Pathophysiology of equine postoperative ileus: effect of adrenergic blockade, parasympathetic stimulation and metoclopramide in an experimental model

Evaluation of pharmacokinetics of metoclopramide administered via subcutaneous bolus and intravenous constant rate infusion to adult horses

OBJECTIVE

To determine the pharmacokinetics (PK) of metoclopramide administered via intravenous continuous rate infusion (IV CRI) and subcutaneous (SC) bolus and evaluate for gastrointestinal motility and adverse side effects.

STUDY DESIGN

Experimental study; randomized, crossover design.

ANIMALS

Six healthy adult horses.

METHODS

Each horse received metoclopramide via IV CRI (0.04 mg/kg/h for 24 h) and SC bolus (0.08 mg/kg once), with ≥1 week washout period between. Plasma was analyzed by UPLC-MS/MS. Compartmental modeling was used to determine PK parameters for each treatment; nonparametric superposition was used to simulate multiple SC bolus regimens. Gastrointestinal motility and evidence of adverse effects were monitored.

RESULTS

Tmax (h) for SC bolus was 0.583 ± 0.204 versus 17.3 ± 6.41 for IV CRI, while Cmax (ng/mL) was 27.7 ± 6.38 versus 43.6 ± 9.97, respectively. AUC (h × ng/mL) was calculated as 902 ± 189 for 24 h IV CRI versus 244 ± 37.4 simulated for 0.08 mg/kg SC bolus every 8 h. Simulations revealed similar exposure between groups with administration of 0.96 mg/kg/day SC bolus, divided into three, four, or six doses. SC bolus bioavailability was estimated as 110 ± 11.5%. No clear trends in motility alteration were identified. No adverse effects were noted.

CONCLUSION

Repeated SC boluses of metoclopramide at 0.08 mg/kg would result in lower total drug exposure and Tmax than IV CRI administration but would be highly bioavailable.

CLINICAL SIGNIFICANCE

Higher and/or more frequent SC bolus doses are needed to achieve a similar AUC to IV CRI. No adverse effects were noted; however, evaluation of alternative dosing strategies is warranted.

The effects of bit chewing on borborygmi, duodenal motility, and gastrointestinal transit time in clinically normal horses

OBJECTIVE

To determine the influence of bit chewing on gastrointestinal transit in clinically normal horses.

STUDY DESIGN

Prospective crossover designed study.

ANIMALS

Six healthy adult horses.

METHODS

Horses were assigned randomly to treatment (apple flavored bit) and control (no-bit) groups and studied for 2 × 1-week trial periods with a 2-week washout period between trials. Horses were fasted for 24 h and slowly refed over 3 days. The bit was placed for 20 min every 6 h. Duodenal contractions and borborygmi auscultations were evaluated every 12 h, approximately 5 min following bit placement. Gastrointestinal total transit time (GI TTT) was measured by administering 200 colored beads via stomach tube and then collected in the manure until 50% and 80% were recovered. Measured variables were compared using an ANOVA or Wilcoxon signed-rank test and the P value was noted.

RESULTS

The GI TTT was shortened in the bit chewing group (median: 106.37 h, range: 70-171 h) compared to the no-bit group (median: 170.1 h, range: 149-186 h) (P = .0156) at 80% bead passage (only 4/6 horses passed 80%). Borborygmi (P = .8193), duodenal contractions (P = .2605), and 50% bead passage (P = .0781) showed no differences.

CONCLUSION

Bit chewing was safe, inexpensive, and well tolerated. Bit chewing shortened GI TTT and might be an adjunct therapy to augment GI TTT. Further clinical studies are warranted.

CLINICAL SIGNIFICANCE

Ileus is a common complication following equine abdominal surgery with no current consistently successful treatment. Bit chewing may be a simple and inexpensive way to augment progressive GI motility.

Continuous fluid infusion per rectum compared with intravenous and nasogastric fluid administration in horses

BACKGROUND

Rectal fluid administration may offer a simple, safe and inexpensive alternative to intravenous or nasogastric fluid therapy in equine clinical cases.

OBJECTIVES

To evaluate the tolerance and effects of rectally administered fluid and compare the measurements of haemodilution and intravascular volume with those during nasogastric and intravenous fluid administration.

STUDY DESIGN

Randomised controlled experimental trial.

METHODS

Six clinically normal Standardbred geldings were used in a 4-way crossover study: each received three different fluid treatments (intravenous, nasogastric and rectal) at 5 mL/kg/h for 6 h and underwent a control (no treatment) with water and feed withheld. Bodyweight was measured at baseline and 6 h. Packed cell volume (PCV), total solids (TS), albumin, electrolytes, lactate, urine specific gravity, vital parameters, gastrointestinal borborygmi and central venous pressure were measured every 2 h.

RESULTS

Rectal fluid administration with plain water was well tolerated and caused clinical chemistry changes consistent with haemodilution, indicating absorption. Mean (95% confidence interval) PCV decreased from 40% [40-42] at 0 h to 35% [34-36] at 6 h during rectal fluid treatment (P<0.001), similar to decreases in PCV occurring also with i.v. and nasogastric (NGT) treatment (P<0.001). The TS also decreased with i.v. and rectal fluid (P<0.001). There was a decrease in bodyweight in the control (P<0.001) but not with any of the fluid treatments.

MAIN LIMITATIONS

A small sample size of healthy, euhydrated horses and a relatively short duration of fluid administration was used.

CONCLUSIONS

Rectal fluid administration requires clinical evaluation, but may offer an inexpensive, safe alternative or adjunct to i.v. fluid administration, particularly when administration via NGT is not possible or contraindicated. The Summary is available in Portuguese - see Supporting Information.

Retrospective evaluation of the effect of intravenous fluid administration on development of postoperative reflux in horses with colic (2004-2012): 194 horses

OBJECTIVE

To evaluate the role of intravenous fluid volume and electrolyte supplementation on the development of postoperative reflux (POR) in horses undergoing celiotomy for colic.

DESIGN

Case-control study spanning 2004-2012 for horses undergoing celiotomy for colic.

SETTING

University teaching hospital.

ANIMALS

Sixty-seven client-owned horses >1 year of age with POR were each matched to 2 controls with similar surgical lesions that did not demonstrate POR.

MEASUREMENTS AND MAIN RESULTS

Survival was significantly lower in cases (65.7%) than controls (96.1%). Factors found to be associated with POR included decreased net fluid volume administered on day 1 postoperatively, increased age, and performing a resection and anastomosis. Mean time until onset of POR was 20.4 hours postoperatively. PCV was significantly higher immediately following surgery and at 24 hours postoperatively in horses that developed POR compared with matched controls. There was no association between electrolyte values at presentation or administration of potassium, calcium, or magnesium in the postoperative period and the subsequent development of reflux.

CONCLUSIONS

In the perioperative period, IV fluid volume overload and electrolyte abnormalities were not contributing factors in the development of POR in this population of surgical colic patients. Close postoperative monitoring with consideration and correction of pre- and postoperative fluid deficits is recommended.

In vivo and in vitro evaluation of the effects of domperidone on the gastrointestinal tract of healthy horses

OBJECTIVE

To determine the effects of domperidone on in vivo and in vitro measures of gastrointestinal tract motility and contractility in healthy horses.

SAMPLE

18 adult horses and tissue samples from an additional 26 adult horses.

PROCEDURES

Domperidone or placebo paste was administered to healthy horses in a 2-period crossover study. Gastric emptying was evaluated after oral administration of domperidone paste (1.1 or 5.0 mg/kg) or placebo paste by means of the acetaminophen absorption test in 12 horses. Frequency of defecation, weight of feces produced, fecal moisture, and stomach-to-anus transit time of microspheres were evaluated after administration of domperidone paste (1.1 mg/kg) or placebo paste in 6 horses. The effect of domperidone on smooth muscle contractile activity in samples of duodenum, jejunum, ileum, or colon obtained from 26 horses immediately after euthanasia (for nonsystemic medical problems) was investigated.

RESULTS

Oral administration of 5.0 mg of domperidone/kg increased peak plasma acetaminophen concentration and area under the curve, indicating increased gastric emptying. Administration of 1.1 mg of domperidone/kg had no effect on gastric emptying, transit time, defecation frequency, or amount and moisture of excreted feces. Contractile activities of circular and longitudinal muscle strips from the duodenum, jejunum, ileum, or colon were not altered by domperidone. Dopamine increased contractile activity of longitudinal muscle strips but not that of circular muscle strips from the midjejunum. Domperidone decreased the dopamine-induced contractile activity of midjejunal longitudinal muscle strips.

CONCLUSIONS AND CLINICAL RELEVANCE

The potential beneficial effects of domperidone in horses with ileus need to be evaluated in horses with decreased gastric emptying or adynamic ileus.

In vivo and in vitro effects of neostigmine on gastrointestinal tract motility of horses

OBJECTIVE

To determine the response to neostigmine of the contractile activity of the jejunum and pelvic flexure and the effects of a continuous rate infusion (CRI) of neostigmine in horses.

ANIMALS

7 adult horses and tissue from 12 adult horses.

PROCEDURES

A CRI of neostigmine (0.008 mg/kg/h) or placebo was administered to 6 horses in a crossover study design. Gastric emptying was evaluated by the acetaminophen test. The frequency of defecation and urination and the consistency and weight of feces were recorded throughout the experiment. The effect of neostigmine on smooth muscle contractile activity was evaluated in tissues from the jejunum and pelvic flexure. The effect of neostigmine and acetylcholine after incubation with muscarinic receptor antagonists (atropine and DAU 5884) and an acetylcholinesterase inhibitor (edrophonium) was also investigated in vitro.

RESULTS

No difference was observed between neostigmine and placebo for time to reach peak plasma acetaminophen concentration and absorption rate constant. A CRI of neostigmine increased fecal production and frequency of urination. Neostigmine induced a dose-dependent increase of contractile amplitude in jejunum and pelvic flexure muscle strips. Incubation of muscle strips with atropine and DAU 5884 inhibited the response to acetylcholine and neostigmine. Incubation of smooth muscle strips from the jejunum with edrophonium increased the response to acetylcholine and had no effect on the response to neostigmine in vitro.

CONCLUSIONS AND CLINICAL RELEVANCE

A CRI of neostigmine increased fecal production and urination frequency in horses. A CRI of neostigmine did not decrease gastric emptying. Neostigmine stimulated contractile activity of jejunum and pelvic flexure smooth muscle strips in vitro.

Application of the lactose 13C-ureide breath test for measurement of equine orocaecal transit time

REASONS FOR PERFORMING STUDY

Application of the lactose (13) C-ureide breath test (LUBT) for measurement of equine orocaecal transit time (OCTT) has not been reported previously. The ability to assess OCTT noninvasively, and to investigate its relationship to gastric emptying rate and small intestinal transit, would be of both clinical and research value.

OBJECTIVES

1) Assessment of the LUBT in healthy horses, with comparison of induced versus noninduced test protocols. 2) Application of a new dual stable isotope breath test (lactose (13) C-ureide and (13) C-octanoic acid) for gastrointestinal transit measurement.

HYPOTHESIS

The LUBT will allow quantification of equine OCTT, and test efficacy will be enhanced by prior administration of lactose (12) C-ureide as shown in vitro. The dual tracer breath test will permit simultaneous measurement of gastric emptying, OCTT and small bowel transit times.

METHODS

Induced and noninduced LUBTs were performed in 3 healthy mature horses in randomised order using a standard test meal and protocol. Combined LUBT and (13) C-octanoic acid breath tests ((13) C-OABT) were performed in 4 individuals on 4 occasions at weekly intervals. Expiratory isotopic recovery was modelled to allow generation of gastric emptying data, small bowel transit times and caecal transit parameters.

RESULTS

The induction protocol for the LUBT increased the rate and magnitude of expiratory (13) CO(2) significantly. Mean ± s.d. values for OCTT, caecal lag phase (t(lag) ) and caecal t(1/2) using the induced LUBT were 3.24 ± 0.65 h, 5.62 ± 1.22 h and 6.31 ± 1.21 h, respectively. Dual stable isotope tests resulted in the production of 2 discrete peaks in expiratory (13) CO(2) in 15/16 tests from which gastric t(1/2), OCTT and small bowel transit (SBT) parameters could be calculated.

CONCLUSIONS

The induced LUBT provides a reliable noninvasive measure of equine OCTT and can be paired with the (13) C-OABT to provide further information about small intestinal motility.

Colic after swimming exercise in racehorses: an investigation of incidence, management, surgical findings and outcome

The incidence of colic following swimming exercise (SC) in a racehorse population examined over 3 years was 0.08%. Following 167,977 swims, 136 cases were reported. Colic resolved spontaneously in 2 horses, medically in 129 or surgically in 5. In a separate study, 21 horses were subject to exploratory laparotomy for SC. Findings included non-strangulating displacements in 7 horses, strangulating displacements in 7, a combination of non-strangulating and strangulating displacements in 1, prominent gaseous intestinal distension in 5 and no abnormalities in 1 horse. A poor response to analgesia prompted surgery in all cases. Rectal palpation and/or abdominal ultrasound was performed in 16 of 21 cases and abnormalities were found in 14 of these. Of the 21 surgical cases, 20 (95.2%) survived to discharge and 18 (90%) raced postoperatively.

Effects of α2-adrenergic drugs on small intestinal motility in the horse: an in vitro study

The effects of selective α(2)-agonists (xylazine, detomidine and medetomidine) and antagonists (yohimbine and atipamezole) on in vitro small intestine motility in the horse were evaluated. Samples of equine jejunum were placed in isolated organ baths and drug-induced modifications of motility were measured by means of an isotonic transducer. All tested α(2)-agonists dose-dependently reduced both spontaneous and electrically-evoked phasic contractions. Conversely, α(2)-antagonists were ineffective when tested alone, and showed a heterogeneous and dose-independent ability to inhibit agonist activity. In particular, the antagonism exerted by higher concentrations of both yohimbine and atipamezole against α(2)-agonists was weaker than when lower concentrations were used. The data are indicative of the presence of both pre- and post-synaptic α(2)-adrenoceptors with inhibitory activity on equine jejunum motility, and support a possible therapeutic utility of these drugs in horse intestinal disorders associated with hypermotility.

Postoperative ileus: pathogenesis and treatment

Surgical manipulation of the intestines activates intestinal macrophages that release cytokines and nitric oxide, which results in inhibition of intestinal motility. Subsequent infiltration of circulating leukocytes into the intestinal wall contributes to cytokine and nitric oxide release and exacerbates ileus. Other factors contributing to ileus are endotoxemia; edema of the intestine wall subsequent to excessive fluid therapy; hypocalcemia; and long abdominal incisions. Because treatment of ileus with prokinetic drugs has not proven to be very effective, efforts should be directed at reducing its severity. Strategies which reduce the severity of ileus include pretreatment with a nonsteroidal anti-inflammatory drug, minimizing the length of the abdominal incision, reducing intestinal manipulation, intraoperative lidocaine infusion, correction of hypocalcemia, limiting the volume of intravenous fluids to prevent intestinal edema, and administration of alpha(2) antagonists.

Evaluation of equine cecal motility by ausculation, ultrasonography and electrointestinography after jejunocecostomy

OBJECTIVE

Horses often suffer reduced intestinal motility after jejunocecostomy. Therefore, accurate evaluation of intestinal motility is important for the prevention, diagnosis and treatment of this condition. The purpose of this study was to evaluate intestinal motility in horses after jejunocecostomy using three different methods, i.e. auscultation, ultrasonography and electrointestinography.

ANIMALS

Six healthy thoroughbreds were used in this study. They were subjected to jejunocecostomy.

PROCEDURE

Bowel sounds in the right paralumbar fossa were assigned a score of 0-3 for intestinal motility evaluation by auscultation, and the number of cecal contractions during a 3-min period were counted by ultrasonography. Electrointestinography (EIG) was used to measure percutaneous potential of the cecum.

RESULTS

We identified three specific postoperative periods: the period of reduced intestinal motility (postoperative day 1 to day 2), in which intestinal motility declined, the unstable period (day 3 to day 7), in which intestinal motility partially recovered, and the full recovery period (day 8 to day 31), in which intestinal motility returned to preoperative state. Careful management was found to be especially important during the period of reduced intestinal motility and the unstable period. We found that, in healthy horses that underwent jejunocecostomy, it takes approximately one month for the cecum to return to normal motility patterns observed before surgery.

CONCLUSION

We have shown in this study that evaluation of intestinal motility after jejunocecostomy in horses by EIG is more objective and provides more details than evaluation by auscultation or ultrasonography.

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.

Effect of intraluminal distension or ischemic strangulation obstruction of the equine jejunum on jejunal motilin receptors and binding of erythromycin lactobionate

OBJECTIVE

To determine whether inflammation of the jejunum of horses decreases the number of motilin receptors and amounts of motilin receptor mRNA and alters erythromycin lactobionate binding affinity to the motilin receptor in jejunal tissues.

SAMPLE POPULATION

Jejunal segments in 6 adult horses.

PROCEDURE

Each horse was anesthetized, and a ventral median celiotomy was performed; 2 segments of jejunum underwent a sham operation, 2 segments underwent ischemic strangulation obstruction (ISO), and 2 segments underwent intraluminal distension (ILD). Treatments were maintained for 120 minutes. From each segment, full-thickness biopsy samples were collected and smooth-muscle homogenates were prepared. Affinity and distribution of motilin binding to these preparations were determined by use of iodine 125 (125I)-labeled synthetic porcine motilin. Via displacement experiments, competition between 125I-labeled motilin and erythromycin lactobionate for binding to motilin receptors in the different segments was investigated. A quantitative real-time PCR technique was used to assess motilin receptor mRNA content in the muscle preparations.

RESULTS

Compared with the ISO or ILD segments, the number of motilin receptors was significantly higher in the sham-operated segments; ILD segments contained the lowest number of motilin receptors. The expression of motilin receptor mRNA was significantly decreased in ILD segments but not in ISO segments. Erythromycin lactobionate displacement of 125I-labeled motilin from motilin receptors did not differ significantly among the jejunal segments.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that downregulation and decreased production of motilin receptors in inflamed jejunal tissue contribute to the altered prokinetic response to erythromycin in horses with gastrointestinal disease.

In vitro effects of bethanechol on equine gastrointestinal contractility and functional characterization of involved muscarinic receptor subtypes

The goal of this study was to investigate the effect of bethanechol (BeCh) on contractility patterns of smooth muscle preparations of equine duodenum descendens, jejunum, caecum and pelvic flexure in vitro. Concentration-response relationships were developed for BeCh using in vitro assays with and without preincubation of muscarinic (M) receptor antagonists for M2 and M3 receptors. BeCh induced a significant, concentration-dependent increase in contractile response in equine intestine in specimens with circular orientation. The maximal effect was largest for jejunal specimens with no difference in EC50 within the different locations investigated. The M2 antagonist, AF-DX 116, caused a rightward shift of the concentration-response curve and the M3 antagonist, 4-DAMP (1,1-dimethyl-4-diphenylacetoxypiperidinium iodide), almost completely inhibited the effect of BeCh over the entire concentration-response curve. These data provide evidence that, although the effect of BeCh is predominantly mediated by M3 receptors, M2 muscarinic receptors also play a role in BeCh-induced contraction in specimens of equine intestine. The involvement of other muscarinic receptor subtypes cannot be excluded. Further studies are necessary to understand the effect of BeCh in vivo including diseased animals.

Post operative neutrophilic inflammation in equine small intestine after manipulation and ischaemia

REASONS FOR PERFORMING STUDY

Post operative ileus (POI) remains an important cause of post operative morbidity and mortality in the horse. However, clinical progression of naturally occurring cases of POI in both horse and man does not entirely support the 'neurogenic' hypothesis as the sole mechanism of POI; and the hypothesis that inflammation plays a major role at 12-24 h after surgery requires validation.

HYPOTHESIS

An inflammatory infiltrate in the muscularis externa and myenteric plexus of equine jejunum is present 18 h following a period of ischaemia.

METHODS

Samples of normal jejunum, jejunum from the proximal resection margins of clinical cases and jejunum obtained 18 h after 1 or 2 h ischaemia or manipulation alone were evaluated for neutrophil infiltration. Samples obtained 18 h after surgery were additionally evaluated for leucocyte activation using calprotectin immunohistochemistry. Results were evaluated by ANOVA and P < 0.05 was considered significant.

RESULTS

Significant neutrophilic inflammation was identified in the samples from the proximal resection margins of clinical cases compared to uninjured jejunum. In experimental cases, neutrophilic inflammation appeared to be increased further by 18 h and was identified through all intestinal layers, particularly in the serosa, fascial planes around circular and longitudinal muscle fibres, and myenteric plexus. This elevated level of neutrophilic inflammation was mirrored by an increased number of calprotectin-positive cells in these intestinal layers, indicating leucocyte activation.

CONCLUSIONS

Significant neutrophilic inflammation occurs in equine jejunal myenteric layers 18 h after surgery.

POTENTIAL RELEVANCE

This neutrophilic inflammation coincides with the clinical time point at which POI is identified and may indicate that inflammatory pathways, rather than solely neurogenic pathways, are responsible for POI in the horse.

Recent advances in equine abdominal surgery

Laparoscopy is a minimally invasive procedure that has applications as a diagnostic, therapeutic and prognostic technique. Specialized equipment is necessary to perform equine laparoscopy, and there is a large range of instruments, both disposable and non-disposable available. Laparoscopic procedures described include ovariectomy, cryptorchidectomy, adhesiolysis and herniorrhaphy. Laparoscopy can be performed in a standing or dorsally recumbent position, depending on surgeon preference, patient status and the procedure to be performed. Stapling equipment is frequently used in gastrointestinal surgery in horses. Advantages include decreased surgical time and a decrease in the risk of contamination. Stapling equipment is often used in creating anastomoses, both in the large and small intestines, as well as in vessel ligation. New surgical techniques intended to decrease adhesion formation include the use of carboxymethylcellulose and bioresorbable patches. Indwelling abdominal drains can be used for peritoneal lavage following surgery and also appear to decrease the risk of adhesion formation. Improvements in post-operative care, including the treatment of post-operative ileus and endotoxaemia can significantly improve the outcome of horses that have undergone surgery for abdominal disorders. Recommendations for the use of prokinetic agents in horses with ileus vary widely. Prokinetic agents include local anaesthetics, macrolide antimicrobials, cholinergic agonists and dopamine antagonists. Endotoxaemia is common in horses following surgery for gastrointestinal disorders. The antibiotic polymyxin B binds to the circulating endotoxin molecule, decreasing its half-life in the intra-vascular space and reducing associated inflammation. This drug appears to be an effective and affordable treatment option for horses with endotoxaemia. The use of specific cyclooxygenase inhibitors in veterinary medicine have been studied recently. Selective cyclooxygenase-2 inhibitors may provide comparable anti-inflammatory and analgesic properties to the non-selective non-steroidal anti-inflammatory drugs. These drugs appear to have similar clinical effectiveness and will hopefully minimize deleterious side effects. The optimal healing of ventral midline incisions in horses is related to many factors including appropriate suture patterns and bite size, in addition to appropriate post-operative exercise recommendations. Recent advances in surgical techniques and post-operative care should decrease the morbidity and mortality associated with abdominal surgery. This article provides an overview of some of these advances.

Are so many adrenergic receptor subtypes really present in domestic animal tissues? A pharmacological perspective

Adrenergic receptors (ARs) are the cellular membrane binding sites through which natural catecholamines and sympathomimetic drugs exert their physiological and pharmacological effects. In recent decades, studies to clarify the distribution and function of ARs have been performed mostly on cultured cells, laboratory animals and human target tissues, but little is known about these aspects in domestic animals. This review focuses on AR structure, classification and signalling pathways and on AR subtype distribution in target tissues of some domestic animals, namely dogs, horses and bovines. In these species, different alpha- and beta-AR subtypes have been characterized and the functions controlled by the adrenergic systems have been studied. In the dog, the role played by the adrenergic system in the pathogenesis of cardiovascular disorders and in the modulation of canine aggression has roused particular interest. In dogs affected by dilated cardiomyopathy a significant down-regulation of beta-ARs has been observed both in the heart and circulating lymphocytes. This finding confirms the involvement of the adrenergic system in the pathogenesis and progression of the disorder and suggests new therapeutic strategies. In the horse, AR distribution has been studied in the cardiac, respiratory and gastrointestinal systems as well as in digital veins and arteries. The cardiac beta-ARs in healthy horses seem to be predominantly represented by the beta(1) subtype. In this species, heart failure may increase the expression of the beta(2) subtype, rather than causing AR down-regulation. Different beta- and alpha-AR subtypes have been characterized in the smooth muscle of equine ileum. The sympathetic relaxation of equine ileum smooth muscle seems to depend mainly on beta(3)-AR subtype activation, with minor involvement of the beta(2) subtype. In the respiratory tract, regional differences have been evidenced in the functionality of beta-AR subtype. The beta(2) subtype predominates in all segments but the beta(2) subtype-mediated adenyl cyclase response is tissue-dependent, with higher activity in tracheal membranes than bronchial or pulmonary ones. Both alpha- and beta-AR subtypes are present in the genital tract of cows. Bovine ovarian and myometrial cell membranes express higher concentrations of beta(2)-ARs than the beta(1) subtype, whereas as far as alpha-ARs are concerned, a single class of alpha(1)-ARs and two distinct classes of alpha(2)-AR binding sites have been discriminated. Interestingly, it has been observed that the activation of the sympathetic system could play an important role in the pathogenesis of bovine ovarian cysts as suggested by the modifications in beta-AR levels in the hypophysis and ovary of cows affected by ovarian cysts. In this species, the phenomenon of down-regulation has been well studied in different organs of veal calves treated with clenbuterol as a "partitioning agent". Since differences exist in AR distribution among species, data obtained in laboratory animals or in human beings cannot be extrapolated to domestic animals and further investigation on AR subtypes in domestic animal tissues is necessary.

Evaluation of the clinical efficacy of prokinetic drugs in the management of post-operative ileus: Can retrospective data help us?

The objectives of the study were to determine whether retrospective data can be used to answer questions about the efficacy of prokinetic agents when used to treat horses with post-operative ileus (POI). We describe prevalence and mortality of POI with reference to treatment with four prokinetic agents. By combining data from two Hospitals a study population of 55 horses with POI following pedunculated lipoma obstruction (PLO) was established. Univariable and multivariable associations were determined between short term survival and potential explanatory variables. With death as outcome in multivariable models, breed and hospital were significantly associated with outcome but the use of prokinetic agents was not (P=0.15). However, sample size estimates indicate the low power of this study to detect differences in outcome. It was not possible definitively to evaluate the efficacy of prokinetics as treatment for POI following PLO using retrospective data. The data were suggestive of limited efficacy of prokinetics as treatment for POI. It is postulated that the identified association between hospital and survival reflects differences in clinician decision making. The study highlights the need for further prospective studies using randomised clinical trials to evaluate accurately the efficacy of prokinetic agents. This report illustrates difficulties with performing retrospective analysis of clinical data to determine the efficacy of treatment regimes.

Tegaserod (HTF 919) stimulates gut motility in normal horses

REASONS FOR PERFORMING STUDY

It has been shown that the selective 5-HT4 receptor agonist tegaserod induces an increase in frequency and amplitude of contractions in isolated muscle preparations of equine ileum and pelvic flexure.

OBJECTIVES

To investigate the effects of tegaserod on gut motility and transit of spheres in normal horses.

METHODS

Six mature Freiberger horses were kept under standardised conditions. Effects of tegaserod (0.02 mg/kg bwt i.v. b.i.d. for 2 days) or vehicle on intestinal transit of barium-filled spheres, defaecation and gut sounds were studied in a cross-over design. Spheres were given via stomach tube prior to the first dosing of tegaserod or vehicle. Faeces were collected every 3 h and spheres eliminated were identified radiologically in the faeces.

RESULTS

Tegaserod significantly accelerated the gastrointestinal (GI) transit time of spheres and increased the frequency of defaecation and scores of gut sounds compared to vehicle. The compound was well tolerated; no effects on behaviour, body temperature, heart rate, respiratory rate and clinical laboratory data were observed.

CONCLUSIONS

Tegaserod efficaciously stimulated motility and accelerate GI transit in healthy horses.

POTENTIAL RELEVANCE

Tegaserod may offer therapeutic potential in horses suffering from impaction or paralytic ileus.

Distribution of mRNA that codes for subtypes of adrenergic receptors in the gastrointestinal tract of dairy cows

OBJECTIVE

To describe the distribution of mRNA that codes for 9 subtypes of adrenergic receptors 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 9 subtypes of adrenergic receptors in the bovine gastrointestinal tract (alpha1A, alpha1B, alpha1D, alpha2AD, alpha2B, alpha2C beta1, beta2, and beta3) 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 mRNA contents of adrenergic receptors in the bovine digestive tract were low (range, 0.00006% to 5.04% of GAPDH). Distribution of receptor subtypes was similar in all tissues, with lowest expression of alpha1D receptors, followed by alpha2B, alpha2C, beta3, alpha1B, alpha1A, beta1, and beta2 in the abomasum, whereas alpha2AD and beta2 in the intestines were highest. In comparison with the intestines, relative concentrations of mRNA for receptors beta2 and beta3 were significantly lower in the abomasum.

CONCLUSIONS AND CLINICAL RELEVANCE

Relative concentrations of mRNA that code for adrenergic receptors differed among receptor subtypes and among locations in the bovine gastrointestinal tract. Comparison of these values established in healthy cattle with results for cows with motility disorders, such as abomasal displacement and cecal dilatation, may lead to improved therapeutic or prophylactic approaches for these diseases.

Alpha2-adrenergic regulation of NO production alters postoperative intestinal smooth muscle dysfunction in rodents

Alpha2-adrenergic receptor activation plays an important role in the development of postoperative ileus. Alpha2-adrenergic receptors also regulate nitric oxide (NO) production by the mononuclear phagocyte system. We have previously shown that intestinal manipulation leads to a significant increase in NO production by infiltrating monocytes within the intestinal muscularis. The purpose of this study was to investigate whether alpha2-adrenergic blockade with yohimbine would alter postsurgical intestinal smooth muscle dysfunction and NO production by infiltrating monocytes and macrophages within the intestinal muscularis. Rats underwent small bowel intestinal manipulation with or without yohimbine. In vivo gastrointestinal transit and in vitro jejunal circular muscle contractility was measured 24 h postoperatively. RT-PCR was used to detect inducible NO synthase (iNOS) expression. NO levels in tissue culture supernatants were measured. Immunohistochemistry was used to localize alpha2-adrenergic receptor expression in the intestinal muscularis. Yohimbine significantly decreased manipulation-induced delay in gastrointestinal transit and reversed the postoperative decrease in intestinal muscle contractility. Intestinal manipulation resulted in significant iNOS mRNA induction in the intestinal muscularis, which was markedly attenuated after yohimbine treatment. Yohimbine also significantly decreased the postoperative increase in NO released into intestinal muscularis tissue culture supernatant. Immunohistochemistry identified alpha2-adrenergic receptors on monocytes recruited postoperatively into the intestinal muscularis. This study demonstrates that alpha2-adrenergic receptor stimulation of the inflamed postoperative intestinal muscularis plays a significant role in aggravating postoperative ileus through an enhanced induction of iNOS mRNA and increased release of NO from manipulated intestinal muscularis.

Survey of Prokinetic Use in Horses with Gastrointestinal Injury

OBJECTIVE

To report prokinetic strategies used to manage horses after gastrointestinal surgery.

DESIGN

Electronic questionnaire.

SAMPLE POPULATION

Diplomates of the American College of Veterinary Surgeons (ACVS) who perform equine intestinal surgery.

PROCEDURE

A survey (21 questions) designed to determine use of prokinetic agents was sent electronically to 112 ACVS Diplomates known to perform equine intestinal surgery. Several clinical scenarios were also described to determine which, if any, prokinetic agent respondents would select.

RESULTS

Responses were obtained from 58 (52%) surgeons from 44 clinics. Selection of prokinetic agent for specific gastrointestinal conditions was relatively uniform whereas there was considerable variation in dose administered. For postoperative ileus (POI) associated with most intestinal lesions, 2% lidocaine was most commonly selected. Other prokinetics in decreasing frequency of use were erythromycin lactobionate, metoclopramide, and cisapride. Prokinetic agents were more commonly administered after small intestine strangulating obstructions and less commonly for large intestinal lesions. No novel agents were identified by respondents.

CONCLUSIONS

Prokinetic drugs are commonly used for the management and/or attenuation of POI in horses, but dosages and routes of administration are variable.

CLINICAL RELEVANCE

Although prokinetics are commonly used for management of POI in horses there is clearly a need for more controlled studies to define efficacious dosing and a need to develop new prokinetic drugs.

Clinical application of prokinetics

Intestinal stasis or ileus is a significant cause of mortality and morbidity in horses and has been attributed to a variety of causes, including loss of intrinsic or extrinsic electrical activity, incoordination of contractile activity from regional stimuli, and dissociation between electrical and mechanical activity. Proposed mechanisms include systemic shock, electrolyte disturbances, persistent luminal distention, ischemia, inflammation, peritonitis, endotoxemia, and anesthesia. Because the cause of ileus is likely multifactorial, a variety of pharmaceutics have been used to target specific causes. Prokinetics are defined as agents that facilitate or enhance the net movement of feed material down the length of the intestinal tract and do not simply produce an uncoordinated increase in local contractile activity. The primary objective of pharmaceutic intervention is to augment the pathways that stimulate motility or attenuate the inhibitory neurons that predominantly suppress activity. The objective of this article is to summarize the actions of prokinetic agents available and suggest clinical applications.

Prevention and treatment of gastrointestinal adhesions

As with many aspects of clinical medicine, there is yet to be a single or definitive cure for postoperative adhesion formation. Current methods of prevention target risk factors predisposing horses to adhesion formation. Systemic pharmacologic therapies, such as antimicrobials, nonsteroidal anti-inflammatory drugs, Salmonella antiserum, and hyperimmune plasma, help to reduce abdominal inflammation and minimize the effects of endotoxemia. Intra-abdominal or systemic heparin aids in enhancing peritoneal fibrinolysis. Prokinetic therapy promotes early postoperative return of intestinal motility, minimizing the propensity for adhesion formation between apposing adynamic segments of intestine. Mechanical separation of potentially adhesiogenic serosal and peritoneal surfaces is commonly achieved with use of abdominal lavage, protective coating solutions, and barrier membranes. Ongoing and future research is directed toward a better understanding of the local effects of intestinal trauma and the corresponding response of the fibrinolytic system. Recognition of horses at high risk for adhesion formation helps to guide the equine surgeon to an appropriate perioperative and intraoperative plan for adhesion prevention, including good surgical technique and a combination of adjunct therapies.

Binding of radiolabeled porcine motilin and erythromycin lactobionate to smooth muscle membranes in various segments of the equine gastrointestinal tract

OBJECTIVE

To identify and characterize motilin receptors in equine duodenum, jejunum, cecum, and large colon and to determine whether erythromycin lactobionate competes with porcine motilin for binding to these receptors.

SAMPLE POPULATION

Specimens of various segments of the intestinal tracts of 4 adult horses euthanatized for reasons unrelated to gastrointestinal tract disease.

PROCEDURE

Cellular membranes were prepared from smooth muscle tissues of the duodenum, jejunum, pelvic flexure, and cecum. Affinity and distribution of motilin binding on membrane preparations were determined by use of 125I-labeled synthetic porcine motilin. Displacement studies were used to investigate competition between 125I-labeled synthetic porcine motilin and erythromycin lactobionate for binding to motilin receptors in various segments of bowel.

RESULTS

Affinity of 125I-labeled synthetic porcine motilin for the equine motilin receptor was estimated to be 6.1nM. A significantly higher number of motilin receptors was found in the duodenum than in the pelvic flexure and cecum. The jejunum had a significantly higher number of motilin receptors than the cecum. Erythromycin lactobionate displacement of 125I-labeled porcine motilin from the equine motilin receptor did not differ significantly among various segments of bowel.

CONCLUSIONS AND CLINICAL RELEVANCE

Motilin receptors were found in the duodenum, jejunum, pelvic flexure, and cecum of horses. The highest number of motilin receptors was in the duodenum, and it decreased in more distal segments of bowel. Erythromycin lactobionate competed with motilin binding in the equine gastrointestinal tract. This suggests that 1 of the prokinetic actions of erythromycin in horses is likely to be secondary to binding on motilin receptors.

The effects of xylazine, detomidine, acepromazine and butorphanol on equine solid phase gastric emptying rate

The aim of this study was to measure the effects of specific commonly used sedative protocols on equine solid phase gastric emptying rate, using the 13C-octanoic acid breath test (13C-OABT). The gastric emptying of a standard 13C-labelled test meal was measured once weekly in 8 mature horses over two 4 week treatment periods. Each horse acted as its own control. In treatment Period 1, saline (2 ml i.v.), xylazine (0.5 mg/kg i.v.), detomidine (0.01 mg/kg i.v.) or detomidine/butorphanol combination (0.01/0.02 mg/kg i.v.) was administered in randomised order after ingestion of the test meal. During treatment Period 2, test meal consumption was followed by saline, xylazine (1.0 mg/kg i.v.), or detomidine (0.03 mg/kg i.v.) administration, or preceded by acepromazine (0.05 mg/kg i.m.) in randomised order. The 13C:12C ratio of sequential expiratory breath samples was determined by isotope ratio mass spectrometry, and used to measure the gastric half-emptying time, t 1/2, and duration of the lag phase, t lag, for each of the 64 tests. In treatment Period 1, detomidine/butorphanol prolonged both t 1/2 and t lag with respect to xylazine 0.5 mg/kg and the saline control (P < 0.05). In Period 2, detomidine 0.03 mg/kg delayed each parameter with respect to saline, acepromazine and xylazine 1.0 mg/kg (P < 0.001). Xylazine 1.0 mg/kg also lengthened t lag relative to the saline control (P = 0.0004), but did not cause a significant change in t 1/2. Comparison of treatment periods showed that the inhibitory effect of detomidine on gastric emptying rate was dose related (P<0.05). These findings may have clinical significance for case selection when these agents are used for purposes of sedation and/or analgesia.

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.

Effects of intestinal ischemia on in vitro activity of adjacent jejunum in samples obtained from ponies

OBJECTIVE

To determine whether intestinal ischemia would alter activity of the jejunum in vitro or alter staining characteristics for certain types of enteric neurotransmitters.

SAMPLE POPULATION

Jejunal samples obtained from 10 ponies.

PROCEDURE

Jejunal samples were obtained from locations proximal and distal to an area of small intestine made ischemic for 60 minutes. A portion of each sample was stained to detect substance P-like immunoreactivity, cholinergic and adrenergic neurons, and nitric oxide synthase. Portions of the remaining samples were suspended in muscle baths. General activity patterns (frequency and amplitude of contraction), responses to neuronal depolarization induced by electrical field stimulation (EFS), and responses to 1 microM norepinephrine (NE) were compared with responses of a normal section of small intestine obtained prior to ischemic insult.

RESULTS

Staining patterns were not altered. Proximal and distal sections had evidence of decreased contractility, compared with the normal section. Contraction frequency also was decreased, and distal sections had lower contraction frequency than proximal sections. Relaxation responses were decreased in distal sections. Responses to NE differed significantly for distal and proximal sections, compared with normal sections.

CONCLUSIONS AND CLINICAL RELEVANCE

Short-term ischemia can significantly affect adjacent bowel. Contractile and relaxation responses are impaired. Discrepancies in intestinal motility patterns and alterations in response to NE for sections proximal and distal to ischemic intestine could lead to clinical ileus or slowed transit of ingesta.

Effect of romifidine on gastrointestinal motility, assessed by transrectal ultrasonography

A technique of transrectal ultrasonography was developed to investigate the effects of romifidine 80 and 120 microg/kg bwt on intestinal motility in the horse. Motility of the small intestine, caecum and left ventral colon were assessed following injection of romifidine and a saline control, using a blinded, cross-over study design in 6 horses. Measurements were taken at 15, 30, 60, 120, 180 and 240 min after drug administration. There was a slight nonsignificant decrease in motility in the control group over the 4 h study period. Both doses of romifidine produced a marked decrease in gastrointestinal motility and were associated with the presence of reduced (nonpropulsive) contractions. Transrectal ultrasonography proved suitable for monitoring changes in the type and frequency of intestinal motility in the horse.

Characterization of alpha-adrenoceptor subtypes in smooth muscle of equine ileum

OBJECTIVE

To determine the concentration and binding characteristics of alpha-adrenoceptor subtypes in smooth muscle cell membranes of equine ileum.

SAMPLE POPULATION

Segments of longitudinal and circular smooth muscle from the ileum of 8 male and 8 female adult horses.

PROCEDURE

Distribution of alpha-adrenoceptor subtypes was assessed by use of radioligand binding assays incorporating [3H]-prazosin and [3H]-rauwolscine, highly selective alpha1- and alpha2-adrenoceptor antagonists, respectively. Characterization of adrenoceptor subtypes was performed by use of binding inhibition assays.

RESULTS

On the basis of binding affinity for specific radioligands, low- and high-affinity alpha1- and alpha2-adrenoceptors were detected. Concentration of low-affinity alpha2-adrenoceptors was significantly greater in male horses, compared with females. Competition studies confirmed the specificity of the radioligands used in the binding assays. Alpha1-adrenoceptors of both subtypes in male and female horses had a higher affinity for prazosin than phentolamine, whereas yohimbine did not compete with the radioligand for binding. For alpha2-adrenoceptors regardless of subtype, potency of inhibition elicited by each drug varied between sexes. In males, yohimbine was a more potent inhibitor than phentolamine, which was more potent than prazosin. In females, yohimbine was more potent than prazosin, which was more potent than phentolamine.

CONCLUSIONS AND CLINICAL RELEVANCE

High- and low-affinity alpha1- and alpha2-adrenoceptors were detected in smooth muscle of equine ileum. Because alpha-adrenoceptor subtypes, particularly alpha2-adrenoceptors, are involved in the regulation of gastrointestinal tract function, characterization of these receptors may represent the basis for development of new therapeutic strategies for the control of gastrointestinal disturbances in horses.

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

OBJECTIVE

To determine effects of cisapride and 5-hydroxytryptamine (5-HT) on the jejunum of horses.

SAMPLE POPULATION

Jejunal muscle strips from 8 horses.

PROCEDURE

Muscle strips were suspended in isolated muscle baths. Isometric stress responses to 5-HT and cisapride, with and without specific antagonists, were determined.

RESULTS

Muscle strips incubated with atropine and tetrodotoxin responded to 5-HT and cisapride with an increase in contractile force. The 5-HT caused a concentration-dependent increase in contractile amplitude, with a maximum response (Emax) of 1,151+/-214 g/cm2 and a molar concentration that induces contractile force equal to 50% of maximum response (EC50) of 0.028+/-0.002 microM. Prior incubation with the 5-HT2 antagonist ketanserin decreased the Emax (626 +/-147 g/cm2) and potency (EC50, 0.307+/-0.105 microM) of 5-HT Prior incubation with the 5-HT3 antagonist tropisetron decreased the efficacy (Emax, 894+/-184 g/cm2) to 5-HT Cisapride also caused a concentration-dependent increase in contractile amplitude, with an Emax of 331+/-82 g/cm2 and an EC50 of 0.302+/-0.122 microM. Prior incubation with ketanserin decreased the Emax (55+/-17 g/cm2) and potency (EC50, 0.520+/-0.274 microM) of cisapride.

CONCLUSION AND CLINICAL RELEVANCE

Stimulatory effects of 5-HT and cisapride on circular smooth muscle of equine jejunum are mediated primarily through a noncholinergic effect. The effects of 5-HT are mediated, at least partially, by 5-HT2 and 5-HT3 receptors, whereas the effects of cisapride are mediated primarily by 5-HT2 receptors. This may impact treatment of horses with postoperative ileus.

In vitro effects of erythromycin, lidocaine, and metoclopramide on smooth muscle from the pyloric antrum, proximal portion of the duodenum, and middle portion of the jejunum of horses

OBJECTIVE

To evaluate effects of erythromycin, lidocaine, and metoclopramide on smooth muscle of the pyloric antrum (PA), proximal portion of the duodenum (PD), and middle portion of the jejunum (MJ) of horses. Sample Population-Strips of smooth muscle from 7 horses.

PROCEDURE

Isolated muscle strips were suspended in a bath and attached to isometric force transducers. Once stable spontaneous contractions were observed, agents were added. Isometric stress responses were compared with the amplitude of spontaneous contractions.

RESULTS

A single dose of erythromycin to the PA increased contractile amplitude (CA) for the longitudinal smooth muscle (mean +/- SEM, 76+/-16 g/cm2) but decreased CA for circular smooth muscle (-79+/-23 g/cm2). The inhibitory effect was decreased by tetrodotoxin, N(G)-nitro-L-arginine methyl ester, and a vasoactive intestinal peptide antagonist. Erythromycin increased CA for the MJ, which was maximal at 10(-4)M (171+/-36 g/cm2). Lidocaine increased CA for the PD, which was maximal at 10(-4) M (60+/-5 g/cm2). Metoclopramide increased the CA, which was maximal at 10(-4) M for the PA (75+/-26 g/cm2), PD (279+/-33 g/cm2), and MJ (456+/-59 g/cm2).

CONCLUSIONS

Regional differences in responses to erythromycin, lidocaine, and metoclopramide were evident in the gastrointestinal tract of horses. Metoclopramide increased CA in all tissues used, whereas erythromycin inhibited CA in circular smooth muscle but stimulated CA in longitudinal smooth muscle from the PA. Inhibition is caused by stimulation of inhibitory nerves and is mediated, in part, by nitric oxide and vasoactive intestinal peptide.

Intracellular microelectrode recording to characterize inhibitory neuromuscular transmission in jejunum of horses

OBJECTIVE

To evaluate electrical activity of jejunal circular muscle in horses and characterize electrical responses to stimulation by intrinsic inhibitory neurons.

SAMPLE POPULATION

Portions of jejunum obtained from horses euthanatized for reasons other than gastrointestinal tract disease.

PROCEDURE

Isolated circular muscle preparations were perfused with oxygenated modified Krebs solution. Glass microelectrodes were used for intracellular recording of membrane potentials from single smooth muscle cells. Electrical activity and responses to electrical field stimulation (EFS) of intrinsic neurons in the presence of guanethidine and atropine were recorded. Mediators of responses to nerve stimulation were also evaluated, using N-nitro-L-arginine methyl ester (L-NAME) and apamin.

RESULTS

Mean resting membrane potential (RMP) was 41.5+/-1.8 mV. Small membrane potential oscillations were observed in muscle cells. Single or multiple action potentials were often superimposed on the peaks of these oscillations. Spontaneous oscillations and action potentials were blocked by nifedipine. Transient hyperpolarizations of smooth muscle cell membrane potentials (inhibitory junction potentials [IJP]) were observed in response to electrical field stimulation. The IJP evoked by stimulus trains consisted of an initial fast component followed by a slow component. The L-NAME did not have a significant effect on RMP and did not significantly affect the fast component of IJP at any stimulus frequency tested. In contrast, L-NAME abolished the slow component of IJP observed after trains of pulses. In the continued presence of L-NAME, apamin had no significant effect on RMP but effectively reduced the fast component of IJP.

CONCLUSIONS AND CLINICAL RELEVANCE

Findings suggest that inhibitory neurotransmitters supplying equine jejunum act through different ionic mechanisms. Understanding these mechanisms may suggest new therapeutic targets for treatment of motility disorders.