title: Alpha 2-adrenoceptor agonists enhance vasoconstrictor responses to alpha 1-adrenoceptor agonists in the rat tail artery by increasing the influx of Ca2+.
abstract: 1. The alpha 2-adrenoceptor agonists TL99 (2-(N N-dimethyl)amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene) and UK14304 (5-bromo-6-[2-imidazoline-2-yl-aminol]-quinoxaline), in concentrations that are less than 1% of those producing vasoconstriction, enhance vasoconstrictor responses to noradrenaline and phenylephrine in isolated perfused preparations of the rat tail artery. 2. The enhancing effect was abolished when Ca2+ was absent and by the calcium channel blocking drug diltiazem. 3. alpha 2-Adrenoceptor agonists had no effect on the component of the responses to noradrenaline and phenylephrine that is attributable to mobilization of intracellular Ca2+, but enhanced the component attributable to influx of extracellular Ca2+. 4. These results suggest that the enhancing effect of alpha 2-adrenoceptor agonists on responses of the rat tail artery to alpha 1-adrenoceptor agonists involves an increase in Ca2+-influx into smooth muscle cells through Ca2+ channels that are opened when alpha 2-adrenoceptors are activated.
authors.affiliation: Department of Pharmacology, University of Melbourne, Victoria, Australia.
authors.forename: X H
authors.initials: XH
authors.lastname: Xiao
authors.forename: M J
authors.initials: MJ
authors.lastname: Rand
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D001158:Arteries / Q000187:drug effects; D000068438:Brimonidine Tartrate; D002118:Calcium / Q000378:metabolism*; D004110:Diltiazem / Q000494:pharmacology; D066298:In Vitro Techniques; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000378:metabolism; D009638:Norepinephrine / Q000494:pharmacology; D010477:Perfusion; D010656:Phenylephrine / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D012039:Regional Blood Flow / Q000187:drug effects; D013623:Tail / Q000098:blood supply; D013764:Tetrahydronaphthalenes / Q000494:pharmacology; D014661:Vasoconstriction / Q000187:drug effects*
chemical_list: D000316:Adrenergic alpha-Agonists; D011810:Quinoxalines; D013764:Tetrahydronaphthalenes; D010656:Phenylephrine; D000068438:Brimonidine Tartrate; C016893:2-(N,N-dimethylamino)-6,7-dihydroxytetralin; D004110:Diltiazem; D002118:Calcium; D009638:Norepinephrine