Topic:Platelets
Platelets are small, anucleated cell fragments derived from megakaryocytes in the bone marrow, playing a key role in hemostasis and thrombosis in horses. They contribute to the formation of blood clots by adhering to the site of vascular injury, aggregating with one another, and facilitating the coagulation process. In equine medicine, platelet count and function are important parameters in assessing bleeding disorders and thrombotic conditions. Variations in platelet count can indicate underlying health issues, such as inflammation, infection, or bone marrow disorders. This page compiles peer-reviewed research studies and scholarly articles that explore the physiology, pathology, and clinical implications of platelets in equine health.
A morphometric study of bone marrow megakaryocytes in foals infected with equine infectious anemia virus. Morphometric evaluation of bone marrow core biopsies was used to determine megakaryocyte (MK) numbers and MK size in nine foals with equine infectious anemia virus (EIAV)-induced thrombocytopenia. Both immunocompetent normal foals and foals with severe combined immunodeficiency (SCID) were used. Platelet counts were made three times weekly following viral infection. Bone marrow core biopsies were taken from the ilium of each foal prior to experimental infection, immediately after the onset of thrombocytopenia, and at necropsy. All foals developed thrombocytopenia by 23 days postinfection. The ...
Platelet-activating factor and not thromboxane A2 is an important mediator of endotoxin-induced platelet aggregation in equine heparinised whole blood in vitro. Endotoxin has previously been shown to induce platelet aggregation in equine heparinised whole blood. This study aimed to determine whether platelet-activating factor or products of cyclo-oxygenase metabolism (thromboxane A2 or prostaglandins) were important in mediating the response of platelets to endotoxin. The effects of the following drugs on endotoxin-induced aggregation were investigated: aspirin, flunixin meglumine and carprofen (non-steroidal anti-inflammatory drugs); CV-3988 and WEB2086 (platelet-activating factor receptor antagonists); quinacrine (phospholipase A2 inhibitor). The ef...
The inhibition of adenylate cyclase in equine platelets by collagen and by platelet-activating factor. Equine platelet aggregation was stimulated by collagen fibres or platelet-activating factor. The action of both ligands was blocked by forskolin or prostaglandin E(1) agents which are known to activate adenylate cyclase. Equine platelet membranes were found to contain adenylate cyclase activity which was inhibited in dose-dependent fashion by both collagen and platelet-activating factor. Platelet-activating factor-induced inhibition was antagonised by WEB2086.
Protein tyrosine phosphorylation in equine platelets: the effect of stimulation by thrombin and platelet-activating factor (PAF). Protein tyrosine phosphorylation (PTP) in thrombin- and platelet-activating factor (PAF)-stimulated equine platelet activation was investigated in the absence and presence of 2 protein tyrosine kinase inhibitors (PTKIs), methyl 2,5-dihydroxycinnamate (MDHC) and genistein. Washed equine platelets aggregated irreversibly in response to thrombin or PAF in an agonist concentration dependent fashion. MDHC produced an MDHC concentration and time dependent inhibitory effect on rate and extent of thrombin- and PAF-induced aggregations, whereas the effect of genistein on the same parameters was only ge...
Actions of PAF receptor antagonists in horses with the allergic skin disease sweet itch. Platelet activating factor (PAF) mimics the effects of Culicoides antigen by inducing oedema and inflammatory cell accumulation in the dermis of horses with the allergic skin disease, sweet itch. PAF could therefore contribute to antigen-induced inflammatory changes in these horses. We now report that intravenous administration of the PAF receptor antagonist WEB 2086 (3 mg kg-1), at a dose that inhibited the vascular and cellular responses to PAF in sweet itch horses, reduced Culicoides antigen-induced oedema at 1 h by 73% and at 8 h by 71% (p < 0.05). Neutrophil accumulation and eosinophil...
Comparison of the effects of low-molecular-weight and unfractioned heparin in horses. Thirty healthy male horses were allotted to 3 groups and treated blindly during 4 days. Group-1 horses received unfractioned calcium heparin (100 IU/kg of body weight, SC, q 12 h). Group-2 horses received a single dose of a low-molecular-weight heparin (50 anti-Xa IU/kg, SC) every morning, and a similar volume of saline solution every evening. Group-3 horses received the vehicle (saline solution), SC, every 12 hours. Citrated and EDTA-anticoagulated blood samples were collected before starting the medication (T-0) and once daily 3 hours after each morning injection (T-3, T-27, T-51, and T-75)....
Myeloid and megakaryocytic hypoplasia in related standardbreds. Myeloid and megakaryocytic bone marrow hypoplasia in association with moderate to profound neutropenia was observed in 8 young Standardbred horses sired by the same stallion; 7 horses were intermittently thrombocytopenic. Evaluation of serial neutrophil counts in 2 horses suggested that a cyclic variation in neutrophil numbers was present, that lymphocyte numbers increased when neutrophil counts decreased, and that platelet counts decreased when neutrophil counts decreased. Preliminary bone marrow cultures indicated that myeloid progenitor cells were present and that these cells were able to r...
Prothrombotic events in the prodromal stages of acute laminitis in horses. Prothrombotic changes occurring in the prodromal stages of carbohydrate-induced laminitis were investigated. Hemostatic alterations were evaluated by determining platelet counts, platelet survival, activated partial thromboplastin time, one-stage prothrombin time, and monocyte procoagulant activity. Thrombosis of vessels in the hoof wall was evaluated by contrast arteriography and histologic examination. Of 5 horses, 4 became lame between 28 and 52 hours after carbohydrate administration. Mean platelet count in laminitis-affected horses was lower throughout the prodromal stages of laminitis, c...
The effects of tyrphostins B42 and B46 on equine platelet function and protein tyrosine phosphorylation. The effects of the protein tyrosine kinase inhibitors tyrphostins B42 and B46 on equine platelet function and protein tyrosine phosphorylation (PTP) were assessed. Tyrphostins B42 and B46 (both at 100 microM concentration) produced significant inhibition of thrombin-stimulated equine platelet aggregation. The effect of tyrphostin B46 was also time-dependent. The same concentration of these inhibitors produced very little or no inhibition of platelet-activating factor (PAF)-induced aggregation. The effects of tyrphostins B42 and B46 on thrombin- and PAF-stimulated PTP were generally similar, al...
Hemostatic indices in healthy foals from birth to one month of age. Hemostatic indices were determined in 45 healthy light breed foals, from birth to 1 month of age, and in 20 healthy adult (> 2 years of age) light breed horses. Blood samples were obtained from each foal at 4 ages: 1) < 24 hours, 2) 4-7 days, 3) 10-14 days, and 4) 25-30 days. The following hemostatic indices were determined: platelet count; prothrombin and activated partial thromboplastin times; activity concentrations of protein C, antithrombin III, plasminogen, alpha-2 antiplasmin, tissue plasminogen activator, and plasminogen activator inhibitor-1; plasma protein C antigen and fibrino...
Kinetics, dose response, tachyphylaxis and cross-tachyphylaxis of vascular leakage induced by endotoxin, zymosan-activated plasma and platelet-activating factor in the horse. Vascular leakage induced by intradermal injection of endotoxin, zymosan-activated plasma (ZAP) and platelet-activating factor (PAF) was measured in nine Thoroughbreds using 125-iodine human serum albumin (125I-HSA) as a marker in the blood. ZAP and PAF produced dose-dependent increases in vascular permeability with the maximum occurring within the first 15 min after injection. The vascular leakage induced by endotoxin was also dose-dependent, but the maximum occurred 2 h after intradermal injection. Intradermal sites previously injected with endotoxin were refractory to a second injection of e...
Morphologic changes of the ascending colon during experimental ischemia and reperfusion in ponies. The morphologic changes following ascending colon volvulus result from the interaction of inflammatory and coagulation mediators. The objective of this study was to establish a quantifiable histopathologic scoring system to evaluate the serial pathomorphologic changes during ischemia and reperfusion. Such a scoring system could then be applied to subsequent studies designed to attenuate bowel lesions by regulating activity of individual mediators. Ten normal, healthy adult ponies were randomly divided into two equal groups. Following anesthesia and a 30-minute stabilization period, the colon o...
Clinical pathology and hemostatic abnormalities in experimental African horsesickness. Infection of naive North American horses with 10(4) cell culture infectious doses (CCID50) of virulence variants of African horsesickness virus (AHSV), designated AHSV/4SP, AHSV/9PI, and AHSV/4PI, reproduced three classical forms of African horsesickness: acute (pulmonary), subacute (cardiac), and febrile, respectively. Distinct clinicopathologic and hemostatic abnormalities were associated with each form of disease. Hemostatic abnormalities included increased concentration of fibrin degradation products and prolongation of prothrombin, activated partial thromboplastin, and thrombin clotting t...
Characterisation of a membrane receptor on ruminants and equine platelets and peripheral blood leukocytes similar to the human integrin receptor glycoprotein IIb/IIIa (CD41/61). This paper describes two anti-glycoprotein IIb/IIIa or CD41/61 murine monoclonal antibodies (Co.35E4 and Co.2oA1). The cellular distribution and apparent molecular weight of the antigen detected by these antibodies is consistent with their reaction with ruminant and equine glycoprotein IIb/IIIa. Biochemical analysis of the equine molecule using sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) revealed bands of 24, 100 and 110 kDa under reducing conditions and 115 and 80 kDa under nonreducing conditions. Biochemical analysis of ruminant antigen revealed that the 24 kDa band...
Platelet activating factor mimics antigen-induced cutaneous inflammatory responses in sweet itch horses. Hypersensitivity responses to biting flies such as Culicoides are believed to be the cause of sweet itch, a seasonal intensely pruritic skin condition of horses. Little is known about the mediators released by antigen in the skin of affected horses. In the present study the cutaneous vascular and cellular responses to intradermally injected platelet activating factor (PAF) have been characterised in sweet itch cases during the active phase of the disease and compared with those of Culicoides antigen extract. Histamine was used as a positive control in vascular permeability studies. Responses w...
The amino Acid sequence glutamine-628 to valine-646 within the A1 repeat domain mediates binding of von Willebrand factor to bovine brain sulfatides and equine tendon collagen. von Willebrand Factor (vWF) is a multifunctional glycoprotein in plasma and vascular subendothelial matrix which plays a major role in cellular adhesion. vWFdependent adhesion of platelets to the subendothelium at high shear rates involves a specific platelet membrane receptor, the glycoprotein (GP) Ib-IX complex. We have previously purified a 39/34-kiloDalton (kDa) dispase fragment of vWF (Leu-480/Val-481 to Gly-718) and demonstrated that this fragment contains the binding site for the GP Ib-IX complex [Andrews R K, et al. Biochemistry 1989; 28: 8326-83361. vWF also mediates agglutination of ...
Endotoxin-induced platelet aggregation in heparinised equine whole blood in vitro. Endotoxaemia is a leading cause of death among horses. Thrombocytopenia is a common finding in clinical and experimentally-induced cases of endotoxaemia and can lead to coagulopathies, including disseminated intravascular coagulopathy which is usually fatal. In this study it was shown that endotoxin (3 ng ml-1 to 25 micrograms ml-1) can aggregate equine platelets in heparinised whole blood in vitro. The endotoxin-induced aggregation (EIA) was shown to be dependent on the presence of leucocytes in the blood and did not occur when detoxified endotoxin was used, suggesting that lipid A was necess...
The effects of aspirin and paracetamol on the aggregation of equine blood platelets. The responses of equine blood platelets in citrated platelet-rich plasma to arachidonic acid, U44069 (prostaglandin endoperoxide analogue), adenosine 5'-diphosphate, platelet-activating factor or collagen were investigated by turbidimetric aggregometry. Pre-treatment of the platelets with aspirin (1 mmol/l) or paracetamol (1.3 mmol/l) abolished shape change and aggregation in response to arachidonic acid; decreased the rate of aggregation in response to collagen, with no separate effect on shape change; had no marked effect on aggregation caused by the other agonists; but in no case transforme...
Microvascular thrombosis associated with onset of acute laminitis in ponies. The hypothesis that equine laminitis is caused by thrombosis of vessels in the laminar corium (dermis) was investigated. Hemostatic alterations were evaluated by determining platelet count, platelet survival, platelet adhesiveness to vascular subendothelium, activated clotting time, and whole blood recalcification time. Thrombosis of vessels in the hoof wall was evaluated by scintigraphic studies of the hoof wall after administration of indium-111 (111In)-labeled platelets, contrast arteriography, and histologic examination. Platelet count remained constant before and at the onset of lameness;...
Histologic and ultrastructural changes after large-colon torsion, with and without use of a specific platelet-activating factor antagonist (WEB 2086), in ponies. The role of platelet-activating factor (PAF) in mediating the colonic damage that develops after large-colon torsion was studied in 14 ponies. Morphologic changes in areas of the ascending colon and selected abdominal and thoracic viscera after 1 hour of large-colon torsion and 3 to 5 hours of reperfusion were determined, as well as the protective effects of systemic administration of a specific PAF antagonist (WEB 2086). Ponies were selected then allocated at random and in equal numbers to 2 groups that received 1 of 2 treatments prior to induction of large-colon torsion: group 1--control (sa...
[Total Ca contents in blood cells and plasma of the irradiated animals]. It was found that the total Ca content in erythrocytes from irradiated sheep and horses increased by 10-40% from 1st till 5th days after irradiation, the intracellular Mg content being unchanged. More than 2-fold increase in total Ca content in lymphocytes during first 15 days was revealed, while in platelets only a trend for a growth in acute period of radiation disease was detected. On the contrary, no reliable changes in total Ca content in plasma after irradiation were observed. This fact indicates that Ca accumulation in blood cells after whole-body irradiation is due to disturbance in in...
Metallothionein in platelets. The zinc content in platelets from rabbits, humans and horses was determined, and the levels of zinc were found to be significantly higher (3 micrograms/10(10) cells) than those in other peripheral blood cells. About 70% of the zinc in the supernatants of platelet lysates could be detected. From the results of gel filtration analysis, the zinc in platelet lysates was found to be bound with a low-molecular-weight protein (MW 6,000-8,000) detected as metallothionein (MT) on the basis of antigenic properties determined by enzyme-linked immunoassay and immunoblotting analysis using monoclonal anti...
Diagnosis of EDTA-dependent pseudothrombocytopenia in a horse. Thrombocytopenia in horses may be idiopathic or secondary to chronic infectious or inflammatory diseases (eg, equine infectious anemia, lymphosarcoma), drug administration, bone marrow depression, myelophthisic disease, or disseminated intravascular coagulation. This report describes EDTA-dependent pseudothrombocytopenia in a horse. Platelet counts for blood containing EDTA were consistently less than reference range, but platelet counts of blood containing heparin were within reference range. When thrombocytopenia is diagnosed in horses without clinical evidence of a bleeding tendency, EDTA-d...
Drugs affecting the hematologic system of the performance horse. Pharmacologic alterations in the hematologic and rheologic properties of blood may have an important effect on transport and delivery of oxygen to working muscle during exercise. This article briefly reviews erythropoiesis, hematologic and rheologic responses to training and exercise, and the influence of these alterations on exercise performance. The hemorrheologic and performance effects of hematinics, hematopoietic stimulants, and alterations in blood rheology are discussed. The effects of exercise on blood coagulation, fibrinolysis and platelet function, and the effects of drugs that alter...
A comparison of the actions of platelet activating factor (PAF) antagonists WEB 2170 and WEB 2086 in the horse. The effects of the selective platelet activating factor (PAF) receptor antagonist WEB 2170 on PAF-induced responses in equine cells and tissues have been examined and compared with those of WEB 2086. In initial experiments WEB 2170 was shown to inhibit in vitro platelet aggregation in a dose-dependent, competitive reversible manner (pA2 = 7.21). Co-administration of the antagonists with either PAF or histamine also inhibited PAF, but not histamine, induced wheal formation and PAF-induced neutrophil accumulation in vivo in equine skin. Intravenous (i.v.) administration of both drugs at a dose o...
Early neutrophil but not eosinophil or platelet recruitment to the lungs of allergic horses following antigen exposure. Previous studies have shown that bronchoalveolar lavage fluid from horses with allergic respiratory disease and showing clinical symptoms contains increased numbers of neutrophils. In some cases, the eosinophil count is also increased. In this study the time course of changes in lung function and the accumulation of radiolabelled leucocytes and platelets in the lungs of allergic and normal horses has been examined during a 7 hr allergen exposure. Antigen challenge had no effect on pleural pressure or the distribution of radiolabelled neutrophils, eosinophils or platelets in normal horses. In c...
Production and characterization of a monoclonal antibody recognizing a cytoplasmic antigen of equine mononuclear phagocytes. An IgG1 mouse monoclonal antibody, designated 1.646, is described which recognizes a cytoplasmic antigen of equine mononuclear phagocytes. Indirect fluorescent antibody staining of peripheral blood leukocytes reveals a granular cytoplasmic staining, predominantly in adherent blood mononuclear cells. Indirect fluorescent antibody staining is positive for alveolar and peritoneal macrophages. In some horses, a few neutrophils are also stained. In equine tissue samples stained by immunohistochemistry, the distribution of positive cells is consistent with the distribution of tissue macrophages. The...
Effect of a specific platelet-activating factor antagonist on cardiovascular and peripheral cellular responses to colonic ischemia and reperfusion in anesthetized ponies. The role of platelet-activating factor in mediating the cardiovascular and peripheral cellular responses to large-colon ischemia and reperfusion, was explored in anesthetized ponies. A specific platelet-activating factor (PAF) antagonist (WEB 2086) was administered to a group of 6 ponies, and another 6 ponies (controls) were given an equivalent volume of saline solution, prior to 1 hour of large-colon torsion. After correction of the torsion, ponies were monitored during the reperfusion period. Significant (P < 0.05) hypotension and metabolic acidosis developed in all ponies after correction o...
Administration of a receptor antagonist for platelet-activating factor during equine endotoxaemia. Platelet-activating factor (PAF) is an important mediator of endotoxaemia and various PAF receptor antagonists prevent many of the adverse effects of experimental endotoxaemia in laboratory animals. In this study a specific PAF receptor antagonist was used to investigate the role of PAF in equine endotoxaemia. At an interval of not greater than 10 days, 6 horses were each challenged with endotoxin and endotoxin with concurrent administration of SRI 63-441, a PAF receptor antagonist. The order of the treatments was randomised. Clinical signs, serum biochemical and coagulation profiles, and plat...