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.