Immunocytochemical localization of gamma-glutamyltranspeptidase, GP-2 and amylase in the rat exocrine pancreas: the concept of zymogen granule membrane recycling after exocytosis.

Abstract

We localized gamma-glutamyltranspeptidase (GGT) in the rat pancreas by immunocytochemistry using the protein A-gold technique. The enzyme was found in the apical and zymogen granule (ZG) membranes of the pancreatic acinar cell. With ZG at the onset of exocytosis, labeling was seen over membrane, whereas content was unreactive. In the acinar lumen, the enzyme was generally associated with small vesicles previously described as "pancreasomes." This observation corroborates a recent proposal that a membrane-shedding process is associated with exocytosis in the exocrine pancreas. It also implies that some elements of the ZG membrane are not recycled after exocytosis. The cellular distribution of GGT was compared with GP2, another glycoprotein component of the ZG membrane, and differences in localization indicate different fates for these two proteins. Indeed, GP2 shows a strong signal with the basolateral membrane, whereas in the case of GGT the signal is barely detectable. The reverse situation is observed on the apical plasma membrane, GGT producing a much stronger signal than GP2. The failure to detect GGT in lysosomal structures, combined with the fact that some endocytic-like vacuoles in the vicinity of the apical plasma membrane give a positive reaction, supports the view that some GGT molecules are recycled in the ZG membrane after exocytosis. Our observations clearly demonstrate that a fraction of the protein components of the ZG membrane are not recycled after exocytosis, raising new questions regarding the concept of membrane recycling associated with regulated secretion.