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Ultrastructural study on the venous sphincter in the sublobular vein of the canine liver.

Although the existence of venous sphincters has been demonstrated in the sublobular veins of the canine liver, the role it plays in the regulation of liver blood flow is still uncertain. In the present study, I examined the fine structures of the venous sphincters treated with four kinds of drugs (epinephrine, histamine, isoproterenol, and histamine releaser) by conventional electron microscopy and by scanning electron microscopy using microvascular corrosion casts. Intravenous administration of epinephrine, histamine, and histamine releaser (compound 48/80) resulted in a strong constriction at the small branches (100-400 micron in caliber) of the sublobular veins, while the treatment with isoproterenol showed dilatation in the same branches. When treated with compound 48/80, both the endothelial-specific granules (Weibel-Palade granules) and the mast cell's granules of the sublobular veins showed swelling and became transparent reducing electron density. In contrast, the shape and electron density of the granules did not change when the veins were dilated. The results suggest that the small branches of the sublobular veins have extremely important functions for the regulation of the liver blood flow under normal conditions and that the component parts in the Weibel-Palade granules and/or mast cell's granules may be involved in the constriction of the sphincter muscles.