Applying the colloidal iron method for ultrastructur al detection of SH and SS groups, these reactive groups were demonstrated in the zymogen granules or materials in the serous cells of the rat submandibular glands and the pancreatic exocrine cells. Based on their specific reactivity with SH and SS groups, the reaction products in the secretory materials were examined under several experimental conditions. One hour after pilocarpine injection, the ductular spaces of the submandibular glands contained a relatively large amount of the discharged zymogen material showing a clear reactivity. When the rats were sacrificed after 24 hours of fasting and subsequent 2 hours of food intake, the pancreatic exocrine cells showed marked SH and SS reactivity in the areas of the Golgi apparatus, as well as in the zymogen material just released into ductules and in the zymogen granules. The finding that the zymogen material was being discharged across villi, suggested a secretion mechanism of the diacrine mode in addition to the exocytosis mode.
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