Distinctive characteristics of the splenic vein in the dog. Its morphological and pharmacological discontinuities with the portal vein and splenic capsule.

Vascular characteristics of the splenic vein are entirely different from the portal vein in the dog. We compared the responsiveness of the splenic capsule, which contains abundant smooth muscle with the portal and splenic veins. With respect to potentiation of acetylcholine-induced contraction by neostigmine, the relaxation response to isoproterenol and the contractile response to 5-hydroxytryptamine, the splenic capsule differed from the splenic vein, but was rather similar to the portal vein. A site of morphological and pharmacological discontinuity between the portal vein and the splenic vein occurred at the junction of the splenic vein and the left gastric vein to form the gastrosplenic vein. This junction may coincide with the site of connection of the embryonic splenic vein, a newly formed efferent vein of the spleen, with the portal vein, which was reported by Miki [cf. Bevan et al.: 4th Int. Symp. on Vascular Neuroeffector Mechanisms; Raven Press, New York 1983]. On the splenic side, histological examination revealed that smooth muscle of the splenic capsule and trabeculae are interconnected, but did not extend to the splenic vein. At openings of the trabecular veins into the 'intrasplenic' portion of the splenic vein the trabecular smooth muscle continued to the extended capsule. The site of change in pharmacological responses was also found to be located between the trabeculae and the intrasplenic portion of the splenic vein. Thus the splenic vein can be regarded as a distinct segment of vessel inserted between the portal vein and the spleen. This may reflect differences in embryogenesis of their smooth muscle.