Effects of imidazoline and nonimidazoline alpha-adrenergic agents, including xylazine, medetomidine, yohimbine, tolazoline, and atipamezole, on aggregation of bovine and equine platelets

Objective-To investigate effects of various imidazoline and nonimidazoline alpha-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 alpha-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 alpha-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 alpha(2)-adrenoceptor agonist clonidine had an inhibitory effect on ADP- and collagen-induced aggregation of bovine platelets. The alpha(2)-adrenoceptor antagonists phentolamine and yohimbine also inhibited collagen-induced aggregation of bovine platelets. Noradrenaline, other alpha-adrenoceptor agonists (xylazine, oxymetazoline, and medetomidine), and alpha-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 alpha(2)-adrenoceptor agonist oxymetazoline submaximally inhibited collagen-induced aggregation of equine platelets, and the alpha(2)-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 alpha(2)-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. (Am J Vet Res 2013;74:395-402)