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A comparison of 5‐hydroxytryptamine receptors mediating contraction in rabbit aorta and dog saphenous vein: evidence for different receptor types obtained by use of selective agonists and antagonists

1 Using recently available selective agonists and antagonists we have examined further our postulate (Apperley et al., 1980) that 5‐hydroxytryptamine (5‐HT) mediates contraction of dog saphenous vein via a different 5‐HT receptor type from that in the rabbit aorta. 2 In the rabbit isolated aorta, ketanserin and spiperone were potent, specific, competitively‐acting antagonists of the contractile effects of 5‐HT. 3 In contrast, in the dog isolated saphenous vein neither ketanserin nor spiperone caused any rightward displacement of concentration‐response curves to 5‐HT although the maximum response was reduced by about 10%. 4 In the rabbit aorta 5‐carboxamidotryptamine (5‐CONH2‐T) was a weak agonist whilst the 5‐N,N‐dimethyl and 5‐N‐ethyl derivatives were even weaker or inactive. The contractile effect of 5‐CONH2‐T in the rabbit aorta was potently and competitively antagonized by ketanserin. 5 In contrast, in the dog saphenous vein 5‐CONH2‐T and its 5‐N,N‐dimethyl and 5‐N‐ethyl derivatives were all potent agonists. The contractile effect of 5‐CONH2‐T was not markedly affected by ketanserin. 6 The profile of action of ketanserin and spiperone in the rabbit aorta is consistent with the view that 5‐HT2 receptors mediate contraction in this preparation. However, the 5‐HT receptor mediating contraction in the dog saphenous vein appears to be ‘5‐HT1‐like’, sharing a number of characteristics with the 5‐HT1 recognition site identified from [3H]‐5‐HT ligand binding studies in brain tissue. tissue.