Innervation of the canine inferior vena cava. Distribution of adrenergic and cholinergic excitatory fibers among the embryologically distinct segments.

The innervation of three embryologically distinct segments of the canine inferior vena cava was investigated. These segments were termed A (supradiaphragm), B-C (intrahepatic and that between liver and renal veins), and D (infrarenal). Strips were cut from these segments, and their isometric tensions were recorded. Transmural electrical stimulation induced contractile responses in circular strips from segment B-C (66.5% of the maximum norepinephrine-induced response) and in those from D (14.4%), but not in A. These responses almost completely disappeared in the presence of phenoxybenzamine. In segment B-C, however, the remaining small contraction was markedly enhanced by neostigmine and abolished by atropine. The same phenomenon was also observed in the contraction remaining after reserpinization. Longitudinal strips from segment C responded similarly. Concentration-response curves of circular strips for acetylcholine were shifted by neostigmine markedly to the left only in segment B-C, while no significant shift occurred in A and D. It was concluded that the adrenergic innervation is remarkably dense in B-C, sparse in D, and probably lacking in A. In addition, a cholinergic excitatory innervation is present in segment B-C.