In vitro effects of 5-hydroxytryptamine and cisapride on the circular smooth muscle of the jejunum of horses

Localization of the 5-hydroxytryptamine 4 receptor in equine enteric neurons and extrinsic sensory fibers

BACKGROUND

Serotonin plays a pivotal role in regulating gut motility, visceral sensitivity, and fluid secretion via specific receptors. Among these receptors, 5-HT₄ exerts a prominent control on gut motor function. Although the prokinetic effect exerted by 5-HT₄ agonists is well known, the cellular sites of 5-HT₄ expression remain poorly understood in large mammals, e.g., horses. In this study, we evaluated the distribution of 5-HT₄ in the horse intestine and in foals with enteric aganglionosis, reminiscent of human Hirschsprung's disease.

METHODS

The intestine and spinal ganglia were obtained from three healthy horses and two foals with hereditary ileocolonic aganglionosis. Tissues were processed for immunohistochemistry using a specific antibody to 5-HT₄ and a variety of neuronal markers. Myenteric and submucosal plexus 5-HT₄ -immunoreactive (IR) neurons were quantified as relative percentage (mean±SD) to the total number of neurons counted. Furthermore, the density of 5-HT₄ -IR nerve fibers was evaluated in the mucosa and tunica muscularis.

KEY RESULTS

The 5-HT₄ immunoreactivity was localized to large percentages of myenteric neurons ranging from 28±9% (descending colon) to 63±19% (ileum), and submucosal neurons ranging from 54±6% (ileum) to 68±14% (duodenum). The 5-HT₄ -immunoreactivity was co-expressed by some substance P-IR (SP-IR) spinal ganglion neurons and extrinsic sensory fibers of aganglionic foals.

CONCLUSIONS & INFERENCES

The presence of 5-HT₄ in many enteric and extrinsic sensory neurons and nerve fibers provides solid morphological evidence of the cellular sites of 5-HT₄ expression in horses. The evidence of SP-IR sensory neurons positive for 5-HT₄ suggests its role in visceral sensitivity.

The 5-HT4 receptor agonist prucalopride does not facilitate cholinergic neurotransmission in circular and longitudinal smooth muscle preparations of equine mid-jejunum

BACKGROUND

Postoperative ileus (POI) remains an important cause of death in horses. The recently developed selective 5-HT₄ receptor agonists such as prucalopride target 5-HT₄ receptors on myenteric cholinergic neurons to enhance acetylcholine release and GI motility. No clearcut in vitro evaluation whether highly selective 5-HT₄ receptor agonists enhance submaximal cholinergic neurotransmission towards the muscle layer has been performed in horses.

OBJECTIVES

To identify functional 5-HT₄ receptors in equine jejunum.

STUDY DESIGN

In vitro experimental study.

METHODS

Circular and longitudinal smooth muscle strips (mid-jejunum) were mounted in organ baths between 2 platinum electrodes allowing electrical field stimulation (EFS). To delineate the conditions to obtain purely cholinergic responses, voltage-response curves were studied. To investigate the influence of prucalopride and 5-HT, submaximal cholinergic contractions at a single voltage were induced.

RESULTS

In circular and longitudinal strips, EFS induced voltage-dependent neurogenic on-contractions when the bathing medium contained a NO-synthesis inhibitor and apamin to prevent inhibitory responses to NO and ATP. Contractions at a voltage inducing 50% of maximal amplitude were cholinergic, as they were blocked by atropine. These contractions were not influenced by prucalopride (up to 3μM), even in the presence of the phosphodiesterase inhibitor isobutyl-methyl-xanthine to inhibit breakdown of the second messenger of 5-HT₄ receptors, cAMP. Also the full 5-HT₄ receptor agonist 5-HT did not influence the EFS-induced submaximal cholinergic contractions. Moreover, prucalopride did not influence muscle tone continuously enhanced with KCl.

CONCLUSIONS

There are no functional 5-HT₄ receptors on myenteric cholinergic neurons nor muscular 5-HT₄ receptors in equine jejunum.

Effects of Single-Dose Prucalopride on Intestinal Hypomotility in Horses: Preliminary Observations

Abnormalities of gastrointestinal motility are often a challenge in horses; however, the use of prokinetic drugs in such conditions must be firmly established yet. For this reason we carried out a preliminary study on the effects of prucalopride on intestinal motor activity of horses with gut hypomotility. The effect of prucalopride per os by oral dose syringe (2 mg/100 kg body weight) was assessed by abdominal ultrasound (evaluating duodenal, cecal, and colonic motor activity) in six horses with gut hypomotility. After administration of prucalopride, a significant increase of contractile activity was found in the duodenum at 30 minutes (p = 0.0005), 60 minutes (p = 0.01) and 90 minutes (p = 0.01), whereas in the cecum and in the left colon the increase was only present at 60 minutes (p = 0.03, and p = 0.02, respectively). No changes from baseline heart and respiratory rate or behavior side effects were observed after administration of the drug and throughout the observation period. Prucalopride may be a useful adjunct to the therapeutic armamentary for treating hypomotile upper gut conditions of horses. Dosing information is however needed to establish its actual clinical efficacy and its proper effects on the large bowel in these animals.

Clinical Research Abstracts of the British Equine Veterinary Association Congress 2015

REASONS FOR PERFORMING STUDY

Selective 5-HT4 receptor agonists such as prucalopride are used as human prokinetics, since activation of 5-HT4 receptors on intestinal cholinergic neurons facilitates acetylcholine release. 5-HT4 receptors, linked to adenylyl cyclase, act via generation of cAMP. None of the 4 in vitro studies on 5-HT in horses provided evidence for neuronal 5-HT4 receptors, but none used the protocol as described in human studies [1-4].

OBJECTIVES

To investigate whether functional 5-HT4 receptors are present in the equine small intestine.

STUDY DESIGN AND METHODS

In vitro organ bath set up, applying electrical field stimulation (EFS) in longitudinal and circular smooth muscle strips.

RESULTS

Results were similar in both muscle layers. In the presence of 0.3 mmol/l NG-Nitro-L-arginine methyl ester and 0.3 μmol/l apamine, excluding effects of the inhibitory transmitters NO and ATP, EFS induced voltage-dependent on-contractions; these were neurogenic as they were abolished by 3 μmol/l tetrodotoxin. At a voltage inducing 50% of the maximal amplitude, the submaximal EFS-induced contractions were cholinergic as atropine (1 μmol/l) abolished them. Prucalopride (0.3 μmol/l) did not increase the amplitude of these submaximal EFS-induced contractions. Even in the presence of the nonselective phosphodiesterase inhibitor IBMX, previously shown to enhance the effect of neuronal 5-HT4 receptors by inhibiting breakdown of their 2nd messenger cAMP [5], prucalopride (3 μmol/l) had no influence. Also 5-HT (10 μmol/l), a full agonist at 5-HT4 receptors, tested in the presence of methysergide and granisetron to exclude interaction with other 5-HT receptor subtypes, did not enhance EFS-induced submaximal contractions.

CONCLUSIONS

There is no evidence for presence of 5-HT4 receptors on the cholinergic neurons of the equine small intestine. These results question the application of 5-HT4 prokinetic drugs in horses. Ethical animal research: Research ethics committee oversight not currently required by this conference: the study was performed on material collected at an abattoir. Sources of funding: None. Competing interests: None declared.

Peritoneal adhesions in human and veterinary medicine: from pathogenesis to therapy. A review

Any peritoneal inflammatory process consequent to infections or surgical injuries may induce abdominal adhesion formation. Peritoneal adhesions are connective laciniae that develop among abdomino-pelvic organs that limit physiologic visceral motion. Consequently, fertility may be impaired, and intestinal obstruction and pelvic pain may develop, mainly in subjects that had undergone gynaecological surgery. This review illustrates the pathogenic steps of adhesiogenesis and the therapeutic scenario that evolved over the years to tackle the threat of peritoneal adhesions, both in domestic animals and in women.

Expression and function of 5-HT7 receptors in smooth muscle preparations from equine duodenum, ileum, and pelvic flexure

In horses, gastrointestinal (GI) disorders occur frequently and cause a considerable demand for efficient medication. 5-Hydroxytryptamine receptors (5-HT) have been reported to be involved in GI tract motility and thus, are potential targets for treating functional bowel disorders. Our studies extend current knowledge on the 5-HT(7) receptor in equine duodenum, ileum and pelvic flexure by studying its expression throughout the intestine and its role in modulating contractility in vitro by immunofluorescence and organ bath experiments, respectively. 5-HT(7) immunoreactivity was demonstrated in both smooth muscle layers, particularly in the circular one, and within the myenteric plexus. Interstitial cells of Cajal (ICC), identified by c-Kit labeling, show a staining pattern similar to that of 5-HT(7) immunoreactivity. The selective 5-HT(7) receptor antagonist SB-269970 increased the amplitude of contractions in spontaneous contracting specimens of the ileum and in electrical field-stimulated specimens of the pelvic flexure concentration-dependently. Our in vitro experiments suggest an involvement of the 5-HT(7) receptor subtype in contractility of equine intestine. While the 5-HT(7) receptor has been established to be constitutively active and inhibits smooth muscle contractility, our experiments demonstrate an increase in contractility by the 5-HT(7) receptor ligand SB-269970, suggesting it exerting inverse agonist properties.

Evaluation of the effect of ranitidine on gastroduodenal contractile activity and gastric emptying in horses

OBJECTIVE

To determine the effect of ranitidine on gastric emptying in horses.

ANIMALS

11 adult horses.

PROCEDURES

In vitro, isolated muscle strips from the pyloric antrum and duodenum of 5 horses were suspended in baths and attached to isometric force transducers. Once stable spontaneous contractions were observed, ranitidine or diluent was added at cumulative increasing concentrations. Isometric stress responses were compared. In vivo, 6 horses were assigned to a group in a prospective randomized crossover study design with a wash-out period of 2 weeks between trials. Ranitidine (2.2 mg/kg) or saline (0.9% NaCl) solution was administered IV, and 15 minutes later, acetaminophen (20 mg/kg), diluted in 400 mL of water, was administered via nasogastric tube to evaluate the liquid phase of gastric emptying. Serum acetaminophen concentration was measured at several time points for 3 hours by use of liquid chromatography tandem mass spectrometry. Frequency of defecation was recorded during the 3 hours of the study.

RESULTS

Ranitidine increased the contractile activity of the pyloric antrum smooth muscle at a concentration of 10(-4) M. No significant effect of ranitidine on plasma kinetics of acetaminophen was identified. Frequency of defecation did not differ between groups.

CONCLUSIONS AND CLINICAL RELEVANCE

Ranitidine did increase gastric motility in vitro, but no effect on liquid phase gastric emptying was identified in healthy horses by use of the acetaminophen absorption model. Results do not support the use of ranitidine to promote gastric emptying.

Contractile effects of 5-hydroxytryptamine (5-HT) in the equine jejunum circular muscle: functional and immunohistochemical identification of a 5-HT1A-like receptor

REASONS FOR PERFORMING STUDY

Prokinetic drugs used to treat gastrointestinal ileus in man have equivocal results in horses. In man, prokinetic drugs have 5-hydroxytryptamine4(5-HT4) receptors as their target, but little is known about the 5-HT-receptor subtypes in the equine small intestine.

OBJECTIVE

Functional and immunohistochemical identification of the serotonin receptor subtype(s) responsible for the 5-HT induced contractile response in the equine circular jejunum.

METHODS

Isometric organ-bath recordings were carried out to assess spontaneous and drug-evoked contractile activity of equine circular jejunum. Histological investigations by immunofluorescence analyses were performed to check for presence and localisation of this functionally identified 5-HT receptor subtype.

RESULTS

Tonic contractions were induced by 5-HT in horse jejunal circular muscle. Tetrodotoxin, atropine and NG-nitro L-arginine did not modify this response. A set of 5-HT receptor subtype selective antagonists excluded interaction with 5-HT1B, 1D, 2A, 3, 4 and 7 receptors. The selective 5-HT1A receptor antagonists WAY 100635 and NAN 190 caused a clear rightward shift of the concentration-response curve to 5-HT. The contractile effect of 5-CT, that can interact with 5-HT1A, 1B, 1D, 5 and 7 receptors was also antagonised by WAY 100635, identifying the targeted 5-HT receptor as a 5-HT1A-like receptor. Immunohistology performed with rabbit polyclonal anti-5-HT1A receptor antibodies confirmed the presence of muscular 5-HT1A receptors in the muscularis mucosae, and both longitudinal and circular smooth muscle layers of the equine jejunum.

CONCLUSIONS

Contractile responses in equine jejunal circular smooth muscle induced by 5-HT involves 5-HT1A-like receptors.

Effects of mosapride citrate, metoclopramide hydrochloride, lidocaine hydrochloride, and cisapride citrate on equine gastric emptying, small intestinal and caecal motility

OBJECTIVE

Although extensive work has been done to elucidate the beneficial and unfavorable effects of gastrointestinal prokinetic agents in humans, little is known on the effects of these agents in horses. In this study, we compared the effects of mosapride, metoclopramide, cisapride, and lidocaine on equine gastric emptying, jejunal and caecal motility and evaluated these agents' adverse drug reactions (ADRs).

ANIMALS

Seven healthy adult Thoroughbreds.

PROCEDURE

Mosapride 1.0mg/kg and 2.0mg/kg, metoclopramide 0.2mg/kg, and cisapride 1.0mg/kg were dissolved in 100mL distilled water for oral administration. Lidocaine 1.3mg/kg was mixed with 500 mL saline for a 30-min intravenous infusion. Oral administration of 100mL distilled water was used as control. Gastric emptying was evaluated using (13)CO(2) breath test, and jejunal and caecal motility was assessed by electrointestinography.

RESULTS

The present study demonstrates that mosapride at doses of 1.0mg/kg and 2.0mg/kg facilitates gastric emptying in horses. Improved jejunal motility was observed following administration of mosapride (1.0mg/kg and 2.0mg/kg), metoclopramide (0.2mg/kg), and cisapride (1.0mg/kg). Similarly, improved caecal motility was observed following administration of mosapride (2.0mg/kg).

CONCLUSIONS AND CLINICAL RELEVANCE

This study shows that among the prokinetic agents studied here, only mosapride (2.0mg/kg) promotes jejunal and caecal motility in horses. Considering mosapride ADRs profile, it is believed that this compound is useful in the treatment of diseases associated with decreased GI motility, including postoperative ileus.

Demonstration of 5-HT3 receptor function and expression in the mouse bladder

The aim of this study was to demonstrate the presence of 5-HT(3) receptors in the mouse bladder and to determine their location. Bladder strips from female mice were set up in gassed Krebs-Henseleit solution at 37 degrees C and contractions recorded in response to electrical field stimulation (8 Hz, 60 V, 0.5-ms pulse duration) applied for 2 s every 50 s. The potentiating effects of 5-hydroxytryptamine (5-HT) were recorded (in the presence of 1-microM methysergide and 1-microM GR125487 to isolate the 5-HT(3) receptor response), and contractions were expressed as a percentage of the response to 0.1-M KCl. Responses to (5-HT) were also obtained in the presence of the 5-HT(3) receptor antagonist, ondansetron. RT-PCR was used to detect the expression of the 5-HT(3A) and 5-HT(3B) subunit transcripts of the mouse 5-HT(3) receptor. 5-HT and 5-HT(3) receptor agonists caused concentration-dependent increases in the force of neurogenic contractions without affecting the baseline tone. The rank order of potency was: meta-chloro-phenylbiguanide (m-CPB) = 5-HT > 2-methyl-5-HT (2m5-HT) = 1-phenylbiguanide (1-PBG). The respective pEC(50) values were: 6.42 +/- 0.2 = 5.95 +/- 0.19 > 5.35 +/- 0.12 = 5.14 +/- 0.13. m-CPB acted as a full agonist (E (max) = 40.65 +/- 3.81% KCl), but both 2m5-HT and 1-PBG acted as lower potency partial agonists. Ondansetron (30, 100, 300 nM) caused concentration-related rightward displacements to the concentration-effect curve to 5-HT. Nonlinear regression analysis of the effect of the ondansetron concentrations on the pEC(50) values produced a pK(B) value of 8.29 +/- 0.22. Desensitization of sensory nerves to the contractile effect of capsaicin (10 microM for 60 min) did not alter the ability of 5-HT to potentiate neurogenic contractions. 5-HT (3 microM) inhibited contractions induced by direct muscle stimulation (lignocaine, 300 microM and 10-ms pulse width). m-CPB also caused the same effect with a pIC(50) of 6.62 +/- 0.10 and an E (max) of 48.03 +/- 2.25%. The concentration-response curve to m-CPB was shifted rightwards by ondansetron (1 microM) giving an apparent pK(B) value of 8.15 +/- 0.33. mRNA for both the 5-HT(3A) and 5-HT(3B) receptor subunits was detected in the detrusor as well as the mucosa with a greater relative expression of the 5-HT(3A) subunit in both layers. This study demonstrates that 5-HT mediates enhanced neurogenic contractions of the mouse bladder muscle by an action at 5-HT(3) receptors located prejunctionally on nonsensory nerve elements. Additionally, an inhibitory postjunctional population of the 5-HT(3) receptor was identified. The presence of the 5-HT(3) receptor was confirmed by the expression of both 5-HT(3A) and 5-HT(3B) receptor subunits of the 5-HT(3) receptor.

Contractile effects of 5-hydroxytryptamine and 5-carboxamidotryptamine in the equine jejunum

The use of human prokinetic drugs in colic horses leads to inconsistent results. This might be related to differences in gastrointestinal receptor populations. The motor effects of 5-hydroxytryptamine (5-HT; serotonin) on the equine mid-jejunum were therefore studied. Longitudinal muscle preparations were set up for isotonic measurement. 5-HT induced tonic contractions with superimposed phasic activity; these responses were not influenced by tetrodotoxin and atropine, suggesting a non-neurogenic, non-cholinergic pathway. The 5-HT receptor antagonists GR 127935 (5-HT(1B,D)), ketanserin (5-HT(2A)), SB 204741 (5-HT(2B)), RS 102221 (5-HT(2C)), granisetron (5-HT(3)), GR 113808 (5-HT(4)) and SB 269970 (5-HT(7)) had no influence on the 5-HT-induced response; the 5-HT(1A) receptor antagonists NAN 190 (pK(b)=8.13+/-0.06) and WAY 100635 (pK(b)=8.69+/-0.07), and the 5-HT(1,2,5,6,7) receptor antagonist methysergide concentration-dependently inhibited the 5-HT-induced contractile response. The 5-HT(1,7) receptor agonist 5-carboxamidotryptamine (5-CT) induced a contractile response similar to that of 5-HT; its effect was not influenced by tetrodotoxin and atropine, and SB 269970, but antagonised by WAY 100635. 8-OHDPAT, buspiron and flesinoxan, which are active at rat and human 5-HT(1A) receptors, had no contractile influence. These results suggest that the contractile effect of 5-HT in equine jejunal longitudinal muscle is due to interaction with muscular 5-HT receptors, which cannot be characterised between the actually known classes of 5-HT receptors.

Distribution of mRNA that codes for 5-hydroxytryptamine receptor subtypes in the gastrointestinal tract of dairy cows

OBJECTIVE

To describe the distribution of mRNA that codes for 8 subtypes of 5-hydroxytryptamine receptors (5-HTRs) in the digestive tract of dairy cows.

SAMPLE POPULATION

Fresh full-thickness wall specimens from the abomasum (fundus, corpus, and antrum), ileum, cecum, proximal loop of ascending colon, and 4 locations of the spiral colon collected from 10 healthy cows at slaughter.

PROCEDURE

Concentrations of mRNA that code for 5-HTR subtypes (5-HTR1A. 5-HTR1B, 5-HTR1D, 5-HTR1F, 5-HTR2A, 5-HTR2B, 5-HTR2C, and 5-HTR4) in the bovine digestive tract were measured by use of a quantitative real-time reverse transcription-polymerase chain reaction assay. Results were reported in relation to mRNA expression of the housekeeping gene glyceraldehyde phosphate dehydrogenase (GAPDH).

RESULTS

Mean relative mRNA concentrations for 5-HTR were low (range, 0% to 1.32% of GAPDH), and mRNA that codes for 5-HTR1A was not detected. In the abomasum, mRNA expression was highest for 5-HTR1B and 5-HTR2B, followed by subtypes 1F 2A, 1D, and 4, whereas 5-HTR2C was not detected. In intestinal samples, concentrations of subtypes 1B, 2B, and 4 were highest, followed by 1D, 1F, 2A, and 2C. Relative concentrations of mRNA that code for 5-HTR2A were significantly higher in the abomasum than the intestines, but lower for 5-HTR2B, 5-HTR2C, and 5-HTR4.

CONCLUSIONS AND CLINICAL RELEVANCE

Relative concentrations of mRNA that code for 5-HTRs differ among locations in the gastrointestinal tract of cattle. Understanding differences in the distribution of 5-HTRs in healthy cattle and cattle with gastrointestinal tract disease may lead to improved therapeutic approaches for abomasal and cecal motility disorders.

In vitro effects of cisapride, metoclopramide and bethanechol on smooth muscle preparations from abomasal antrum and duodenum of dairy cows

The objective of this study was to investigate the effects of cisapride (CIS), metoclopramide (MET) and bethanechol (BET) on contractility parameters from smooth muscle preparations of the abomasal antrum and proximal duodenum of cows. Smooth muscle preparations were harvested shortly post-mortem from 42 healthy dairy cows, and concentration-response curves were performed by cumulative application of the drugs. Cisapride and MET did not have any significant effect on the contractility parameters studied, while BET induced a significant, concentration-dependent increase in basal tone (BT), mean amplitude (Amean), and area under the curve (AUC) in smooth muscle preparations from the abomasal antrum, but not from the duodenum. The effect of BET on BT was more pronounced in specimens with longitudinal orientation while the maximal obtainable effect (Vm) in Amean was more pronounced in circular-oriented preparations. Atropine (1 x 10-5 m) significantly inhibited the effect of BET, whereas pre-incubation with hexamethonium or tetrodotoxin (TTX) had no effect, suggesting that the effect was mediated by cholinergic receptors on the smooth muscle. The results may be relevant to diseases or disorders associated with gastric emptying and gastric hypomotility. Further investigations are warranted to investigate the potential ability of BET to enhance abomasal emptying of adult dairy cows.

Modifiers of gastrointestinal motility of cattle

Little clinically relevant, evidence-based data about the effect of motility modifiers on the GI tract of cattle are currently available. Additionally, some of the published results seem to be contradictory. Three main facts explain this apparent discrepancy: (1) Results may not be transferred from one species to another, because presence, concentration, location, distribution, and function of specific receptors may differ significantly among species. (2) The lack of a significant effect on smooth muscle preparations in vitro does not necessarily exclude a certain drug's motility-modifying property on affected animals in vivo. Certain drugs bind to receptors outside the myenteric plexus. Others, such as lidocaine and adrenergic-, dopaminergic-, and opioid-antagonists increase only GI motility, if inhibitory reflexes or a hyperactive state of the inhibitory sympathetic nervous system preexists. (3) Effects of motility modifiers as found in healthy experimental animals in vivo may not be similar to those found in spontaneously diseased animals. Accurate and reliable data on the effect of modifiers of GI motility of cattle will be obtained only from double-blinded, evidence-based, in vivo studies on spontaneously affected animals. Because well-documented results from such studies are extremely rare, intensive research in this field is warranted in the future.

Modification of spontaneous contractility of smooth muscle preparations from the bovine abomasal antrum by serotonin receptor agonists

The effects of serotonin (5-hydroxytryptamine, 5-HT), a 5-HT2-receptor agonist, alpha-methyl-5-hydroxytryptamine (alpha-M-5-HT) and RS-67506, a 5-HT4 receptor partial agonist, on spontaneous contractility of bovine abomasal smooth muscle preparations were investigated in vitro. Preparations from the abomasal antrum of freshly slaughtered healthy dairy cows were cut parallel to the longitudinal fibres, suspended in isolated organ baths, and concentration-response curves were performed by cumulative application of the 5-HT receptor agonists. Blockade of 5-HT2-induced response was tested with atropine and hexamethonium. Serotonin evoked a significant increase in the area under curve (AUC), whilst the 5-HT2 receptor agonist alpha-M-5-HT significantly increased the AUC and resting tone (RT). RS-67506 induced a significant increase in AUC and RT and a significant decrease in the maximum force. The effect of alpha-M-5-HT was mediated by a muscarinic cholinergic pathway, as the effect of alpha-M-5-HT was inhibited in the presence of atropine but not hexamethonium. It is concluded that 5-HT2 and 5-HT4 receptors are present in the bovine abomasal antrum. Muscarinic receptors are involved in the increase in RT seen after 5-HT2 receptor stimulation.

Expression of the ether-a-go-go (ERG) potassium channel in smooth muscle of the equine gastrointestinal tract and influence on activity of jejunal smooth muscle

OBJECTIVE

To determine whether ether-a-go-go (ERG) potassium channels are expressed in equine gastrointestinal smooth muscle, whether ERG channel antagonists affect jejunal muscle contraction in vitro, and whether plasma cisapride concentrations in horses administered treatment for postoperative ileus (POI) are consistent with ERG channels as drug targets.

SAMPLE POPULATION

Samples of intestinal smooth muscle obtained from 8 horses free of gastrointestinal tract disease and plasma samples obtained from 3 horses administered cisapride for treatment of POI.

PROCEDURE

Membranes were prepared from the seromuscular layer of the duodenum, jejunum, ileum, cecum, large colon, and small colon. Immunoblotting was used to identify the ERG channel protein. Isolated jejunal muscle strips were used for isometric stress response to ERG channel blockers that included E-4031, MK-499, clofilium, and cisapride. Plasma concentrations of cisapride were determined in 3 horses administered cisapride for treatment of POI after small intestinal surgery.

RESULTS

Immunoblotting identified ERG protein in all analyzed segments of the intestinal tract in all horses. The selective ERG antagonist E-4031 caused a concentration-dependent increase in jejunal contraction. Clofilium, MK-499, and cisapride also increased jejunal contraction at concentrations consistent with ERG channel block; effects of E-4031 and cisapride were not additive. Peak plasma cisapride concentrations in treated horses were consistent with ERG block as a mechanism of drug action.

CONCLUSIONS AND CLINICAL RELEVANCE

The ERG potassium channels modulate motility of intestinal muscles in horses and may be a target for drugs. This finding may influence development of new prokinetic agents and impact treatment of horses with POI.

5-Hydroxytryptamine mediated contractions in isolated preparations of equine ileum and pelvic flexure: pharmacological characterization of a new 5-HT(4) agonist

The effects of 5-hydroxytryptamine (5-HT), HTF 919, a new 5-HT(4) agonist, and the antagonists SB 203-186 (5-HT(4)) and tropisetron (5-HT(3)) on intestinal motility were tested in vitro on isolated preparations of horse ileum and pelvic flexure. Concentration-response curves were created by cumulative application of the agonists with or without preincubation of the antagonists. The 5-HT preparation induced a concentration-dependent contraction in equine ileum and pelvic flexure. The results indicate that 5-HT receptors are present in all parts of equine intestine investigated in this study. Tropisetron was found to act as a noncompetitive antagonist in all locations of the equine intestine. SB 203-106 was confirmed as an antagonist to 5-HT in the equine ileum circular muscle, in pelvic flexure circular and longitudinal muscle. Nevertheless, a discernible increase of smooth muscle contractions caused by HTF 919 could only be observed in pelvic flexure. In accordance with an earlier study in the guinea pig, in the equine gut HTF 919 acted as a partial agonist for the 5-HT(4) receptor with an affinity constant in the nanomolar range. It is concluded that 5-HT receptors, and especially their subtypes, may represent a promising target for the treatment and prevention of gastrointestinal (GI) motility disorders in horses.

Use of an extracorporeal circuit to evaluate effects of intraluminal distention and decompression on the equine jejunum

OBJECTIVE

To use an extracorporeal circuit to evaluate effects of intraluminal distention on the jejunum of healthy horses.

SAMPLE POPULATION

2 jejunal segments from each of 5 horses.

PROCEDURE

Jejunal segments were harvested and maintained in an extracorporeal circuit. One segment was subjected to distention (intraluminal pressure, 25 cm H2O) followed by decompression, and 1 segment was maintained without distention. The influence of distention-decompression on vascular resistance was calculated. Mucosal permeability was evaluated by measuring the clearance of albumin from blood to lumen. After distention and decompression, tissue specimens were collected for histomorphologic evaluation. In addition, the contractile response of the circular smooth muscle layer was determined following incubation with 3 prokinetic agents.

RESULTS

Intestinal vascular resistance increased during intraluminal distention and returned to baseline values after decompression. Albumin clearance rate increased after distention, compared with baseline and control values. Histologic examination of the distended segments revealed grade-1 and -2 lesions of the mucosal villus. Edema and hemorrhage were evident in the submucosa and muscular layers. Mesothelial cell loss, edema, and hemorrhage were also evident in the serosa. Mucosal surface area and villus tip height decreased and submucosal volume increased in the distended tissue. Compared with responses in control specimens, distention decreased the contractile response induced by cisapride, erythromycin, and metoclopramide.

CONCLUSIONS AND CLINICAL RELEVANCE

Intraluminal distention of the jejunum followed by decompression increased mucosal permeability and injury and decreased responses to prokinetic agents. Horses with intraluminal intestinal distention may have a decreased response to prokinetic agents.