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

The study examined the effects of three alpha2-adrenergic receptor antagonists – yohimbine, tolazoline, and atipamezole – on the pharmacodynamics of detomidine, a drug commonly administered sublingually to horses, revealing that the antagonists have a temporary and incomplete effect on the impacts created by detomidine.

Study Design

• The researchers utilized a randomized crossover design with four treatment groups and nine healthy adult horses.
• The four treatment groups varied the combination of drugs administered to the horses – detomidine by itself, detomidine followed by yohimbine, detomidine followed by tolazoline, and detomidine followed by atipamezole.
• Each horse underwent all treatments, with at least one week between each to curtail overlap effects.
• A technique called liquid chromatography-mass spectrometry was used to analyze blood samples from the horses for the presence and concentration of yohimbine, atipamezole and tolazoline.
• The researchers monitored the horses’ behavioral effects, heart rate, rhythm, and levels of glucose, packed cell volume, and plasma proteins during the treatment.

Findings

• The distance from the horse’s chin to the ground increased after giving the antagonists, meaning the horse’s head was lifted. However, this effect was transient, as it returned to normal within an hour.
• The administration of detomidine reduced the horses’ heart rates (bradycardia) and increased occurrence of irregular heartbeats (atrioventricular blocks). These heart complications were only temporarily or partially mitigated by the antagonists.
• Administering tolazoline resulted in a surge in the horse’s packed cell volume and glucose concentrations. Atipamezole was subjectively observed to increase urination frequency (though not volume).

Conclusion

• The study concluded that at the dosages used, the three alpha2-adrenergic antagonists resulted in temporary and incomplete effects on the cardiac and behavioral consequences brought about by detomidine.