EFFECT OF MAGNESIUM ON PARATHYROID CELLS - EVIDENCE FOR 2 SENSING RECEPTORS OR 2 INTRACELLULAR PATHWAYS

It currently remains controversial as to the intracellular mechanisms coupled to the inhibition of parathyroid hormone (PTH) secretion that are modulated by extracellular divalent cations. To study mechanisms r esponsible for regulation of PTH release by cations, we investigated t he effect of Mg2+ on cytosolic Ca2+ levels ([Ca2+](i)) and PTH secreti on in single isolated bovine parathyroid cells. Addition of 9.0 mM Mg2 + evoked a spike of [Ca2+](i) in similar to 80% of cells in the presen ce of extracellular Ca2+. Mg2+ decreased steady-state [Ca2+](i), which represented inhibition of influx of extracellular Ca2+ in 13-78% of c ells. The percentage of cells that had a decline in steady-state [Ca2](i) after exposure to Mg2+ was dependent on the extracellular Ca2+ co ncentration. The effect of Mg2+ on intracellular Ca2+ response was dos e dependent. Extracellular Mg2+ inhibited PTH secretion in cells that showed decline in steady-state [Ca2+](i), although cells that showed a spike after addition of Mg2+ secreted more PTH than cells that did no t show a spike. The spike of [Ca2+](i) and decline in steady-state [Ca 2+](i) that occur in response to extracellular Mg2+ may be caused inde pendently by two distinct mechanisms that differentially regulate secr etion of PTH.