Endocytosis of parotid salivary proteins by striated duct cells in streptozotocin-diabetic rats

Salivary gland striated duct cells play an important role in the modification of primary saliva by secretion and reabsorption of electrolytes, and secretion of glycoproteins. Recent observations have shown that in the rat parotid gland these cells are able to internalize exogenous proteins, e.g., horseradish peroxidase and ferritin, from the ductal lumen. In rats made diabetic by injection of streptozotocin, dense vacuoles and crystalloids are present in the apical cytoplasm of parotid striated duct cells. In this study we utilized electron microscopic immunocytochemistry to determine if these vacuoles and crystalloids contain acinar secretory proteins. At various times after induction of diabetes by streptozotocin (65 mg/kg), the parotid glands were fixed in a glutaraldehyde-formaldehyde mixture, postfixed in OsO4, and embedded in epoxy resin. Thin sections were immunolabeled with antibodies to protein B1 (Ball et al., 1988) and alpha-amylase (Baum et al., 1982) using a modification of the Protein A-gold technique (Bendayan and Duhr, 1986). With antibody to B1, label was localized in the secretory granules of acinar and intercalated duct cells of both normal and diabetic rats. In striated duct cells of diabetic rats, label was present over the electron-dense vacuoles but not over the crystalloids. Since crystalloids appear to form within the vacuoles, their lack of reactivity may indicate degradation of the internalized protein. The same distribution of label was found with antibody to amylase except for the intercalated duct granules, which were unlabeled in both control and diabetic animals. These results demonstrate that striated duct cells take up salivary proteins from the lumen and that the endocytosis of some secretory proteins from the saliva may be a significant function of these cells in certain pathological conditions.