Non-adrenergic inhibitory response in the rat portal vein.

Transmural electrical stimulation depressed the spontaneous activity of the longitudinal muscle of the rat portal vein in modified Krebs solution containing guanethidine (5 X 10(-6)M). This non-adrenergic inhibitory response was frequency-dependent and recovered slowly after cessation of stimulation. Tetrodotoxin (5 X 10(-7)M) but not atropine (2 X 10(-7)M) completely prevented this electrically-induced inhibitory response, indicating that it is due to a stimulation of non-adrenergic, non-cholinergic inhibitory nerves. Of the putative transmitters tested, only vasoactive intestinal polypeptide (VIP) mimicked the depression of the spontaneous activity elicited by transmural electrical stimulation. Using an indirect immunofluorescence technique, VIP-immunoreactive nerve fibres were observed in the wall of the rat portal vein. Such fibres were located both in the adventitia close to the longitudinal muscle layer and between the longitudinal and circular muscle layers. These observations suggest a possibility that VIP acts as a transmitter substance of vasodilator nerves in the rat portal vein.