title: Possible subdivision of postsynaptic alpha-adrenoceptors mediating pressor responses in the pithed rat.
abstract: Additional evidence has been obtained indicating a possible subclassification of postsynaptic alpha-adrenoceptors into alpha 1 - and alpha 2 -subtypes. The pressor responses to the alpha-adrenoceptor agonists L-phenylephrine and guanfacine were quantified after i.v. administration to pithed rats. The alpha-sympatholytic drug yohimbine (1 mg/kg) displaced both dose-response curves to the right, but the effect was greatest for guanfacine. After prazosin (0.1 mg/kg) a 53-fold shift to the right was noticed for the dose-response characteristic of L-phenylephrine. Prazosin antagonized the effect of only the higher doses of guanfacine. The findings indicate that L-phenylephrine and prazosin preferentially interact with alpha 1 -adrenoceptors as agonist and antagonist, respectively. Yohimbine proved less selective than prazosin, but preferentially blocks postjunctional alpha 2 -adrenoceptors in the vascular wall. The results obtained with guanfacine may be interpreted to indicate that this drug acts on alpha 2 -adrenoceptors at lower doses and additionally stimulates alpha 1 -adrenoceptors at higher ones. Preliminary findings with corynanthine and rauwolscine support this interpretation.
mesh_terms: D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D008297:Male; D010656:Phenylephrine / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D051381:Rats; D011941:Receptors, Adrenergic / Q000502:physiology*; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D013116:Spinal Cord / Q000502:physiology; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D015016:Yohimbine; D011224:Prazosin