Alpha-adrenoceptors in equine digital veins: Evidence for the presence of both alpha(1) and alpha(2)-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 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 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 pK(b) values of 8.49 +/- 0.18 and 5.23 +/- 0.14, respectively. Yohimbine (0.1 mu 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 nM) 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.