title: Characterization of postjunctional alpha-1 and alpha-2 adrenoceptors activated by exogenous or nerve-released norepinephrine in the canine saphenous vein.
abstract: Experiments were designed to characterize alpha-1 and alpha-2 adrenoceptor-mediated effects in the canine saphenous vein. Rings of saphenous vein were mounted for isometric tension recording in physiological saline solution. Contractile responses evoked by alpha-1 adrenoceptor agonists, cirazoline or St 587 were inhibited by alpha-1 antagonists, prazosin (pA2 = 7.9) or phenoxybenzamine, but were relatively resistant to the alpha-2 adrenoceptor antagonist rauwolscine. The responses to alpha-2 adrenoceptor agonists, xylazine or B-HT 920, were relatively resistant to prazosin or phenoxybenzamine but were antagonized by rauwolscine (pA2 = 8.7). After phenoxybenzamine, the alpha-2 agonists, M-7, guanfacine, UK 14304, B-HT 920 and xylazine evoked similar maximal increases in tension which were considerably smaller (approximately 50%) than that attained by alpha-1 adrenoceptor stimulation. The different concentration-effect characteristics of these responses were also revealed using norepinephrine. Prazosin produced a biphasic effect on the concentration-response curve of norepinephrine, being more potent against responses above 50% of the maximum (pA2 = 7.9) compared to lower increases in tension (pA2 = 6.2). After alpha-1 adrenoceptor blockade with prazosin, rauwolscine was more effective against responses below 50% of the maximum, compared to higher increases in tension. The results suggest that the alpha-1 and alpha-2 adrenoceptor-mediated concentration-effect curves to norepinephrine are almost coincident and that alpha-2 adrenergic stimulation produces only partial activation of the vascular smooth muscle.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000818:Animals; D003000:Clonidine / Q000031:analogs & derivatives / Q000494:pharmacology; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D005260:Female; D007093:Imidazoles / Q000494:pharmacology; D008297:Male; D008729:Methoxamine / Q000494:pharmacology; D009638:Norepinephrine / Q000097:blood*; D010643:Phenoxybenzamine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*; D012501:Saphenous Vein; D013564:Sympathetic Nervous System / Q000378:metabolism*; D014439:Tyramine / Q000494:pharmacology; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D007093:Imidazoles; D011942:Receptors, Adrenergic, alpha; D010643:Phenoxybenzamine; C030655:St 587; D010656:Phenylephrine; D015016:Yohimbine; D008729:Methoxamine; D003000:Clonidine; C014282:cirazoline; D009638:Norepinephrine; D014439:Tyramine; D011224:Prazosin