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 p arathyroid cells. Dissociated calf parathyroid cells were permeabilize d with streptolysin-O (SLO) to provide an in vitro model system to exa mine Ca2+-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 secreti on from SLO-permeabilized cells in a dose-dependent manner from 0.1-10 0 mu M. In the absence of GTP-gamma S there mas no relationship betwee n the ambient Ca2+ concentration and the rate of PTH secretion. Howeve r, in the presence of GTP gamma S intracellular Ca2+ inhibited PTH sec retion with an EC(50) of approximately 0.1 mu M, corresponding to phys iological intracellular Ca2+ levels. Thus, the addition of GTP gamma S to SLO-permeabilized parathyroid cells reconstituted the inverse rela tionship between extracellular Ca2+ and PTH secretion that Is observed in vivo and in intact cells. The data indicate that this effect is me diated at least in part by heterotrimeric guanosine triphosphatases. I n addition, calcium/calmodulin-dependent protein kinase II appears to mediate low Ca2+-dependent PTH secretion from these cells.