Nitric oxide is involved in 5‐HT‐induced relaxations of the guinea‐pig colon ascendens in vitro

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Abstract
In the guinea‐pig colon ascendens, 5‐hydroxytryptamine (5‐HT) induces contractions, mediated by 5‐HT2, 5‐HT3 and 5‐HT4 receptors, and relaxations, through a 5‐HT1 receptor subtype, that triggers the release of an inhibitory neurotransmitter. Nitric oxide (NO) is one of the main candidates of NANC inhibitory neurotransmission in the gut. The aim of this study was to establish whether NO is involved in 5‐HT‐induced relaxations of the guinea‐pig colon ascendens. Antagonists to block the contractile responses to 5‐HT via 5‐HT2, 5‐HT3 and 5‐HT4 receptors were present throughout the experiments and methacholine was administered to precontract the strips. Under these conditions, 5‐HT concentration‐dependently induced relaxations from 10 nm onwards (EC50 = 258 (172–387) nm). The relaxations were inhibited by metergoline (10 nm) and methiothepine (100 nm) and abolished by tetrodotoxin (TTX, 320 nm). Guanethidine (3.2 μm) did not affect them. NG‐nitro‐l‐arginine (l‐NNA) inhibited the responses to 5‐HT (IC50 = 18.7 (13.3–26.3) μm); at the highest 5‐HT concentration a maximum inhibition of about 75% was observed with 320 μm l‐NNA. This inhibition was reversed with l‐arginine. Relaxations to glyceryl trinitrate (GTN) were not inhibited by l‐NNA. Haemoglobin (32 μm) inhibited the relaxations to 5‐HT and GTN, but not those to isoprenaline (Iso). Methylene blue (10 μm) inhibited the relaxations to 5‐HT but did not affect those caused by GTN or Iso. It is concluded that 5‐HT induces relaxations that involve NO. We also confirmed that 5‐HT induces these relaxations via (a) 5‐HT1 receptor subtype(s), located on neurones.