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.