BAFILOMYCIN A1, A SPECIFIC INHIBITOR OF V-TYPE H-ATPASES, INHIBITS THE ACIDIFICATION OF ENDOCYTIC STRUCTURES AND INHIBITS HORSERADISH-PEROXIDASE UPTAKE IN ISOLATED RAT SINUSOIDAL ENDOTHELIAL-CELLS()

The role of vacuolar type H+-ATPases (v-ATPases) and pH gradient betwe en the endocytic compartments and cytoplasm in the endocytosis of hors eradish peroxidase, a mannose-terminated glycoprotein, was investigate d morphologically in isolated rat sinusoidal endothelial cells. Toward this purpose a specific inhibitor of v-ATPases, bafilomycin Al, was u sed to inhibit v-ATPases in the vacuolar system. Uptake of horseradish peroxidase was examined by electron microscopy. Fluorescent staining by acridine orange showed that bafilomycin Al inhibited the acidificat ion of the endocytic compartments. Horseradish peroxidase was taken up via mannose receptors and was distributed in the endocytic structures in the isolated sinusoidal endothelial cells. Uptake of horseradish p eroxidase was significantly inhibited by bafilomycin Al, and this find ing was confirmed by morphometrical analysis. These results suggest th at: a) v-ATPases are necessary for acidification of the endocytic comp artments in the sinusoidal endothelial cells and b) the pH gradient be tween the endocytic compartments and the cytoplasm that is generated b y v-ATPases is necessary for the receptor-mediated endocytosis of a ma nnose-terminated glycoprotein, horseradish peroxidase.