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phosphatidylserine and exposure of platelet receptors for factors VIIIa and Va. 8 Phosphatidylserine is a phospholipid with a negative charge that can be found primarily on the inner platelet membrane during a resting state. With platelet activation, an ATP

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in American Journal of Veterinary Research

factor 6,11,12,15,16,18 or contact activators 11,18 and simple recalcification of citrated blood. 9,10,13,14,16 Recalcification in the absence of a trigger for coagulation allows for thrombin formation as a downstream effect of activation of the

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in American Journal of Veterinary Research

Platelet activation has been reported for a variety of conditions in dogs, such as IMHA, nephrotic syndrome, parvoviral enteritis, heartworm disease, lymphosarcoma and other malignancies, and sepsis. 1–8 In humans, excessive platelet activation

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in American Journal of Veterinary Research

Abstract

Objective

To determine the potential usefulness of tests for detection of platelet activation and platelet-leukocyte aggregates in horses.

Samples

Blood from 3 healthy Thoroughbreds.

Procedures

Microscopic and flow cytometric assays were used to evaluate spontaneous platelet aggregation, platelet activation, and platelet-leukocyte aggregates. Platelet activation was detected by evaluation of binding of anti-human fibrinogen to unactivated and ADP-, thrombin-, thrombin agonist receptor peptide-, and platelet activating factor-activated platelets. Platelet-leukocyte aggregates were evaluated microscopically and by flow cytometric determination of leukocyte fluorescence that resulted from binding of fluorescently labeled platelets to leukocytes.

Results

Equine platelets readily aggregated spontaneously when blood was stirred at low, medium, and high speeds. Compared with unactivated platelets, activated platelets had a marked increase in the percentage of cells with increased fluorescence intensity and in mean fluorescence intensity. Unactivated platelets formed aggregates with neutrophils and monocytes, but not with lymphocytes. Activation of platelets resulted in a calcium-dependent increase in platelet-leukocyte aggregates.

Conclusions

Flow cytometric techniques can be used to detect in vitro platelet activation and platelet-leukocyte aggregates in horses.

Clinical Relevance

Flow cytometric techniques may be useful for detection of prothrombotic disorders in horses. (Am J Vet Res 1997;58:823–827)

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in American Journal of Veterinary Research

blood-gas analyzers. Complete evaluation of coagulation is still often performed at a traditional laboratory. Conventional test tubes and handheld analyzers are able to determine the activated coagulation time. Several point-of-care coagulation analyzers

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in American Journal of Veterinary Research

minimize activation of the clotting cascade and result in the most accurate measurements. 1–3 With the increased reliance on clotting times in human medicine, techniques have been developed to collect blood samples by use of indwelling arterial catheters 4

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in American Journal of Veterinary Research

Abstract

Objectives

To determine whether platelets become activated and form platelet-platelet or platelet-neutrophil aggregates, or both, when subjected to shear.

Sample Population

Blood obtained from 3 Thoroughbreds.

Procedures

Blood, with PCV adjusted to 32 (low hematocrit) or 60 (high hematocrit)%, was subjected to shear rates of 11.25, 22.5, 45, 90, 225, and 750/s for 3 minutes by use of a cone-plate viscometer. Flow cytometric techniques were used to identify activated platelets, platelet-platelet aggregates, and platelet-neutrophil aggregates.

Results

Shear resulted in decreased platelet count, increased mean platelet volume, platelet activation, and formation of platelet-platelet and platelet-neutrophil aggregates. These changes occurred at lower shear rates in blood with high hematocrit. Platelet-neutrophil aggregate formation was inhibited by blocking P-selectin, but not CD11/CD18 receptors.

Conclusions

Shear-induced platelet activation and aggregate formation occur at physiologic shear rates.

Clinical Relevance

Shear-induced platelet activation may explain the exercise-associated platelet-neutrophil aggregates observed in Thoroughbreds undergoing treadmill exercise. (Am J Vet Res 1998;59:1243-1246)

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in American Journal of Veterinary Research

Abstract

Objectives

To determine whether platelets become activated and form platelet-neutrophil aggregates during near-maximal treadmill exercise in horses.

Animals

4 Thoroughbreds.

Procedure

Horses were subjected to 4 standardized exercise tests on a treadmill, and blood samples were collected before exercise, at treadmill speed of 12 m/s, and 5 minutes after exercise. Flow cytometric techniques were used to identify activated platelets, and flow cytometric and microscopic techniques were used to identify platelet-neutrophil aggregates.

Results

Platelet-neutrophil aggregates increased from 2.8 ± 0.4% at rest to 17.2 ± 1.1% and 14.7 ± 1.6% during and after exercise, respectively. Platelet activation was not detected during or after exercise.

Conclusions

Platelet-neutrophil aggregates consistently form during strenuous exercise in horses. (Am J Vet Res 1998;59:393–396)

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in American Journal of Veterinary Research

Abstract

Objective

To establish the existence of platelet-derived proteins in equine plasma, with the future goal of developing an assay for the detection of in vivo platelet activation.

Animals

5 mature healthy horses.

Procedure

Platelet-rich plasma and platelet-poor plasma were prepared from anticoagulated blood. Platelets were separated from plasma proteins by gel filtration, then activated with 0.5 μM platelet-activating factor. Protease inhibitors were added, and the released platelet proteins were harvested. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis was performed on the released platelet proteins and platelet-poor plasma, and the resultant silver-stained bands were compared. Immunoblot analysis was performed on released platelet proteins, using an antibody to human thrombospondin; human platelet-derived proteins served as the positive control for the antibody.

Results

Released platelet proteins in the presence of β-mercaptoethanol (reduced samples) contained several proteins that were not observed in plasma including (mean ± SEM) 194 ± 2, 159 ± 2, 151 ±2, 104 ± 2, and 95 ± 1 kd. Immunoblots of released platelet proteins had a prominent 180 ± 2-kd protein in reduced samples that was recognized by an antibody to human thrombospondin, and with prolonged color development, 2 additional less prominent proteins (166 ± 1 and 155 ± 1 kd) were observed.

Conclusions

Several proteins are released from activated equine platelets that are not detectable in normal equine plasma. Thrombospondin is one of the high molecular mass proteins released by activated equine platelets.

Clinical Relevance

An assay can be developed for detection of thrombospondin in equine plasma and may be useful for detection of in vivo platelet activation in horses. (Am J Vet Res 1997;58:954–960)

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in American Journal of Veterinary Research

SUMMARY

Objectives

To determine whether platelets are hyperaggregable or form platelet-neutrophil aggregates during the prodromal stages of acute laminitis of ponies.

Animals

Healthy adult ponies: 8 experimental and 6 control.

Procedures

Acute laminitis was induced by oral administration of corn starch and wood flour to 8 ponies, and indices of platelet activation were evaluated. Blood samples were collected before and at 4, 8, 12, 24, 28, and 32 hours after carbohydrate administration, and PCV, total plasma protein concentration, platelet count, activated clotting time, whole blood recalcification time, spontaneous platelet aggregation, ex vivo platelet aggregation responses, and platelet-neutrophil aggregates were determined. When lameness was first detected, ponies were euthanatized and arteriography and histologic examination of hooves were performed.

Results

Carbohydrate overload was associated with hyperaggregability of platelets throughout the prodromal stages of laminitis and increased numbers of platelet-neutrophil aggregates. Reduction of blood supply to affected hooves was variable, and blood clots were found in 6 of 11 laminitis-affected hooves.

Conclusions

Platelets were hyperaggregable throughout the prodromal stages of carbohydrate-induced laminitis and formed platelet-neutrophil aggregates. Platelet-neutrophil aggregates may initiate or contribute to development of acute laminitis.

Clinical Relevance

Anti-platelet therapy may be useful for treatment of acute alimentary laminitis in horses. (Am J Vet Res 1997;58:1376–1380)

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in American Journal of Veterinary Research