Effects of three antagonists on selected pharmacodynamic effects of sublingually administered detomidine in the horse

Objective: To describe the effects of alpha₂-adrenergic receptor antagonists on the pharmacodynamics of sublingual (SL) detomidine in the horse. Study design: Randomized crossover design. Animals: Nine healthy adult horses with an average age of 7.6 ± 6.5 years. Methods: Four treatment groups were studied: 1) 0.04 mg kg^-1 detomidine SL; 2) 0.04 mg kg^-1 detomidine SL followed 1 hour later by 0.075 mg kg^-1 yohimbine intravenously (IV); 3) 0.04 mg kg^-1 detomidine SL followed 1 hour later by 4 mg kg^-1 tolazoline IV; and 4) 0.04 mg kg^-1 detomidine SL followed 1 hour later by 0.12 mg kg^-1 atipamezole IV. Each horse received all treatments with a minimum of 1 week between treatments. Blood samples were obtained and plasma analyzed for yohimbine, atipamezole and tolazoline concentrations by liquid chromatography-mass spectrometry. Behavioral effects, heart rate and rhythm, glucose, packed cell volume (PCV) and plasma proteins were monitored. Results: Chin-to-ground distance increased following administration of the antagonists, however, this effect was transient, with a return to pre-reversal values as early as 1 hour. Detomidine induced bradycardia and increased incidence of atrioventricular blocks were either transiently or incompletely antagonized by all antagonists. PCV and glucose concentrations increased with tolazoline administration, and atipamezole subjectively increased urination frequency but not volume. Conclusions and clinical relevance: At the doses administered in this study, the alpha₂-adrenergic antagonistic effects of tolazoline, yohimbine and atipamezole on cardiac and behavioral effects elicited by SL administration of detomidine are transient and incomplete.