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

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. 5‐HT (0.1–100 μ m ) produced a concentration‐dependent inhibition of the excitatory NANC response with 50.9 ± 5.0% ( n = 5, P &lt; 0.01) inhibition at 100 μ m . This inhibition was not significantly affected by the 5‐HT 2 antagonist, ketanserin (1 μ m ) 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 μ m ) was effective in blocking bronchoconstriction evoked by activation of 5‐HT 2A receptors on airway smooth muscle. In the presence of ketanserin (1 μ m ) 5‐HT (100 μ m ) evoked an inhibition of 57.4 ± 5.9% ( n = 5, P &lt; 0.01) with an EC 50 of 0.57 μ m . Inhibition evoked by 5‐HT (0.1–100 μ m ) was unaffected by the α‐adrenoceptor antagonist phentolamine (1 μ m ), the β 2 ‐adrenoceptor antagonist, ICI 118551 (0.1 μ m ), the 5‐HT 1A/B antagonist, cyanopindolol (1 μ m ) or the 5‐HT 3/4 antagonist, ICS 205–930 (1 μ m ). Methiothepin (0.1 μ m ) produced an insurmountable inhibition of the effect of 5‐HT (0.1–100 μ m ), reducing the maximum inhibition produced by 5‐HT (100 μ m ) to 30.2 ± 5.0% ( n = 5, P &lt; 0.001) and suggesting a non‐competitive antagonism. Methiothepin inhibited the effect of 5‐HT (10 μ m ) in a concentration‐dependent manner with an IC 50 of 81 n m . Selective 5‐HT receptor agonists were also tested on excitatory NANC responses. 5‐Carboxamidotryptamine (5‐CT, 0.1–100 μ m ) was the most potent, producing a concentration‐dependent inhibition with an EC 50 of 0.13 μ m . Calculation of approximate IC 25 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) &gt; α‐methyl‐5‐hydroxytryptamine (α‐Me‐5HT). Sumatriptan, 5‐methoxytryptamine (5‐MeOT) and 2‐methyl‐5‐hydroxytryptamine (2‐Me‐5HT) were essentially inactive with IC 25 &gt; 100 μ m . 5‐HT (10 μ m ) did not significantly affect contractile responses to exogenously applied substance P (1 n m − 10 μ m ). The effect of 5‐HT was unchanged after incubation with the nitric oxide (NO) synthase inhibitor l ‐N G ‐nitroarginine methyl ester ( l ‐NAME, 100 μ m ). However, pretreatment with charybdotoxin (ChTX, 0.1–30 n m ), a blocker of the large conductance Ca 2+ ‐activated K + channel (K + Ca ), produced a concentration‐dependent inhibition of the effect of 5‐HT (10 μ m ). 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.