A protocol was developed for preparation of platelet concentrates (PC) to support thrombocytopenic dogs. Four clinically normal dogs with platelet counts that ranged from 200 to 330 X 10(9) platelets/L were used as donors. One unit (450 ml) of blood was collected by venipuncture into a double blood bag. Whole blood (WB) was centrifuged for 4 minutes at 1,000 X g (braking time = 2 minutes, 30 seconds) to prepare platelet-rich plasma (PRP). The PRP was expressed into the satellite bag and was centrifuged for 10 minutes at 2,000 X g (braking time = 2 minutes, 36 seconds). The platelet-poor plasma was expressed, leaving 40 to 70 ml of plasma and the pelleted platelets in the satellite bag. The resulting PC was left undisturbed for 60 minutes to promote disaggregation, and the platelets were then resuspended by gentle manual agitation. Forty-eight PC were prepared. Mean (+/- SD) platelet yield from WB to PRP was 78 +/- 13)% (range, 35 to 97%); yield from PRP to PC was 94 (+/- 6) % (range, 75 to 100%); and overall yield (PC from WB) was 74 (+/- 13) % (range, 36 to 91%). Mean PC platelet count was 8.0 (+/- 3.0) X 10(10) platelets/PC (range, 2.3 to 13.4 X 10(10) platelets/PC). The WBC content was 0.1 to 2.3 X 10(9) platelets/PC, representing 3 to 74% of WBC in the WB. Hematocrit was 0.1 to 26.2%. Results of bacterial and fungal culturing were negative.

The effects of exogenous platelet-activating factor (PAF) were determined in anesthetized ponies. Administration of PAF induced a decrease in cardiac index that resulted in systemic hypotension. This was followed by tachycardia, hypertension, and a return of cardiac index to baseline. Pulmonary arterial pressure increased markedly because of pulmonary vasoconstriction. Exogenous PAF also caused leukopenia and thrombocytopenia. The specific PAF receptor antagonist (WEB 2086) blocked all PAF-induced changes. Flunixin meglumine, a cyclooxygenase inhibitor, abolished the pulmonary hypertension and tachycardia, and attenuated the systemic hypotension but did not change the PAF-induced peripheral cellular changes. The PAF antagonist also inhibited platelet aggregation induced by PAF in vitro. The PAF-induced changes are similar to those reported after endotoxin exposure in horses.

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 afl ponies after correction of colonic torsion, cardiac index increased initially, but then decreased significantly (P < 0.05) over the study period. Mean times between correction of torsion and onset of cardiac failure and death were not different between groups. Significant (P < 0.05) thrombocytopenia developed during the reperfusion period in control ponies, but not in WEB-treated ponies. Blood leukocyte concentration in control ponies was more variable and significantly (P < 0.05) decreased immediately upon reperfusion, compared with that in WEB-treated ponies. We conclude that although the cardiovascular responses to colonic ischemia and reperfusion are not prevented by use of a specific PAF-antagonist, specific peripheral cellular responses are mediated by PAF.

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 pro-fibrinolytic 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 a 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. All cats had normal clot retraction in dilute blood, which indicated adequate platelet numbers and function; however, clots failed to lyse in vitro. To the authors knowledge, this observation, at present, lacks adequate explanation. Development of marked taurine deficiency and altered in vitro results of anticoagulant activities and some platelet function tests did not result in clinical manifestations in our cats. Results of our study do not conclusively document a pathophysiologic role of taurine depletion in the formation or persistence of thrombi.

Changes in platelet indices (platelet count and platelet size) and PCV associated with thyroid disease were studied in 7 dogs with hypothyroidism and 21 cats with hyperthyroidism that were admitted to the veterinary teaching hospital. Compared with control (euthyroid) dogs, dogs with hypothyroidism had higher platelet count (P = 0.003), smaller platelet size (P = 0.01), and lower PCV (P = 0.02). Comparison of the group of hyperthyroid cats with a group of similarly aged, clinically normal cats with normal thyroxine values indicated that the group of hyperthyroid cats had significantly (P = 0.03) higher mean platelet size than did control cats, but differences were not found in mean platelet count or PCV. Results of this investigation indicate that the changes in platelet size reported in human beings with thyroid endocrinopathies also are found in animals so-affected. Although the pathogenesis of platelet abnormalities in animals with thyroid derangement is unclear and likely is multifactorial, the observed relation between platelet and erythrocyte production in this group of dogs is consistent with reports of an inverse relation between thrombocytopoiesis and erythropoiesis in iatrogenically hyperthyroid mice and in mice exposed to hypoxia.