Endocytic Pathways at the Lateral and Surfaces of Exocrine Acinar Cells Basal Cell

In parotid acinar cells, horseradish peroxidase (HRP) administered via the main excretory duct is endocytosed from the apical cell surface in smooth Cor ring-shaped vesicles (Oliver, C. and A. R. Hand. 1979. J. Cell BioL 76:207). These vesicles ult imately fuse with lysosomes adjacent to the Golgi apparatus. The present investigation extends these findings and examines the uptake and fate of intravenously injected HRP from the lateral and basal cell surfaces of resting and stimulated parotid and pancreatic acinar cells from rats and mice. Isoproterenol and pilocarpine were used to stimulate the parotid gland and the pancreas, respectively. HRP was internalized in smooth and coated vesicles primarily in areas of membrane infoldings. Both the number of coated vesicles and the amount of tracer internalized increased markedly fol lowing secretagogue administration. In both resting and stimulated cells, the HRP was rapidly sequestered in a unique system of basally located lysosomes that possess trimetaphosphatase activity, but not acid phosphatase activity. At 1-3 h after HRP administration, reaction product was also found in multivesicular bodies, vesicles, and lysosomes adjacent to the Golgi apparatus. With time, more HRP was localized in Golgi-associated lysosomes. By 6-7 h, tubules in the apical cytoplasm of stimulated cells contained HRP reaction product. When native ferritin was administered retrogradely and HRP injected intravenously, both tracers could be localized in the same lysosome after 4-5 h, indicating that material taken in from all cell surfaces mixes in Golgi-associated lysosomes. The results of this study suggest that two separate and distinct endocytic pathways exist in exocrine acinar cells: one involves membrane retrieval from the apical cell surface; and the other is a st imulat ion-dependent process at the lateral and basal cell surfaces. In secretory cells, membrane retrieval from the cell surface may aid the cells in maintaining a relatively constant size and shape (2, 19) despite the addition of considerable membrane to the surface during secretion, as well as providing a source of membrane for new secretory granule formation (13, 15, 23-25, 42). In cells as highly polarized as exocrine acinar cells, membrane retrieval could be restricted primarily to the apical cell surface, where secretion occurs and membrane insertion is most extensive. Previous studies employing electron-dense markers (13, 15, 29) have shown that membrane is internalized from the apical cell surface in small endocytic vesicles (see Fig. 4, reference 29). Intracellularly, this membrane may be degraded in lysosomes or recycled during secretory granule production. However, the uptake of membrane from the lateral and basal cell surfaces has not been thoroughly investigated. In the present investigation, the uptake and fate of intravenously injected horseradish peroxidase (HRP) was followed in resting and stimulated pancreatic and parotid acinar cells from rats and mice. Endocytosis at the lateral and basal surfaces was distinct from that at the apical surface, with tracer being initially sequestered in a unique lysosomal system at the base of the cells. Ultimately, the HRP was localized in typical secondary lysosomes adjacent to the Golgi apparatus. Furthermore, endocytosis at the lateral and basal surfaces proved to be largely dependent on secretagogue administration. MATERIALS AND METHODS Adult male and female Wistar Furth rats and National Institutes of Health (NIH) Swiss mice were used. A solution (l mg/gm body weight [b.wt]) of horseradish peroxidase (Type II or Type VI; Sigma Chemical Company, St. Louis, MO) in sterile saline was injected intravenously via the saphenous vein. Some animals also received a retrograde infusion of native ferritin (20 mg/ml; Polysciences, Tu~ ]OURNAL Of CeLL BIOLOGY • VOLUMe 95 OCTOBer 1982 154-161 154 © T h e Rockefe l le r Un ive rs i t y Press • 0 0 2 1 9 5 2 5 / 8 2 / 1 0 / 0 1 5 4 / 0 8 $ 1 0 0 on Jne 6, 2017 D ow nladed fom Published October 1, 1982