Search

Background Hyperinsulinemia is associated with equine laminitis, and digital lamellar inflammation in equine metabolic syndrome‐associated laminitis (EMSAL) is modest when compared with sepsis‐associated laminitis. Objectives To characterize digital lamellar inflammation in horses in a euglycemic‐hyperinsulinemic clamp (EHC) model of laminitis. Animals Sixteen healthy adult Standardbred horses. Methods Prospective experimental study. Horses underwent EHC or saline infusion (CON) for 48 hours or until the onset of Obel grade 1 laminitis. Horses were euthanized, and digital lamellar tissue was collected and analyzed via polymerase chain reaction (pro‐inflammatory cytokine and chemokine genes—CXCL1, CXCL6, CXCL8, IL‐6, MCP‐1, MCP‐2, IL‐1β, IL11, cyclooxygenases 1 and 2, tumor necrosis factor alpha [TNF‐α], E‐selectin, and ICAM‐1), immunoblotting (phosphorylated and total signal transducer and activator of transcription 1 [STAT1], STAT3, and p38MAPK), and immunohistochemistry (markers of leukocyte infiltration: CD163, MAC387). Results Lamellar mRNA concentrations of IL‐1β, IL‐6, IL‐11, COX‐2, and E‐selectin were increased; the concentration of COX‐1 was decreased; and concentrations of CXCL1, CXCL6, MCP‐1, MCP‐2, IL‐8, TNF‐α and ICAM‐1 were not significantly different in the EHC group compared to the CON group (P ≤ .003). Lamellar concentrations of phosphorylated STAT proteins (P‐STAT1 [S727], P‐STAT1 [Y701], P‐STAT3 [S727], and P‐STAT3 [Y705]) were increased in the EHC group compared to the CON group, with phosphorylated STAT3 localizing to nuclei of lamellar basal epithelial cells. There was no change in the lamellar concentration of P‐p38 MAPK (T180/Y182), but the concentration of total p38 MAPK was decreased in the EHC samples. There was no evidence of notable lamellar leukocyte emigration. Conclusions and Clinical Importance These results establish a role for lamellar inflammatory signaling under conditions associated with EMSAL.

Summary An 8-year-old Arabian stallion weighing 361 kg presented to Louisiana State University Veterinary Teaching Hospital with a 3-month history of weight loss, exercise intolerance, long hair coat and recent history of seizures and aimless wandering in the pasture. An initial presumptive diagnosis of pituitary pars intermedia dysfunction (PPID) was made based on clinical signs. The initial examination revealed weight loss and loss of body condition (BCS 3/9), hypertrichosis, muscle wasting and reluctance to move when prompted. A neurological examination revealed dull mentation with no evidence of proprioceptive deficits in the limbs. Mild hyperglycaemia and a stress leucogram were noted on initial biochemical panel and haematology, respectively. Plasma adrenocorticotrophic hormone (ACTH) concentrations before and after thyrotropin releasing hormone (TRH) stimulation were markedly increased. Rapid slice computed tomography (CT) scan of the head before and after contrast revealed a large mass in the region of the pituitary gland suggestive of macroadenoma causing PPID. Prior to imaging, treatment consisted of supportive nursing care. Due to size of the pituitary gland (measuring 4.6 × 4.6 × 3.8 cm) and the presence of seizure-like activity and dull mentation, the stallion was subjected to euthanasia. A necropsy was not performed. Pituitary macroadenomas in horses affected with PPID, who show neurological signs such as seizure-like activity, dull mentation and aimless wandering, might have a poor prognosis and treatment with pergolide mesylate might not reduce pituitary gland size or relieve clinical signs. A CT scan is indicated in horses with neurological signs suspected of PPID to further evaluate pituitary gland size and surrounding structures and rule out other causes to better assess prognosis.

Background Laminitis has a considerable impact on the equine industry. Endocrinopathic laminitis is the most common form and affected horses often have hyperinsulinaemia due to an underlying metabolic disorder. Objectives The aim of this study was to determine if insulin weakens the structural integrity of digital lamellae and to develop an ex vivo model for the study of hyperinsulinaemia-induced lamellar failure. Study design Ex vivo experiment. Methods Biomechanical testing was used to assess the structural integrity of lamellar explants exposed to either medium alone (control) or medium supplemented with insulin. Lamellar explants comprised of hoof wall, lamellar tissue and distal phalanx were harvested from four adult horses with no evidence of inflammatory disease or pre-existing disease of the digit. Following an equilibration period, explants were incubated in medium or medium supplemented with insulin (2.5 μg/ml) for 8 h prior to biomechanical testing to obtain load (N), stress (MPa), elongation to failure (mm), and Young's modulus (MPa) for each explant. Significant differences were assessed using a mixed linear model with horses as a random factor and control or insulin-treated group as a fixed factor. Results Lamellar explants incubated in medium supplemented with insulin failed at significantly lower load (P = 0.0001) and lower stress (P = 0.001) and had greater elongation to failure (P = 0.02). Main limitations In addition to the ex vivo nature of the study, location-dependent variability in explant structural integrity and variable diffusion of nutrients due to explant size may have been limitations. However, the study design attempted to account for these limitations through random assignment of explants to treatment groups independent of location and by evaluating stress to failure. Conclusions Insulin weakens the structural integrity of equine lamellar explants and an ex vivo model for evaluation of hyperinsulinaemia-induced lamellar failure was established. The summary is available in Spanish - see Supporting Information.

Changes in cardiopulmonary function, hematology, serum chemistry and peritoneal fluid were examined in six healthy, pharmacologically restrained, mature horses during and after abdominal insufflation with CO2 (15 mm Hg) during standing laparoscopy. Each horse underwent standing left flank exploratory laparoscopy (LFL) and a sham procedure (SLFL) (cannula placement without insufflation) in a randomized cross-over design with at least 4 weeks between procedures. Heart rate (HR), respiratory rate (RR), cardiac output (thermodilution) (CO), right atrial pressure (RAP), systemic arterial blood pressures (ABP), pulmonary arterial pressure (PAP) and arterial blood gases (ABG) were measured. Cardiac index (CI), systemic vascular resistance (SVR) and pulmonary vascular resistance (PVR) were derived from measured parameters. Baseline measurements were the average of measurements taken every 15 minutes for 60 minutes. Following baseline measurements, samples for complete blood count (CBC), serum chemistry analysis (SC) and peritoneal fluid (PF) analysis were obtained. The horses were then sedated with detomidine (0.02 mg kg−1 IV) followed in 5 minutes by butorphanol (0.02 mg kg−1 IV) and the flank was prepared for laparoscopy. Cardiopulmonary data and blood gas samples were obtained 5 minutes postdetomidine sedation, 10 minutes after butorphanol administration and at 0, 5, 15, 30, 45, and 60 minutes after abdominal insufflation with CO2 (15 mm Hg) (LFL) or instrumentation without insufflation (SLFL). CBC, SC, and PF analyses were repeated at the conclusion of insufflation and desufflation and 24 hours after the conclusion of each procedure. Significant decreases in HR, CO, and CI and significant increases in MRAP, SVR and PVR occurred immediately after detomidine administration. There were no clinically significant effects of abdominal insufflation with CO2 on any measure of cardiopulmonary function, including blood gases. There was a significant increase in peritoneal fluid total nucleated cell count 24 hours following LFL. Abdominal insufflation with CO2 (15 mm Hg) for 1 hour during the standing laparoscopy does not cause significant alterations in cardiopulmonary function, but does cause a mild inflammatory response within the peritoneal cavity.

Laminitis is an extremely painful condition resulting in damage to the soft tissues anchoring the third phalanx to the hoof, which can result in life-threatening debilitation. Specific therapy is not available. The most important principles of therapy include aggressive nutritional and medical management of primary disease processes, cryotherapy, anti-inflammatory therapy, pain management, and biomechanical support. This review focuses on the principles of evidenced-based therapies.

6-week-old, Arabian-cross male foal was referred to the Veterinary Teaching Hospital at the Louisiana State University School of Veterinary Medicine for a 1-week history of progressive weakness and intermittent right thoracic limb lameness. The foal had a normal birth, was healthy, had been active, and had not exhibited any abnormalities of gait before presentation. The current owner had acquired the mare just before foaling. The foal presented in sternal recumbency and was unable to rise without assistance. Physical examination identified a quiet, moderately responsive foal with dull mentation and body weight of 64 kg. Rectal temperature was 101.0°F, heart rate was 64 beats/min, and respiratory rate was 28 breaths/min. Mucous membranes were tacky, there was mild decreased skin turgor, and capillary refill time (CRT) was 2 seconds. There was a grade II/VI holosystolic murmur over the left heart base that was considered functional. Increased bronchovesicular sounds were present bilaterally. Lameness (grade 1/5) was evident in the right thoracic limb with pain on palpation and manipulation of the caudal aspect of the distal phalangeal and elbow joints. The lameness resolved after injection of lidocaine (Lidocaine 2%, VetOne a ) (2,000 U) into the elbow joint. Joint fluid analysis and radiographs of the distal phalangeal and elbow joints were considered normal. Spinal radiographs, evaluated because of rear limb weakness, also were considered normal. Radiographs of the thorax disclosed ventral alveolar opacities, consistent with aspiration pneumonia. The foal was somnolent on neurological examination and in sternal recumbency, but when assisted it/he could stand and walk. The lower lip drooped bilaterally and the palpebral fissures were decreased bilaterally because of upper eyelid ptosis. Normal palpebral reflexes were

Equine recurrent uveitis (ERU) has been linked to leptospirosis in Europe; however, regional differences exist in reports from the United States. The objective of this study was to investigate the role of intraocular leptospiral infections in horses with ERU in the Southern United States. Blood and ocular fluid samples were collected from horses with ERU and normal controls. Leptospira serology was performed using microscopic aggluti- nation test (MAT). Aqueous and vitreous humor samples were obtained and submitted for aerobic and Leptospira culture, polymerase chain reaction (PCR), and MAT. Twenty-one control horses (40 eyes) and 31 ERU horses (46 eyes) were available. Serology was avail- able for 48 of 52 horses: 16 of 21 control and 23 of 27 affected horses were positive for at least one serovar; bratislava was the most common serovar identified. Bacillus sp. and Micrococcus sp. were cultured from one control horse' se ye;Streptococcus sp. (n ¼ 1) and Leptospira (n ¼ 6) were cultured from the eyes of six ERU horses. Leptospira isolates belonged to serogroup pomona (n ¼ 4) and grippotyphosa (n ¼ 2). Polymerase chain re- action results were positive in 14 of 31 (45%) horses with ERU; no control horses were positive by PCR (P ¼ .0001). Microscopic agglutination test was positive for 17 of 24 ERU horses (71%) and one 21 (4.7%) normal horses (P < .0001). Horses with ERU had a high prevalence of Leptospira infection based on PCR and MAT results from intraocular fluids compared with control horses. The diagnosis of intraocular infections was not aided by serology and required specific invasive sampling of ocular fluid. Leptospira infection should be considered as a cause of ERU in the Southern United States.

Hyperinsulinemia leads to insulin resistance of blood vessels and interferes with circulation of laminae of the equine foot. The objective of the study was to compare the digital vessel responses of clinically healthy and laminitic horses with and without experimental induction of insulin resistance. Vessel segments (artery and vein) collected after euthanasia were cut into 3-mm wide rings and were prepared in tissue baths containing Tyrode solution for response studies. Two rings of each vessel type from each horse were used as nonresistant, and two were made insulin resistant. Insulin resistance was induced by incubating the rings with 10-μM bath concentration of insulin. Then, all rings were contracted with phenylephrine (5 μM). When the contraction reached a plateau, insulin (10 μM) was added, and the response monitored for 30 minutes. The response to insulin was calculated as a percentage of the phenylephrine response. The results showed that the responses of the nonresistant rings differed significantly between the groups. However, when all rings were made insulin resistant, the significance between the groups disappeared. In the laminitic group, responses of resistant and nonresistant rings did not differ, whereas in the healthy group, the vessel responses differed significantly. In all experiments, arterial and venous rings followed the same pattern, but the magnitudes were greater in arterial rings. The findings suggest that the vessel responses to insulin are altered in laminitis which could be due to insulin resistance.

Objective—To establish an in vivo method for matrix metalloproteinase (MMP)-2 and MMP-9 induction in horses via IV administration of lipopolysaccharide (LPS) and to evaluate the ability of doxycycline, oxytetracycline, flunixin meglumine, and pentoxifylline to inhibit equine MMP-2 and MMP-9 production. Animals—29 adult horses of various ages and breeds and either sex. Procedures—In part 1, horses received an IV administration of LPS (n = 5) or saline (0.9% NaCl) solution (5). Venous blood samples were collected before and at specified times for 24 hours after infusion. Plasma was harvested and analyzed for MMP-2 and MMP-9 activities via zymography. In part 2, horses received doxycycline (n = 5), oxytetracycline (5), flunixin meglumine (5), or pentoxifylline (4) before and for up to 12 hours after administration of LPS. Plasma was obtained and analyzed, and results were compared with results from the LPS-infused horses of part 1. Results—Administration of LPS significantly increased MMP-2 and MMP-9 activities in the venous circulation of horses. All MMP inhibitors significantly decreased LPS-induced increases in MMP activities but to differing degrees. Pentoxifylline and oxytetracycline appeared to be the most effective MMP-2 and MMP-9 inhibitors, whereas doxycycline and flunixin meglumine were more effective at inhibiting MMP-2 activity than MMP-9 activity. Conclusions and Clinical Relevance—IV administration of LPS to horses caused increased venous plasma activities of MMP-2 and MMP-9. These MMP activities were reduced by pentoxifylline and oxytetracycline, suggesting that further evaluation of these medications for treatment and prevention of MMP-associated diseases in horses is indicated.

Vascular changes have been observed both in the carbohydrate overload (CHO) and black walnut extract (BWE) models of acute sepsis-related laminitis (SRL) laminitis, and therefore it has been suggested that vasoconstriction or dysfunction may occur during the prodromal stage of laminitis, resulting in a decreased blood flow to the lamellar tissues. A number of drugs with vasoactive properties have been used in the treatment of laminitis; these drugs have been used most in SRL, where lamellar ischemia was thought to be the central event resulting in lamellar failure during the early stages of laminitis. Acetylpromazine (acepromazine) is a phenothiazine derivative and is a frequently used tranquilizer in the horse. Phenoxybenzamine is another phenothiazine derivative, which has also been used in the prevention and treatment of acute laminitis. Domperidone is a benzimidazole derivative which exhibits similar receptor antagonist profiles to acepromazine and phenoxybenzamine. Others include pentoxifylline, isoxsuprine hydrochloride, glyceryl trinitrate (GTN) and dimethyl sulfoxide (DMSO).

Summary Reasons for performing study: One of the causes of equine laminitis is hyperinsulinaemia, which may be associated with endothelial dysfunction and insulin resistance of vessels. Hypothesis and objectives: Insulin resistance can be induced in palmar digital vessels by continued exposure to insulin in vitro. The objective was to evaluate this in vitro model for future studies. Methods: Palmar digital vessel segments were collected immediately after euthanasia from horses with normal insulin/glucose blood values. Four arterial and 4 venous rings (3 mm wide) were prepared and each ring mounted in a tissue bath, containing Tyrode's solution at 37°C, 2 g tension was applied and the rings allowed to equilibrate for 45 min. Of the 4 rings of each vessel type, one was used as a control. One each of the remaining 3 rings was used for incubation with insulin (to induce resistance), wortmannin (to block PI3-kinase) and PD-098059 (to block MAP-kinase), respectively, for 30 min. After the incubation period, the rings were contracted with phenylephrine. When the response reached a plateau, a single dose of insulin was added to the baths and the response of each ring monitored for 30 min. Results: Insulin relaxed the control rings and those treated with PD 098059 but contracted those pretreated with insulin and wortmannin. Normal relaxation responses of the rings were converted to contractions by insulin resistance. Insulin resistance was confirmed by the qualitative response of insulin-incubated and wortmannin-incubated rings. Conclusions: This study demonstrated successful induction of insulin resistance in both arterial and venous rings. It also suggested that the MAP-kinase pathway plays a minor role in controlling vasomotor tone under normal physiological conditions. Potential relevance: The study suggests that the induction of insulin resistance in equine palmar digital vessel rings is reliable and provides a good in vitro model for studying the vascular insulin resistance which may occur in equine laminitis.

All cases of laminitis are characterized by failure of the attachment of the epidermal cells of the epidermal laminae to the underlying basement membrane of the dermal laminae despite the diversity of diseases that underlie the syndrome. The preponderance of evidence supports roles for inflammation, metabolic derangement, endothelial and venous dysfunction, and matrix degradation as causes of laminitis. Inflammation, oxidant stress, and matrix degradation may be factors common to each of these mechanisms that lead to the laminar damage of laminitis. The understanding of the pathophysiology and progression of the disease is incomplete, and this limits efforts to prevent and treat this devastating disease successfully. However, scientific investigations are occurring at a phenomenal rate and shedding light on the pathophysiologic events involved with laminitis.

Laminitis is frustrating for veterinarians because current knowledge and understanding of the pathophysiology and progression of the disease are incomplete, limiting efforts to prevent and treat this devastating disease successfully. However, scientific investigations have recently occurred at a phenomenal rate shedding light on the pathophysiologic events involved with laminitis. Development of acute laminitis often follows other primary diseases; therefore, the mechanisms involved in the pathogenesis of laminitis are most likely numerous and interrelated. On the basis of the discussion from the 2007 Havemeyer Meeting, inflammation, metabolic disease, and endothelial and vascular dysfunction are considered pivotal events in the development of laminitis.

To date, there is a substantial amount of data to support the hypotheses that vascular and enzymatic changes are ongoing in experimental laminitis. Furthermore, there is substantial in vitro evidence that the enzymatic changes weaken the dermo-epidermal attachments leading to mechanical failure of the hoof-bone interface of the equine digit. However, investigators of both the vascular and enzymatic theories have, to date, been unable to substantiate the effects of these pathophysiological changes in vivo on laminar tissues of horses afflicted with experimentally induced or naturally acquired laminitis. In addition, the effects of laminitis-inducing treatment have not been prevented or reversed by treatment with an MMP inhibitor or a vasoactive antagonist. It is possible that there is simultaneous activation of the vascular and enzymatic pathways and/or other inflammatory processes. Moreover, the third theory involving mechanical factors cannot be discounted simply because strong evidence for vascular and enzymatic changes exists. It is common for horses with severe musculoskeletal disease affecting weightbearing on a limb to develop laminitis in the contralateral limb. It remains to be determined what factors are responsible for initiation of laminitis in these individuals. Evidence has not been presented that precludes the possibility of coincident occurrence of vascular and enzymatic changes. In fact, many of the inflammatory mediators (e.g. interleukin-1beta) found in laminitic tissues can concurrently stimulate synthesis of vasoactive substances and activate MMPs. Because enzymatic action on proteins is largely dependent on the concentrations of proteins and enzyme, the enzymatic theory is not dependent upon increased delivery of enzymes via increased capillary flow. Likewise, because vascular changes can alter tissue function via increased capillary flow and oedema formation, the vascular theory is not dependent upon decreased capillary flow. It is true that naturally acquired laminitis is widely variable in severity and predisposing diseases. Therefore, most probably there are multiple mechanisms involved in the initiation and propagation of the pathophysiologic cascade(s) and, therefore, successful intervention will necessitate multiple treatment modalities.

Objective— To determine the magnitude and duration of effects of acepromazine administered intramuscularly (IM) on digital and systemic hemodynamic variables in clinically healthy horses. Study Design— Experimental study. Animals— Healthy adult horses (n=12). Methods— An ultrasonic Doppler flow probe was surgically implanted around the medial palmar digital artery before the study. Catheters were inserted in the transverse facial artery, lateral palmar digital artery, and jugular vein. A treatment group (n=6) was administered 0.04 mg/kg body weight of acepromazine IM; control horses (n=6) were administered an equivalent volume of saline IM. Palmar digital blood flow, and digital and facial arterial pressures were measured at baseline and for 6 hours after administration. Venous blood was collected for measurement of packed cell volume (PCV). Results— Horses administered acepromazine had significantly lower facial arterial pressure compared with control horses administered saline. Palmar digital arterial blood flow in acepromazine-treated horses was not significantly different from that in control horses but increased significantly post-administration, compared with the respective baseline values for acepromazine-treated horses. PCV significantly decreased in horses administered acepromazine compared with their respective baseline value. Conclusion— IM acepromazine causes hypotension and increases palmar digital blood flow over time but the magnitude of the effect on digital blood flow was not sufficient to yield differences compared with saline-treated horses. Clinical Relevance— IM acepromazine has a modest effect on palmar digital blood flow, facial arterial pressures and PCV in healthy horses with minimal sedation.

Objective—To quantify changes in endothelium-derived factors and relate those changes to various aspects of digital hemodynamics during the prodromal stages of carbohydrate overload (CHO)-induced laminitis in horses. Animals—20 adult horses without abnormalities of the digit. Procedures—Digital and jugular venous blood samples were collected at 1-hour intervals (for assessment of endothelin-1 [ET-1] immunoreactivity and measurement of glucose, insulin, and nitric oxide [NO] concentrations) or 4-hour intervals (CBC and platelet-neutrophil aggregate assessment) for 8 hours or 16 hours after induction of CHO-associated laminitis in horses treated with an ET-1 antagonist. Effects of treatment, collection site, and time and the random effects of horse on each variable were analyzed by use of a repeated-measures model. Where treatment and collection site had no significant effect, data were combined. Results—Compared with baseline values, CHO resulted in changes in several variables, including a significant increase from baseline in digital blood ET-like immunoreactivity at 11 hours; digital blood ET-like immunoreactivity was significantly greater than that in jugular venous blood at 8, 9, 11, and 12 hours. Digital and jugular venous blood concentrations of glucose increased from baseline significantly at 3, 4, and 5 hours; insulin concentration increased significantly at 5 hours; and the number of platelet-neutrophil aggregates increased significantly at 12 hours. Conclusions and Clinical Relevance—In horses, concurrent increases in venous blood ET-1 immunoreactivity, insulin and glucose concentrations, and platelet-neutrophil aggregates support a role of endothelial dysfunction in the pathogenesis of CHO-induced laminitis.

Objective—To characterize the in vitro response of equine cecal longitudinal smooth muscle (CLSM) to endothelin (ET)-1 and assess the role of ETA and ETB receptors in those ET-1–induced responses. Animals—36 horses without gastrointestinal tract disease. Procedure—To determine cumulative concentrationresponse relationships, CLSM strips were suspended in tissue baths containing graded concentrations of ET-1 (10–9 to 10–6M) with or without BQ-123 (ETA receptor antagonist); with or without IRL-1038 (ETB receptor antagonist); or with both antagonists at concentrations of 10–9, 10–7, and 10–5M. To determine the percentage change in baseline tension of CLSM, the areas under the curve during the 3-minute periods before and after addition of each dose were compared . Also, the effects of ET-1 and a combination of selective ETA and ETB receptor antagonists on electrically evoked contractions were studied. Results—ET-1 caused sustained increases in CLSM tension in a concentration-dependent manner. Contractile responses to ET-1 were not significantly inhibited by either BQ-123 or IRL-1038 alone at any concentration; however, responses were significantly inhibited by exposure to the antagonists together at a concentration of 10–5M. Electrical field stimulation did not change the spontaneous contractile activity of CLSM and did not significantly alter the tissue response to ET-1, BQ-123, or IRL-1038. Conclusions and Clinical Relevance—Results indicated that ET-1 has a contractile effect on equine CLSM that is mediated via ETA and ETB receptors. In vitro spontaneous contractions of equine CLSM apparently originate in the smooth muscle and not the enteric nervous system. (Am J Vet Res 2005;66:1202–1208)

Objective—To determine and compare the number, type, location, and distribution of apoptotic epidermal cells in the laminae of clinically normal horses and horses with laminitis. Sample Population—Formalin-fixed samples of digital lamellar tissue from 47 horses (including clinically normal horses [controls; n = 7], horses with acute [4] and chronic [7] naturally acquired laminitis, and horses with black walnut extract-induced [11] or carbohydrate overload-induced [18] laminitis). Procedure—Blocks of paraffin-embedded lamellar tissues were stained for DNA fragmentation with the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) technique. Differential immunohistochemical staining for caspases 3 and 14 were used to confirm apoptosis. Results—The number of TUNEL-positive epidermal cells per 0.1 mm of primary laminae was significantly greater in the acute laminitis group than in the other groups. In the acute laminitis group, there were 17 and 1,025 times as many TUNEL-positive basal layer cells and keratinocytes, respectively, compared with the control group. Apoptosis of TUNEL-positive basal layer cells was confirmed by results of caspase 3 immunohistochemical staining. The TUNEL-positive keratinocytes did not stain for caspases 3 or 14. Conclusions and Clinical Relevance—The large number of apoptotic basal layer cells detected in the lamellar tissue of horses with acute naturally acquired laminitis suggests that apoptosis may be important in the development of acute laminitis. The role of the large number of TUNEL-positive keratinocytes detected in the interface of primary and secondary epidermal laminae of horses with acute laminitis remains to be elucidated. ( Am J Vet Res 2004;65:578–585)

Objective—To evaluate systemic effects of IV infusion of ATP-MgCl2 subsequent to infusion of a low dose of endotoxin in horses. Animals—12 adult horses. Procedure—Horses were administered endotoxin (lipopolysaccharide [LPS]) or saline (0.9% NaCl) solution, IV, during a 30-minute period. Immediately thereafter, horses in each group were infused IV with ATP-MgCl2 or saline solution. Two weeks later, horses were administered the opposite solution (LPS or saline solution), but it was followed by the same infusion as 2 weeks previously (ie, ATP-MgCl2 or saline solution). Cardiopulmonary and clinicopathologic variables, cytokine activity, and endothelin (ET) concentrations were recorded. Results—IV infusion of ATP-MgCl2 after administration of a low dose of endotoxin failed to attenuate the cardiopulmonary, clinicopathologic, and cytokine alterations that develop secondary to endotoxin exposure. The combination of LPS and ATP-MgCl2 potentiated pulmonary hypertension, leukopenia, and neutropenia when compared with the combination of LPS and saline solution. The combination of LPS and ATP-MgCl2 resulted in thrombocytopenia. Endothelin concentration was increased in jugular venous and pulmonary arterial plasma in horses receiving LPS and ATP-MgCl2. Similar increases were not observed with LPS and saline solution. Conclusions and Clinical Relevance—Administration of ATP-MgCl2 did not protect horses from systemic effects of experimentally induced endotoxemia. Furthermore, the use of ATP-MgCl2 during endotoxemia may worsen the cardiopulmonary and clinicopathologic status of affected horses. Because ATP and other adenine nucleotides are released from cells during shock, their potential role in the development of hemodynamic derangements, leukocyte adherence, and coagulopathies during endotoxemic episodes warrants further investigation. (Am J Vet Res 2004;65: 225–237)

The study objective was to evaluate the effectiveness of active and passive immunotherapy in horses affected with recurrent airway obstruction (RAO), using a newly developed peptide and antibodies to the peptide. This peptide is part of an endogenous protein and has a sequence homology to some common environmental allergens. RAO is a type-I immediate hypersensitivity response involving reaginic antibodies (IgE and IgG), and is a model for human asthma. Our hypothesis is that a protective immune response to asthma can be induced by active vaccination or by passive immunotherapy. Five RAO-affected horses with a history of seasonal RAO, transpulmonary pressure >15 cm of H2O, and a clinical score (CS) of 5 and above were selected. Either of the two doses of the antibody, 0.1 mg/kg or 0.2 mg/kg, was administered subcutaneously to 2 horses each. A third dose 0.3 mg/kg was given to the fifth horse. The peptide (0.1 mg/kg) was used to vaccinate two additional RAO-horses. After closely watching them for 2 days, th...

Objective—To characterize alterations in systemic and local colonic hemodynamic variables associated with IV infusion of ATP-MgCl2 in healthy anesthetized horses. Animals—12 adult horses. Procedure—Six horses were given ATP-MgCl2, IV, beginning at a rate of 0.1 mg of ATP/kg of body weight/min with incremental increases until a rate of 1.0 mg/kg/min was achieved. The remaining 6 horses were given an equivalent volume of saline (0.9% NaCl) solution over the same time period. Colonic and systemic hemodynamic variables and colonic plasma nitric oxide (NO) concentrations were determined before, during, and after infusion. Results—Infusion of ATP-MgCl2 caused a rate-dependent decrease in systemic and colonic vascular resistance, principally via its vasodilatory effects. A rate of 0.3 mg of ATP/kg/min caused a significant decrease in systemic and colonic arterial pressure and colonic vascular resistance without a significant corresponding decrease in colonic arterial blood flow. Consistent alterations in NO concentrations of plasma obtained from colonic vasculature were not detected, despite profound vasodilatation of the colonic arterial vasculature. Conclusions and Clinical Relevance—Results revealed that IV infusion of ATP-MgCl2 may be beneficial in maintaining colonic perfusion in horses with ischemia of the gastrointestinal tract, provided a sufficient pressure gradient exists to maintain blood flow. (Am J Vet Res 2001;62:1240–1249)

We describe the echocardiographic findings in a 4-day-old thoroughbred foal with an aortico-pulmonary septal defect. The foal had labored breathing, cyanotic mucous membranes and a continuous grade 5/6 heart murmur with point of maximal intensity over the base of the heart on the right side. Echocardiographically, there was a large communication between the aorta and the pulmonary artery just dorsal to the base of the heart. The cardiac anomaly seen during the echocardiographic exam was confirmed at necropsy where a large communication between the two great vessels was observed. These findings correlate with previous studies in humans, dogs, and cats. The possible failure in the embryologic development that led to this unusual cardiac anomaly is discussed.

Damage to the intestinal mucosal barrier sustained during the compensated stages of shock may result in bacterial translocation and systemic accumulation of toxins, leading to sepsis and multiple organ failure. Neutrophils are known to be an important component of endotoxin and cytokine-induced tissue injury. The purpose of this study, therefore, was to test the hypothesis that neutrophil infiltration and MPO production accompany endotoxin and TNFα induced intestinal mucosal dysfunction.To evaluate mucosal dysfunction, jejunal blood-to-lumen clearances of chromium 51-labeled ethylenediaminetetraacetate (51Cr-EDTA) were measured in cats administered fetal calf serum (controls), endotoxin, TNFα, or polyclonal anti-TNFα antibody followed by endotoxin. Neutrophil accumulation in the jejunal mucosa was evaluated by histologic enumeration and measurement of jejunal myeloperoxidase (MPO) activity.Endotoxin and TNFα induced mucosal dysfunction in the proximal jejunum is accompanied by neutrophil accumulation in the jejunal mucosa as indicated by histologic enumeration and increased MPO activity. Pretreatment with anti-TNFα polyclonal antibody significantly decreased endotoxin-induced increases in jejunal MPO activity. The results of this study support the contribution of neutrophils to endotoxin and TNFα induced mucosal dysfunction.

Hyperinsulinemia causes laminitis experimentally and is a risk factor for naturally occurring laminitis. The aim of this study was to investigate the effects of insulin on laminar vascular relaxation and to induce insulin-associated vascular dysfunction in vitro. Relaxation responses of isolated laminar arterial and venous rings to acetylcholine and insulin were evaluated. To alter vascular function in response to insulin, all vessel rings were incubated with insulin or vehicle, submaximally contracted, administered insulin again and relaxation responses recorded. Laminar arteries were also incubated with the mitogen-activated protein kinase (MAPK) inhibitor, PD-98059. Relaxation in response to acetylcholine was not different between arteries and veins, but veins relaxed less in response to insulin than arteries. In arteries incubated with insulin, the subsequent relaxation response to insulin was blunted. Veins had minimal relaxation to insulin regardless of incubation. Arteries incubated with PD-98059 relaxed more in response to insulin than arteries not exposed to PD-98059, indicating that MAPK plays a role in maintenance of basal tone in laminar arteries. A differing response of laminar veins and arteries to insulin-induced relaxation may be important in understanding the link between hyperinsulinemia and laminitis. In vitro induction of vascular dysfunction in response to insulin in laminar arteries may be useful for testing therapeutic interventions and for understanding the pathophysiology of laminitis.

The digital laminae is a two layer tissue that attaches the distal phalanx to the inner hoof wall, thus suspending the horse's axial skeleton in the hoof capsule. This tissue fails at the epidermal:dermal junction in laminitic horses, causing crippling disease. Basal epithelial cells line the laminar epidermal:dermal junction, undergo physiological change in laminitic horses, and lose versican gene expression. Versican gene expression is purportedly under control of the canonical Wnt signaling pathway and is a trigger for mesenchymal-to-epithelial transition; thus, its repression in laminar epithelial cells of laminitic horses may be associated with suppression of the canonical Wnt signaling pathway and loss of the epithelial cell phenotype. In support of the former contention, we show, using laminae from healthy horses and horses with carbohydrate overload-induced laminitis, quantitative real-time polymerase chain reaction, Western blotting after sodium dodecylsulfate polyacrylamide gel electrophoresis, and immunofluorescent tissue staining, that positive and negative regulatory components of the canonical Wnt signaling pathway are expressed in laminar basal epithelial cells of healthy horses. Furthermore, expression of positive regulators is suppressed and negative regulators elevated in laminae of laminitic compared to healthy horses. We also show that versican gene expression in the epithelial cells correlates positively with that of β-catenin and T-cell Factor 4, consistent with regulation by the canonical Wnt signaling pathway. In addition, gene and protein expression of β-catenin correlates positively with that of integrin β4 and both are strongly suppressed in laminar basal epithelial cells of laminitic horses, which remain E-cadherin(+)/vimentin(-), excluding mesenchymal transition as contributing to loss of the adherens junction and hemidesmosome components. We propose that suppression of the canonical Wnt signaling pathway, and accompanying reduced expression of β catenin and integrin β4 in laminar basal epithelial cells reduces cell:cell and cell:basement membrane attachment, thus, destabilizing the laminar epidermal:dermal junction.

Tumor necrosis factor α (TNFα) occupies a pivotal role in the development of shock and tissue injury during endotoxemia and septicemia, and may be an important trigger in the pathogenesis of endotoxin-induced intestinal mucosal dysfunction. This study investigated the contribution of TNFα to endotoxin-induced mucosal dysfunction and the efficacy of polyclonal anti-TNFα antibody in preventing endotoxin-induced mucosal dysfunction. To evaluate mucosal dysfunction, jejunal blood-to-lumen clearances of chromium 51-labeled ethylenediaminetetraacetate ([51Cr]-EDTA) were measured in cats administered fetal calf serum (controls), endotoxin, TNFα, or polyclonal anti-TNFα antibody and endotoxin. Serum TNFα activity was determined using a modified in vitro cytotoxicity bioassay using the murine fibrosarcoma cell line, WEHI-164 clone 13. Endotoxin and TNFα induced jejunal mucosal dysfunction as indicated by increases in [51 Cr]-EDTA clearance. Mucosal dysfunction was accompanied by marked increases in serum TNFα activity. Furthermore, pretreatment with polyclonal anti-TNFα antibody prevented endotoxin-induced mucosal dysfunction and markedly reduced the associated increase in serum TNFα activity. The findings of this study suggest that TNFα is an important mediator of endotoxin-induced mucosal epithelial barrier dysfunction.

The effects of an intravenous infusion of endotoxin (750 μg/kg) or an equal volume of saline solution (control) on the proximal and distal jejunal permeability and blood flow were evaluated in cats. In 8 cats, proximal and distal jejunal segments were isolated and mucosal clearance of 51Cr-ethylenediaminetetraacetic acid (EDTA) was measured. The mucosal clearance in the distal loop was not altered by endotoxin infusion in 6 cats given endotoxin or 2 cats given saline solution. In 6 cats given endotoxin, the proximal jejunal segment exhibited a 10-fold increase in mucosal permeability. In contrast, mucosal clearance remained stable in the proximal jejunal loop in 2 cats infused with saline solution. In a second group of 13 cats (7 cats infused with endotoxin; 6 cats infused with saline solution), blood flow to the proximal and distal segments was measured. Either endotoxin (n = 7) or saline solution (n = 6) did not significantly alter blood flow to the proximal and distal jejunum in these experiments. Samples of proximal and distal jejunum were collected from 12 cats (6 cats infused with endotoxin and 6 cats infused with saline solution). There was significantly more epithelial necrosis in the endotoxin treated cats than in the saline treated cats. Neutrophil infiltration was greater in the jejunal segments of endotoxin treated cats than in the jejunal segments of saline treated cats. In the endotoxin treated cats, there was significantly greater necrosis in the proximal jejunal segment than in the distal jejunal segment. There were no significant differences in numbers of neutrophils in the proximal and distal jejunal segments. These results demonstrate that the proximal jejunum is more sensitive to endotoxin-induced increases in mucosal permeability than is the distal jejunum. The increases in mucosal permeability in the proximal jejunum were not accompanied by significant reductions in jejunal arterial blood flow. Endotoxemia induced neutrophil infiltration to the proximal and distal jejunum.

Objective—To determine the effects of clenbuterol, at a dosage of up to 3.2 μg/kg for 14 days, PO, on skeletal and cardiac muscle in healthy horses undergoing treadmill exercise. Animals—12 healthy horses from 3 to 10 years old. Procedures—Horses were randomly assigned to a control group (n = 6) or clenbuterol group (6) and received either saline (0.9% NaCl) solution or clenbuterol, PO, every 12 hours for 14 days. Horses were subjected to submaximal treadmill exercise daily during treatment. Muscle biopsy specimens were collected before and after treatment for determination of apoptosis. Echocardiographic measurements, serum clenbuterol and cardiac troponin I concentrations, and serum activities of creatine kinase and aspartate aminotransferase were measured before, during, and after treatment. Jugular venous blood samples were collected every 3 days during treatment. Echocardiography was repeated every 7 days after beginning treatment. Response variables were compared between treatment groups and across time periods. Results—No significant effect of clenbuterol or exercise on response variables was found between treatment and control groups at any time point or within groups over time. Conclusions and Clinical Relevance—Results did not reveal any adverse effects of treatment with an approved dose of clenbuterol on equine cardiac or skeletal muscle in the small number of horses tested.

Carbohydrate overload models of equine acute laminitis are used to study the development of lameness. It is hypothesized that a diet-induced shift in cecal bacterial communities contributes to the development of the pro-inflammatory state that progresses to laminar failure. It is proposed that vasoactive amines, protease activators and endotoxin, all bacterial derived bioactive metabolites, play a role in disease development. Questions regarding the oral bioavailability of many of the bacterial derived bioactive metabolites remain. This study evaluates the possibility that a carbohydrate-induced overgrowth of potentially pathogenic cecal bacteria occurs and that bacterial translocation contributes toward the development of the pro-inflammatory state. Two groups of mixed-breed horses were used, those with laminitis induced by cornstarch ( n = 6) or oligofructan ( n = 6) and non-laminitic controls ( n = 8). Cecal fluid and tissue homogenates of extra-intestinal sites including the laminae were used to enumerate Gram-negative and -positive bacteria. Horses that developed Obel grade2 lameness, revealed a significant overgrowth of potentially pathogenic Gram-positive and Gram-negative intestinal bacteria within the cecal fluid. Although colonization of extra-intestinal sites with potentially pathogenic bacteria was not detected, results of this study indicate that cecal/colonic lymphadenopathy and eosinophilia develop in horses progressing to lameness. It is hypothesized that the pro-inflammatory state in carbohydrate overload models of equine acute laminitis is driven by an immune response to the rapid overgrowth of Gram-positive and Gram-negative cecal bacterial communities in the gut. Further equine research is indicated to study the immunological response, involving the lymphatic system that develops in the model.

A 5-day-old Thoroughbred colt was presented with profuse watery diarrhea, hypovolemic shock, and a patent urachus. Despite intensive medical therapy, the colt was euthanized 15 d later due to poor clinical response. Necropsy revealed a small intestinal structural abnormality that formed a closed jejunal ring. Although rare, intestinal malformations should be considered in neonatal foals with clinical signs resembling enteritis.

The role of arachidonic acid metabolites in the forestomach stasis induced by Escherichia coli endotoxin was evaluated. Six adult Holstein cows received saline solution; endotoxin at 1, 10, and 100 ng/kg of body weight; flunixin meglumine at 1.1 mg/kg of body weight; and flunixin meglumine at 1.1 mg/kg plus endotoxin at 100 ng/kg. The frequency of reticulorumen contractions, mental attitude, body temperature, respiratory rate, heart rate, and plasma concentration of prostaglandin E2, prostacyclin, and thromboxane were evaluated. Administration of saline solution and endotoxin at 1 ng/kg had no significant effects. Administration of endotoxin at 10 ng/kg did not cause significant clinical effects or alter reticulorumen contractions but enhanced synthesis of thromboxane. Administration of endotoxin at 100 ng/kg caused mild clinical signs of stasis, reduced the frequency of reticulorumen contractions, and enhanced synthesis of thromboxane and prostacyclin. Reticulorumen stasis was not accompanied by an increase in the plasma concentration of prostaglandin E2. Flunixin meglumine abolished endotoxin-induced reticulorumen stasis, tachycardia, and synthesis of arachidonic acid metabolites. Reticulorumen stasis during bovine endotoxemia is caused either by enhanced synthesis of an arachidonic acid metabolite other than prostaglandin E2 or by local synthesis of prostaglandin E2.

Background: Summer pasture-associated recurrent airway obstruction (SPA-RAO), a seasonal airway obstructive disease of horses, is characterized by clinical exacerbation after exposure to pasture during warm months of the year. Endothelin (ET)-1, potent bronchoconstrictor, mitogen, secretagogue, and proinflammatory mediator, has been implicated in the pathogenesis of asthma and equine heaves. Hypothesis: Immunoreactive ET-1 concentrations increase during clinical exacerbation and return to basal values during periods of disease remission. Animals: Twelve horses, 6 affected with SPA-RAO and 6 nonaffected. Methods: Prospective, observational study. Bronchoalveolar lavage fluid (BALF), arterial and venous plasma samples, and clinical variables were obtained from affected horses during clinical exacerbation and remission. Samples and data of nonaffected horses were collected during the summer and winter on dates similar to affected horses. Immunoreactive ET-1 was determined using a commercial ELISA. Results: The median and range ET-1 concentrations (pg/ml) in arterial (1.3, 0.7–1.8) and venous (1.3, 1.2–1.7) plasma and in BALF (0.3, 0.2–0.4), and pulmonary epithelial lining fluid (PELF) (25.5, 21–50) were greater in affected horses during clinical exacerbation compared with remission (P < .01). The concentrations of immunoreactive ET-1 were greater in affected horses during clinical exacerbation compared with nonaffected horses (P < .05). Conclusions and Clinical Importance: During clinical exacerbation of SPA-RAO, ET-1 is increased in circulation and pulmonary secretions. Intervention with ET receptor antagonists should provide further information on the role of ET-1 in SPA-RAO.

To determine the magnitude and duration of effects of acepromazine administered intramuscularly (IM) on digital and systemic hemodynamic variables in clinically healthy horses.Experimental study.Healthy adult horses (n=12). Methods- An ultrasonic Doppler flow probe was surgically implanted around the medial palmar digital artery before the study. Catheters were inserted in the transverse facial artery, lateral palmar digital artery, and jugular vein. A treatment group (n=6) was administered 0.04 mg/kg body weight of acepromazine IM; control horses (n=6) were administered an equivalent volume of saline IM. Palmar digital blood flow, and digital and facial arterial pressures were measured at baseline and for 6 hours after administration. Venous blood was collected for measurement of packed cell volume (PCV).Horses administered acepromazine had significantly lower facial arterial pressure compared with control horses administered saline. Palmar digital arterial blood flow in acepromazine-treated horses was not significantly different from that in control horses but increased significantly post-administration, compared with the respective baseline values for acepromazine-treated horses. PCV significantly decreased in horses administered acepromazine compared with their respective baseline value.IM acepromazine causes hypotension and increases palmar digital blood flow over time but the magnitude of the effect on digital blood flow was not sufficient to yield differences compared with saline-treated horses.IM acepromazine has a modest effect on palmar digital blood flow, facial arterial pressures and PCV in healthy horses with minimal sedation.

The goals of this study were to determine the concentration-response (C-R) relationship of endothelin-1 (ET-1), compare 2 ET-receptor antagonists and determine the antagonist concentrations that block the vasomotor effects of ET-1, and compare the effectiveness of ET-1 and previously studied vasoconstrictors in equine palmar digital arterial and venous rings in vitro. Vessel rings from 8 nonlaminitic horses were placed in Tyrode's solution, 1 side fixed to the floor of an organ bath and the other side fixed to a force-displacement transducer. Two separate studies were conducted: (I) incubation with a single ET-receptor antagonist (PD142893 or PD145065 at a concentration of 10(-7), 10(-6), or 10(-5) M), followed by determination of an ET-1 C-R curve (using concentrations of 10(-10) to 10(-6) M) for medial vessel rings; and (II) comparison of ET-1 with norepinephrine and histamine (10(-10) to 10(-6) M) and comparison of contractile responses of medial and lateral vessel rings. In study I, ET-1 administration caused pronounced and sustained concentration-dependent contraction of vessel rings; these contractile responses were decreased by 10(-5) M PD142893 and were completely blocked by 10(-5) M PD145065. Venous rings had greater apparent maximum contraction in response to ET-1 than arterial rings. In study II, the relative sensitivity of norepinephrine was found to be equivalent to that of ET-1, whereas that of histamine was lower. No significant differences were observed between responses of medial versus lateral vessel rings. Thus, ET-1 is a potent vasoconstrictor of equine palmar digital arteries and veins, and the ET-receptor antagonist PD145065 is more effective than PD142893 in inhibiting these contractile effects in vitro.

Objective—To evaluate changes in digital vascular function in horses with carbohydrate overload (CHO)-induced laminitis and determine the effects of an endothelin (ET) receptor antagonist and nitroglycerin on laminitis-associated vascular dysfunction. Animals—20 adult horses without abnormalities of the digit. Procedures—Hemodynamic variables were recorded before (baseline) and hourly after all horses were administered a CHO ration via nasogastric tube. In 4 groups of 5 horses each, saline (0.9% NaCl) solution or ET receptor antagonist (10−5M in digital blood) was administered into the digital arterial circulation according to 1 of 2 schedules. During anesthesia, blood flow; arterial, venous, and capillary pressures; and total, precapillary, and postcapillary resistances were measured in an isolated perfused digit of each horse. In all groups, nitroglycerin was infused (10−5M in digital blood), and digital microvascular assessments were repeated. Results—The CHO caused a significant decrease in right atrial pressure by 14 hours that was not affected by administration of saline solution or ET receptor antagonist. In isolated digits of anesthetized horses, CHO resulted in a significant decrease in digital blood flow associated with a significant increase in total and postcapillary resistances. Treatment with the ET receptor antagonist and nitroglycerin caused a significant decrease in total resistance. Postcapillary resistance was significantly decreased following treatment with the ET receptor antagonist but was not altered by treatment with nitroglycerin. Conclusions and Clinical Relevance—Treatment with an ET receptor antagonist and nitroglycerin resulted in significant improvement in vascular resistance in isolated perfused digits of anesthetized horses with CHO-induced laminitis.

Objective— To measure and compare palmar digital venous plasma nitric oxide (NO) concentrations and digital arterial blood flow after application of topical nitroglycerine (NTG). Study Design— Experimental study. Animals— Healthy adult horses (n=8). Methods— Digital blood flow was measured by an ultrasonic Doppler flow probe surgically implanted around the medial palmar digital artery. Blood was collected from a catheter placed in the medial palmar digital vein for quantification of NO. NTG patches, NTG ointment or control patches were placed over the palmar digital vessels at the level of the fetlock. Two horses had an intra-arterial infusion of an NTG solution into the medial palmar digital artery in a pilot study. Results— Digital arterial blood flow did not change significantly with application of the NTG patches, NTG ointment, or control patches. There were no statistically significant or biologically important changes in digital venous NO concentrations across time or between treated and control horses. In the pilot study, digital arterial blood flow and palmar digital venous NO concentrations increased with intra-arterial infusion of NTG. Conclusions— In clinically healthy horses, digital arterial blood flow and digital venous plasma NO concentrations did not change significantly with application of the NTG patches/ointment. These treatments are unlikely to have an effect on the digital vasculature of laminitic horses, however, further investigation is warranted. Clinical Relevance— Although NTG patches have been used as a method of decreasing vasomotor tone and improving digital blood flow in horses with laminitis, this study provides evidence in healthy conscious horses that this treatment is not effective in altering digital blood flow.

Objective—To characterize the in vitro response of circular and longitudinal myometrial layers of the uterine horn (CMLH and LMLH, respectively) of horses to endothelin (ET)-1 by use of specific ETA (BQ-123) and ETB (IRL-1038) receptor antagonists. Sample Population—Uteruses from 10 nongravid mares in anestrus. Procedure—Muscle strips from the CMLH and LMLH were suspended in tissue baths and connected to force-displacement transducers interfaced with a polygraph. Strips were incubated for 45-minute intervals with no antagonist (control specimens), and 3 concentrations (10–9, 10–7, and 10–5M) of BQ-123, IRL- 1038, or BQ-123 and IRL-1038 before concentrationresponse curves to ET-1 were generated. Contractile response to cumulative concentrations of ET-1 (10–9 to 10–6M) was quantified by measuring change in the area under the curve (AUC) for the 3-minute period after each ET-1 dose. Results—ET-1 caused concentration-dependent contraction of the CMLH and LMLH specimens. Application of BQ-123 decreased AUC values for both layers. Application of IRL-1038 increased the AUC value for LMLH specimens but did not affect the CMLH value. The combination of BQ-123 and IRL-1038 decreased the AUC value for LMLH tissue and increased that for CMLH tissue. Conclusions and Clinical Relevance—ET-1 causes contraction of the CMLH and LMLH in nongravid horses. In both layers, ETA receptors mediate contraction but the role of ETB receptors remains unclear. In the LMLH, ETA receptors have a dominant role; the presence of another receptor or receptor subtype within this layer is suggested. These findings support a physiologic role for ET-1 in uterine contractility. (Am J Vet Res 2005;66:1094–1100)

Objective—To examine the secretory response (in the presence and absence of prostaglandin inhibition) in vitro and structural alterations of colonic mucosa in horses after intragastric administration of black walnut extract (BWE). Animals—14 adult horses. Procedure—Seven horses were administered BWE intragastrically and monitored for 11 hours. Tissue samples were obtained from the right ventral, left ventral, and right dorsal colons (RVC, LVC, and RDC, respectively) of the 7 BWE-treated and 7 control horses. Tissue samples were examined via light microscopy, and the extent of hemorrhage, edema, and granulocytic cellular infiltration (neutrophils and eosinophils) was graded. Colonic mucosal segments were incubated with or without flunixin meglumine (FLM) for 240 minutes; spontaneous electrical potential difference and short-circuit current (Isc) were recorded and used to calculate mucosal resistance. Results—Colonic tissues from BWE-treated horses (with or without FLM exposure) had an overall greater Isc during the 240-minute incubation period, compared with tissues from control horses. The resistance pattern in RVC, LVC, and RDC samples (with or without FLM exposure) from BWE-treated horses was decreased overall, compared with control tissues (with or without FLM exposure). Histologically, colonic mucosal tissues from BWE-treated horses had more severe inflammation (involving primarily eosinophils), edema, and hemorrhage, compared with tissue from control horses. Conclusions and Clinical Relevance—In horses, BWE administration appears to cause an inflammatory response in colonic mucosal epithelium that results in mucosal barrier compromise as indicated by decreased mucosal resistance with presumed concomitant electrogenic chloride secretory response, which is not associated with prostaglandin mediation. (Am J Vet Res 2005;66:443–449)