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

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. Airway preparations from young guinea-pigs of either sex always contracted when exposed to 5-HT, an effect that was blocked by methysergide. Airway muscle preparations from old animals exhibited a wide range of responses to 5-HT, namely, no effect, relaxation or contraction. 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. 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.