Quantitative changes of membranes in rat parathyroid cells related to variations of serum calcium.

Parathyroid glands of rats were exposed to low, normal, or high serum calcium concentration by infusion of either ethylenglycol-bis-(2-aminoethylether)-tetraacetic acid (EGTA), NaCl, or CaCl2 for 90 minutes. Electron microscopic morphometry revealed that the cell volume remained constant, whereas the cell surface increased significantly after EGTA infusion and decreased significantly after CaCl2 infusion compared with parathyroid cells of NaCl-infused rats. The surface of the Golgi complex including the surrounding vesicles increased significantly after CaCl2 infusion. Significant changes were not obvious in the volume and surface of the nucleus or in the surface of the rough endoplasmic reticulum. These results suggest, first, that parathyroid hormone is released by exocytosis leading to increased cell surface within 90 minutes. Second, it is considered likely that plasma membrane constituents are retrieved by endocytosis and incorporated into the Golgi complex supporting the idea of membrane recycling.