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

Surgical Service, Veterans Administration Connecticut Healthcare System, and the Department of Surgery and Cell Biology, Yale University School of Medicine, New Haven, Connecticut 06250-8062

Address all correspondence and requests for reprints to: Lisa M. Matovcik, Ph.D., Department of Surgery, P.O. Box 208062, New Haven, Connecticut 06520-8062. E-mail: matovcik{at}aol.com

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 (GTPS) increased PTH secretion from SLO-permeabilized cells in a dose-dependent manner from 0.1–100 µM. In the absence of GTPS there was no relationship between the ambient Ca²⁺ concentration and the rate of PTH secretion. However, in the presence of GTPS, intracellular Ca²⁺ inhibited PTH secretion with an EC₅₀ of approximately 0.1 µM, corresponding to physiological intracellular Ca²⁺ levels. Thus, the addition of GTPS 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 Ca²⁺-dependent PTH secretion from these cells.

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