Synthesis of membrane and secretory glycoproteins by the acinar pancreatic cells as visualized by radioautography.

Abstract

Young rats were injected with 3H-fucose and killed at 10 min, 1, 4 and 24 h thereafter. Samples of the pancreas were processed for light- and electron-microscopic radioautography and then analyzed quantitatively. Fucose label was taken up in the supranuclear region of acinar pancreatic cells that were maximally labeled at 1 h after injection. Between 1 and 24 h after injection there was a very marked decrease in the concentration of label in the pancreatic acini. Radioautography at the ultrastructural level demonstrated that the earliest time interval the silver grains indicating the sites of newly synthesized glycoproteins were predominantly observed over the Golgi apparatus of the acinar cells. The quantitative analysis indicated that condensing vacuoles were labeled from 10 min to 4 h after injection. At 1 h the concentration of label in the Golgi apparatus was greatly diminished while it was significantly higher in the secretory (zymogen) granules. At this interval the secretion product visualized in the lumen of the acini and of the ducts was found to be labeled. At 4 h both the zymogen granules and the plasma membrane were significantly labeled. Further analysis carried out on the zymogen granules indicated that fucose-labeled glycoproteins were located in the granule membrane as well as in the granule content. In conclusion, 3H-fucose is added to glycoproteins in the Golgi apparatus of the acinar cells. Then, labeled glycoproteins are collected within condensing vacuoles and zymogen granules and released into the acinar lumen as secretion products. This latter event, which almost certainly is a consequence of exocytosis, may also account for the addition of granule-membrane glycoproteins to the plasma membrane.