Objective—To determine the relationship between
plasma β-endorphin (EN) concentrations and exercise
intensity and duration in horses.
Animals—8 mares with a mean age of 6 years
(range, 3 to 13 years) and mean body weight of 450 kg.
Procedure—Horses were exercised for 20 minutes
at 60% of maximal oxygen consumption (O2max)
and to fatigue at 95% O2max. Plasma EN concentrations
were determined before exercise, after a 10-
minute warmup period, after 5, 10, 15, and 20 minutes
at 60% O2max or at the point of fatigue (95%
O2max), and at regular intervals after exercise.
Glucose concentrations were determined at the
same times EN concentrations were measured.
Plasma lactate concentration was measured 5 minutes
Results—Maximum EN values were recorded 0 to
45 minutes after horses completed each test.
Significant time and intensity effects on EN concentrations
were detected. Concentrations were significantly
higher following exercise at 95% O2max,
compared with those after 20 minutes of exercise at
60% O2max (605.2 ± 140.6 vs 312.3 ± 53.1 pg/ml).
Plasma EN concentration was not related to lactate
concentration and was significantly but weakly correlated
with glucose concentration for exercise at
both intensities (r = 0.21 and 0.30 for 60 and 95%
Conclusions and Clinical Relevance—A critical
exercise threshold exists for EN concentration in
horses, which is 60% O2max or less and is related
to exercise intensity and duration. Even under conditions
of controlled exercise there may be considerable
differences in EN concentrations between
horses. This makes the value of comparing horses
on the basis of their EN concentration questionable.
(Am J Vet Res 2000;61:969–973)
Objective—To investigate the effects of formaldehyde
fixation on equine platelets using flow cytometric
methods to evaluate markers of platelet activation.
Sample Population—Blood samples from 6
Procedure—The degree of fluorescence associated
with binding of fluorescein isothiocyanate (FITC)-conjugated
anti-human fibrinogen antibody and FITCannexin
V in unactivated and adenosine diphosphate
(ADP)-, platelet activating factor (PAF)-, and A23187-
activated platelet samples in unfixed and 0.5, 1.0, and
2.0% formaldehyde-fixed samples was assessed by
use of flow cytometry.
Results—In samples incubated with FITC-anti-human
fibrinogen antibody prior to fixation, addition of 2.0%
formaldehyde resulted in a 30% increase in total fluorescence
in ADP- and PAF-activated samples and a
60% increase in A23187-activated samples. Fixation
for 24 hours prior to addition of antibody resulted in
reduced fluorescence of samples containing antihuman
fibrinogen antibody for all 3 concentrations of
formaldehyde in PAF-activated samples. The addition
of all 3 concentrations of formaldehyde after incubation
with FITC-annexin V resulted in significant
increases in fluorescence in unactivated and activated
platelet samples. As length of fixation time increased,
there was a gradual increase in fluorescence that was
significant at 24 hours.
Conclusion and Clinical Relevance—Because fixation with 2.0% formaldehyde
results in significant changes in fluorescence in
activated platelet samples containing anti-fibrinogen
antibody, lower concentrations of formaldehyde
should be used to fix equine platelet samples.
Formaldehyde-fixed platelet samples should be analyzed
within 12 hours of fixation to avoid artifactual
increases in fluorescence. Fixation of samples containing
FITC-annexin V should be avoided because of
significant increases in fluorescence that may interfere
with interpretation of results. (Am J Vet Res
Objective—To investigate the potential use of fluorescent-
labeled annexin V, anti-human fibrinogen antibody,
and anti-human thrombospondin antibody for
detection of the activation of equine platelets by use
of flow cytometry.
Sample Population—Platelets obtained from 6
Procedure—Flow cytometry was used to assess
platelet activation as indicated by detection of binding
of fluorescent-labeled annexin V, anti-human fibrinogen
antibody, and anti-thrombospondin antibody
to unactivated and ADP-, collagen-, platelet
activating factor (PAF)-, and A23187-activated equine
platelets. Human platelets were used as control
samples. Determination of 14C-serotonin uptake and
release was used to assess the extent of platelet
Results—Anti-human thrombospondin antibody
failed to bind to equine platelets. Annexin V bound to
platelets activated with PAF or A23187 when
platelets had undergone secretion. Anti-human fibrinogen
antibody bound to ADP-, PAF-, and A23817-
activated platelets, but binding was not dependent
on platelet secretion. The extent of binding of anti-fibrinogen
antibody was less in equine platelets, compared
with that for human platelets, despite maximal
Conclusions and Clinical Relevance—Activation of
equine platelets can be detected by use of fluorescent-
labeled annexin V and anti-human fibrinogen
antibody but not by use of anti-human thrombospondin
antibody. These flow cytometric techniques
have the potential for detection of in vivo
platelet activation in horses at risk of developing
thrombotic disorders. (Am J Vet Res 2002;63:513–519)
Objective—To investigate the effects of sodium citrate,
low molecular weight heparin (LMWH), and
prostaglandin E1 (PGE1) on aggregation, fibrinogen
binding, and enumeration of equine platelets.
Sample Population—Blood samples obtained from
Sample Population—Blood samples obtained from
Procedure—Blood was collected into syringes in the
ratio of 9 parts blood:1 part anticoagulant.
Anticoagulants used were sodium citrate, LMWH,
sodium citrate and LMWH, or 300 nM PGE1/ml of
anticoagulant. Platelet aggregation in response to
ADP, collagen, and PGE1 was assessed, using optical
aggregometry. Platelet activation was evaluated,
using flow cytometry, to detect binding of fluorescein-
conjugated anti-human fibrinogen antibody.
Plasma concentration of ionized calcium was measured,
using an ion-selective electrode.
Results—Number of platelets (mean ± SEM) in samples
containing LMWH (109.5 ± 11.3 × 103 cells/µl)
was significantly less than the number in samples
containing sodium citrate (187.3 ± 30.3 × 103 cells/µl).
Increasing concentrations of sodium citrate resulted
in reductions in platelet aggregation and plasma concentration
of ionized calcium. Addition of PGE1 prior
to addition of an agonist inhibited platelet aggregation
in a concentration-dependent manner, whereas addition
of PGE1 4 minutes after addition of ADP resulted
in partial reversal of aggregation and fibrinogen binding.
Conclusion and Clinical Relevance—A high concentration
of sodium citrate in blood samples
decreases plasma concentration of ionized calcium,
resulting in reduced platelet aggregation and fibrinogen
binding. Platelets tend to clump in samples collected
into LMWH, precluding its use as an anticoagulant.
Platelet aggregation and fibrinogen binding can
be reversed by PGE1, which may result in underestimation
of platelet activation. (Am J Vet Res 2001;
Objectives—To establish maximum oxygen consumption
(O2max) in ponies of different body
weights, characterize the effects of training of short
duration on O2max, and compare these effects to
those of similarly trained Thoroughbreds.
Animals—5 small ponies, 4 mid-sized ponies, and 6
Procedure—All horses were trained for 4 weeks.
Horses were trained every other day for 10 minutes
on a 10% incline at a combination of speeds equated
with 40, 60, 80, and 100% of O2max. At the beginning
and end of the training program, each horse performed
a standard incremental exercise test in which
O2max was determined. Cardiac output (), stroke
volume (SV), and arteriovenous oxygen content difference
(C [a-v] O2) were measured in the 2 groups of
ponies but not in the Thoroughbreds.
Results—Prior to training, mean O2max for each
group was 82.6 ± 2.9, 97.4 ± 13.2, and 130.6 ± 10.4
ml/kg/min, respectively. Following training, mean
O2max increased to 92.3 ± 6.0, 107.8 ± 12.8, and
142.9 ± 10.7 ml/kg/min. Improvement in O2max was
significant in all 3 groups. For the 2 groups of ponies,
this improvement was mediated by an increase in ;
this variable was not measured in the Thoroughbreds.
Body weight decreased significantly in the
Thoroughbreds but not in the ponies.
Conclusions and Clinical Relevance—Ponies have a
lower O2max than Thoroughbreds, and larger ponies
have a greater O2max than smaller ponies. Although
mass-specific O2max changed similarly in all groups,
response to training may have differed between
Thoroughbreds and ponies, because there were different
effects on body weight. (Am J Vet Res 2000;
Objective—To determine the cardiovascular and respiratory
effects of water immersion in horses recovering
from general anesthesia.
Animals—6 healthy adult horses.
Procedure—Horses were anesthetized 3 times with
halothane and recovered from anesthesia while positioned
in lateral or sternal recumbency in a padded
recovery stall or while immersed in a hydropool.
Cardiovascular and pulmonary functions were monitored
before and during anesthesia and during recovery
until horses were standing. Measurements and
calculated variables included carotid and pulmonary
arterial blood pressures (ABP and PAP, respectively),
cardiac output, heart and respiratory rates, arterial
and mixed venous blood gases, minute ventilation,
end expiratory transpulmonary pressure (PendXes),
maximal change in transpulmonary pressure
(ΔPtpmax), total pulmonary resistance (RL), dynamic
compliance (Cdyn), and work of breathing ().
Results—Immersion in water during recovery from
general anesthesia resulted in values of ABP, PAP, PendXes, ΔPtpmax, RL, and that were significantly greater and values of Cdyn that were significantly less,
compared with values obtained during recovery in a padded stall. Mode of recovery had no significant
effect on any other measured or calculated variable.
Conclusions and Clinical Relevance—Differences in
pulmonary and cardiovascular function between horses
during recovery from anesthesia while immersed
in water and in a padded recovery stall were attributed
to the increased effort needed to overcome the
extrathoracic hydrostatic effects of immersion. The
combined effect of increased extrathoracic pressure
and PAP may contribute to an increased incidence of
pulmonary edema in horses during anesthetic recovery
in a hydropool. (Am J Vet Res 2001;62:1903–1910)