Alpha-adrenoceptors in equine digital veins: evidence for the presence of both alpha1 and alpha2-receptors mediating vasoconstriction.
Rings of equine digital vein examined under conditions of isometric tension recording constricted to alpha-adrenoceptor agonists with an order of potency of 5-bromo-6-[2-imidazolin-2-yl-amino]-quinoxaline bitartrate (UK 14304) = noradrenaline > 6-Allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo-(4,5-d) azepine (BHT-920) > phenylephrine > dopamine > methoxamine. The maximum force generated was greatest for the non-selective agonist noradrenaline and lowest for the alpha2-selective agonist BHT-920 with the other agonists between these two extremes. Selective inactivation of alpha1-adrenoceptors (achieved by treating yohimbine-protected tissues with phenoxybenzamine) reduced the maximum responses of all agonists, the EC50 values of UK 14304, BHT-920 and noradrenaline and increased the EC50 values of phenylephrine and methoxamine. Prazosin (30 nM) had no inhibitory effect on responses to low concentrations of BHT-920 and UK 14304 and caused competitive inhibition of responses to phenylephrine and noradrenaline giving pKb values of 8.49 +/- 0.18 and 8.23 +/- 0.14, respectively. Yohimbine (0.1 microM) caused significant competitive inhibition of responses to BHT-920 and noradrenaline with calculated pKb values of 8.43 +/- 0.11 for BHT-920 and 7.43 +/- 0.31 for noradrenaline and non-competitive inhibition of responses to UK 14304. 2-[2-methoxy-1,4-benzodioxan-2-yl]-2-imidazoline (RX 821002; 10 nM) caused competitive inhibition of responses to BHT-920 (pKb 9.04 +/- 0.27) and dopamine (pKb 8.2 +/- 0.2). These data indicate that equine digital veins possess both post-synaptic alpha1 and alpha2-adrenoceptors.
Elliott
J
J
Department of Veterinary Basic Sciences, Royal Veterinary College, London, UK.
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001381:Azepines / Q000494:pharmacology; D001667:Binding, Competitive; D000068438:Brimonidine Tartrate; D004298:Dopamine / Q000494:pharmacology; D006736:Horses / Q000378:metabolism*; D019329:Idazoxan / Q000031:analogs & derivatives / Q000494:pharmacology; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000187:drug effects; D008729:Methoxamine / Q000494:pharmacology; D009131:Muscle, Smooth, Vascular / Q000187:drug effects; D009638:Norepinephrine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D018340:Receptors, Adrenergic, alpha-1 / Q000187:drug effects / Q000378:metabolism*; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects / Q000378:metabolism*; D014661:Vasoconstriction / Q000187:drug effects*; D014680:Veins / Q000187:drug effects / Q000378:metabolism*; D015016:Yohimbine / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D001381:Azepines; D011810:Quinoxalines; D018340:Receptors, Adrenergic, alpha-1; D018341:Receptors, Adrenergic, alpha-2; D010656:Phenylephrine; D015016:Yohimbine; D000068438:Brimonidine Tartrate; C024763:talipexole; C058525:2-methoxyidazoxan; D008729:Methoxamine; D004298:Dopamine; D009638:Norepinephrine; D011224:Prazosin; D019329:Idazoxan