Endocytosis of native and glycosylated bovine serum albumin by duct cells of the rat parotid gland

Summary The ability of duct cells of the rat parotid gland to internalize bovine serum albumin (BSA) and several glycosylated albumins (glucosamide, galactosamide, fucosamide, lactosyl, p-aminophenyl-N-acetyl-D-glucosamide, p-aminophenyl-N-acetyl-D-mannopyranoside, p-aminophenyl-N-acetyl-D-galactosamide) was investigated. The various BSA preparations were infused into the gland via the main excretory duct, after which the tissues were fixed and prepared for light and electron microscopy. Immunolocalization of native BSA, as well as the glycosylated BSAs, was performed on thin sections, using an unlabeled antibody to BSA followed by protein A-colloidal gold. Gold particles were present over the lumina of both intercalated ducts and striated ducts, and over small endocytic structures and large vacuoles in the apical cytoplasm of both duct cell types. Endocytosis of the glycosylated BSAs by duct cells was greater than native BSA. Fucosylamide-BSA and mannopyranoside-BSA were taken up to a greater extent than the other glycosylated BSAs. Uptake by intercalated duct cells was greater than by striated duct cells, was independent of the concentration of the glycosylated BSA, and was reduced by an excess of the corresponding sugar. Striated duct cells showed some damage by the glycosylated BSAs that was concentration-dependent, and which was reduced in the presence of an excess of the corresponding sugar. These results suggest that endocytosis by salivary gland duct cells may involve specific recognition of carbohydrate residues and that the endocytosis of acinar secretory proteins observed in certain conditions may be due to increased and/or altered protein glycosylation.