Regulation of arachidonic acid production by intracellular calcium in parathyroid cells: effect of extracellular phosphate.

The action of extracellular calcium on the calcium receptor in parathyroid cells results in activation of phospholipase C (PLC), PLD, and PLA(2). The PLA(2)-arachidonic acid (AA) intracellular signaling pathway mediates inhibition of parathyroid hormone (PTH) secretion. In addition, stimulation of the calcium receptor produces increases in intracellular calcium levels. It was demonstrated that high extracellular phosphate levels reduce the production of AA, a mechanism by which phosphate may stimulate PTH secretion. The objective was to determine, in parathyroid tissue, whether AA production is stimulated by increases in intracellular calcium levels and to investigate whether the decreased AA production induced by high extracellular phosphate levels could be modified by increases in intracellular calcium levels. Experiments were performed in vitro using parathyroid tissue. The intracellular calcium level was increased by incubation with an ionophore (A23187), which increases calcium influx across the cell membrane, or thapsigargin, which releases calcium from intracellular stores. The phosphate concentration in the medium was normal (1 mM) or high (4 mM). The response to calcium was evaluated by incubation with 0.6 or 1.35 mM calcium concentrations. AA production by parathyroid tissue was measured by gas chromatography. In parathyroid tissue incubated with either a calcium ionophore or thapsigargin, there was an increase in AA production, together with inhibition of PTH secretion, suggesting that PLA(2) is activated by the elevation in intracellular calcium levels. Therefore, the effect of intracellular calcium level elevation on AA production in the presence of high extracellular phosphate levels was evaluated. The results demonstrate that, despite high phosphate levels in the medium, both the ionophore and thapsigargin were capable of inducing a marked increase in AA production, which was associated with a decrease in PTH secretion. In conclusion, in parathyroid tissue, AA levels can be regulated by an ionophore and thapsigargin, both of which increase cytosolic calcium concentrations. The stimulation of PTH secretion by high phosphate levels can be prevented by increases in intracellular calcium levels.