The effectiveness of α2‐adrenoceptor activation increases from the distal to the proximal part of the veins of canine limbs

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Abstract
1 The effectiveness of α1- and α2-adrenoceptor activation was compared at different levels of the saphenous and cephalic vein of the dog in vitro. 2 Helically cut strips were used to determine concentration-response curves to phenylephrine, noradrenaline, UK-14, 304 (5-bromo-6-(imidazoline-2-ylamino)-quinoxaline) and B-HT 920 (2-amino-6-allyl-5,6,7,8-tetra-hydro-4H-(thiazo)-4,5-d-azepine). The effect of prazosin and yohimbine on these curves was also studied. 3 At the distal level, the maximum response to UK-14,304 amounted to 33 and 50% of those to noradrenaline in the saphenous and cephalic vein, respectively, while at the proximal level the maximum response to UK-14,304 amounted to 72 and 78% of those to noradrenaline, in the saphenous and cephalic vein, respectively. 4 In both vessels, the results obtained with B-HT 920 were very similar to those for UK-14,304. 5 The pD2 values for UK-14,304 — which were identical at the three levels of both vessels — and the pA2 values for the antagonism exerted by either prazosin or yohimbine against the responses to UK-14,304 indicate that the α2-adrenoceptors are identical at the different levels of both vessels. 6 These results show that the effectiveness of α2-adrenoceptor stimulation increases from the distal to the proximal regions of canine limb veins. Apparently, this is due to a greater density of α2-adrenoceptors in the proximal regions. 7 Yohimbine is much more potent against phenylephrine distally than proximally in both vessels. However, after 30 nm phenoxybenzamine — a concentration which eliminates the vast majority of α1-adrenoceptors without affecting α2-adrenoceptors — yohimbine became equally potent at both levels, suggesting that the difference existing before phenoxybenzamine depended on α1-adrenoceptors. Hence it is concluded that α1-adrenoceptors in distal and proximal portions may differ.