5-Hydroxytryptamine (5-HT) has been demonstrated to cause both constriction and relaxation of guinea pig airways, partly through direct action on airway smooth muscle and partly through postganglionic facilitation of cholinergic neurotransmission. We performed an in vitro study to investigate whether 5-HT can modulate the noncholinergic contraction in guinea pig airways due to release of neuropeptides from airway sensory nerves. In the presence of atropine (1 microM), ketanserin (10 microM), and indomethacin (10 microM), 5-HT (0.1–100 microM) produced concentration-dependent inhibition of electrical field stimulation-induced noncholinergic contraction with maximal inhibition of approximately 72 +/- 4%. Tropisetron (ICS-205–930, 1 microM), a 5-HT3 and 5-HT4 receptor antagonist, was unable to prevent the inhibition produced by 5-HT. Methiothepin (1–100 nM), a 5-HT1 and 5-HT2 receptor antagonist, produced a concentration-dependent inhibition of the effect of 5-HT (1 microM) with a 50% inhibition concentration value of 66 nM. 5-HT (100 microM) had no effect on the cumulative concentration-response relationship to exogenous substance P (10 nM-10 microM). The concentration of agonist causing 35% inhibition of the noncholinergic contraction (EC35) was calculated, and a rank order of potency was established: 5-carboxamidotryptamine (EC35 = 0.24 microM) > 5-HT (EC35 = 0.77 microM) > 8-hydroxy-2-(dipropylamino)tetralin (EC35 = 8.1 microM) > sumatriptan (EC35 = 18 microM). We conclude that 5-HT concentration dependently modulates noncholinergic contraction in guinea pig airways in vitro by a prejunctional mechanism. This effect is probably mediated through a 5-HT1-like receptor; however, the exact subtype remains to be elucidated.
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