1 The 5-hydroxytryptamine (5-HT) receptors mediating contraction of human isolated pulmonary artery rings were investigated. Responses to the agonists 5-carboximidotryptamine (5-CT, non-selective 5-HT, agonist), sumatriptan (5-HTD-like receptor agonist), 5-HT and 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT, 5-HTA receptor agonist) were studied. Responses to 5-HT and sumatriptan in the presence of the antagonists, methiothepin (non-selective 5-HT,+2-receptor antagonist), ketanserin (5-HT2A receptor antagonist) and the novel antagonist, GR55562 (5-HTID receptor antagonist) were also studied. 2 All agonists contracted human pulmonary artery ring preparations in the following order of potency 5-CT > 5-HT = sumatriptan > 8-OH-DPAT. Maximum responses to 5-HT, 5-CT and sumatriptan were not significantly different. 3 Methiothepin 1 nM and 10 nM, but not 0.1 nM reduced the maximum contractile responses to 5-HT but did not alter tissue sensitivity to 5-HT. Methiothepin 0.1 nM, 1 nM and 10 nM had a similar effect on responses to sumatriptan.4 The 5-HT2A receptor antagonist ketanserin (10 nM, 100 nM and 1 gM) also reduced the maximum contractile response to both 5-HT and sumatriptan without affecting tissue sensitivity to these agonists. 5 The novel 5-HTlD receptor antagonist, GR55562, inhibited responses to 5-HT and sumatriptan in a true competitive fashion. 6 The results suggest that the human pulmonary artery has a functional population of 5-HTID-like receptors which are involved in the contractile response to 5-HT.

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