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Substance P-immunoreactive nerve fibres were localized by the indirect immunohistochemical method in the adventitia and the adventitial-medial border of large peripheral arteries and veins of the rat. Arteries showed a richer substance P-containing innervation than veins. The superior mesenteric artery was densely innervated, whereas no substance P-containing fibres were found around the carotid artery. Substance P produced a vasoconstriction of the veins, but was basically without effect on arteries, although with the carotid artery a dose-dependent relaxation was observed. The absence of a correlation between the degree of innervation of the blood vessels and their responsiveness to exogenous substance P suggests that there nerves do not subserve a vasomotor function. The depletion of substance P immunoreactivity from nerves in arteries and veins by capsaicin suggest that substance P-containing vascular nerves are primarily sensory in nature.

The adrenergic innervation of the rat portal vein is subdivided into two plexuses: the adventitial and the medial. Nerve fibres containing immunoreactive substance P (SP) are present in the adventitial and medial plexuses, and extend into the longitudinal and circular muscle layers. Vasoactive intestinal polypeptide (VIP) immunofluorescence is restricted to the adventitial plexus. The catecholamine modulation of the intrinsic rhythmical contractions of the portal vein may be complemented by SP released from intramural nerves and VIP released from the adventitial nerves.

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The reactivity of longitudinal and circular muscle of the rat portal vein to noradrenaline, acetylcholine, 5-hydroxytryptamine, substance P, angiotensin II and neurotensin was compared. Longitudinal muscle was prepared as longitudinal strips and circular muscle was studied as transversally cut rings. Longitudinal muscle was more sensitive than circular muscle to acetylcholine, 5-hydroxytryptamine and angiotensin II, whereas both muscle layers were equally sensitive to noradrenaline and substance P. Circular muscle was generally unresponsive to neurotensin. Differential sensitivity of longitudinal and circular muscle layers suggests that these two muscle layers do not necessarily operate in unison.