Reconstitution of calcium-regulated parathyroid hormone secretion from streptolysin-O-permeabilized parathyroid cells by guanosine 5'-O-(thio)triphosphate.

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 Ca(2+)-dependent regulation of hormone secretion. PTH release from these cells was energy dependent and increased by cytosolic cofactors. Guanosine 5'-O-(thio)triphosphate (GTP gamma S) increased PTH secretion from SLO-permeabilized cells in a dose-dependent manner from 0.1-100 microM. In the absence of GTP gamma S there was no relationship between the ambient Ca2+ concentration and the rate of PTH secretion. However, in the presence of GTP gamma S, intracellular Ca2+ inhibited PTH secretion with an EC50 of approximately 0.1 microM, corresponding to physiological intracellular Ca2+ levels. Thus, the addition of GTP gamma S 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 mediated at least in part by heterotrimeric guanosine triphosphatases. In addition, calcium/calmodulin-dependent protein kinase II appears to mediate low Ca(2+)-dependent PTH secretion from these cells.