title: Fluoride stimulates the accumulation of inositol phosphates, increases intracellular free calcium, and inhibits parathyroid hormone release in dispersed bovine parathyroid cells.
abstract: The stimulation of polyphosphoinositide (PPI) turnover is associated with cellular activation and hormone secretion in numerous systems. GTP-binding proteins appear to couple receptors to phospholipase-C-mediated PPI breakdown. We assessed the effects of fluoride, an activator of GTP-binding proteins, on inositol phosphate accumulation, intracellular free Ca2+ [(Ca2+)i], cAMP content, and PTH release in dispersed bovine parathyroid cells. Sodium fluoride (5-30 mM) produced marked dose-dependent increases in inositol phosphates. With anion exchange HPLC, we confirmed that 30 mM fluoride stimulated a rapid increase in 1,4,5-inositol trisphosphate, a potent Ca2+-mobilizing compound. Using the Ca2+-sensitive probe fura-2, we determined that 30 mM fluoride increased [Ca2+]i from 339 +/- 9 to 650 +/- 39 nM (n = 8) within 30-60 sec at 1 mM extracellular Ca2+. After the depletion of extracellular Ca2+ by the addition of 1 mM EGTA, 30 mM fluoride increased [Ca2+]i 45 +/- 9% (n = 4), indicating that fluoride can mobilize intracellular Ca2+ stores. Fluoride (1-30 mM) also inhibited PTH release in dose-dependent fashion. Fluoride (30 mM) produced 72.8 +/- 4.2% suppression of maximal low Ca2+-stimulated PTH release comparable to the 83.7 +/- 3.7% inhibition by 2.0 mM extracellular Ca2+. Since changes in both [Ca2+]i and cAMP regulate PTH release, we measured the effect of fluoride on intracellular cAMP. Fluoride did not detectably change basal cAMP content, but it reduced forskolin-stimulated increases in cAMP. We conclude that fluoride may activate at least two GTP-dependent processes in parathyroid cells, resulting in PPI breakdown and cAMP accumulation. While both may contribute to the fluoride-induced suppression of PTH release, our findings suggest that the stimulation of PPI turnover leads to inhibition of PTH secretion.
authors.affiliation: Endocrine Research Unit, San Francisco Veterans Administration Medical Center, California 94121.
authors.forename: D M
authors.initials: DM
authors.lastname: Shoback
authors.forename: J M
authors.initials: JM
authors.lastname: McGhee
mesh_terms: D000818:Animals; D001572:Benzofurans; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D002851:Chromatography, High Pressure Liquid; D000242:Cyclic AMP / Q000378:metabolism; D005456:Fluorescent Dyes; D016257:Fura-2; D019204:GTP-Binding Proteins / Q000502:physiology; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000378:metabolism*; D010280:Parathyroid Glands / Q000187:drug effects / Q000378:metabolism*; D010281:Parathyroid Hormone / Q000378:metabolism*; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols / Q000378:metabolism; D012969:Sodium Fluoride / Q000494:pharmacology*; D013050:Spectrometry, Fluorescence; D013403:Sugar Phosphates / Q000378:metabolism*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
chemical_list: D001572:Benzofurans; D005456:Fluorescent Dyes; D007295:Inositol Phosphates; D010281:Parathyroid Hormone; D018129:Phosphatidylinositol Phosphates; D010716:Phosphatidylinositols; D013403:Sugar Phosphates; D010566:Virulence Factors, Bordetella; C044259:inositol 1,4-bis(phosphate); D015544:Inositol 1,4,5-Trisphosphate; D012969:Sodium Fluoride; D000242:Cyclic AMP; D019204:GTP-Binding Proteins; D002118:Calcium; D016257:Fura-2