Paradoxical block of parathormone secretion is mediated by increased activity of G alpha subunits

The paradox of blunted parathormone (PTH) secretion in patients with severe hypomagnesemia has been known for more than 20 years, but the underlying m echanism is not deciphered, We determined the effect of low magnesium on in vitro PTH release and on the signals triggered by activation of the calciu m sensing receptor (CaSR), Analogous to the in vivo situation, PTH release from dispersed parathyroid cells was suppressed under low magnesium, In par allel, the two major signaling pathways responsible for CaSR triggered bloc k of PTH secretion, the generation of inositol phosphates, and the inhibiti on of cAMP were enhanced, Desensitization or pertussis toxin-mediated inhib ition of CaSR-stimulated signaling suppressed the effect of low magnesium, further confirming that magnesium acts within the axis CaSR-G-protein, Howe ver, the magnesium binding site responsible for inhibition of PTH secretion is not identical with the extracellular ion binding site of the CaSR, beca use the magnesium deficiency-dependent signal enhancement was not altered o n CaSR receptor mutants with increased or decreased affinity for calcium an d magnesium, By contrast, when the magnesium affinity of the G alpha subuni t was decreased, CaSR activation was no longer affected by magnesium, Thus, the paradoxical block of PTH release under magnesium deficiency seems to b e mediated through a novel mechanism involving an increase in the activity of G alpha subunits of heterotrimeric G-proteins.