Characterisation of the cardiovascular pharmacology of medetomidine in the horse and sheep.

This research examines the effects of the drug medetomidine on the cardiovascular function of sheep and horses, concluding that the drug primarily influences alpha2-adrenoceptors in both animals. The cardiovascular effects of medetomidine in horses, however, show complexity likely due to other factors, including central alpha2-adrenoceptor regulation and possible implication of other receptor types.

Methodology and Experimentation

• The researchers administered medetomidine to both sheep and horses at a dose rate of 5 microg kg(-1).
• They monitored and recorded the heart rate and blood pressure of the subjects to track the cardiovascular response.
• Beyond medetomidine, the researchers also experimented with prazosin—an alpha1 adrenoceptor antagonist, and methoxamine—an alpha1 adrenoceptor agonist, to test their effects on the cardiovascular responses of sheep and horses.
• They also experimented with L-659,066—a specific alpha2 adrenoceptor antagonist which does not cross the blood-brain barrier—and observed its impact on the cardiovascular responses in both animals.

Key Findings

• Administration of medetomidine resulted in bradycardia (slowing of the heart rate) and a biphasic blood pressure response, leading to an initial phase of hypertension (high blood pressure) followed by hypotension (low blood pressure).
• Prazosin did not affect the cardiovascular response to medetomidine while it efficientely inhibited the response to methoxamine.
• L-659,066 diminished the medetomidine-induced bradycardia but had no impact on the cardiovascular response to methoxamine. Moreover, L-659,066 also reduced the hypertension induced by medetomidine in sheep to a greater extent than in horses.

Conclusion

• The findings suggest that both alpha1 and alpha2 adrenoceptors play significant roles in controlling cardiovascular function in horses and sheep.
• In sheep, medetomidine seems to act solely on alpha2 adrenoceptors.
• The cardiovascular effects of medetomidine in horses appear to be more complex and could be influenced not only by the direct stimulation of peripheral alpha2 adrenoceptors but also by the regulation of central alpha2 adrenoceptors or effects on other unspecified receptor subtypes.