title: Characterisation of the cardiovascular pharmacology of medetomidine in the horse and sheep.
abstract: 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.
authors.affiliation: Department of Farm Animal and Equine Medicine and Surgery, The Royal Veterinary College, The University of London, Hatfield, Herts.
authors.forename: C E
authors.initials: CE
authors.lastname: Bryant
authors.forename: J
authors.initials: J
authors.lastname: Thompson
authors.forename: K W
authors.initials: KW
authors.lastname: Clarke
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000008:administration & dosage / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects*; D006339:Heart Rate / Q000187:drug effects*; D006736:Horses / Q000502:physiology*; D007093:Imidazoles / Q000008:administration & dosage / Q000494:pharmacology*; D007275:Injections, Intravenous; D020926:Medetomidine; D011224:Prazosin / Q000494:pharmacology; D011807:Quinolizines / Q000494:pharmacology; D012756:Sheep / Q000502:physiology*
chemical_list: D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D007093:Imidazoles; D011807:Quinolizines; C055732:vatinoxan; D020926:Medetomidine; D011224:Prazosin