Correlation of rhythmic variations in subcellular structures of rat hepatocytes, pancreatic acinar cells, and islet B cells.

Correlation of variations in subcellular structures of hepatocytes, pancreatic acinar cells, and islet B cells was examined morphometrically at six time points during 24 hr, using 24 Wistar male rats housed under standardized conditions (LD 12:12; light from 0600 to 1800). The morphometric features examined were subject to strong variations during the 24-hr period. The volume and surface densities of rough endoplasmic reticulum in hepatocytes and acinar cells were highest at 0400, when those of secretory granules in B cells were lowest. At 0400, the surface density of the plasma membranes of B cells facing the intercellular spaces exhibited its highest value. The volume density of glycogen granules in hepatocytes was highest 4 hr after the secretory granules of B cells showed the lowest value. The volume density of glycogen granules in hepatocytes decreased during the light cycle, being lowest at 2000, while that of secretory granules in acinar cells increased during the light cycle, being highest at 1600. At 1600 and 2000, the volume and surface densities of secretory granules were high in B cells. These results suggest that insulin secreted from B cells stimulates protein synthesis in hepatocytes and pancreatic acinar cells, but not directly the synthesis of glycogen granules and secretion of zymogen granules.