title: Effects of prazosin and piperoxan on central cardiovascular actions of St 91 in cats.
abstract: Intravenous administration of St 91 (2-(2,6-diethylphenylimino)-imidazolidine hydrochloride) (8 and 16 micrograms/kg) to anaesthetized cats, produced a dose-related but brief rise in blood pressure accompanied by bradycardia. The bradycardia was biphasic. An initial rapid fall in heart rate accompanied the pressor response and was followed by a less pronounced residual bradycardia that long outlasted the pressor response. Prazosin (5 micrograms/kg i.v.) reduced both the mean resting blood pressure and the maximum pressor response to St 91 (8 and 16 micrograms/kg i.v.) and unmasked a small secondary hypotension. The bradycardia was not diminished. Atropine (1 mg/kg i.v.) did not alter the pressor response to St 91 (16 micrograms/kg i.v.) but depressed the initial bradycardia accompanying the pressure rise. The residual bradycardia was not reduced. Piperoxan (150 micrograms/kg i.v. or 10 micrograms/kg i.a. vert.), given after St 91 (16 micrograms/kg i.v.) abolished the residual bradycardia whereas pretreatment with piperoxan (150 micrograms/kg, intracisternally) inhibited both the initial and the residual bradycardia. St 91 (8 and 16 micrograms/kg i.a. vert.) caused hypotension and bradycardia. Prazosin (5 micrograms/kg i.a. vert.) inhibited the hypotension only, but the same dose i.v. was ineffective. Piperoxan (150 micrograms/kg i.v. or 10 micrograms/kg i.a. vert.) given 15 min after St 91 (16 micrograms/kg i.a. vert.) reduced both the hypotension and bradycardia. The results show that the long-lasting residual bradycardia caused by i.v. St 91 is due to activation of central alpha-adrenoceptors. Selective inhibition of the St 91 (i.a. vert.) hypotension by prazosin may indicate that different receptors mediate hypotension and bradycardia but more likely shows that the central pathways differ.
mesh_terms: D000818:Animals; D001285:Atropine / Q000494:pharmacology; D001794:Blood Pressure / Q000187:drug effects; D001921:Brain / Q000187:drug effects; D002415:Cats; D003000:Clonidine / Q000031:analogs & derivatives* / Q000494:pharmacology; D005260:Female; D006339:Heart Rate / Q000187:drug effects; D006439:Hemodynamics / Q000187:drug effects*; D007269:Injections, Intra-Arterial; D007275:Injections, Intravenous; D008297:Male; D010880:Piperidines / Q000494:pharmacology*; D010883:Piperoxan / Q000494:pharmacology*; D011224:Prazosin / Q000494:pharmacology*; D011799:Quinazolines / Q000494:pharmacology*; D014711:Vertebral Artery
chemical_list: D010880:Piperidines; D011799:Quinazolines; C011266:ST 91; D001285:Atropine; D010883:Piperoxan; D003000:Clonidine; D011224:Prazosin