Electron-microscopic studies on the relationship between the frequency of parathyroid storage granules and serum calcium levels in the rat.

In the rat parathyroid, the mean number of storage granules (NSG) per chief cell has been electron-microscopically studied and correlated with the mean serum calcium level (SCL). In animals given 4% CaCl2 plus vitamin D2 for 3 days. SCL is significantly elevated and NSG is increased. When these animals are injected with 2% EDTA, SCl is lowered to 8 mg/dl, but NSG is not affected; in those injected with 4% EDTA, however, SCL declines to a minimum (5.8 mg/dl) after 30 min, and NSG is also decreased. Control SCL are 8.9 mg/dl. These results indicate that storage granules may not be released until SCL is depressed to a certain level. In rats 3 weeks after castration, the chief cells show hyperplastic changes and SCl is at a low concentration (8.0 mg/dl). NSG, however, remains almost within control limits. Castrated animals injected with 4% EDTA show a hypocalcemia and a decrease in NSG, but NSG gradually recovers over a period of 6 h. These data suggest that storage granules can be produced even under lower calcium concentrations. It is concluded that storage granules may be constantly produced and stored, and are released only as an emergency supply of hormone.