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Alpha‐adrenoceptors in equine digital veins: Evidence for the presence of both alpha 1 and alpha 2 ‐receptors mediating vasoconstriction
Author(s) -
ELLIOTT J.
Publication year - 1997
Publication title -
journal of veterinary pharmacology and therapeutics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.527
H-Index - 60
eISSN - 1365-2885
pISSN - 0140-7783
DOI - 10.1046/j.1365-2885.1997.00078.x
Subject(s) - prazosin , yohimbine , chemistry , methoxamine , phenylephrine , agonist , alpha (finance) , endocrinology , medicine , alpha 2 adrenergic receptor , phenoxybenzamine , idazoxan , pharmacology , receptor , antagonist , biochemistry , biology , construct validity , nursing , blood pressure , patient satisfaction
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 alpha 2 ‐selective agonist BHT‐920 with the other agonists between these two extremes. Selective inactivation of alpha 1 ‐adrenoceptors (achieved by treating yohimbine‐protected tissues with phenoxybenzamine) reduced the maximum responses of all agonists, the EC 50 values of UK 14304, BHT‐920 and noradrenaline and increased the EC 50 values of phenylephrine and methoxamine. Prazosin (30 n M ) 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 pK b values of 8.49 ± 0.18 and 8.23 ± 0.14, respectively. Yohimbine (0.1 μ M ) caused significant competitive inhibition of responses to BHT‐920 and noradrenaline with calculated pK b 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 n M ) caused competitive inhibition of responses to BHT‐920 (pK b 9.04 ± 0.27) and dopamine (pK b 8.2 ± 0.2). These data indicate that equine digital veins possess both post‐synaptic alpha 1 and alpha 2 ‐adrenoceptors.