Effects of fluoride on parathyroid hormone secretion and intracellular second messengers in bovine parathyroid cells

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Abstract
Fluoride ion (F−) alone or in conjunction with aluminum (Al3+) has been shown to stimulate the activity of guanine nucleotide-binding proteins (G proteins) in cell membrane preparations from a variety of cell types and in intact hepatic cells. Several studies have indicated that G proteins are involved in the regulation of parathyroid hormone (PTH) secretion. Intracellular second messengers which modulate PTH secretion (e.g., cAMP) have also been found to be regulated by G proteins. We have, therefore, employed F− as a probe to investigate the possible role of G proteins in the modulation of PTH release and the intracellular second messengers that have been implicated in the control of PTH secretion. F− produces a dose-dependent inhibition of PTH release with a maximal inhibitory effect (67%) at 5 mM. F− exerts its inhibitory effect within 5 min and the degree of suppression of PTH secretion gradually increases over 1 hr. F− (5 mM) inhibits PTH secretion at 0.5 mM Ca2+ to the level observed with 2 mM Ca2+ alone; moreover, the effects of F− and high Ca2+ are not additive. While 1 mM F− suppresses PTH secretion by only 21%, and 10 μM Al3+ has virtually no effect at all, together they inhibit PTH release approximately to the level (63% inhibition) observed with 5 mM NaF alone. In the presence of 10−5 M dopamine, F− produces a concentration-dependent inhibition of cAMP accumulation (0.684 ± 0.033 pmoles/105 cells at 0 mM F− vs. 0.256 ± 0.048 at 5 mM F−). However, the F−-induced decrease in cAMP cannot account for the inhibition of PTH release by this agent, since addition of methylisobutylxanthine (10−4 M) by F− -treated cells raises intracellular cAMP content above that of control cells but fails to reverse the inhibition of PTH release. The cytosolic calcium concentration in Fura-2-loaded cells increases from 210 ± 20 nM to 340 ± 44 nM after 5 mM F− was added to incubation media. Prior removal of extracellular Ca2+ by EGTA totally blocks the F−-induced rise in cytosolic Ca2+ without preventing the inhibition of PTH release by NaF. F− also produces a time- and dose-dependent increase in the accumulation of IP, IP2, and IP3 in cells prelabeled with [3H]inositol and incubated with 10 mM Li+, consistent with activation of phospholipase C. We conclude that F− is a potent inhibitor of PTH secretion. Its action is potentiated by Al3+, consistent with mediation via G protein(s). F− also mimics the effects of high Ca2+ on several intracellular second messengers in parathyroid cells (e.g., cAMP, cytosolic Ca2+, and IP3), presumably through activating G proteins, although the changes in these mediators may not be directly linked to inhibition of secretion. The tumor promoter TPA, an activator of protein kinase C, reverses the inhibition of PTH release by 2 mM Ca2+ but has no effect on the inhibition of secretion by F−, suggesting that F− may also affect G proteins related to secretion at a locus distal to the protein kinase C system.
Funding Information
  • National Institutes of Health (AM 36796, AM 36801, HD 18653, HD 07277)