MORPHOLOGICAL ALTERATIONS IN ENDOTHELIAL-CELLS ASSOCIATED WITH THE RELEASE OF VON-WILLEBRAND-FACTOR AFTER THROMBIN GENERATION IN-VIVO

von Willebrand factor (vWF) is synthesized by endothelial cells and st ored in endothelium-specific granules, the Weibel-Palade (WP) bodies. The release of vWF from endothelial cells in vitro in response to secr etagogues such as thrombin is considered to result in the loss of WP b odies through the fusion of the WP bodies with the plasma membrane. Bi ochemical and morphological techniques, including transmission (TEM) a nd scanning (SEM) electron microscopy, were used to examine the plasma profile of VWF in parallel with morphological alterations in endothel ial cells associated with the generation of thrombin in vivo. There wa s a rapid loss of high-molecular-weight multimers of the circulating v WF, with full recovery within 1 hour. Simultaneously, TEM demonstrated that the endothelial cells lost WP bodies and became severely vacuola ted; this was associated with the appearance of craters in the endothe lial surface on SEM. Release of stored VWF in WP bodies seemed to foll ow the fusion of multiple rather than individual WP bodies, with the r esulting vacuole fusing and rupturing through the plasmatic membrane. Within 1 hour there was increased morphological evidence of metabolic organelle activity associated with replacement of WP bodies, presumabl y due to de novo synthesis of the basic protomer and its packaging in high-molecular-weight multimeric form in the storage organelles.