Parathyroid hormone secretion does not respond to changes of free calcium in electropermeabilized bovine parathyroid cells, but is stimulated with phorbol ester and cyclic AMP

The secretion of parathyroid hormone (PTH) is suppressed in bovine parathyroid cells by raised extracellular Ca2+], and 12-0-tetradecanoyl-phorbol-13-acetate (TPA) stimulates the release of PTH from cells suppressed by high extracellular Ca2+]. Extracellular and cytosolic free Ca2+] are proportionally related in intact cells. To assess the role of cytosolic free Ca2+] on PTH secretion, bovine parathyroid cells were rendered permeable by brief exposure to an intense electric field. PTH secretion was comparable at 40 nM, 500 nM, 5 microM, 28 microM, 0.5 mM and 2 mM Ca2+] (release of total cellular PTH 3.7 +/- 0.5%, 3.9 +/- 0.4%, 3.4% +/- 0.3%, 3.9 +/- 0.4%, 3.1 +/- 0.3%, 3.5 +/- 0.7%, respectively), but the secretion was stimulated twofold (P less than 0.05 vs. control) in a dose and ATP dependent manner with TPA (100 nM) and cyclic AMP (1 mM). As a result, free Ca2+] in the range of those observed in intact cells during regulation of PTH secretion by changes of extracellular Ca2+] did not affect the release of PTH in permeabilized cells. The Ca2+] independent stimulation of PTH release by TPA and cyclic AMP indicates that changes of cytosolic free Ca2+] may represent a secondary event not related to the regulation of PTH secretion.