Endocytic pathways at the lateral and basal cell surfaces of exocrine acinar cells.

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Abstract
In parotid acinar cells, horseradish peroxidase (HRP) administered via the main excretory duct is endocytosed from the apical cell surface in smooth C- or ring-shaped vesicles. These vesicles ultimately fuse with lysosomes adjacent to the Golgi apparatus. The uptake and fate of i.v. injected HRP from the lateral and basal cell surfaces of resting and stimulated parotid as well as pancreatic acinar cells from rats and mice were examined. 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 following 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 i.v., 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. Two separate and distinct endocytic pathways probably exist in exocrine acinar cells: one involves membrane retrieval from the apical cell surface and the other is a stimulation-dependent process at the lateral and basal cell surfaces.