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 characterize the in vitro effects of
oxytocin, acepromazine, xylazine, butorphanol,
detomidine, dantrolene, isoproterenol, and terbutaline
on skeletal and smooth muscle from the
Animals—14 adult horses without digestive tract disease.
Procedure—Circular and longitudinal strips from
the skeletal and smooth muscle of the esophagus
were suspended in tissue baths, connected to
force-displacement transducers interfaced with a
physiograph, and electrical field stimulation was
applied. Cumulative concentration-response curves
were generated for oxytocin, acepromazine,
xylazine, detomidine, butorphanol, isoproterenol,
terbutaline, and dantrolene. Mean maximum twitch
amplitude for 3 contractions/min was recorded and
compared with predrug-vehicle values for the
skeletal muscle segments, and area under the
curve (AUC) for 3 contractions/min was compared
with predrug-vehicle values for the smooth muscle
Results—No drugs caused a significant change in
skeletal muscle response. In smooth muscle, isoproterenol,
terbutaline, and oxytocin significantly
reduced AUC in a concentration-dependent manner.
Maximum reduction in AUC was 69% at 10–4M for
isoproterenol, 63% at 10–5M for terbutaline, and
64% at 10–4M for oxytocin.
Conclusions and Clinical Relevance—Isoproterenol,
terbutaline, and oxytocin cause relaxation of the
smooth muscle portion of the esophagus. The clinical
relaxant effects on the proximal portion of the esophagus
reported of drugs such as oxytocin, detomidine,
and acepromazine may be the result of centrally mediated
mechanisms. (Am J Vet Res 2002;63:1732–1737)
Objective—To compare effects of oxytocin, acepromazine
maleate, xylazine hydrochloride-butorphanol
tartrate, guaifenesin, and detomidine hydrochloride
on esophageal manometric pressure in horses.
Animals—8 healthy adult horses.
Procedure—A nasogastric tube, modified with 3
polyethylene tubes that exited at the postpharyngeal
area, thoracic inlet, and distal portion of the
esophagus, was fitted for each horse. Amplitude,
duration, and rate of propagation of pressure waveforms
induced by swallows were measured at 5, 10,
20, 30, and 40 minutes after administration of oxytocin,
detomidine, acepromazine, xylazine-butorphanol,
guaifenesin, or saline (0.9% NaCl) solution.
Number of spontaneous swallows, spontaneous
events (contractions that occurred in the absence of
a swallow stimulus), and high-pressure events (sustained
increases in baseline pressure of > 10 mm
Hg) were compared before and after drug administration.
Results—At 5 minutes after administration, detomidine
increased waveform amplitude and decreased
waveform duration at the thoracic inlet. At 10 minutes
after administration, detomidine increased waveform
duration at the thoracic inlet. Acepromazine administration
increased the number of spontaneous events
at the thoracic inlet and distal portion of the esophagus.
Acepromazine and detomidine administration
increased the number of high-pressure events at the
thoracic inlet. Guaifenesin administration increased
the number of spontaneous events at the thoracic
inlet. Xylazine-butorphanol, detomidine, acepromazine,
and guaifenesin administration decreased the
number of spontaneous swallows.
Conclusions and Clinical Relevance—Detomidine,
acepromazine, and a combination of xylazine butorphanol
had the greatest effect on esophageal motility
when evaluated manometrically. Reduction in spontaneous
swallowing and changes in normal, coordinated
peristaltic activity are the most clinically relevant
effects. (Am J Vet Res 2002;63:1738–1744)
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:
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 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
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 
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 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.
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.