Reconstitution of Calcium-Regulated Parathyroid Hormone Secretion from Streptolysin-O-Permeabilized Parathyroid Cells by Guanosine 5′-O-(Thio)Triphosphate1

Intracellular Ca2+ levels determine the amount of PTH secretion from parathyroid cells. Dissociated calf parathyroid cells were permeabilized with streptolysin-O (SLO) to provide an in vitro model system to examine Ca2+-dependent regulation of hormone secretion. PTH release from these cells was energy dependent and increased by cytosolic cofactors. Guanosine 5′-O-(thio)triphosphate (GTPγS) increased PTH secretion from SLO-permeabilized cells in a dose-dependent manner from 0.1–100 μm. In the absence of GTPγS there was no relationship between the ambient Ca2+ concentration and the rate of PTH secretion. However, in the presence of GTPγS, intracellular Ca2+ inhibited PTH secretion with an EC50 of approximately 0.1 μm, corresponding to physiological intracellular Ca2+ levels. Thus, the addition of GTPγS to SLO-permeabilized parathyroid cells reconstituted the inverse relationship between extracellular Ca2+ and PTH secretion that is observed in vivo and in intact cells. The data indicate that this effect is m...