Effects of endothelin-1 on vasoactivity and its synthesis, storage, and acting sites in the rat superior mesenteric vasculature: an ultrastructural and immunocytochemical study.

Vasoactivity after treatment with endothelin (ET)-1 and immunoreactivity for ET-1 and its receptors were investigated in the rat superior mesenteric vasculature (SMV). By measurements of corrosion cast images of the SMVs, it was seen that ET-1 induces remarkable vasocontraction of the distal arterial branches, consisting of small arteries and arterioles, and localized vasoconstriction throughout the venous branches which possess localized medial thickenings. Immunoreactivity for ET-1 was preferentially seen along the endothelia of the proximal arterial branches. Cisterns of the rough endoplasmic reticulum and Weibel-Palade (WP) bodies of the endothelial cells were immunoreacted. Immunoreactivity for the ETa receptor was preferentially seen on the media of the distal arterial branches. These findings indicate that endothelial cells of the proximal arterial branches synthesize ET-1 and store it in the WP bodies. Because WP bodies are involved in the release of ET-1, this suggests that this endogenous ET-1, which is released from the proximal arterial branches, may be involved in the regulation of blood flow through the distal arterial branches by mediation of the ETa receptor. In addition, it seems likely that ET-1-induced vasoconstriction of the venous branches may act to impel the portal blood flow.