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 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
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
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)
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 
and chronic  naturally acquired laminitis, and horses
with black walnut extract-induced  or carbohydrate
overload-induced  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 identify differentially expressed genes in pulmonary tissues of horses affected with summer pasture-associated obstructive pulmonary disease (SPAOPD), which is a form of recurrent airway obstruction (RAO), compared with those of unaffected horses.
Animals—6 horses with SPAOPD-RAO and 6 unaffected (healthy) horses.
Procedures—Horses were assigned to 2 groups on the basis of medical history, clinical score, and transpleural pressure. Total RNA from each of the 5 lung lobes of each of the 6 SPAOPD-RAO–affected horses was extracted and pooled. Similarly, total RNA from unaffected horses was pooled. Differential display (DD) PCR assay was performed, and differentially expressed bands were purified and cloned into a plasmid vector. Plasmids were extracted from recombinant colonies, and purified DNA was sequenced. Genes of interest for RAO pathogenesis were identified. Real-time PCR assay was performed to confirm findings for the DD PCR assay.
Results—18 differentially expressed genes (17 upregulated and 1 downregulated) were identified. Three genes of particular interest were found to be altered (2 upregulated and 1 downregulated) in horses with SPAOPD-RAO by use of real-time PCR assay, and these findings matched the differential expression found by use of the DD PCR assay.
Conclusions and Clinical Relevance—SPAOPD-RAO in horses is a multifactorial, complex disease involving several genes. Upregulated genes, particularly β2-microglobulin, and the downregulated secretoglobin gene can serve as marker genes that may help to identify SPAOPD-RAO at an early age.
Objective—To characterize the bioavailability and pharmacokinetics of oral and injectable formulations of methadone after IV, oral, and intragastric administration in horses.
Animals—6 healthy adult horses.
Procedures—Horses received single doses (each 0.15 mg/kg) of an oral formulation of methadone hydrochloride orally or intragastrically or an injectable formulation of the drug orally, intragastrically, or IV (5 experimental treatments/horse; 2-week washout period between each experimental treatment). A blood sample was collected from each horse before and at predetermined time points over a 360-minute period after each administration of the drug to determine serum drug concentration by use of gas chromatography–mass spectrometry analysis and to estimate pharmacokinetic parameters by use of a noncompartmental model. Horses were monitored for adverse effects.
Results—In treated horses, serum methadone concentrations were equivalent to or higher than the effective concentration range reported for humans, without induction of adverse effects. Oral pharmacokinetics in horses included a short half-life (approx 1 hour), high total body clearance corrected for bioavailability (5 to 8 mL/min/kg), and small apparent volume of distribution corrected for bioavailability (0.6 to 0.9 L/kg). The bioavailability of methadone administered orally was approximately 3 times that associated with intragastric administration.
Conclusions and Clinical Relevance—Absorption of methadone in the small intestine in horses appeared to be limited owing to the low bioavailability after intragastric administration. Better understanding of drug disposition, including absorption, could lead to a more appropriate choice of administration route that would enhance analgesia and minimize adverse effects in horses.
Objective—To determine the feasibility of performing serial laminar and skin biopsies on sedated horses and whether sampling affected adjacent tissues.
Procedures—Laminar tissues were harvested via biopsy through the hoof wall from healthy conscious horses via sedation and regional anesthesia. Eight specimens were collected at 4 time points during 24 hours from a single foot. Laminar biopsy specimens were harvested with a 6-mm-diameter biopsy punch after burring through the horny corium to the stratum medium. Skin biopsy specimens were collected from an area proximal to the coronary band. All tissues were examined via light microscopy. Total RNA was extracted and quantified, and gene expression analysis was completed for 2 housekeeping genes and the inflammatory mediator cyclooxygenase-2.
Results—Laminar and skin biopsies yielded adequate specimens for histologic and gene expression evaluation. There was no extension of inflammation or detectable damage to adjacent tissues during the 24-hour period in either laminar or skin specimens as judged via histologic findings and cyclooxygenase-2 expression. Lameness and discomfort induced by the procedure were minimal.
Conclusions and Clinical Relevance—Laminar biopsy provided a satisfactory method of collecting laminar specimens and allowed serial sampling of individual horses.
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.