Role of guanine nucleotide binding protein, cytosolic calcium and cAMP in fluoride-induced suppression of PTH secretion.

In the present studies, we used fluoride and pertussis toxin, potent modulators of guanine nucleotide binding proteins (G proteins), to examine the role of G proteins, cytosolic calcium ([Ca]i) and cAMP in the regulation of PTH secretion from dispersed bovine parathyroid cells. NaF suppressed PTH secretion and cAMP content and increased [Ca]i levels in a dose-dependent manner. Prior removal of extracellular calcium with EGTA completely blocked the NaF-induced increase in [Ca]i, but did not prevent the NaF-induced inhibition of PTH secretion and cAMP content. Pretreatment with 10(-5) M verapamil or 10(-4) M diltiazem blocked neither the NaF-induced suppression of PTH secretion and cAMP content nor the increase in [Ca]i. Manganese chloride (10(-4) M) significantly inhibited the NaF-induced increase in [Ca]i, but did not block the NaF-induced suppression of PTH secretion and cAMP content. Pertussis toxin blocked neither the NaF-induced increase in [Ca]i nor suppression of PTH secretion and cAMP content. Our data suggest that (1) NaF might stimulate a calcium channel resulting in the increase in [Ca]i by acting on a G protein in a manner resistant to the inhibition by pertussis toxin; (2) the NaF-induced increase in [Ca]i is not directly linked to the suppression of PTH secretion, and (3) the NaF-induced suppression of PTH secretion might be explained at least in part by the decrease in cell cAMP content.