Inhibition of excitatory non‐adrenergic non‐cholinergic bronchoconstriction in guinea‐pig airways in vitro by activation of an atypical 5‐HT receptor

1 The effect of 5‐hydroxytryptamine (5‐HT) was studied on excitatory neurally mediated non‐adrenergic non‐cholinergic (NANC) contractions evoked by electrical field stimulation (EFS) in guinea‐pig isolated bronchi. 2 5‐HT (0.1–100 μm) produced a concentration‐dependent inhibition of the excitatory NANC response with 50.9 ± 5.0% (n = 5, P < 0.01) inhibition at 100 μm. This inhibition was not significantly affected by the 5‐HT2 antagonist, ketanserin (1 μm) when inhibitions (± ketanserin) at each concentration of 5‐HT were compared by unpaired t tests; however, this concentration appeared to produce a leftward shift (∼ 10 fold) of the 5‐HT concentration‐inhibition curve. Ketanserin (1 μm) was effective in blocking bronchoconstriction evoked by activation of 5‐HT2A receptors on airway smooth muscle. In the presence of ketanserin (1 μm) 5‐HT (100 μm) evoked an inhibition of 57.4 ± 5.9% (n = 5, P < 0.01) with an EC50 of 0.57 μm. 3 Inhibition evoked by 5‐HT (0.1–100 μm) was unaffected by the α‐adrenoceptor antagonist phentolamine (1 μm), the β2‐adrenoceptor antagonist, ICI 118551 (0.1 μm), the 5‐HT1A/B antagonist, cyanopindolol (1 μm) or the 5‐HT3/4 antagonist, ICS 205–930 (1 μm). 4 Methiothepin (0.1 μm) produced an insurmountable inhibition of the effect of 5‐HT (0.1–100 μm), reducing the maximum inhibition produced by 5‐HT (100 μm) to 30.2 ± 5.0% (n = 5, P < 0.001) and suggesting a non‐competitive antagonism. Methiothepin inhibited the effect of 5‐HT (10 μm) in a concentration‐dependent manner with an IC50 of 81 nm. 5 Selective 5‐HT receptor agonists were also tested on excitatory NANC responses. 5‐Carboxamidotryptamine (5‐CT, 0.1–100 μm) was the most potent, producing a concentration‐dependent inhibition with an EC50 of 0.13 μm. Calculation of approximate IC25 values (concentration of the agonist required to give a 25% inhibition of the excitatory NANC response) gave a rank order of potency 5‐CT ≥ 5‐HT ≫ 8‐hydroxy‐dipropylaminotetralin (8‐OH‐DPAT) > α‐methyl‐5‐hydroxytryptamine (α‐Me‐5HT). Sumatriptan, 5‐methoxytryptamine (5‐MeOT) and 2‐methyl‐5‐hydroxytryptamine (2‐Me‐5HT) were essentially inactive with IC25 > 100 μm. 6 5‐HT (10 μm) did not significantly affect contractile responses to exogenously applied substance P (1 nm − 10 μm). 7 The effect of 5‐HT was unchanged after incubation with the nitric oxide (NO) synthase inhibitor l‐NG‐nitroarginine methyl ester (l‐NAME, 100 μm). However, pretreatment with charybdotoxin (ChTX, 0.1–30 nm), a blocker of the large conductance Ca2+‐activated K+channel (K+Ca), produced a concentration‐dependent inhibition of the effect of 5‐HT (10 μm). 8 5‐HT evokes a concentration‐dependent inhibition of e‐NANC bronchoconstriction in guinea‐pig isolated bronchi but does not affect cumulative concentration‐dependent contractile responses to substance P, suggesting that inhibition is via a prejunctional receptor. Effects of selective antagonists and agonists suggest that an atypical 5‐HT receptor mediates this inhibition. The inhibitory effect of 5‐HT does not involve the production of NO, but may involve the opening a ChTX‐sensitive K+Ca channel. These data suggest that an atypical 5‐HT receptor inhibits the release of neuropeptides from sensory C fibres and may act as other inhibitory neuromodulators via the opening of a common K+channel.