Possible involvement of actin filaments in the contraction and dilatation of the hepatic sinusoidal endothelial fenestrae.

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Heterogeneity of the sinusoidal endothelial fenestrae (SEF) in the hepatic lobule was investigated by scanning electron microscopy using liver tissue perfused with 1.2% glutaraldehyde under a physiological pressure and primary monolayer-cultured sinusoidal endothelia. Two types of the SEF, sieve plate-like small pores and relatively large scattered pores, were evident in the zone 1 of the hepatic lobule, whereas only the former were present in the zone 3. Actin filaments were proved in the cytoplasm of the sinusoidal endothelium, particularly around the SEF and underneath the endothelial plasma membrane by the indirect immunofluorescence antibody method, transmission electron microscopic observations of the uranyl acetate en bloc preparations following heavy meromyosin decorations and of whole cell mount preparations of the isolated cultured sinusoidal endothelia. Treatment with cytochalasin B, an actin depolymerizing agent both in vivo and in vitro, was resulted in the dilatation and fusion of the SEF around which actin filaments were found to be disassembled and granular with focal aggregation of actin filaments in the sinusoidal endothelium. These observations indicate that the actin filaments would be involved in the contraction and dilatation of the SEF and the endothelial cell as a whole, possibly regulating the sinusoidal blood flow as well as the blood-hepatocyte exchange.
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