Objective: To investigate effects of various imidazoline and nonimidazoline α-adrenergic agents on aggregation and antiaggregation of bovine and equine platelets. Sample: Blood samples obtained from 8 healthy adult cattle and 16 healthy adult Thoroughbreds. Procedures: Aggregation and antiaggregation effects of various imidazoline and nonimidazoline α-adrenergic agents on bovine and equine platelets were determined via a turbidimetric method. Collagen and ADP were used to initiate aggregation. Results: Adrenaline, noradrenaline, or α-adrenoceptor agents alone did not induce changes in aggregation of bovine or equine platelets or potentiate ADP- or collagen-induced platelet aggregation. Adrenaline and the α2-adrenoceptor agonist clonidine had an inhibitory effect on ADP- and collagen-induced aggregation of bovine platelets. The α2-adrenoceptor antagonists phentolamine and yohimbine also inhibited collagen-induced aggregation of bovine platelets. Noradrenaline, other α-adrenoceptor agonists (xylazine, oxymetazoline, and medetomidine), and α-adrenoceptor antagonists (atipamezole, idazoxan, tolazoline, and prazosin) were less effective or completely ineffective in inhibiting ADP- and collagen-induced aggregation of bovine platelets. The imidazoline α2-adrenoceptor agonist oxymetazoline submaximally inhibited collagen-induced aggregation of equine platelets, and the α2-adrenoceptor antagonist idazoxan, along with phentolamine and yohimbine, also inhibited collagen-induced aggregation of equine platelets. The imidazoline compound antazoline inhibited both ADP- and collagen-induced aggregation of equine platelets. Conclusions and Clinical Relevance: Several drugs had effects on aggregation of platelets of cattle and horses, and effective doses of ADP and collagen also differed between species. The α2-adrenoceptor agonists (xylazine and medetomidine) and antagonists (tolazoline and atipamezole) may be used by bovine and equine practitioners without concern for adverse effects on platelet function and hemostasis.

Objective-To evaluate the platelet activation response before and after treatment with clopidogrel in horses. Animals-12 healthy adult mares. Procedures-In a masked study, horses (6/group) were randomly allocated to alternately receive placebo or clopidogrel via nasogastric tube at a loading dose of 4 mg/kg followed by 2 mg/kg every 24 hours. Blood samples were collected before and 72 hours after initiation of treatment for ADP- and collagen-induced light transmission aggregometry; determination of closure time in collagen-ADP cartridges; modified thrombelastography for comparison of maximal amplitudes generated by kaolin, reptilase, and reptilase plus ADP activation; and flow cytometric tests to detect platelet fibrinogen binding, P-selectin expression, and phosphatidylserine externalization before and after ex vivo stimulation with thrombin, convulxin, thrombin with convulxin, and calcium ionophore. Results-Clopidogrel administration induced a significant decrease in mean aggregation response to 5μM and 10μM ADP stimulation; however, 2 horses had resistance to clopidogrel's inhibitory action. Significant differences after clopidogrel treatment were not found in any other tests of platelet function. Conclusions and Clinical Relevance-Assays using commercially available reagents were configured to measure different variables of the platelet activation response; however, clopidogrel's platelet inhibitory action was only detected by ADP-induced light transmission aggregometry. Results also suggested that horses, like humans, have interindividual variability in response to clopidogrel that may influence the drug's clinical efficacy as an antiplatelet agent.

Objective-To determine whether soybean oil emulsion has an in vitro effect on platelet aggregation and thromboelastography in blood samples obtained from healthy dogs. Animals-12 healthy adult dogs. Procedures-Blood samples were collected from each dog into tubes containing EDTA, hirudin, or sodium citrate for a CBC, collagen- and ADP-induced impedance aggregometry, or thromboelastography, respectively. Whole blood platelet aggregation, determined with ADP or collagen agonists, was measured in blood samples containing hirudin and final lipid concentrations of 0, 1, 10, and 30 mg/mL. The thromboelastographic variables R (reaction time), K (clotting time), α angle, and maximum amplitude were evaluated in blood samples containing sodium citrate and final lipid concentrations equivalent to those used for assessment of platelet aggregation. Results-Median maximum ADP- and collagen-induced platelet aggregation in blood samples containing 1, 10, or 30 mg of lipid/mL did not differ significantly from the value for the respective lipid-free blood sample. Maximum amplitude determined via thromboelastography was significantly reduced in blood samples containing 10 and 30 mg of lipid/mL, compared with findings for lipid-free blood samples. Values of other thromboelastographic variables did not differ, regardless of lipid concentrations. Conclusions and Clinical Relevance-Maximum amplitude determined via thromboelastography in canine blood samples was significantly affected by the addition of lipid to final concentrations that are several orders of magnitude higher than clinically relevant lipid concentrations in dogs. Lipid treatment appears to have no significant effect on hemostatic variables in dogs, although clinical studies should be performed to confirm these in vitro findings.