Evaluation of nitric oxide as an inhibitory neurotransmitter in the equine ventral colon

OBJECTIVE: To determine the role of nitric oxide and an apamin-sensitive nonadrenergic noncholingeric inhibitory transmitter on contractility of the ventral colon of horses. SAMPLE POPULATION: Strips of the circular and longitudinal muscle layers and taenia of the ventral colon from 14 horses. PROCEDURE: Muscle strips were suspended in tissue baths and attached to force transducers. Contractile activity of circular, longitudinal, and taenia muscle strips in response to electrical field stimulation was measured after addition of apamin and a nitric oxide inhibitor, N-nitro-L-arginine methyl ester (L-NAME). RESULTS: Electrical field stimulation reduced contractile activity in the circular muscle layer and taenia but not the longitudinal muscle layer. Addition of L-NAME significantly reduced inhibitory contractile activity at all frequencies for the circular muscle layer, whereas a significant effect was evident for the taenia only at the highest frequency. The combination of L-NAME and apamin resulted in a significant reduction in inhibition of the taenia at all frequencies but for circular muscle only at lower frequencies. CONCLUSIONS AND CLINICAL RELEVANCE: Nitric oxide and an apamin-sensitive neurotransmitter appear to mediate a component of inhibitory transmission in the circular muscle and taenia, but not the longitudinal muscle layer, of the equine ventral colon. Nitric oxide has a role in regulating contractile activity of the equine ventral colon, and nitric oxide synthase inhibitors may be useful in horses with ileus of the large colon.     

In vitro investigation of the effect of prostaglandins and nonsteroidal anti-inflammatory drugs on contractile activity of the equine smooth muscle of the dorsal colon, ventral colon, and pelvic flexure

OBJECTIVES: To determine the in vitro effect of prostaglandin E2 (PGE2), PGF2alpha, PGI2; and nonsteroidal anti-inflammatory drugs (NSAID; ie, flunixin meglumine, ketoprofen, carprofen, and phenylbutazone) on contractile activity of the equine dorsal colon, ventral colon, and pelvic flexure circular and longitudinal smooth muscle. ANIMALS: 26 healthy horses. PROCEDURE: Tissue collected from the ventral colon, dorsal colon, and pelvic flexure was cut into strips and mounted in a tissue bath system where contractile strength was determined. Incremental doses of PGE2, PGF2alpha,, PGI2, flunixin meglumine, carprofen, ketoprofen, and phenylbutazone were added to the baths, and the contractile activity was recorded for each location and orientation of smooth muscle. RESULTS: In substance P-stimulated tissues, PGE2 and PGF2alpha enhanced contractility in the longitudinal smooth muscle with a decrease or no effect on circular smooth muscle activity. Prostaglandin I2 inhibited the circular smooth muscle response with no effect on the longitudinal muscle. The activity of NSAID was predominantly inhibitory regardless of location or muscle orientation. CONCLUSIONS AND CLINICAL RELEVANCE: In the equine large intestine, exogenous prostaglandins had a variable effect on contractile activity, depending on the location in the colon and orientation of the smooth muscle. The administration of NSAID inhibited contractility, with flunixin meglumine generally inducing the most profound inhibition relative to the other NSAID evaluated in substance P-stimulated smooth muscle of the large intestine. The results of this study indicate that prolonged use of NSAID may potentially predispose horses to develop gastrointestinal tract stasis and subsequent impaction.     

In vitro investigation of the effects of cyclooxygenase-2 inhibitors on contractile activity of the equine dorsal and ventral colon

OBJECTIVE: To evaluate the effect of 2 cyclooxygenase (COX)-2 inhibitors on contractile activity of the circular smooth muscle layer of the equine dorsal and ventral colon. SAMPLE POPULATION: Samples of the dorsal and ventral colon obtained from 10 healthy horses. PROCEDURE: Full-thickness tissue samples were collected from the dorsal colon in the area of the diaphragmatic flexure and the ventral colon in the area of the sternal flexure. Samples were cut into strips oriented along the fibers of the circular muscle layer and mounted in a tissue bath system for determination of contractile strength. Incremental amounts of etodolac, nabumetone, and indomethacin were added, and contractile activity was recorded. RESULTS: Response of the dorsal and ventral colon to nonsteroidal anti-inflammatory drugs (NSAIDs) was variable. Indomethacin induced the greatest reduction in contractile activity, followed by nabumetone. For etodolac, the difference from baseline values was only significantly reduced at the highest concentration used (1 X 10(5)M) for the ventral colon. CONCLUSIONS AND CLINICAL RELEVANCE: The NSAIDs that are designed to target the COX-2 isoform appeared to have variable effects on the contractile activity of the equine dorsal and ventral colon. Etodolac appeared to have the least effect on contractile activity, compared with the effects attributable to nabumetone, and would potentially have the fewest adverse effects relative to motility of the dorsal and ventral colon.     

Evaluation of substance P as a neurotransmitter in equine jejunum

OBJECTIVE: To determine whether substance P (SP) functions as a neurotransmitter in equine jejunum. SAMPLE POPULATION: Samples of jejunum obtained from horses that did not have lesions in the gastrointestinal tract. PROCEDURE: Jejunal smooth muscle strips, oriented in the plane of the circular or longitudinal muscle, were suspended isometrically in muscle baths. Neurotransmitter release was induced by electrical field stimulation (EFS) delivered at 2 intensities (30 and 70 V) and various frequencies on muscle strips that were maintained at low tension or were under contraction. A neurokinin-1 receptor blocker (CP-96,345) was added to baths prior to EFS to interrupt SP neurotransmission. Additionally, direct effects of SP on muscle strips were evaluated, and SP-like immunoreactivity was localized in intestinal tissues, using indirect immunofluorescence testing. RESULTS: Substance P contracted circularly and longitudinally oriented muscle strips. Prior treatment with CP-96,345 altered muscle responses to SP and EFS, suggesting that SP was released from depolarized myenteric neurons. Depending on orientation of muscle strips and stimulation variables used, CP-96,345 increased or decreased the contractile response to EFS. Substance P-like immunoreactivity was detected in the myenteric plexus and circular muscle layers. CONCLUSIONS AND CLINICAL RELEVANCE: Substance P appears to function as a neurotransmitter in equine jejunum. It apparently modulates smooth muscle contractility, depending on preexisting conditions. Effects of SP may be altered in some forms of intestinal dysfunction. Altering SP neurotransmission in the jejunum may provide a therapeutic option for motility disorders of horses that are unresponsive to adrenergic and cholinergic drugs.     

Adrenergic, cholinergic, and nonadrenergic-noncholinergic intrinsic innervation of the jejunum in horses.

OBJECTIVE: To determine the major neurotransmitters that regulate contractile activity in the jejunum of horses. SAMPLE POPULATION: Jejunal specimens from 65 horses without gastrointestinal tract lesions. PROCEDURE: Jejunal smooth muscle strips, oriented in the plane of the circular or longitudinal muscular layer, were suspended isometrically in muscle baths. Neurotransmitter release was induced by electrical field stimulation (EFS) delivered at 30 and 70 V intensities and at various frequencies on muscle strips maintained at low or high muscle tone. To detect residual nonadrenergic-noncholinergic neurotransmission, the response of muscle to EFS in the presence of adrenergic and cholinergic blockade was compared with the response in the presence of tetrodotoxin. RESULTS: Atropine (ATR) decreased the contractile response of muscle strips to EFS under most conditions. However, ATR increased the contractile response of high-tone circular muscle. Adrenergic blockade generally increased the muscle responses to 30 V EFS and in high-tone longitudinal muscle but decreased contractile responses in high-tone circular muscle. Tetrodotoxin significantly altered the responses to EFS, compared with adrenergic and cholinergic receptor blockade. CONCLUSIONS: Acetylcholine and norepinephrine appear to be important neurotransmitters regulating smooth muscle contractility in the equine jejunum. They induce contraction and relaxation, respectively, in most muscle preparations, although they may cause opposite effects under certain conditions. In addition, nonadrenergic-noncholinergic excitatory and inhibitory influences were detected. CLINICAL RELEVANCE: Acetylcholine or norepinephrine release within the myenteric plexus of horses may alter gastrointestinal motility.     

奶牛输卵管平滑肌前列腺素类受体的分析研究

本研究选用了前列腺素类化合物(Prostaglandin E2（PGE2）、Prostaglandin F2α（PGF2α）、Prostaglandin D2（PGD2)）和受体选择性激动剂（Butaprost、U-46619）,用多道生理信号采集系统测定其对体外分离的奶牛输卵管峡部及壶腹部平滑肌收缩的影响,以此初步揭示受体的种类。结果发现,PGE2和Butaprost均抑制了奶牛输卵管峡部、壶腹部的自发性收缩运动,呈现明显的浓度依存性舒张反应;PGF2α在高浓度(1～10 μmol/L)下,使峡部和壶腹部的自发性收缩运动有了显著增强;PGD2在低浓度时轻微抑制了壶腹部平滑肌的收缩,但在高浓度(1～10 μmol/L)时加强了峡部和壶腹部平滑肌的收缩;U-46619从低浓度便引起了输卵管峡部和壶腹部平滑肌的强烈收缩。结果表明,EP4、EP2、FP、DP和TP受体同时存在于奶牛输卵管平滑肌上,能与内源性前列腺素类结合并参与调节输卵管平滑肌的自发性收缩。     

Nitrergic inhibition of tachykininergic neuro-muscular transmission via cyclic GMP in the hamster ileum

The present study was designed to explore the inhibitory mechanism by nitric oxide (NO) of the tachykininergic neuro-muscular transmissions in the hamster ileum. In the presence of guanethidine (1 μM), atropine (0.5 μM), nifedipine (0.1 μM) and apamin (100 nM), electrical field stimuli (EFS; 0.5 ms duration, 15 V) evoked non-adrenergic, non-cholinergic excitatory junction potentials (EJPs) in circular smooth muscle cells. The EJPs were markedly inhibited by the tachykinin NK1 receptor antagonists [D-Pro(4), D-Trp(7,9)]-SP(4-11) (3 μM). Both the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME, 200 μM) and the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ, 10 μM), did not affect on the resting membrane potentials, but enhanced the tachykininergic EJPs. In the presence of L-NAME (200 μM), exogenously applied NO (10 μM) and the membrane permeable analogue of guanosine 3',5'-cyclic monophosphate (cGMP), 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP, 3 mM), significantly inhibited the tachykininergic EJPs. Application of EFS (0.5 msec duration, 15 V) with trains of 20 pulses at 20 Hz increased amount of released substance P (SP). The release of SP was further increased by the treatment of L-NAME or ODQ, but markedly reduced by exogenously applied NO and 8-Br-cGMP. These results suggest that the endogenous NO may inhibit the tachykininergic neuro-muscular transmissions by the decrease of SP release from the tachykininergic neurons, possibly through a guanylate cyclase-cGMP-dependent mechanism in the hamster ileum.     

Alteration of nitrergic neuromuscular transmission as a result of acute experimental colitis in rat

Nitric oxide (NO) is a non-adrenergic, non-cholinergic neurotransmitter found in the enteric nervous system that plays a role in a variety of enteropathies, including inflammatory bowel disease. Alteration of nitrergic neurons has been reported to be dependent on the manner by which inflammation is caused. However, this observed alteration has not been reported with acetic acid-induced colitis. Therefore, the purpose of the current study was to investigate changes in nitrergic neuromuscular transmission in experimental colitis in a rat model. Distal colitis was induced by intracolonic administration of 4% acetic acid in the rat. Animals were sacrificed at 4 h and 48 h post-acetic acid treatment. Myeloperoxidase activity was significantly increased in the acetic acid-treated groups. However, the response to 60 mM KCl was not significantly different in the three groups studied. The amplitude of phasic contractions was increased by N^ω-nitro-L-arginine methyl ester (L-NAME) in the normal control group, but not in the acetic acid-treated groups. Spontaneous contractions disappeared during electrical field stimulation (EFS) in normal group. However, for the colitis groups, these contractions initially disappeared, and then reappeared during EFS. Moreover, the observed disappearance was diminished by L-NAME; this suggests that these responses were NO-mediated. In addition, the number of NADPH-diaphorase positive nerve cell bodies, in the myenteric plexus, was not altered in the distal colon; whereas the area of NADPH-diaphorase positive fibers, in the circular muscle layer, was decreased in the acetic acid-treated groups. These results suggest that NO-mediated inhibitory neural input, to the circular muscle, was decreased in the acetic acid-treated groups.     

In vitro investigation of the effects of nonsteroidal anti-inflammatory drugs, prostaglandin E2, and prostaglandin F2alpha on contractile activity of the third compartment of the stomach of llamas

OBJECTIVE: To determine the in vitro effect of prostaglandin (PG) E2, PGF2alpha, and the nonsteroidal anti-inflammatory drugs (NSAIDs) indomethacin, ketoprofen, and nabumetone on the contractile strength of the circular smooth muscle layer of the third compartment of the stomach of llamas. SAMPLE POPULATION: Specimens of the third compartment obtained from 5 healthy adult llamas. PROCEDURE: Full-thickness tissue samples were collected from the third compartment immediately after euthanasia. Specimens were cut into strips oriented along the circular muscle layer and mounted in a tissue bath system. Incremental amounts of ketoprofen, nabumetone, indomethacin, PGE2, and PGF2alpha were added, and contractile strength (amplitude of contractions) was recorded. RESULTS: Generally, PGE2 reduced contractile strength of the circular smooth layer of the third compartment, whereas PGF2alpha, increased the strength of contractions. The activity of the NSAIDs was generally excitatory in a concentration-dependent manner, although significant changes were induced only by administration of indomethacin. CONCLUSIONS AND CLINICAL RELEVANCE: On isolated smooth muscle strips of the third compartment of llamas, exogenous PGE2 and PGF2alpha had a variable effect on contractile strength. Administration of the NSAIDs did not inhibit contractility and would not be likely to induce stasis of the third compartment in the absence of an underlying disease process.     

Mechanical stretch-induced activation of skeletal muscle satellite cells is dependent on nitric oxide production in vitro

Mechanical stretch induces activation of cultured quiescent satellite cells and the activation response is owing to rapid release of hepatocyte growth factor (HGF) from its extracellular association with satellite cells and its subsequent presentation to the c-met receptor. We provide new evidence that the stretch activation is dependent on nitric oxide (NO) production. Stretch activation could be abolished by the addition of N ^G-nitro- L -arginine methyl ester (L-NAME), a competitive inhibitor of NO synthesis, but not by N ^G-nitro- D -arginine methyl ester hydrochloride, a less active enantiomer of L-NAME. Adding HGF to the L-NAME culture restored the activation response, indicating that L-NAME does not directly inhibit satellite cell activation, but acts upstream from the HGF release. In addition, immunoblots of satellite cell lysate revealed the presence of nitric oxide synthase. These experiments suggest that NO is involved in linking mechanical perturbation of satellite cells to chemical signaling responsible for HGF release from its sequestration in vitro .     

Endothelin mediated contraction of equine laminar veins

REASONS FOR PERFORMING STUDY: Endothelin-1 (ET-1) may be a key mediator in the pathogenesis of laminitis, but endothelin-mediated responses in the venous microcirculation of the equine foot have yet to be fully characterised. OBJECTIVES: To characterise the response of equine laminar veins to ET-1 and evaluate the ET-1 receptor subtypes that mediate this response. METHODS: Small veins (150-500 microns) draining the equine digital laminae from healthy horses and ponies subjected to euthanasia at an abattoir were investigated using wire myography. Concentration response curves were constructed for ET-1 in the presence of ETA (BQ123) and ETB (BQ788) receptor antagonists, and L-NAME, a nitric oxide synthase blocker. The selective ETB receptor agonist BQ3020 was investigated alone and following incubation with L-NAME, with or without BQ788. RESULTS: Endothelin-1 contraction of laminar veins was significantly inhibited by BQ123 but not by BQ788. In the presence of L-NAME, sensitivity of laminar veins to ET-1 was enhanced 4-fold, and further addition of BQ788 did not alter this increased sensitivity. BQ3020 induced no venoconstriction; however, in the presence of L-NAME, it caused contraction of veins with approximately 30% of the efficacy of ET-1. The action of BQ3020 in the presence of L-NAME was abolished by BQ788. CONCLUSIONS: Both ETA and ETB receptors are involved in the net tonic response to ET-1 in normal laminar veins. A population of ETB receptors may be present on the vascular endothelium and on smooth muscle of laminar veins, and the action of ET-1 at these 2 sites is likely to be approximately equal and opposite. POTENTIAL RELEVANCE: Our results clarify the function of the ET-1 receptor subtypes in laminar veins from healthy horses. Further study of ET-1 receptors in laminitic horses is therefore warranted.     

The teniae of the equine intestinal tract.

At several locations along the equine cecum and colon, the outer longitudinal portion of the tunica muscularis is gathered into discrete bands of smooth muscle and connective tissue called "teniae". In this study, the disposition of the teniae ceci and coli was traced along the equine intestinal tract. It was discovered that, in several instances, arrays of teniae converge toward the valves and sphincters which separate the various intestinal compartments. The teniae may also provide support for and directionality to, peristaltic contraction waves. The tissue proportions of the teniae vary in different locations. The tenia libera lateralis of the ventral colon is rich in elastic connective tissue, while that of the right dorsal colon is primarily composed of smooth muscle. This may reflect the different responsibilities of these two compartments. The teniae are innervated and their smooth muscle cells are joined by many gap junctions. The connective tissue constituents afford intestinal support while yielding to intestinal distension. The smooth muscle and neural elements may foster active tenial participation in peristalsis. This premise must be tested by electrophysiological experimentation. Further experimentation is also necessary to ascertain whether injury to the teniae might predispose a horse to colic.     

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.     

Effects of xylazine on canine coronary artery vascular rings

OBJECTIVE: To determine the effects of xylazine on canine coronary artery smooth muscle tone. SAMPLE POPULATION: Hearts of 26 healthy dogs. PROCEDURE: Dogs were anesthetized with pentobarbital, and vascular rings of various diameters were prepared from the epicardial coronary arteries. Vascular rings were placed in tissue baths to which xylazine was added (cumulative concentrations ranging from 10(-10) to 10(-4) M), and changes in vascular ring tension were continuously recorded. Effects of the nitric oxide inhibitor NG-nitro-L-arginine methyl ester (L NAME; 5 mM), the alpha1-adrenoceptor antagonist prazosin (10 mM), and the alpha2-adrenoceptor antagonist atipamezole (10 mM) on xylazine-induced changes in vascular ring tension were determined. Results were expressed as percentage of maximal contraction for each vascular ring preparation. RESULTS: Xylazine induced vasoconstriction of small (< 500-microm-diameter) and medium (500- to 1,000-microm-diameter) vascular rings but not of large (> 1,000-microm-diameter) rings. For large vascular rings, L-NAME, atipamezole, and prazosin did not significantly affect the contractile response to xylazine. For small vascular rings, the contractile response following addition of xylazine to rings treated with L-NAME was not significantly different from the contractile response following addition of xylazine to control rings, except at a xylazine concentration of 10(-6) M. Xylazine-induced vasoconstriction of small vascular rings was blocked by atipamezole, but the addition of prazosin had no effect on xylazine-induced vasoconstriction. CONCLUSIONS AND CLINICAL RELEVANCE: Results suggest that xylazine increases smooth muscle tone of small canine coronary arteriesand that this effect is predominantly mediated by stimulation of alpha2adrenoceptors.     

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.     

Cisapride Stimulates Contraction of Idiopathic Megacolonic Smooth Muscle in Cats

We have previously shown that cisapride, a substituted piperidinyl benzamide, stimulates contraction of healthy feline colonic smooth muscle. The purpose of the present investigation was to determine the effect of cisapride on feline idiopathic megacolonic smooth muscle function. Longitudinal smooth muscle strips from ascending and descending colon were obtained from cats with idiopathic megacolon, suspended in a 1.5 mM Ca²⁺-HEPES buffer solution (37°C, 100% O₂, pH 7.4), attached to isometric force transducers, and stretched to optimal muscle length (L₀). Control responses were obtained at each muscle site with acetylcholine (10–⁸ to 10 ⁴ M), substance P (10–¹¹ to 10^-7 M), or potassium chloride (10 to 80 mM). Muscles were then stimulated with cumulative (10–⁹ to 10–⁶ M) doses of cisapride in the absence or presence of tetrodotoxin (10–⁶ M) and atropine (10–⁶ M), or in a 0 calcium HEPES buffer solution. In cats with idiopathic megacolon, cisapride stimulated contractions of longitudinal smooth muscle from both the ascending and the descending colon. Cisapride-induced contractions were similar in magnitude to those induced by substance P and acetylcholine in the ascending colon, but were less than those observed in the descending colon. Cisapride-induced contractions in megacolonic smooth muscle were only partially inhibited by tetrodotoxin and atropine, but were virtually abolished by removal of extracellular calcium. We concluded that cisapride-induced contractions of feline megacolonic smooth muscle are largely smooth muscle mediated and dependent on influx of extracellular calcium. Cisapride-induced contractions in megacolonic smooth muscle are only partially dependent on enteric cholinergic nerves. Thus, cisapride may be useful in the treatment of cats with idiopathic megacolon.     

Pharmacological modulation of postprandial colonic motor activity in the pony

The contractile activity of the equine large intestine exhibited a biphasic response to feeding: enhancement of migrating complexes passing along the colon and an increase of 50% in cyclic variations in smooth muscle at intervals of 20 min on the left ventral colon for a period of 5 to 7 h postfeeding. The cholinergic agonist, bethanechol (50 micrograms/kg subcutaneously), induced both the migrating complexes and the cyclic variations at intervals of 10-15 min. In contrast, the intra-arterial infusion of PGF2 alpha (3 micrograms/kg/min) increased the contractile activity during infusion, but without inducing distinct patterns of activity. Atropine but not indomethacin or flunixin pre-treatment prevented the effects of postprandial, cholinergic and PGF2 alpha stimulation of colonic motility, suggesting that the gastrocolonic reflex involved mainly cholinergic stimulation of the caecum and replicated colon, including the prostaglandin F2 alpha excitatory effects.     

Role of Endothelium and Nitric Oxide in the Response of Equine Colonic Arterial Rings to Vasoconstrictor Agents

Objective- To determine the in vitro contractile responses of equine colonic arteries to angiotensin II, histamine, serotonin, norepinephrine, prostaglandin F_2α, vasopressin, and a thromboxane-B₂-analogue. Study Design- The tension generated in colonic arterial rings placed in organ baths with oxygenated Tyrode's solution at 37°C after exposure to the previously mentioned chemical agents was measured using force-transducers interfaced with a polygraph. Sample Population- Large colon arterial rings collected from eight horses. Methods- The rings were allowed to equilibrate for 45 minutes after applying 2 g tension. Bath solution was replaced and tension reapplied at 15-minute intervals. Cumulative-concentration-responses were determined for concentrations ranging from 10^-8M to 10^-4M on three vessel groups namely endothelium intact, endothelium denuded, and L-NAME treated. The maximal response for each vessel was considered as 100%; responses to lower concentrations were calculated as a percentage of the maximum. The EC₅₀ value was determined for each concentration-response relationship of each agent. Results- Vessels with denuded endothelium or those incubated with L-NAME had greater contractile responses. Angiotensin, histamine, serotonin, and norepinephrine produced greater maximal responses than the other agents. Endothelium denuded rings had lower EC₅₀ values. Responses to norepinephrine and serotonin were affected less by denudation. Conclusion- Endothelium plays an important role in modulating responses of colonic arterial rings to contractile agents. Endothelium-derived vasodilators, other than nitric oxide, may modulate contractile responses of equine colonic arteries. Clinical Relevance- Endothelial damage associated with colonic vovulus may be a major factor for sustained reduced perfusion after surgical correction.     

Role of nitric oxide in regulating the ductus arteriosus caliber in fetal rats

The role of nitric oxide (NO) on the ductus arteriosus (DA) patency was examined in fetal rats at various stages of gestation. N(G)-nitro-L-arginine methyl ester (L-NAME, 50 mg/kg, i.p.), an NO synthase (NOS) inhibitor, or indomethacin (3 mg/kg, p.o.), a cyclooxygenase inhibitor, was administered at 3 hr before cesarean section to pregnant rats ranging from day 17 to day 21 of gestation. Dams were decapitated and the fetuses were obtained by cesarean section. The fetuses were rapidly frozen in an acetone-dry ice mixture. Using rapid-freezing and shaving methods, the calibers of the DA and pulmonary artery were measured. The constrictive effect of L-NAME on the fetal DA caliber was stronger than that of indomethacin in 19-day-old and immature fetuses. In near-term fetuses, the constrictive effects of L-NAME were reduced, while indomethacin caused marked DA constriction. We conclude that endogenous NO may play a major role in regulating the patency of the DA in earlier fetal stages, while dilator prostaglandins may play a greater role in regulating the ductal patency in the near-term fetus.