Medetomidine was administered to sheep and horses at a dose rate of 5 microg kg(-1) (i.v.). Heart rate and blood pressure were recorded.
Medetomidine induced
bradycardia and a biphasic blood pressure response consisting of a transient
hypertension followed by
hypotension. Administration of
prazosin (an alpha1
adrenoceptor antagonist; 100 microg kg(-1), i.v.) had no effect on the cardiovascular response to
medetomidine (5 microg kg(-1), i.v.), but inhibited the cardiovascular response of
methoxamine (an alpha1
adrenoceptor agonist; 75 microg kg(-1), i.v.).
L-659,066 (an alpha2
adrenoceptor antagonist which does not cross the blood brain barrier; 264 microg kg(-1), i.v.) attenuated the
medetomidine induced
bradycardia, but had no effect on the cardiovascular response to
methoxamine. L659,066 also reduced the
medetomidine induced
hypertension in sheep, but had less effect on the horse. It is concluded that both alpha1 and alpha2
adrenoceptors are important in the control of cardiovascular function in horses and sheep.
Medetomidine appears to act on alpha2
adrenoceptors alone in the sheep. The cardiovascular effects of
medetomidine in the horse are complex and may be influenced by central alpha2
adrenoceptor regulation or effects on other receptor subtypes as well as direct stimulation of peripheral alpha2
adrenoceptors.