Clinical, Biochemical, and Molecular Aspects of Glanzmann's Thrombasthenia in Humans and Dogs

Glanzmann's thrombasthenia (GT) is an inherited, intrinsic platelet function defect that involves the platelet glycoprotein complex IIb–IIIa, also known as the fibrinogen receptor and the integrin αIIbβ3. The defect was originally described by Dr. Glanzmann in humans in 1918 as a bleeding disorder that differed clinically from other known coagulopathies. Over the decades that followed, researchers determined the biochemical and molecular basis for the disease in humans. Otterhounds with thrombasthenic thrombopathia, described in the 1960s, were the only animal model that closely resembled the disease described in humans until 1996. At that time, a Great Pyrenees dog was identified with unequivocal clinical and biochemical features of Type I GT. The cDNA encoding for glycoproteins IIb and IIIa were sequenced in normal dogs in 1999, allowing for identification of specific mutations causing Type I GT in both Otterhounds and Great Pyrenees dogs. Knowing the molecular basis for Type I GT in dogs as well as the cDNA sequences in normal dogs should enhance the understanding of structure/function relationships of the αIIbβ3 integrin and provide an excellent animal model for studies aimed at correction of GT in humans. The following review focuses on the structure and function of this platelet receptor and reviews the molecular, biochemical, and clinical aspects of Glanzmann's thrombasthenia in humans and dogs.

Calcium diacylglycerol guanine nucleotide exchange factor I (CalDAG-GEFI) gene mutations in a thrombopathic Simmental calf

Simmental thrombopathia is an inherited platelet disorder that closely resembles the platelet disorders described in Basset Hounds and Eskimo Spitz dogs. Recently, two different mutations in the gene encoding calcium diacylglycerol guanine nucleotide exchange factor I (CalDAG-GEFI) were described to be associated with the Basset Hound and Spitz thrombopathia disorders, and a third distinct mutation was identified in CalDAG-GEFI in thrombopathic Landseers of European Continental Type. The gene encoding CalDAG-GEFI was sequenced using DNA obtained from normal cattle and from a thrombopathic calf studied in Canada. The affected calf was found to have a nucleotide change (c.701 T>C), which would result in the substitution of a proline for a leucine within structurally conserved region two (SCR2) of the catalytic domain of the protein. This change is likely responsible for the thrombopathic phenotype observed in Simmental cattle and underscores the critical nature of this signal transduction protein in platelets.

Characterization of the cDNA Encoding alphaIIb and beta3 in normal horses and two horses with Glanzmann thrombasthenia

Glanzmann thrombasthenia (GT) is an inherited, intrinsic platelet defect characterized by a quantitative or qualitative change in the platelet glycoprotein complex IIb-IIIa (integrin alpha(IIb)beta3). The subunits are encoded by separate genes and both subunits must be expressed for a stable complex to form on the platelet surface; therefore, a defect in either gene can result in GT.

Use of a polymerase chain reaction assay to detect infection with Eperythrozoon wenyoni in cattle

OBJECTIVE

To determine whether a polymerase chain reaction (PCR) assay could be used to detect Eperythrozoon wenyoni in the blood of cattle.

DESIGN

Prospective study.

ANIMALS

95 cattle from various herds in Alabama and Georgia and 96 bulls enrolled in Auburn University's Alabama Beef Cattle Improvement Association Bull Test program.

PROCEDURE

Blood samples were collected by means of venipuncture of the median caudal vein and submitted for a CBC and PCR assay. Blood smears were made immediately after blood collection and examined by means of light microscopy.

RESULTS

Three of 95 cattle from herds in Alabama and Georgia and 5 of 96 bulls enrolled in the Bull Test program had positive PCR assay results. Organisms were seen in blood smears from only 5 of these 8 animals. Organisms were not seen in blood smears from any animals for which results of the PCR assay were negative.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that a PCR assay may be an effective method for detecting E wenyoni infection in cattle and that the PCR assay may be a more sensitive test than evaluation of blood smears.

Molecular and genetic basis for thrombasthenic thrombopathia in otterhounds

OBJECTIVES

To determine the molecular and genetic basis for thrombasthenic thrombopathia in Otterhounds and establish whether the defect would be best classified as type-I Glanzmann's thrombasthenia.

ANIMALS

57 dogs, including 13 affected Otterhounds, 23 carrier Otterhounds, 17 unaffected Otterhounds, and 4 clinically normal unrelated dogs of other breeds.

PROCEDURE

Functional (platelet aggregation, clot retraction, buccal mucosa bleeding time) and biochemical (electrophoresis, flow cytometry, fibrinogen content) analyses were conducted. In addition, first-strand cDNA synthesis from platelet total RNA was performed. Exons of the genes encoding for glycoproteins (GP) IIb and IIIa were amplified in overlapping fashion. The resulting products were excised from agarose gels and sequenced. The sequences obtained were compared with known cDNA sequences for canine GPIIb and GPIIIa.

RESULTS

A single nucleotide change at position G1193 (1100) was detected in exon 12 of the gene encoding for platelet GPIIb in 2 affected Otterhounds. Carrier Otterhounds were heterozygous at this position, and 2 unaffected Otterhounds were unchanged. This nucleotide change would result in substitution of histidine for aspartic acid at position 398 (367) within the third calcium-binding domain of GPIIb.

CONCLUSIONS AND CLINICAL RELEVANCE

These studies suggest that thrombasthenic thrombopathia of Otterhounds is homologous phenotypically and has a similar molecular basis to type-I Glanzmann's thrombasthenia in humans.

Nucleotide sequence of the canine alphaIIb gene from platelet-derived cDNA

OBJECTIVE

To determine the nucleotide sequence of the alphaIIb gene from canine platelet-derived cDNA.

ANIMALS

3 adult dogs.

PROCEDURE

First-strand cDNA was prepared from total RNA isolated from canine platelets. The cDNA was amplified, using specific primers in polymerase chain reaction (PCR), and the nucleotide sequence was obtained from purified PCR products.

RESULTS

Except for the nucleotide at position 694, results of all sequencing reactions of alphaIIb were identical for canine platelet-derived cDNA. Canine alphaIIb had 3 fewer codons than alphaIIb of humans. The nucleotide and deduced amino acid sequences of full-length canine alphaIIb shared > or = 83% similarity with the sequences established for humans. Segments of canine alphaIIb nucleotide and deduced amino acid sequences were > or = 78% similar to alphaIIb associated with 7 functional domains (extracellular, transmembrane, cytoplasmic, and 4 calcium-binding domains) in humans, with the highest degree of similarity correlating with the sequences of the 4 calcium-binding domains. Amino acid residues associated with development of alloantibodies in humans (Met837, Val837, Ile843, Ser843) are not encoded by canine alphaIIb.

CONCLUSIONS AND CLINICAL RELEVANCE

The nucleotide variation at position 694 of canine alphaIIb may represent a polymorphism. The species differences in the alphaIIb sequence may contribute to variations in receptor-li gand interactions. The high degree of alphaIIb sequence conservation of the 4 calcium-binding domains implies functional importance. Some disorders associated with alphaIIbbeta3 in dogs are clinically analogous to diseases in humans, and results indicate that dogs are an appropriate model for the evaluation of gene therapy and other treatments of platelet-associated disorders.

There are at least three genetically distinct small piroplasms from dogs

The 18S nuclear subunit ribosomal RNA (18S rRNA) gene of small piroplasms isolated from dogs from Okinawa (Japan), Oklahoma, North Carolina, Indiana, Missouri, and Alabama, was isolated and sequenced. Phylogenetic analysis of these sequences and comparisons with sequences from other Babesia, Cytauxzoon, and Theileria species revealed that all canine small babesial isolates, with the exception of isolates from California and Spain, were placed in a group containing the Babesia spp. sensu stricto. Within the Babesia spp. sensu stricto, there was support for separating the small canine piroplasms from the large canine piroplasm, Babesia canis. The isolate from California was in a distinct phylogenetic clade, closely related to babesial isolates from wildlife and humans from the Western US. The canine isolate from Spain was closely related to Babesia microti. These results suggest that there are multiple small piroplasm species in dogs. The isolates from the Midwestern and Eastern US and the one from Japan probably represent a single species with wide geographic distribution.

Two genetic defects in alphaIIb are associated with type I Glanzmann's thrombasthenia in a Great Pyrenees dog: a 14-base insertion in exon 13 and a splicing defect of intron 13

Glannzmann's thrombasthenia (GT) is an autosomal recessive bleeding disorder caused by qualitative or quantitative deficiencies of the platelet membrane glycoprotein alphaIIbbeta3. This is the first report of a molecular genetic basis for type I GT in dogs. As previously reported, a thrombasthenic Great Pyrenees dog (dog No. 1) experienced uncontrolled epistaxis despite results of coagulation screening tests, platelet quantitation, and von Willebrand factor quantitation that were within reference ranges. Platelet aggregation was minimal in response to agonists. Flow cytometry, autoradiography, and immunoblot experiments demonstrated either marked reduction or absence of glycoproteins alphaIIb and beta3. In this study, we report the presence of a 14-base insertion in exon 13 and defective splicing of intron 13 in the alphaIIb gene of two thrombasthenic dogs (Nos. 1 and 8). The insertion disrupted the fourth alphaIIb calcium-binding domain, caused a shift in the reading frame and resulted in a premature termination codon. Possible consequences of this mutation include decreased alphaIIb mRNA stability and production of truncated alphaIIb protein that lacks the transmembrane and cytoplasmic domains and a large portion of the extracellular domain. We identified the dam, sire, and three littermates of dog No. 8 as carriers of the alphaIIb mutation. Canine alphaIIb and beta3 genes share significant homology with the genes in human beings, making canine GT an excellent translational model for human GT. A defined molecular basis for canine GT will enhance ongoing gene therapy research and increase the understanding of structure-function relationships of this integrin.

DNA sequence of the canine platelet beta3 gene from cDNA: comparison of canine and mouse beta3 to segments that encode alloantigenic sites and functional domains of beta3 in human beings

The platelet glycoprotein complex alphaIIb beta3 is required for platelet-fibrinogen binding and platelet aggregation. This study was designed to characterize the nucleotide sequence of the canine platelet beta3 gene from cDNA. The nucleotide and deduced amino acid sequences of the canine beta3 gene were 92% and 96% homologous, respectively, with the sequences previously established for the beta3 gene of human beings. Within the beta3 gene, the nucleotide sequence of cDNA prepared from canine platelets shared homology of 89% for the cytoplasmic domain, 93% for the transmembrane domain, 92% for the extracellular domain, 94% for the arginine-glycine-aspartic acid (RGD) binding domain, and 97% for the region associated with Ca2+-dependent stabilization of the alphaIIb beta3 fibrinogen-binding pocket. The deduced amino acid sequence of canine beta3 was 100%, 97%, 96%, and 95% homologous with the cytoplasmic, transmembrane, extracellular, and RGD-binding domains, respectively, and was 100% homologous with the region associated with Ca2+-dependent stabilization of the alphaIIb beta3 fibrinogen-binding pocket of beta3 in human beings. The canine platelet cDNA signal peptide segment of the beta3 gene encodes for 22 amino acids, as compared with 26 amino acids previously reported for human beings. The deduced amino acid sequence of canine beta3 corresponds to the high-frequency allelic form for five of the six alloantigenic sites reportedly associated with human platelets: Leu33Leu40Pro407Arg489Arg636. The apparent amino acid residue in position 143 (Pen alloantigen) of canine platelet beta3 is histidine compared with arginine in human beings. Knowledge of the beta3 gene nucleotide sequence of normal dogs will facilitate the understanding of platelet alphaIIb beta3 structure-function relationships.

Identification of thrombospondin as a high molecular mass protein released from activated equine platelets

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 microM 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 beta-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.

The effects of varying dietary n-6 to n-3 fatty acid ratios on platelet reactivity, coagulation screening assays, and antithrombin III activity in dogs

Thirty beagles were placed on diets containing ratios of n-6 to n-3 fatty acids ranging from 5:1 to 100:1 for 12 weeks to determine the effects of these diets on platelet reactivity, coagulation screening assays, and antithrombin III activity. Although small changes were observed in adenosine diphosphate (ADP)-, collagen-, and arachidonic acid-induced platelet aggregation and 14C-serotonin release, fibrinogen concentrations, and antithrombin III activities during the 12-week study, these changes were not of clinical significance and did not correlate with the varying ratios of n-6 to n-3 fatty acids.

Evaluation of ploidy of mature canine megakaryocytes, using Feulgen staining and microspectrophotometry

OBJECTIVE

To evaluate megakaryocyte size and ploidy, using Feulgen staining and microspectrophotometry, in adult dogs with normal platelet count.

ANIMALS

Group A contained 8 and group B contained 11 adult dogs.

PROCEDURE

Megakaryocytes were evaluated by light microscopy and staged according to maturation status. Stage-III megakaryocytes were measured and mapped for future relocation. Bone marrow aspirates were destained and restained, using the Feulgen method. Previously identified stage-III megakaryocytes were measured for DNA content, using microspectrophotometry.

RESULTS

Megakaryocyte size correlated with ploidy values, and mean sizes within ploidy groups were significantly (P < 0.05) different from each other for both groups. The model ploidy value of stage-III megakaryocytes, which represented 18% of the total megakaryocyte population of the combined groups, was 32N. This is in contrast to results of flow cytometric studies, which indicated that the modal ploidy value for all canine megakaryocytes was 16N.

CONCLUSIONS

Reasons for the disparate results between microspectrophotometric techniques and flow cytometry include maturation stage of the megakaryocyte population evaluated and percentage of megakaryocytes within that maturation stage. Flow cytometric methods, which evaluate all megakaryocytes detectable by antibody, may include cells still capable of DNA synthesis, resulting in a shift in the observed modal ploidy value. Recognition of the difference between canine and human megakaryocyte ploidy distribution is important, particularly in studies in which the dog is used as an animal model for human megakaryocytopoiesis.

Platelets and coagulation. An update

Accepted mechanisms of coagulation are being challenged as new proteins and feedback mechanisms are discovered and evaluated. These new proteins, including tissue factor pathway inhibitor, not only change the way we look at the coagulation mechanism, but also give us potential alternatives in treatment of hemophilia. Gene therapy continues to be a challenging research area, with hemophilia serving as the prototypic hereditary disease target. As acquired disorders of coagulation secondary to immune-mediated events are studied and better understood, our approaches to management of these cases will also change to the benefit of our patients.

Type I Glanzmann's thrombasthenia in a Great Pyrenees dog

An 8-month-old female Great Pyrenees dog with chronic epistaxis and a history of gingival bleeding during shedding of deciduous teeth was evaluated for platelet function. Platelet morphology was normal at both the light and electron microscopic level. Platelet number and mean platelet volume were also normal. Platelet aggregation responses to adenosine diphosphate, collagen, platelet activating factor, and thrombin were markedly reduced, although shape change responses were normal. Clot retraction was markedly impaired. Monoclonal antibody (MoAb) Y2/51, a murine anti-human platelet beta 3 antibody that cross-reacts with canine platelet beta 3, and MoAb 5G11, a murine anti-dog platelet alpha IIb beta 3 antibody, bound minimally to affected dog platelets, as demonstrated by flow cytometry. Binding of MoAb Y2/51 was not detectable by immunoblot. MoAb CAP1, a murine anti-dog fibrinogen receptor-induced binding site antibody, failed to bind to affected dog platelets, as demonstrated by flow cytometry. A reduction in glycoproteins alpha IIb and beta 3 was demonstrated by two-dimensional protein electrophoresis. This is the first reported case of type I Glanzmann's thrombasthenia in the dog that closely resembles the clinical syndrome and the platelet morphology described in type I Glanzmann's thrombasthenia of human beings.

A platelet activation-specific monoclonal antibody that recognizes a receptor-induced binding site on canine fibrinogen

An activation-specific monoclonal antibody (MoAb) termed "Canine Activated Platelet 1" (CAP1) has been developed and partially characterized. Flow cytometric studies of isolated canine platelets, using adenosine diphosphate (ADP) and platelet activating factor (PAF) as agonists, demonstrated that CAPI binding site number was proportional to agonist strength and agonist concentration. MoAb CAP1 binding was diminished by ethylenediamine-tetraacetic acid, suggesting that the antigen was either stabilized by calcium or antigen binding to the platelet surface was mediated by calcium. ADP-activated gel-filtered platelets also demonstrated reduced binding of MoAB CAP1 even in the presence of 1 mM CaCl2. Binding of MoAb CAP1 could be partially restored by activating gel-filtered platelets with PAF, suggesting that the antigen was either present within platelet granule membranes or was exposed after binding of released proteins(s) with a platelet receptor. A monoclonal antibody to human platelet glycoprotein IIIa (GPIIIa), which cross-reacts with canine platelet GPIIIa regardless of platelet activation status, did not inhibit binding of MoAb CAP1. MoAb CAP1 bound to isolated canine fibrinogen captured on polystyrene microtiter plates in the absence of platelet proteins. Immunoblots indicated that MoAb CAP1 recognizes nonreduced fibrinogen as well as a plasmin digest of isolated canine fibrinogen. Results of the present studies suggest that MoAb CAP1 recognizes a receptor-induced binding site on canine fibrinogen.

Detection of activated feline platelets in platelet-rich plasma by use of fluorescein-labeled antibodies and flow cytometry

Platelets contribute to prethrombotic or thrombotic states; however, accepted evaluation methods (i.e., in vitro testing by use of an aggregometer) of platelet function in cats can be difficult because of the large volume of blood required from which platelets are isolated and the potential for platelet activation due to difficult venipunctures in sometimes uncooperative or excited animals. The activation problem also contributes to errors in platelet counts. Platelets from four domestic short haired cats (two males, two females, 2-3 years old) minimally restrained without sedation or anesthesia were evaluated. Blood (5 ml) was collected by jugular venipuncture directly into syringes containing 3.8% trisodium citrate (nine parts blood to one part anticoagulant) plus prostaglandin E1 (3 microM; 0.25, 0.5, 1, or 2 microliters/500 microliters citrate) or 3.8% trisodium citrate alone. Prostaglandin E1, which is a stable metabolite of arachidonic acid with platelet inhibitory properties similar to those of prostaglandin I2, was added to the anticoagulant to prevent activation of platelets during the collection process. Feline platelets exposed to prostaglandin E1 became immediately and persistently nonreactive to agonists, which negated their use in functional studies (aggregation, 14C-serotonin release, binding of fluorescein-conjugated antifibrinogen) but improved platelet counting accuracy. Detection of in vivo activation of platelets in prethrombotic and thrombotic states in humans has been done by identification of activation-dependent molecules on platelet surfaces by use of specific antibody recognition and detection by flow cytometric analysis. Many activation-dependent platelet surface receptor changes are species specific; however, fibrinogen appears to be conserved across species.(ABSTRACT TRUNCATED AT 250 WORDS)

Platelet function and antithrombin, plasminogen, and fibrinolytic activities in cats with heart disease

Platelet function, antithrombin and plasminogen activities, and fibrinolytic capabilities in 11 cats with acquired heart disease were compared with results in 4 healthy cats. Of 11 cats with heart disease, 9 had hyperthyroidism with secondary cardiac dysfunction. One cat with hyperthyroidism had renal disease and heart failure, and of 2 cats with idiopathic hypertrophic cardiomyopathy, 1 also had renal disease. At the time of testing, 3 cats had thromboembolic events associated with the disease. Compared with healthy cats, cats with acquired heart disease had increased activity of antithrombin III, a protein that behaves as an acute-phase reactant. Plasminogen activity was decreased, although not significantly, in cats with acquired heart disease, compared with results in healthy cats. In cats with left ventricular dysfunction, clot retraction was decreased (marginal significance, P = 0.058) and might be attributed, in some cases, to the medications received by the cats. Dilute whole blood clots from all cats failed to lyse in vitro. This observation, at present, lacks adequate explanation. Platelets from cats with acquired heart disease, compared with platelets from healthy cats, had decreased responsiveness (aggregation and [14C]serotonin release) to adenosine diphosphate and increased responsiveness to collagen. Hyperthyroid cats were receiving various drugs (propranolol, atenolol, or diltiazem) to empirically treat clinical signs of disease attributable to cardiac dysfunction. Although numbers of cats in each group were small, definite trends were observed in the results of tests. Platelets from cats receiving atenolol had decreased responsiveness to adenosine diphosphate and unaltered responsiveness to collagen, compared with platelets from healthy cats, and may have decreased risk of thrombus formation.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of an intrinsic platelet function defect in Spitz dogs

A recently identified intrinsic platelet function defect in 2 Spitz dogs is described. Both affected dogs had a history of chronic intermittent bleeding primarily from the nasal, oral, and gastrointestinal mucosa. Platelet aggregation in response to adenosine diphosphate (ADP), collagen, and platelet activating factor (PAF) was absent; however, platelet shape change did occur. Platelets aggregated in response to gamma thrombin, although a delayed onset and a reduced velocity of aggregation were present. Platelet 14C-serotonin release was diminished in response to collagen and PAF. Glycoprotein IIIa was detected on the surface of platelets by flow cytometry. Platelets were morphologically normal under light and electron microscopy. Two male Spitz dogs, related to one of the affected dogs, did not have a bleeding diathesis. Collagen-induced platelet aggregation, however, was diminished in these 2 dogs. This platelet defect most closely resembles the defect described in Basset hounds.

Toxicologic effects of ribavirin in cats

Ribavirin, a broad-spectrum antiviral agent active in vitro against a number of RNA and DNA viruses, has been associated with moderate toxicity in laboratory animals and humans. Clinically, ribavirin has been used effectively in persons primarily to treat life-threatening viral diseases such as acute haemorrhagic fever or viral pneumonia of infants. In order to evaluate the feasibility of using this antiviral agent in cats, the effects of oral (p.o.), intramuscular (i.m.) and intravenous (i.v.) doses of ribavirin in 27 9-month-old specific-pathogen-free cats were evaluated by haematology, clinical chemistries, bone marrow biopsies and histopathology. Ribavirin was administered once daily for 10 consecutive days at a dose of either 11, 22, or 44 mg/kg after which all cats were euthanatized and necropsied. Most cats receiving 22 or 44 mg of ribavirin/kg became anorectic and suffered some degree of weight loss (0.2 to 0.6 kg), and about one-third of the cats developed diarrhoea and/or mucous membrane pallor. Icterus or haemorrhage was not observed. The most profound and consistent haematologic change, particularly among the moderate and high dosage groups regardless of route of administration, was a significant and severe thrombocytopenia (range, 33-78% reduction in mean platelet counts vs. baseline). Other changes, particularly reductions in total WBC and neutrophils and reductions in RBC and PCV, tended to occur at lower ribavirin dosages, but generally they were not statistically significant. Cats given 44 mg of ribavirin/kg i.v. showed significant decreases in leukocyte variables, including total WBC (P = 0.016), neutrophils (P = 0.026) and lymphocytes (P = 0.047). Mild-to-moderate increases in serum alanine aminotransferase and alkaline phosphatase activities occurred at doses of 22 and 44 mg/kg. Evaluation of bone marrow biopsies before and after treatment revealed that cats given 11 mg of ribavirin/kg had mild megakaryocytic (MK) hypoplasia, whereas cats receiving 22 or 44 mg/kg had progressively severe degrees of MK hypoplasia and dysplasia, asynchronous MK maturation, and increased myeloid:erythroid ratio. Pathologic changes in ribavirin-treated cats generally were mild and included primarily enteritis (seven cats) and hepatocellular vacuolation and/or centrilobular necrosis (seven cats). Results of this study in cats indicated that daily administration of ribavirin at a dose range of 11 to 44 mg/kg induced a dose-related toxic effect on bone marrow, primarily on megakaryocytes and erythroid precursors, and at the higher dosages is suppressed numbers of circulating leukocytes.

Platelet, antithrombin, and fibrinolytic activities in taurine-deficient and taurine-replete cats

Cats with cardiomyopathy, especially dilated cardiomyopathy associated with taurine deficiency, often develop systemic thrombi. To investigate the relation of taurine deficiency to formation and persistence of thrombi, cats were made taurine-deficient by consumption of a casein-based taurine-deficient diet, then were evaluated for anticoagulant and profibrinolytic activities and platelet function. The cats served as their own controls in the taurine-replete state; then, values were compared for the taurine-deficient state. Plasma (P < 0.01), blood (P < 0.05), and platelet (P < 0.05) taurine concentrations were decreased markedly after cats consumed the taurine-deficient diet for 6 weeks, compared with baseline concentrations before diet. Compared with the taurine-replete state, taurine deficiency induced significantly (P < 0.05) increased mean antithrombin III activity, no significant change in plasminogen and fibrinolytic activities, and similar clot retraction/lysis test results. Decreased (P < 0.01) adenosine diphosphate (ADP)-induced platelet aggregation and [14C]serotonin release, and slightly increased (P < 0.05) collagen-induced platelet [14C]serotonin release, but unchanged collagen-induced platelet aggregation were observed in taurine-deficient cats, compared with taurine-replete cats. Changes in antithrombin III activity most likely reflected hepatocellular acute-phase reaction, which indicates that taurine deficiency may induce a stress-responsive state. Results of platelet function testing indicate that taurine may modulate platelet responsiveness to physiologic agonists, but not in consistent manner. That platelets from the taurine-deficient cats had decreased responsiveness to ADP, but increased responsiveness to collagen is surprising, because irreversible aggregation is mediated by release of granule-associated ADP after sufficient initial stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of ticlopidine on canine platelets, fibrinogen, and antithrombin III activity

The effect of ticlopidine on platelet function, platelet number, mean platelet volume, antithrombin III activity, and fibrinogen was evaluated in 10 laboratory beagles. Ticlopidine (62 mg/kg) significantly inhibited ADP- and collagen-induced platelet aggregation within 2 days of the beginning of oral administration. Collagen-induced platelet 14C-serotonin release was not inhibited by day 9 of medication but was inhibited by day 20 in two of three beagles given medication for 32 days. Significant increases in mean platelet number were observed on days 2 and 5. The trend toward increased platelet number continued until day 16, at which time platelet number began to decrease toward baseline in three of three dogs treated for 32 days. Mean platelet volume (MPV) was significantly decreased compared to baseline on days 5 and 9. In three dogs treated for 32 days, the lowest MPV was observed on day 9 in two dogs and on day 12 in one dog. Significant changes were not observed in antithrombin III activity or fibrinogen with ticlopidine treatment.

Platelet function, antithrombin-III activity, and fibrinogen concentration in heartworm-infected and heartworm-negative dogs treated with thiacetarsamide

Platelet aggregation and release, platelet number, mean platelet volume, antithrombin-III activity, and fibrinogen concentration were evaluated in heartworm-negative and heartworm-infected dogs at baseline and on days 3, 10, and 21 after treatment with thiacetarsamide. Platelet reactivity was enhanced in a group of dogs naturally infected with Dirofilaria immitis, compared with 2 groups of heartworm-negative dogs, but platelet reactivity was not further enhanced after treatment with thiacetarsamide. A significant decrease in antithrombin-III activity was detected 21 days after treatment. The platelets from a group of laboratory Beagles implanted with 50 adult D immitis displayed enhanced reactivity 6 months after implantation, but by 18 months, platelet reactivity had returned to near, or less than, baseline. Platelet reactivity was enhanced after thiacetarsamide treatment in this group. Thiacetarsamide-associated changes were not observed in platelet number or size; antithrombin-III activity decreased, but the change was not significant. Fibrinogen concentration was increased significantly (P less than 0.05) on day 10. Enhanced adenosine diphosphate (ADP)-induced platelet aggregation was observed on days 3, 10, and 21 after treatment in heartworm-negative dogs. This change was not observed in 6 control Beagles not treated with thiacetarsamide. Although antithrombin-III activity was decreased on day 3 and fibrinogen concentration was increased on day 10, paralleling changes observed in the heartworm-infected dogs, the changes were not statistically significant. In this study, thiacetarsamide was procagulatory in heartworm-negative dogs and may be an important contributing factor to the thromboembolism observed with adulticidal therapy.

Effects of treatment with ticlopidine in heartworm-negative, heartworm-infected, and embolized heartworm-infected dogs

Ticlopidine hydrochloride was evaluated for its effectiveness in inhibiting platelet aggregation and serotonin release in 5 laboratory Beagles before and after heartworm implantation with 7 adult Dirofilaria immitis, and after embolization with 7 dead heartworms to mimic what happens after heartworm adulticide treatment. Five other laboratory Beagles, similarly implanted and embolized with heartworms, were used as nonmedicated controls. During the heartworm-negative stage, the dosage of ticlopidine that inhibited adenosine diphosphate (ADP)-induced platelet aggregation in 5 dogs by at least 50% after 5 days of treatment was 62 mg/kg of body weight once a day. In the same dogs implanted with 7 adult heartworms 21 days previously, mean (+/- SD) ticlopidine dosage required to obtain similar results was 71 (+/- 13) mg/kg given once daily. During the 21 days after dead heartworms were implanted in heartworm-infected dogs, mean ticlopidine dosage was 108 (+/- 35) mg/kg (range, 62 to 150 mg/kg). Ticlopidine treatment was associated with increased platelet numbers in all 5 dogs during the heartworm-negative stage and in 4 of 5 dogs during the heartworm implantation and heartworm embolization stages. Mean platelet volume tended to decrease as platelet numbers increased. At necropsy, gross and histologic pulmonary lesions were less severe in ticlopidine-treated dogs than in nonmedicated control dogs.

Effects of treatment with aspirin or aspirin/dipyridamole combination in heartworm-negative, heartworm-infected, and embolized heartworm-infected dogs

To determine the drug dose required to inhibit platelet reactivity by at least 50%, 2 drug regimens were evaluated in heartworm-negative, heartworm-infected, and heartworm-infected dogs embolized with dead heartworms. Aspirin, or a combination of aspirin and dipyridamole, were administered to 2 groups of Beagles (n = 5 each) for 5 to 9 days; a third group of 5 Beagles served as nontreated controls. For heartworm-negative dogs, mean (+/- SD) aspirin dosage that inhibited collagen-induced platelet reactivity by at least 50% was 6 (+/- 2) mg/kg of body weight given once daily. The aspirin/diphridamole combination dosage was 1 mg of each drug/kg given every 12 hours. All dogs (n = 15) were implanted with 7 adult heartworms each and remedicated (or not treated) beginning at 21 days after heartworm implantation. In heartworm-infected dogs, mean aspirin dosage required to inhibit collagen-induced platelet reactivity greater than or equal to 50% was 10 (+/- 6) mg/kg. Mean dosage of aspirin/dipyridamole combination was 1.6 +/- (0.5) mg of each drug/kg given every 12 hours. When platelet reactivity in response to collagen was determined to be inhibited by at least 50% in all medicated dogs, each dog (n = 15) was embolized with 7 dead adult heartworms to mimic heartworm adulticidal treatment. Platelet reactivity was monitored for 21 days after treatment, and drug dose was adjusted to maintain platelet inhibition by at least 50%. In embolized dogs, mean aspirin dosage was 17 (+/- 14) mg/kg given once daily. Mean dosage of the aspirin/dipyridamole combination was 2.8 (+/- 1.3) mg of each drug/kg given every 12 hours. All dogs (n = 15) were euthanatized 21 days after heartworm embolization. Each lung lobe was evaluated for severity of lesions and presence of organized or fibrinous thrombi. Lesion severity in the aspirin- and aspirin/dipyridamole-treated dogs was not significantly different from that in control dogs.

Potentiation of platelet responses in vitro by feline infectious peritonitis virus

The effect of feline infectious peritonitis virus (FIPV) on platelet aggregation and 14C-serotonin release induced by threshold levels of four agonists (adenosine diphosphate [ADP], collagen, arachidonic acid, and epinephrine) was examined in vitro in ten specific-pathogen-free cats. Purified suspensions of FIPV added to stirred platelet suspensions (virus to platelet ratio equal to 1:320) 1 minute prior to the addition of agonist potentiated the ADP-induced aggregation response by greater than 100% in seven cats. Platelet 14C-serotonin release was increased by greater than 100% in four cats. Collagen-induced platelet aggregation was enhanced in ten cats while collagen-induced 14C-serotonin release was enhanced in eight cats. Potentiation of arachidonic acid-induced platelet aggregation was observed in three cats, two of which demonstrated enhanced platelet 14C-serotonin release. Although epinephrine-induced platelet aggregation was enhanced in five cats, the samples displayed only fine microaggregates. Enhanced 14C-serotonin release from platelets in response to epinephrine was not demonstrated. Interaction with the outer platelet membrane and internalization of viral particles within the surface-connected open canalicular system were demonstrated by electron microscopy within 5 minutes of the addition of virus to platelet suspensions with or without added agonists. Decreasing the virus concentration by ten- or one hundred-fold abolished the potentiating effect observed previously, while increasing the concentration tenfold resulted in direct platelet activation in the absence of agonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced platelet reactivity in cats experimentally infected with feline infectious peritonitis virus

Platelet function was evaluated in six specific-pathogen-free cats prior to and following intraperitoneal inoculation with feline infectious peritonitis virus (FIPV). By 4 days post-inoculation, platelet samples from five of six cats responded with irreversible platelet aggregation to threshold concentrations of adenosine diphosphate (ADP). This was accompanied by enhanced platelet 14C-serotonin release (greater than 10%) in two cats. Compared to one of six baseline samples, five of five post-inoculation samples exhibited microaggregate formation in response to 20 microM epinephrine. Enhanced platelet 14C-serotonin release did not accompany these responses. Enhanced platelet responses to ADP and epinephrine were also observed on day 11 post-inoculation and day 16 (when one cat died) or 21 (the end of the study). Platelet 14C-serotonin release in response to 20 microM epinephrine increased markedly in three of five cats on day 21. Enhanced collagen-induced platelet responses were not demonstrated. Although the mechanism for the enhanced platelet responses observed on day 4 was unknown, a direct effect on the virus on platelets, mononuclear inflammatory cells, and endothelial cells must be considered.

Evaluation of antithrombin-III activity as a coindicator of disseminated intravascular coagulation in cats with induced feline infectious peritonitis virus infection

Six adult specific-pathogen-free cats were inoculated intraperitoneally with a cell culture-adapted strain of feline infectious peritonitis virus. Plasma samples were evaluated for antithrombin-III (AT-III) activities at post-inoculation days (PID) 0, 4, and 11 and at termination on PID 16 (1 cat) or 21 (5 cats). Other hemostatic values evaluated were activated partial thromboplastin times, prothrombin times, thrombin times, fibrinogen, platelet counts, and fibrin/fibrinogen degradation products. Antithrombin-III activity remained within normal or above normal range (89 to 246%) in all cats, with the exception of one cat on PID 4 (AT-III, 70%). Mean baseline AT-III activity for 6 cats at PID 0 was 123%. Mean AT-III activity on PID 4, 11, and 16 or 21 was 98, 162, and 130%, respectively. On PID 4 and 16 or 21, results of coagulation screening tests indicated that all cats had disseminated intravascular coagulation. Histologically, cats also had severe fibrinonecrotizing thrombovasculitis.

Enhanced platelet reactivity in heartworm-infected dogs

Platelet number, mean platelet volume, and platelet function were evaluated in 34 clinically normal dogs and 28 heartworm-infected (HWI) dogs. Mean platelet numbers for dogs of the HWI group was not significantly lower than those for dogs of the control group (214,000 vs 254,000 cells/microliter); however, 6 (21%) HWI dogs had platelet numbers less than 150,000/microliter, compared with only 2 (6%) heartworm-negative dogs. The mean platelet volume was not significantly different (7.8 vs 7.7 fl) between the 2 groups of dogs. Mean platelet aggregation responses to intermediate and low concentrations of collagen (3.0 and 1.5 micrograms) and to high, intermediate, and low concentrations of ADP (25, 10, and 5 microM) were greater in dogs of the HWI group. Mean platelet 14C-serotonin release was also greater in HWI dogs in response to high concentration of ADP (25 microM) and to intermediate concentration of collagen (3.0 micrograms).

The pony as an animal model for vascular implants

This study evaluated the pony as a potentially suitable model for vascular implant research. Healthy, conditioned ponies were randomly assigned to one of three groups: group I, carotid artery autografts (n = 6); group II, e-PTFE carotid interpositional grafts (n = 5); and group III, e-PTFE carotid interpositional grafts plus aspirin (10 mg/kg) and dipyridamole (3.5 mg/kg) drug administration. It was found that autografts remained patent longest (mean = 396.2 days; grafts were still patent at time of writing) followed by group III grafts (157.5 days), with group II grafts remaining patent for the shortest duration (61.1 days), (p less than 0.01). Patency was determined using two-dimensional real-time ultrasonography with Doppler velocimetry and/or arteriography. It was demonstrated that the pony's response to antithrombotic drugs was consistent and comparable to that in other animal models, both with respect to platelet function and affect on patency rate. The combination of the ease of surgical manipulation, drug administration, and platelet function testing, the comparable size of the pony and its heart and blood vessels to that of an adult human, the long life span of ponies, and the patency results of this study have demonstrated that the pony is a valuable animal model for vascular research.

Combined factors IX and XII deficiencies in a family of cats

Combined factors IX and XII deficiencies were detected in a family of cats in which 2 male kittens had bleeding diathesis. The combination of factors IX and XII deficiencies in one male kitten did not appear to exacerbate bleeding when compared with a sole deficiency of factor IX in its male sibling. Neutering of carrier females and affected males was recommended. Blood transfusions before castration of affected males was advised.

Platelet function testing in the pony

Platelet isolation techniques and platelet function were evaluated in 35 adult ponies. Platelet recovery from whole blood was consistent and the preparation of platelet rich plasma was facilitated by an enhanced erythrocyte sedimentation rate. All platelet samples aggregated in response to 10 microM ADP. However, concentrations of ADP as high as 100 microM did not elicit significant 14C-serotonin release. Collagen induced irreversible platelet aggregation and 14C-serotonin release in all samples. The threshold dose for collagen in most ponies was 1.5 micrograms. Arachidonic acid (500 microM) failed to induce irreversible platelet aggregation or 14C-serotonin release in any of the samples evaluated. Pony platelets were nonresponsive to epinephrine (5.5 microM).

Impaired cAMP metabolism associated with abnormal function of thrombopathic canine platelets

The effects of forskolin and 1-Methyl-3-isobutyl-xanthine on thrombopathic platelet function and cyclic AMP accumulation were examined. The concentration of forskolin required to inhibit gamma-thrombin- induced platelet aggregation and secretion by 50% was significantly lower for thrombopathic than for normal platelets. This inhibition was accompanied by a marked elevation of cyclic AMP. 1-Methyl-3-isobutyl-xanthine, alone or in combination with forskolin, augmented both the cyclic AMP accumulation and the inhibition of platelet function. These results demonstrate that cyclic AMP metabolism is abnormal in thrombopathic platelets and imply that cyclic AMP-phosphodiesterase activity is impaired.

Evidence for regulatory control of canine platelet phosphodiesterase

Forskolin, epinephrine, and prostaglandin I2 were used to examine the adenylate cyclase-phosphodiesterase system of intact thrombopathic and normal canine platelets. The results provide indirect support for the hypothesis that the elevation of intraplatelet c-AMP in this unique hereditary defect is due to impaired phosphodiesterase activity. The inhibitory (Nj) and stimulatory (Ns) components of adenylate cyclase appeared functionally intact. Cytosolic fractions of normal and thrombopathic platelets had similar cAMP hydrolytic activities. The failure of intact forskolin-stimulated thrombopathic platelets to return elevated cAMP to non-stimulated levels after 15 min, despite significant phosphodiesterase activity in cytosolic fractions, implies that the platelet isoenzymes are under regulatory control.