Excitatory cholinergic innervation in canine portal and mesenteric veins.

A possibility that the canine portal and mesenteric veins may have an excitatory cholinergic innervation similar to that observed in the middle segment of the inferior vena cava (IVC) was investigated. After adrenergic blockade with prazosin, contractile responses of isolated longitudinal strips of the portal and mesenteric veins to transmural electrical stimulation were greatly diminished. Subsequent anticholinesterase (neostigmine or physostigmine) treatment, however, revealed in the nerve-evoked contraction a component that was antagonized by atropine. The same phenomenon was also observed in circular strips of the portal and mesenteric veins but was absent in those of the external jugular, saphenous, and splenic veins. In accord with the results of nerve stimulation, concentration-response curves of acetylcholine-induced contraction were shifted leftward by neostigmine in strips of the portal and mesenteric veins as well as of the middle segment of IVC. In contrast, no shift was observed in 12 other different sites of the venous system. Cholinesterase staining demonstrated positively stained smooth muscle and nerve fibers in the portal and mesenteric veins and in the middle segment of IVC. These findings indicate the existence of excitatory cholinergic innervation in the portal and mesenteric veins, which may reflect the common embryological origin of smooth muscle of these veins with the intestinal muscle.