Modulation of cholinergic neurotransmission by VIP, VIP-antiserum and VIP-antagonists in dog and cat trachea: VIP plays a role of "double braking" in broncho-constriction.

Exogeous VIP in low concentrations has a prejunctional action inhibiting the excitatory neuroeffector transmission in addition to a direct inhibitory action on the smooth muscle cells, by suppressing transmitter release from the vagus nerve terminals in dog and cat trachea. In the cat trachea, effects of VIP antiserum and VIP antagonists indicate that endogenous VIP may contribute to non-adrenergic non-cholinergic inhibitory response and inhibits acetylcholine (ACh) release from the vagus nerve terminals. Endogenous VIP may play a "double braking" role in broncho-constriction in the cat trachea.