Extrinsic and intrinsic cholinergic systems of the vascular wall.

Immunohistochemical, biochemical and functional studies have revealed two separate cholinergic systems in the arterial vascular wall. Endothelial cells represent the ubiquitous intrinsic, intimal system; they contain the acetylcholine-synthesizing enzyme, choline acetyltransferase, release a choline ester, and contain functional muscarinic receptors. Perivascular autonomic nerve fibres represent the extrinsic, adventitial system. These axons are not ubiquitous but show a highly selective distribution among and even within organs, and utilize co-mediators (NO, neuropeptides) in an organ-specific pattern. We put forward the hypothesis that the intrinsic, intimal system serves as a general regulator of basal vascular tone and wall structure responding to local, luminal stimuli, whereas the perivascular nerve fibres act on top of this basal tone by providing fine tuning in response to reflex activation due to systemic demands.