Alpha-adrenoceptors in equine digital veins: Evidence for the presence of both alpha1 and alpha2-receptors mediating vasoconstriction |
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| Authors: | J ELLIOTT |
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| Institution: | Department of Veterinary Basic Sciences, Royal Veterinary College, Royal College Street, London NW1 0TU, UK |
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| Abstract: | 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 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 pKb 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 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 n M ) 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. |
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