sites were localized in the circular muscle layer and myenteric plexus, and could be reduced in the presence of an excess of EM574. These results indicate that EM574 is a potent motilin receptor agonist in the rabbit gastrointestinal tract.

Motilin and clinical application

Motilin is a regulatory polypeptide of 22 amino acid residues and orginates in motilin cells scattered in the duodenal epithelium of most mammals and chickens. Motilin is released into the general circulation at about 100-min intervals during the interdigestive state and is the most important factor in controlling the interdigestive migrating contractions. Recent studies have revealed that motilin stimulates endogenous release of the endocrine pancreas. Clinical application of motilin as a prokinetic has become possible since erythromycin and its derivatives were proved to be nonpeptide motilin agonists.