Central and peripheral alpha-adrenoceptor actions of amitraz in the dog.
The aim of this study was to determine the contribution of alpha 2- and alpha 1-adrenoceptor agonist activity of the formamidine, amitraz, on peripheral circulation in the dog. Intra-arterial injections of amitraz (0.25-5.0 micrograms/kg) produced a dose-dependent increase in perfusion pressure in the autoperfused hind limbs of methoxyflurane-anaesthetized dogs. A constant blood flow to the hind limbs was maintained using a peristaltic pump. Intravenous phentolamine (0.5 mg/kg), prazosin (35 micrograms/kg) and yohimbine (10 micrograms/kg) in separate experiments antagonized the vasoconstrictor actions of amitraz and produced a parallel shift to the right of the amitraz dose-response curve. Cumulative doses of amitraz (0.5-15 micrograms/kg) given by intracisterna magna (i.c.m.) injections reduced mean arterial pressure and heart rate in a dose-dependent manner. Similar responses were produced by intravenous amitraz but at much higher doses. In separate experiments amitraz-induced hypotension (doses up to 25 micrograms/kg i.c.m.) was prevented by pre-treatment with yohimbine (30 micrograms/kg i.c.m.) but not prazosin (20 micrograms/kg i.c.m.). Both antagonists partially inhibited the bradycardia produced by amitraz. It is concluded that amitraz stimulates alpha 1- and alpha 2-adrenoceptors to produce vascular constriction. The central hypotensive action of amitraz appears to be mediated by alpha 2-adrenoceptors; however, both receptor subtypes appear to be stimulated to produce bradycardia.
Cullen
L K
LK
School of Veterinary Studies, Murdoch University, Western Australia.
Reynoldson
J A
JA
D000818:Animals; D001794:Blood Pressure / Q000187:drug effects*; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D005260:Female; D006339:Heart Rate / Q000187:drug effects*; D006614:Hindlimb / Q000098:blood supply / Q000187:drug effects; D008297:Male; D011224:Prazosin / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects; D012039:Regional Blood Flow / Q000187:drug effects; D014052:Toluidines / Q000037:antagonists & inhibitors / Q000494:pharmacology*
D011942:Receptors, Adrenergic, alpha; D014052:Toluidines; C014983:amitraz; D011224:Prazosin