INHIBITION OF EXCITATORY NONADRENERGIC NONCHOLINERGIC 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 n eurally mediated non-adrenergic non-cholinergic (NANC) contractions ev oked by electrical field stimulation (EFS) in guinea-pig isolated bron chi. 2 5-HT (0.1-100 muM) produced a concentration-dependent inhibitio n of the excitatory NANC response with 50.9 +/- 5.0% (n = 5, P < 0.01) inhibition at 100 muM. This inhibition was not significantly affected by the 5-HT2 antagonist, ketanserin (1 muM) when inhibitions (+/- ket anserin) at each concentration of 5-HT were compared by unpaired t tes ts; however, this concentration appeared to produce a leftward shift ( almost-equal-to 10 fold) of the 5-HT concentration-inhibition curve. K etanserin (1 muM) was effective in blocking bronchoconstriction evoked by activation of 5-HT2A receptors on airway smooth muscle. In the pre sence of ketanserin (1 muM) 5-HT (100 muM) evoked an inhibition of 57. 4 +/- 5.9% (n = 5, P<0.01) with an EC50 of 0.57 muM. 3 Inhibition evok ed by 5-HT (0.1 - 100 muM) was unaffected by the alpha-adrenoceptor an tagonist phentolamine (1 muM), the beta2-adrenoceptor antagonist, ICI 118551 (0.1 muM), the 5-HT1A/B antagonist, cyanopindolol (1 muM) or th e 5-HT3/4 antagonist, ICS 205-930 (1 muM). 4 Methiothepin (0.1 muM) pr oduced an insurmountable inhibition of the effect of 5-HT (0.1 - 100 m uM), reducing the maximum inhibition produced by 5-HT (100 muM) to 30. 2 +/- 5.0% (n = 5, P < 0.001) and suggesting a non-competitive antagon ism. Methiothepin inhibited the effect of 5-HT (10 muM) in a concentra tion-dependent manner with an IC50 of 81 nM. 5 Selective 5-HT receptor agonists were also tested on excitatory NANC responses. 5-Carboxamido tryptamine (5-CT, 0.1 - 100 muM) was the most potent, producing a conc entration-dependent inhibition with an EC50 of 0.13 muM. Calculation o f approximate IC25 values (concentration of the agonist required to gi ve a 25% inhibition of the excitatory NANC response) gave a rank order of potency 5-CT greater-than-or-equal-to 5-HT > > 8-hydroxy-dipropyla minotetralin (8-OH-DPAT) > alpha-methyl-5-hydroxytryptamine (alpha-Me- 5HT). Sumatriptan, 5-methoxytryptamine (5-MeOT) and 2-methyl-5-hydroxy tryptamine (2-Me-5HT) were essentially inactive With IC25> 100 muM. 6 5-HT (10 muM) did not significantly affect contractile responses to ex ogenously applied substance P (1 nM - 10 muM). 7 The effect of 5-HT wa s unchanged after incubation with the nitric oxide (NO) synthase inhib itor L-N(G)-nitroarginine methyl ester (L-NAME, 100 muM). However, pre treatment with charybdotoxin (ChTX, 0.1 - 30 nM), a blocker of the lar ge conductance Ca2+-activated K+channel (K+Ca), produced a concentrati on-dependent inhibition of the effect of 5-HT (10 muM). 8 5-HT evokes a concentration-dependent inhibition of e-NANC bronchoconstriction in guinea-pig isolated bronchi but does not affect cumulative concentrati on-dependent contractile responses to substance P, suggesting that inh ibition is via a prejunctional receptor. Effects of selective antagoni sts and agonists suggest that an atypical 5-HT receptor mediates this inhibition. The inhibitory effect of 5-HT does not involve the product ion of NO, but may involve the opening a ChTX-sensitive K+Ca. channel. These data suggest that an atypical 5-HT receptor inhibits the releas e of neuropeptides from sensory C fibres and may act as other inhibito ry neuromodulators via the opening of a common K+channel.