Response and sensitivity of guinea‐pig airway muscle preparations to 5‐hydroxytryptamine during ontogenesis

1 Responsiveness (g mm−2) and sensitivity (pD2 value) to 5‐hydroxytryptamine (5‐HT) were markedly reduced in isolated tracheal and bronchial tissues from guinea‐pigs during ontogenesis. Responsiveness of the trachea to 5‐HT was depressed much more than that to histamine. 2 Airway preparations from young guinea‐pigs of either sex always contracted when exposed to 5‐HT, an effect that was blocked by methysergide. 3 Airway muscle preparations from old animals exhibited a wide range of responses to 5‐HT, namely, no effect, relaxation or contraction. 4 The contractile effect of 5‐HT in tracheal and bronchial preparations from old animals was always blocked by methysergide, whereas, the relaxant effect was not. 5 These results indicate that there are significant alterations in the response to 5‐HT receptor stimulation in airway muscle preparations from guinea‐pigs during ontogenesis.