The significance of calcium in insulin secretion - Ultrastructural studies on identification and localization of calcium in activated and inactivated B cells of mice

Calcium plays an important role in the release of insulin. When the GBHA [glyoxal bis (2-hydroxyanil)] reaction is employed, calcium can be clearly demonstrated by light microscope in pancreatic islets of mice. The specificity of this finding has now been proved by elemental X-ray analysis. Electron microscopically, certain cations can be visualized by a precipitation technique using potassium pyroantimonate as the precipitating agent. In the B cell of mice this technique reveals a characteristic precipitation pattern. Elemental X-ray analysis suggests that the precipitates contain high amounts of calcium. The pattern of the precipitates changes dependent on the functional state of the B cell. In normoglycemia the deposits are mainly associated with the granule membranes, the cell membranes and the cytoplasmic matrix. In hypoglycemia there is a shift of precipitates into the endoplasmic reticulum and the mitochondria, which are thought to be storage organelles for intracellular calcium. The deposits within the halos of the numerous secretory granules, are diminished. In hyperglycemia there is a marked ion translocation across the cell membrane to its inner surface and particularly into the halos of the secretory granules, while the deposit content of mitochondria and endoplasmic reticulum is decreased. Within the saccules of the secretory granules, the deposits sometimes seem to impregnate a filamentous network, which encloses the secretory granule and cannot be seen by conventional electron microscopical preparations. The morphological data suggest that emiocytosis of hormone granules is associated with a release of cellular calcium. The presented observations in treated and untreated animals extend and support the conceptions on the specific role of calcium within the insulin releasing mechanism of the B cell.