Evidence for 5‐HT7 receptors mediating relaxation of human colonic circular smooth muscle

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Abstract
5‐HT4 receptors mediate relaxation of human colon circular muscle. However, after 5‐HT4 receptor blockade (SB 204070 10 nM), 5‐HT still induced a relaxation (pEC50 6.3). 5‐HT4 receptors were sufficiently blocked, as the curves to 5‐HT obtained in the presence of 10 and 100 nM SB 204070 were indistinguishable. This 5‐HT‐induced relaxation was tetrodotoxin‐insensitive, indicative of a smooth muscle relaxant 5‐HT receptor. This, and the rank order of potency (5‐CT=5‐MeOT=5‐HT) suggested involvement of 5‐HT1 or 5‐HT7 receptors. Mesulergine, a 5‐HT7 receptor antagonist at nanomolar concentrations, and a 5‐HT1 receptor antagonist at micromolar concentrations, competitively antagonized the 5‐HT‐induced relaxation (pKB 8.3) and antagonized the relaxation to 5‐CT. Methysergide antagonized the 5‐HT‐induced relaxation (pA2 7.6). It is concluded that the profile of the smooth muscle inhibitory 5‐HT receptor resembles that of the 5‐HT7 receptor. These data provide the first evidence for functional human 5‐HT7 receptors.British Journal of Pharmacology (1999) 128, 849–852; doi:10.1038/sj.bjp.0702762

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