Composition of guinea pig pancreatic zymogen granules: calcium and magnesium concentration

The role of calcium in exocytosis of zymogen granules is not clear, and the secretion of bivalent cations with zymogen granules must be demonstrated. Trypsinogen autoactivation inside the granules is related to calcium concentration. Calcium and magnesium concentration in pancreatic zymogen granules could play a role in cellular autolysis and subsequent pancreatitis. The calcium and magnesium composition of zymogen granules was not studied previously owing to the difficulty in preparing a pure subcellular fraction. Following the fractionation technique of Meldolesi et al., a pure zymogen granule fraction from guinea pig pancreas was obtained. The mitochondrial contamination was below 5% (as shown by electron microscopy and cytochrome oxydase assay). Ca⁺⁺ and Mg⁺⁺ determinations were performed in pancreatic homogenate and in zymogen granules, using an atomic absorption spectrophotometer. The pancreas contains about 6 mmoles Ca⁺⁺ and 14 mmoles Mg⁺⁺ per gram tissue. The calcium concentration in pancreas is similar to the one observed in other secretory glands like parotid and medullosurrenal. The pancreatic magnesium concentration is higher, meaning probably a high biosynthetic activity. In zymogen granules, calcium concentration is 36 nmoles/mg protein, and magnesium concentration is 9 nmoles/mg protein. The latter is very low, below the Mg⁺⁺ level of other secretory glands. But calcium concentration is high. Filtration on Sephadex G 10 of granule content shows that only a fraction of calcium is bound to the proteins. The rest of calcium is free and may play an important role in the presence of zymogens.