Calcium uptake and inositol trisphosphate-induced calcium release from parathyroid gland membranes

The manner in which changes in Ca2+ are translated by the parathyroid gland into a signal for secretion of its major secretory proteins, parathormone and secretory protein-I, is not understood. In order to gain insight into this mechanism, Ca2+ transport was evaluated in porcine parathyroid gland membranes prepared by homogenization and differential centrifugation. The membranes were considered to be intact and appropriately 'sided' since they contained secretory protein-I and exhibited ATP-dependent uptake of [45Ca2+]. About 1 mol of ATP was hydrolyzed per mol of Ca2+ transported, signifying that the ATPase activity was relevant to the Ca2+ transport system of the membranes. Inositol trisphosphate, which is believed to be a universal signal for generating a change in cytosolic Ca2+ by discharging Ca2+ from internal membranes, caused the parathyroid membranes to release accumulated Ca2+. The membranes were able to partially reaccumulate the discharged Ca2+. The data suggest that intracellular Ca2+ metabolism in the parathyroid cell is similar to that of other secretory cells. If a change in intracellular concentration of this ion is involved in secretion, it may occur locally within the cytosol.