ULTRASTRUCTURAL EVIDENCE OF DIFFERENTIAL SOLUBILITY IN TRITON X-100 OF ENDOTHELIAL VESICLES AND PLASMA-MEMBRANE

Endothelial plasmalemmal vesicles (EV) are distinct membrane-bound structures characteristic for all vascular endothelia, being involved in transcytosis of plasma macromolecules. EV are considered to be similar to the caveolae (characterized by a specific peptide called caveolin) found in other cell types. Caveolin-rich membrane domains were recently isolated from whole lung and chicken gizzard as a Triton X-100 (TX)-insoluble membrane fraction. However, ultrastructural data on the localization of these domains within cells have not yet been reported. We have examined whether EV are TX-insoluble structures. Cultured bovine aortic endothelial cells (BAEC) briefly fixed in paraformaldehyde (10 min, 37 degrees C) were exposed to 0.1% TX for 5 min at 22 degrees C and further subjected to standard electron microscopy procedure. The results showed an extensive solubilization of endothelial-plasmalemma as well as other intracellular membranes. Individual or clusters of EV were not affected by TX extraction, retaining their trilaminar unit membrane appearance and dimensions. Moreover, a crude membrane fraction prepared from unfixed BAEC was also extracted with 1% TX for 20 min at 4 degrees C and the insoluble material was examined by electron microscopy. In this fraction clusters of about 10 membranous vesicles were found. These data suggest that EV and plasma membrane have a different lipid composition; the low TX solubility is a characteristic common to caveolin-rich domains (caveolae) of other cells types and EV, whereas the ultrastructural complexity and intracellular localization of the latter are specific for endothelia. (C) 1995 Academic Press, Inc.