Secretory granules of murine salivary glands. A morphological, biochemical and morphological, biochemical and immunochemical study of submandibular and parotid granules.

Two secretory granular fractions from murine submandibular glands (SM) and one fraction from murine parotid glands (Par) were isolated by centrifugation on two discontinuous sucrose gradients. From the parotid glands the granular fraction was layered on 1.9 M sucrose (ParB), and in addition a second fraction was layered on 2.1 M sucrose (SMc). The contamination of the granular fractions by other cellular organelles was determined. The membranes of both the Parb and SMb granular fraction contained a major protein fraction with a molecular weight of 12-14,000 designated PMC and MMC, respectively. Their total amino acid composition was similar, but not their carbohydrate composition and immunochemical properties. The major protein within the Parb granules was amylase, and in addition AM2-glycoprotein was also present. Both secretory components had exclusively an acinar localization in the parotid glands, indicating the acinar origin of the Parb granules. In the SMb granular fraction, consisting of large secretory granules, amylase activity was detected by enzymatic method, and in addition the nerve growth factor and epidermal growth factor were demonstrated by immunochemical methods. These activities could only be localized in the SM granular tubular (GCT) cells, pointing to a GCT cellular origin of the SMb granules. On the other hand, immunoreactive amylase, AM2-glycoprotein, and submandibular mucin (MSM) were present in the SMc granular fraction. In the SM tissue sections these components were localized in the acinar cells. So, it is likely that the SMc granules are seromucous acinar granules.