Paradoxical Block of Parathormone Secretion Is Mediated by Increased Activity of Gα Subunits

Abstract

The paradox of blunted parathormone (PTH) secretion in patients with severe hypomagnesemia has been known for more than 20 years, but the underlying mechanism is not deciphered. We determined the effect of low magnesium on in vitro PTH release and on the signals triggered by activation of the calcium-sensing receptor (CaSR). Analogous to the in vivosituation, PTH release from dispersed parathyroid cells was suppressed under low magnesium. In parallel, the two major signaling pathways responsible for CaSR-triggered block of PTH secretion, the generation of inositol phosphates, and the inhibition of cAMP were enhanced. Desensitization or pertussis toxin-mediated inhibition of CaSR-stimulated signaling suppressed the effect of low magnesium, further confirming that magnesium acts within the axis CaSR-G-protein. However, the magnesium binding site responsible for inhibition of PTH secretion is not identical with the extracellular ion binding site of the CaSR, because the magnesium deficiency-dependent signal enhancement was not altered on CaSR receptor mutants with increased or decreased affinity for calcium and magnesium. By contrast, when the magnesium affinity of the Gα subunit was decreased, CaSR activation was no longer affected by magnesium. Thus, the paradoxical block of PTH release under magnesium deficiency seems to be mediated through a novel mechanism involving an increase in the activity of Gα subunits of heterotrimeric G-proteins. The paradox of blunted parathormone (PTH) secretion in patients with severe hypomagnesemia has been known for more than 20 years, but the underlying mechanism is not deciphered. We determined the effect of low magnesium on in vitro PTH release and on the signals triggered by activation of the calcium-sensing receptor (CaSR). Analogous to the in vivosituation, PTH release from dispersed parathyroid cells was suppressed under low magnesium. In parallel, the two major signaling pathways responsible for CaSR-triggered block of PTH secretion, the generation of inositol phosphates, and the inhibition of cAMP were enhanced. Desensitization or pertussis toxin-mediated inhibition of CaSR-stimulated signaling suppressed the effect of low magnesium, further confirming that magnesium acts within the axis CaSR-G-protein. However, the magnesium binding site responsible for inhibition of PTH secretion is not identical with the extracellular ion binding site of the CaSR, because the magnesium deficiency-dependent signal enhancement was not altered on CaSR receptor mutants with increased or decreased affinity for calcium and magnesium. By contrast, when the magnesium affinity of the Gα subunit was decreased, CaSR activation was no longer affected by magnesium. Thus, the paradoxical block of PTH release under magnesium deficiency seems to be mediated through a novel mechanism involving an increase in the activity of Gα subunits of heterotrimeric G-proteins. parathormone calcium-sensing receptor intracellular free Mg2+ concentration extracellular free Mg2+concentration guanosine 5′-3O-(thio)triphosphate Parathormone (PTH)1secretion from the parathyroid gland is suppressed by high extracellular calcium and magnesium (1Pollak M.R. Brown E.M. Chou Y.W. Hebert S.C. Marx S.J. Steinman B. Levi T. Seidman C.E. Seidman J.G. Cell. 1993; 75: 1297-1303Abstract Full Text PDF PubMed Scopus (920) Google Scholar). The calcium-sensing receptor (CaSR) is responsible for the calcium-dependent inhibition of PTH secretion (2Ho C. Conner D.A. Pollak M.R. Ladd D.J. Kifor O. Warren H.B. Brown E.M. Nat. Genet. 1995; 11: 389-394Crossref PubMed Scopus (524) Google Scholar). Direct binding of calcium or magnesium activates the CaSR (3Brown E.M. Gamba G. Riccardi D. Lombardi M. Butters R. Kifor O. Sun A. Hediger M.A. Lytton J. Hebert S.C. Nature. 1993; 366: 575-580Crossref PubMed Scopus (2390) Google Scholar). Activation of the CaSR triggers Gαq/Gαi-mediated signaling pathways (4Brown E.M. Physiol. Rev. 1991; 71: 371-411Crossref PubMed Scopus (643) Google Scholar). Several mutations have been identified with increased activation of this receptor (5Pollak M.R. Brown E.M. Estep H.L. McLaine P.N. Kifor O. Park J. Hebert S.C. Seidman C.E. Seidman J.G. Nat. Genet. 1994; 8: 303-307Crossref PubMed Scopus (539) Google Scholar, 6Pearce S.H.S. Bai M. Quinn S.J. Kifor O. Brown E.M. Thakker R.V. J. Clin. Invest. 1996; 98: 1860-1866Crossref PubMed Scopus (170) Google Scholar). CaSR mutants with increased affinity/potency for the agonist calcium and in part enhanced constitutive activity led to permanent inhibition of PTH secretion (7Pearce S.H.S. Williamson C. Kifor O. Bai M. Coulthard M.G. Davies M. McCredie D. Powell H. Lewis-Barned N. Kendall-Taylor P. Brown E.M. Thakker R.V. N. Engl. J. Med. 1996; 335: 1115-1122Crossref PubMed Scopus (510) Google Scholar). Therefore, patients with activated CaSR mutants suffer from hypoparathyroidism. A similar phenotype of blunted PTH secretion is seen in patients with severe magnesium deficiency (8Anast C.S. Mohs J.M. Kaplan S.L. Burns T.W. Science. 1972; 210: 606-608Crossref Scopus (281) Google Scholar, 9Duran M.J. Borst G.C. Osburne R.C. Eil C. Am. J. Med. 1984; 76: 151-154Abstract Full Text PDF PubMed Scopus (25) Google Scholar, 10Mennes P. Rosenbaum R. Martin K. Slatopolsky E. Ann. Intern. Med. 1978; 88: 206-209Crossref PubMed Scopus (30) Google Scholar). This finding is unexpected since the effects of high magnesium on parathyroid hormone secretion are similar to those of calcium, and therefore, low magnesium should be expected to result in increased PTH secretion. And indeed, in contrast to patients, rats respond to severe hypomagnesemia with increased secretion of PTH (11Anast C.S. Forte L.F. Endocrinology. 1983; 113: 184-189Crossref PubMed Scopus (28) Google Scholar, 12Inaba M. Okuno S. Imanishi Y. Nishizawa Y. Morii H. J. Endocrinol. Invest. 1992; 15: 135-142PubMed Google Scholar). It is known that hypomagnesemia reflects intracellular magnesium deficiency (9Duran M.J. Borst G.C. Osburne R.C. Eil C. Am. J. Med. 1984; 76: 151-154Abstract Full Text PDF PubMed Scopus (25) Google Scholar). Thus, the site of magnesium action has been assumed to lie intracellularly (9Duran M.J. Borst G.C. Osburne R.C. Eil C. Am. J. Med. 1984; 76: 151-154Abstract Full Text PDF PubMed Scopus (25) Google Scholar). However, causality between blunted PTH secretion and magnesium deficiency is not established, although the magnesium paradox has been known for more than 20 years (8Anast C.S. Mohs J.M. Kaplan S.L. Burns T.W. Science. 1972; 210: 606-608Crossref Scopus (281) Google Scholar). In search for the mechanism we investigated the relationship between magnesium deficiency, PTH secretion, and CaSR-mediated signaling. We present evidence that increased activity of Gα subunits leads to enhanced signaling mediated by constitutive activation of the human CaSR. Enhanced CaSR-mediated signaling may thus constitute the link between severe hypomagnesemia and blunted PTH secretion. Human embryonic kidney cells (HEK-293) were cultivated and transfected with plasmids encoding the wild-type and the different CaSR mutants (7Pearce S.H.S. Williamson C. Kifor O. Bai M. Coulthard M.G. Davies M. McCredie D. Powell H. Lewis-Barned N. Kendall-Taylor P. Brown E.M. Thakker R.V. N. Engl. J. Med. 1996; 335: 1115-1122Crossref PubMed Scopus (510) Google Scholar, 13Freichel M. Zink-Lorenz A. Holloschi A. Hafner M. Flockerzi V. Raue F. Endocrinology. 1996; 137: 3842-3848Crossref PubMed Scopus (101) Google Scholar, 14Ruat M. Molliver M.E. Snowman A.M. Snyder S.H. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 3161-3165Crossref PubMed Scopus (345) Google Scholar, 15Bai M. Quinn S. Trivedi S. Kifor O. Pearce S.H.S. Pollak M.R. Krapcho K. Hebert S.C. Brown E.M. J. Biol. Chem. 1996; 271: 19537-19545Abstract Full Text Full Text PDF PubMed Scopus (385) Google Scholar) under the control of the cytomegalovirus promotor as described (16AbdAlla S. Zaki E. Lother H. Quitterer U. J. Biol. Chem. 1999; 274: 26079-26084Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar). For mutagenesis of Gαi we used the Escherichia coli expression vector pQE60 containing the Gαi1-cDNA with an internal nucleotide sequence after amino acid 121 encoding a hexahistidine tag. The codon coding for arginine 209 of Gαi1 was exchanged to cysteine by site-directed mutagenesis (16AbdAlla S. Zaki E. Lother H. Quitterer U. J. Biol. Chem. 1999; 274: 26079-26084Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar). For mutagenesis of the human CaSR, an additional restriction site for XhoI was introduced (silent mutation of leucine 276). The mutated cDNAs were sequenced entirely to confirm the identity of the mutants. The Gαs and Gαi1proteins (wild-type and R209C) were expressed in E. coliBL21(DE3) and purified by Ni2+-agarose according to the manufacturer's protocol (Qiagen). Purified proteins were desalted and concentrated by centrifugation through a Centricon with an of the of the purified proteins were in and in a concentration of from was as described J. Biol. Chem. 1984; Full Text PDF PubMed Google Scholar). was expressed in cells with after cells were and of cells were as described (16AbdAlla S. Zaki E. Lother H. Quitterer U. J. Biol. Chem. 1999; 274: 26079-26084Abstract Full Text Full Text PDF PubMed Scopus (100) Google Scholar). were for binding in to control since was not further of cells transfected with the encoding the human CaSR were by centrifugation as described S. Lother H. Quitterer U. Nature. PubMed Scopus Google Scholar). The was with and a concentration of and binding of to and was determined as described S. Lother H. Quitterer U. Nature. PubMed Scopus Google Scholar, U. M.J. Proc. Natl. Acad. Sci. U. S. A. 1999; PubMed Scopus Google Scholar). of cells or of in were with the Gα subunits and for on in and as were with For of nucleotide binding to and were The was by the of The were and different were and by with and was of dispersed parathyroid or of cells were determined as described U. M.J. Proc. Natl. Acad. Sci. U. S. A. 1999; PubMed Scopus Google Scholar) with the cells were with 20 containing the concentration of and in the for to the cells with the Mg2+ concentration was 20 inositol were and cAMP were determined in dispersed parathyroid cells or in cells after the the cells were with 20 containing the concentration of were to with the Mg2+ concentration for with the Mg2+ concentration and with was and the was by the of or as a 20 cAMP was and determined by Human or parathyroid from patients with were in parathyroid cells from parathyroid were by with and as described E.M. S. Endocrinology. PubMed Scopus Google Scholar, A. J. Biol. Chem. Full Text PDF PubMed Google Scholar). For of PTH dispersed parathyroid cells were for in 20 with and as PTH release was for in the The concentration of PTH in the was by human parathyroid hormone and cAMP were determined in dispersed parathyroid cells as described For of in cells in 20 were with in with acid for in a were with and for to were with with and as was with a an of and an between and was determined from the between and as described J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). In patients with severe PTH secretion is (8Anast C.S. Mohs J.M. Kaplan S.L. Burns T.W. Science. 1972; 210: 606-608Crossref Scopus (281) Google Scholar, 9Duran M.J. Borst G.C. Osburne R.C. Eil C. Am. J. Med. 1984; 76: 151-154Abstract Full Text PDF PubMed Scopus (25) Google Scholar, 10Mennes P. Rosenbaum R. Martin K. Slatopolsky E. Ann. Intern. Med. 1978; 88: 206-209Crossref PubMed Scopus (30) Google Scholar). this in paradox is to the release of PTH from parathyroid we determined the effect of low magnesium on PTH vitro on dispersed parathyroid a control for calcium-dependent of the parathyroid from patients with of the CaSR by calcium was as a CaSR agonist the release of PTH with an of when the Mg2+concentration was This is in with the for human or E.M. J.G. Am. J. Med. 71: Full Text PDF PubMed Scopus Google Scholar, C. E. R. S. J. G. J. Endocrinol. PubMed Scopus Google Scholar). magnesium deficiency was by the extracellular Mg2+ to Mg2+ was decreased, PTH secretion was of the extracellular concentration the in in magnesium deficiency PTH to (8Anast C.S. Mohs J.M. Kaplan S.L. Burns T.W. Science. 1972; 210: 606-608Crossref Scopus (281) Google Scholar, 9Duran M.J. Borst G.C. Osburne R.C. Eil C. Am. J. Med. 1984; 76: 151-154Abstract Full Text PDF PubMed Scopus (25) Google Scholar, 10Mennes P. Rosenbaum R. Martin K. Slatopolsky E. Ann. Intern. Med. 1978; 88: 206-209Crossref PubMed Scopus (30) Google and calcium this inhibition of PTH secretion (9Duran M.J. Borst G.C. Osburne R.C. Eil C. Am. J. Med. 1984; 76: 151-154Abstract Full Text PDF PubMed Scopus (25) Google Scholar). The of magnesium for the inhibition of PTH release was and inhibition of PTH release is not seen the Mg2+concentration This finding with in that the magnesium in patients that PTH secretion block are with between and (8Anast C.S. Mohs J.M. Kaplan S.L. Burns T.W. Science. 1972; 210: 606-608Crossref Scopus (281) Google Scholar, 9Duran M.J. Borst G.C. Osburne R.C. Eil C. Am. J. Med. 1984; 76: 151-154Abstract Full Text PDF PubMed Scopus (25) Google Scholar, 10Mennes P. Rosenbaum R. Martin K. Slatopolsky E. Ann. Intern. Med. 1978; 88: 206-209Crossref PubMed Scopus (30) Google Scholar). PTH secretion is by of and are of the CaSR. different signaling pathways are activated by the CaSR, generation of inositol and inhibition of magnesium deficiency, signaling pathways were enhanced in dispersed parathyroid cells of cAMP was to that of the inhibition after with the CaSR agonist calcium and not and inositol were increased to that of the by magnesium or calcium The of the magnesium deficiency-dependent signal enhancement were similar as for the PTH secretion, for the cAMP and for the inositol and that magnesium deficiency similar signaling pathways as those activated by the CaSR. the CaSR for the effect of magnesium deficiency on PTH this we the CaSR on parathyroid cells by calcium Desensitization of the CaSR was by a decreased to the agonist magnesium and calcium after magnesium and calcium PTH release by with a inhibition of PTH release in control cells the effect of magnesium deficiency on PTH release was suppressed similar to the to the CaSR This finding that are a for the effect of magnesium deficiency on PTH the that the site of magnesium action within the axis we further the mechanism of the magnesium paradox in a transfected Direct effects of magnesium on the signaling of the CaSR were with control cells this inositol of cells the CaSR increased to when the Mg2+ concentration in the was decreased from to cells CaSR expression not increase in inositol under The of magnesium was The CaSR was responsible for the increase in inositol under magnesium deficiency, since inositol increased with CaSR expression Thus, the effect of magnesium deficiency on activation of expressed human CaSR the effects of magnesium deficiency on parathyroid signaling. CaSR-mediated inhibition of activity was In activity was by the CaSR. inhibition was with in with or Mg2+ magnesium in inhibition The CaSR-mediated inhibition of cAMP Mg2+ was that of the inhibition by The of magnesium was In contrast, the cAMP of control cells were affected by a in magnesium that the and pathways triggered by constitutive activity of the CaSR were affected by magnesium deficiency in cells as in dispersed parathyroid cells is an agonist of the CaSR. we the extracellular magnesium binding of the CaSR is in the signal enhancement under magnesium CaSR receptor mutants were expressed expression of the different CaSR mutants were in has a decreased for calcium not and 15Bai M. Quinn S. Trivedi S. Kifor O. Pearce S.H.S. Pollak M.R. Krapcho K. Hebert S.C. Brown E.M. J. Biol. Chem. 1996; 271: 19537-19545Abstract Full Text Full Text PDF PubMed Scopus (385) Google Scholar) and magnesium and not the of magnesium in receptor activation was similar between cells mutated and the wild-type CaSR This finding that the extracellular ion binding site of the CaSR for calcium and magnesium is not in the signal enhancement under low magnesium. CaSR mutants with altered activity were activity of increased with the wild-type receptor The signal enhancement under magnesium deficiency in cells was increased with the wild-type receptor the of magnesium in activation was similar between the wild-type CaSR and the mutated a the for the activation of by magnesium was Thus, signaling mediated by the activity of the wild-type CaSR and of is enhanced under magnesium deficiency with similar but the of this enhancement is different and on the activity of the CaSR. further CaSR activity the of the signal enhancement by low magnesium, we wild-type CaSR with in receptor with altered S. Lother H. Quitterer U. Nature. PubMed Scopus Google Scholar, M. Trivedi S. Kifor O. Quinn S.J. Brown E.M. Proc. Natl. Acad. Sci. U. S. A. 1999; PubMed Scopus Google decreased of with wild-type CaSR decreased the affinity/potency for calcium or magnesium In parallel, the signal by the activity of was suppressed to that of the signal of the wild-type receptor and By contrast, the of the magnesium signal enhancement was not Thus, a in the activity of the CaSR is by a in the of the signal enhancement under magnesium deficiency but no in the of magnesium. confirm that magnesium deficiency signaling mediated by the activity of the CaSR. the activity of the CaSR to the magnesium paradox of blunted PTH secretion in the paradox of blunted PTH secretion under severe magnesium deficiency has been in patients but not in rats (11Anast C.S. Forte L.F. Endocrinology. 1983; 113: 184-189Crossref PubMed Scopus (28) Google Scholar, 12Inaba M. Okuno S. Imanishi Y. Nishizawa Y. Morii H. J. Endocrinol. Invest. 1992; 15: 135-142PubMed Google we the activity of the and the human CaSR. the increase in inositol of cells was that of the increase of cells the human CaSR and a in the extracellular magnesium led to an enhancement of the signaling mediated by CaSR with an for magnesium similar to that of the human CaSR However, the of the signal enhancement of the CaSR Mg2+ was that of the human CaSR of the human and the CaSR were expressed in as determined by with Mg2+ and by similar concentration with of and for calcium and magnesium that in PTH secretion under magnesium deficiency for patients and rats with in the activity of the and with in the of the signal enhancement under magnesium The that the of magnesium for the signal enhancement were similar between the human and the CaSR and between different CaSR mutants with different activity or with different affinity for extracellular magnesium binding a magnesium binding site different from the extracellular ion binding site of the CaSR. the extracellular magnesium binding site of the CaSR not to be in the magnesium we magnesium on the intracellular of the CaSR, the this CaSR-mediated activation of was determined by the enhancement of binding to A of the Mg2+concentration from to or the of CaSR-mediated binding of and The for the inhibition of CaSR-stimulated binding was Thus, magnesium the CaSR-mediated activation and the CaSR-stimulated activation of and pathways with similar that magnesium acts within the axis and the CaSR-stimulated binding of to wild-type Gαi and to Gα with from cells as described under in a the magnesium binding site responsible for this inhibition of CaSR activation on the CaSR or on the of magnesium for the inhibition of CaSR activation was not affected by CaSR mutants with increased or decreased affinity for extracellular magnesium or calcium we on the magnesium binding site of the Gα subunit Nature. 1994; PubMed Scopus Google Scholar, PubMed Scopus Google Scholar). magnesium effects on the activation of Gα subunits have not been on from T. J. Biol. Chem. Full Text PDF PubMed Google the that magnesium the nucleotide of proteins Rev. PubMed Scopus Google Scholar) the of a similar mechanism for Gα And indeed, on wild-type magnesium nucleotide with an of By contrast, nucleotide of a Gαi Gα magnesium binding A.M. N. J. Biol. Chem. 1993; Full Text PDF PubMed Google Scholar) was not affected by The a decreased affinity for magnesium, with an of for the inhibition of binding and determined of the inhibition of binding to different purified Gα subunits determined as described under inhibition in a CaSR-stimulated binding was determined for the Gα In contrast to the wild-type CaSR-stimulated binding to Gα not suppressed by Mg2+ with Mg2+ similar and and are with the that the magnesium binding site responsible for the enhancement of CaSR-mediated activation under low magnesium is on the the that the magnesium binding site responsible for the enhancement of CaSR-mediated activation was intracellularly on the Gα we a in the extracellular free was by a in the intracellular free Mg2+ cells were with and was The of cells in with Mg2+ was This is in with the of cells J. Biol. Chem. 1996; 271: Full Text Full Text PDF PubMed Scopus Google Scholar). By contrast, when the cells were for 20 in with the was to This finding that a in to a in the of extracellular the of cells increased to the within are in with that in are by in S. T. J. Physiol. 1991; PubMed Scopus Google Scholar). that effects of magnesium on nucleotide were with a Mg2+ concentration and the in a of is to effects on nucleotide of in further are in the magnesium paradox of blunted PTH secretion, we determined the effect of inhibition on parathyroid cells the effect of high magnesium in PTH release the effect of low magnesium on PTH release that are in the magnesium paradox of PTH The magnesium binding site on the Gα subunit that seems responsible for the enhancement of CaSR-mediated signaling under magnesium deficiency is between Gα subunits Nature. 1994; PubMed Scopus Google Scholar). In with this binding to Gα subunits was by magnesium with similar concentration However, in parathyroid magnesium deficiency enhanced CaSR-mediated the of PTH secretion by was not enhanced under magnesium deficiency proteins are the Gα subunits in the we inhibition of proteins effects of low magnesium on signaling. And indeed, when proteins were by pertussis PTH release was but enhanced under Mg2+ with or Mg2+ Thus, in parathyroid cells the enhancement of signaling under magnesium deficiency is by the increased activity of the magnesium binding site responsible for the enhancement of CaSR-mediated signaling under magnesium deficiency is on the Gα signals mediated by the activity of than the CaSR should be enhanced under magnesium magnesium deficiency the activity of in we determined the effect of magnesium deficiency on signals by the activity of two different the and the were expressed high in cells to increase signaling mediated by receptor by receptor activity of the and the receptor were increased under low magnesium with Mg2+ extracellular magnesium is not an agonist of the and the receptor as of the CaSR, we signals by receptor activation were enhanced under magnesium deficiency as were the signals by constitutive receptor And indeed, signaling of and was enhanced under magnesium deficiency with Mg2+ as by a in the of and that signaling mediated by activation of different is enhanced under magnesium are in with the identified mechanism of magnesium the nucleotide of Gα subunits in effects on activity activity or increase in inositol of cells the or the receptor was determined in with or receptor activity is expressed as of agonist with or and was similar and in a The or increase in inositol of cells the or the receptor was determined in with or receptor activity is expressed as of agonist with or and was similar and of a (3Brown E.M. Gamba G. Riccardi D. Lombardi M. Butters R. Kifor O. Sun A. Hediger M.A. Lytton J. Hebert S.C. Nature. 1993; 366: 575-580Crossref PubMed Scopus (2390) Google Scholar) and the of CaSR mutants from patients with and (1Pollak M.R. Brown E.M. Chou Y.W. Hebert S.C. Marx S.J. Steinman B. Levi T. Seidman C.E. Seidman J.G. Cell. 1993; 75: 1297-1303Abstract Full Text PDF PubMed Scopus (920) Google Scholar, M.R. Brown E.M. Estep H.L. McLaine P.N. Kifor O. Park J. Hebert S.C. Seidman C.E. Seidman J.G. Nat. Genet. 1994; 8: 303-307Crossref PubMed Scopus (539) Google Scholar, 6Pearce S.H.S. Bai M. Quinn S.J. Kifor O. Brown E.M. Thakker R.V. J. Clin. Invest. 1996; 98: 1860-1866Crossref PubMed Scopus (170) Google Scholar, S.H.S. Williamson C. Kifor O. Bai M. Coulthard M.G. Davies M. McCredie D. Powell H. Lewis-Barned N. Kendall-Taylor P. Brown E.M. Thakker R.V. N. Engl. J. Med. 1996; 335: 1115-1122Crossref PubMed Scopus (510) Google Scholar) the relationship between calcium and magnesium and PTH secretion. this the paradox of blunted PTH secretion under magnesium deficiency is an that the of magnesium action was intracellular (9Duran M.J. Borst G.C. Osburne R.C. Eil C. Am. J. Med. 1984; 76: 151-154Abstract Full Text PDF PubMed Scopus (25) Google Scholar). PTH was not affected by magnesium deficiency J. Biol. Chem. Full Text PDF PubMed Google Scholar) and PTH within after magnesium J. Clin. Endocrinol. 1978; PubMed Scopus Google the was to the of PTH secretion. And indeed, we that in vitro PTH secretion from parathyroid cells was under low magnesium to that in the effect of low magnesium was on the axis of the CaSR or inhibition of by pertussis suppressed the effect of magnesium deficiency on PTH secretion, we this of in vitro in a than the parathyroid The of the magnesium signal enhancement of CaSR activity were similar in parathyroid and in The magnesium binding site was on the Gα the of a Gαi with decreased affinity for magnesium the effect of magnesium on CaSR-mediated on CaSR mutants with increased or decreased affinity for magnesium, the of the magnesium signal enhancement not that the enhancement of activation under magnesium deficiency signals mediated by constitutive CaSR The of this enhancement is for the human CaSR than for the CaSR because the human receptor has a constitutive This between the and the human CaSR leads to blunted PTH secretion under severe hypomagnesemia in patients but not in that the magnesium binding site responsible for this effect is on the we the that the human CaSR from the CaSR by an additional magnesium binding site in with the magnesium binding site on the Gα inhibition of nucleotide by nucleotide binding is a of proteins Rev. PubMed Scopus Google Scholar). finding that magnesium the release of on Gα subunits the of magnesium on J. H. A. J. Biol. Chem. 1993; Full Text PDF PubMed Google Scholar). In contrast to has a more than affinity for magnesium than Gα J. H. A. J. Biol. Chem. 1993; Full Text PDF PubMed Google the affinity of magnesium for Gα subunits is in the in the magnesium the activity of heterotrimeric G-proteins. nucleotide of and was affected by magnesium. the mechanism of magnesium the Gα may a novel of magnesium on heterotrimeric G-proteins. magnesium deficiency in signaling mediated by the CaSR although low magnesium nucleotide of different Several of signaling may be responsible for this In proteins are the G-proteins. the enhancement of signaling be This is for parathyroid magnesium effects on signaling are not proteins are The of magnesium effects on signaling has been for secretion than PTH J. Clin. Endocrinol. PubMed Scopus Google Scholar). or effects of magnesium deficiency on in may be to enhanced activation of because receptor are by and F. Am. J. Physiol. 1996; PubMed Google Scholar, S. S. J. PubMed Scopus (30) Google Scholar, N. T. Levi R. PubMed Scopus Google Scholar). magnesium deficiency-dependent signal enhancement of receptor may vitro when are not A. Science. 1983; PubMed Scopus Google Scholar) under of By contrast, CaSR-mediated inhibition of PTH release is of the that of Gαi and no of CaSR-mediated as by the phenotype of CaSR mutations from patients with (7Pearce S.H.S. Williamson C. Kifor O. Bai M. Coulthard M.G. Davies M. McCredie D. Powell H. Lewis-Barned N. Kendall-Taylor P. Brown E.M. Thakker R.V. N. Engl. J. Med. 1996; 335: 1115-1122Crossref PubMed Scopus (510) Google magnesium deficiency in may PTH release to an increase in We of for and for the and of for human