In fed yellow KK mice, the pancreatic B cells showed severe degranulation and diminished content of zinc. In these islets, there were observed small number of secretory granules which could be classified into pale granules and dense granules with their electron densities. Both granules underwent emiocytosis. Zinc was detected in cores of dense granules as fine precipitates of silver sulfide. The pancreatic islets from 14 hours fasted mice were heavily granulated but still low in zinc reaction. By electron microscopy, B cells were filled with pale granules with very low zinc contents. Fortyeight hours fasted mice showed granulated and zinc-rich islets. In their B cells, many dense granules could be observed.
These findings suggest conversion of pale granules into dense ones, on which zinc plays some role. Furthermore, it is likely that premature granules undergo secretion without being matured into dense granules in animals with hyperinsulinemia.