The effect of tachykinins on cholinergic neurotransmission was studied in an innervated tracheal tube preparation isolated from guinea‐pigs anaeshetized with urethane. The tracheal tube was bathed in Krebs‐Henseleit solution containing 5 μ m indomethacin. Neurokinin A (NKA), eledoisin (El) and substance P (SP) caused concentration‐dependent increases in intraluminal pressure (ILP), with an order of potency NKA > El >> SP. Low concentrations of tachykinins, that had little effect on ILP, caused an increase in the contractions elicited by stimulation of the preganglionic vagal nerve fibres and by postganglionic (transmural) stimulation. The order of potency was NKA ≤ El > SP. Contractions induced by exogenous acetylcholine (ACh) were not increased by the tachykinins. The magnitude of the tachykinin‐induced augmentation of responses to nerve stimulation was inversely related to stimulation voltage and frequency. These results suggest that tachykinins act on NK 2 receptors, both on the trachealis muscle and on postganglionic pulmonary parasympathetic nerve terminals. Activation of the neuronal receptors may increase the probability of transmitter release from the nerve terminals.
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