CaSR-mediated interactions between calcium and magnesium homeostasis in mice

Abstract

Calcium (Ca) and magnesium (Mg) homeostasis are interrelated and share common regulatory hormones, including parathyroid hormone (PTH) and vitamin D. However, the role of the calcium-sensing receptor (CaSR) in Mg homeostasis in vivo is not well understood. We sought to investigate the interactions between Mg and Ca homeostasis using genetic mouse models with targeted inactivation of PTH (PTH KO) or both PTH and the calcium-sensing receptor (CaSR) (double knockout, DKO). Serum Mg is lower in PTH KO and DKO mice than in WT mice on standard chow, whereas supplemental dietary Ca leads to equivalent Mg levels for all three genotypes. Mg loading increases serum Mg in all genotypes; however, the increase in serum Mg is most pronounced in the DKO mice. Serum Ca is increased with Mg loading in the PTH KO and DKO mice but not in the WT mice. Here, too, the hypercalcemia is much greater in the DKO mice. Serum and especially urinary phosphate are reduced during Mg loading, which is likely due to intestinal chelation of phosphate by Mg. Mg loading decreases serum PTH in WT mice and increases serum calcitonin in both WT and PTH KO mice but not DKO mice. Furthermore, Mg loading elevates serum 1,25-dihydroxyvitamin D in all genotypes, with greater effects in PTH KO and DKO mice, possibly due to reduced levels of serum phosphorus and FGF23. These hormonal responses to Mg loading and the CaSR's role in regulating renal function may help to explain changes in serum Mg and Ca found during Mg loading.

Fig. 1.

A: effect of 7 days of magnesium (Mg) supplementation on serum Mg in mice receiving plain drinking water. B: effect of 7 days of Mg supplementation on urinary Mg normalized to urinary creatinine (Cr) in mice receiving plain drinking water. C: effect of 7 days of Mg supplementation on serum Mg in wild-type (WT) mice receiving plain water as well as in parathyroid hormone (PTH) knockout (KO) and double knockout (DKO) mice receiving 1.5 and 0.75% calcium (Ca), respectively, in their drinking water. D: effect of 7 days of Mg supplementation on urinary Mg/Cr in WT mice receiving plain water or in PTH KO and DKO mice receiving 1.5 and 0.75% Ca, respectively, in their drinking water. Values are means ± SE (n = 8/genotype). *P < 0.05 for comparison of Mg-loaded values with the respective control values; #P < 0.05 for comparison of WT with PTH KO values; **P < 0.05 for comparison of PTH KO with DKO values. In this and subsequent figures, statistical comparisons of PTH KO and DKO mice with their respective genetic controls, WT and PTH KO mice, were made since comparison of DKO with WT mice is across 2 genetic changes (e.g., loss of both PTH and the CaSR), complicating the interpretation of any observed differences.

Fig. 2.

A: effect of 7 days of Mg supplementation on serum Ca in mice receiving plain drinking water. B: effect of 7 days of Mg supplementation on urinary Ca/Cr in mice receiving plain drinking water. C: effect of 7 days of Mg supplementation on serum Ca in WT mice receiving plain water as well as in PTH KO and DKO mice receiving 1.5 and 0.75% Ca, respectively, in their drinking water. D: effect of 7 days of Mg supplementation on urinary Ca/Cr in WT mice receiving plain water or in PTH KO and DKO mice receiving 1.5 and 0.75% Ca, respectively. Values are means ± SE (n = 8/genotype). *P < 0.05 for comparison of Mg-loaded values with the respective control values; #P < 0.05 for comparison of WT with PTH KO values; **P < 0.05 for comparison of PTH KO to DKO values.

Fig. 3.

A: effect of 7 days of Mg supplementation on serum P_i in mice receiving plain drinking water. B: effect of 7 days of Mg supplementation on urinary P_i/Cr in mice receiving plain drinking water. C: effect of 7 days of Mg supplementation on serum P_i in WT mice receiving plain water as well as in PTH KO and DKO mice receiving 1.5 and 0.75% Ca, respectively, in their drinking water. D: effect of 7 days of Mg supplementation on urinary P_i/Cr in WT mice receiving plain water or in PTH KO and DKO mice supplemented with 1.5 and 0.75% Ca, respectively. Values are means ± SE (n = 8/genotype). *P < 0.05 for comparison of Mg-loaded values with the respective control values; #P < 0.05 for comparison of WT with PTH KO values; **P < 0.05 for comparison of PTH KO with DKO values.

Fig. 4.

A: effects of Mg loading on serum alkaline phosphatase (AP) concentration in WT, PTH KO, and DKO mice receiving plain drinking water. B: effect of 7 days of Mg supplementation on serum AP in WT mice receiving plain water or in PTH KO and DKO mice receiving 1.5 and 0.75% Ca, respectively, in their drinking water. Values are means ± SE (n = 8/genotype). #P < 0.05 for comparison of WT with PTH KO values; **P < 0.05 for comparison of PTH KO with DKO values.

Fig. 5.

A: effects of Mg loading on osteocalcin concentration in WT, PTH KO, and DKO mice receiving plain drinking water. B: effect of 7 days of Mg supplementation on serum osteocalcin in WT mice receiving plain water or in PTH KO and DKO mice receiving 1.5 and 0.75% Ca, respectively, in their drinking water. Values are means ± SE (n = 8/genotype). *P < 0.05 for comparison of Mg-loaded values with the respective control values; #P < 0.05 for comparison of WT with PTH KO values; **P < 0.05 for comparison of PTH KO with DKO values.

Fig. 6.

A: effects of Mg loading on urinary deoxypyridinoline (DPD)/Cr in WT, PTH KO, and DKO mice receiving plain drinking water. B: effect of 7 days of Mg supplementation on urinary DPD/Cr in WT mice receiving plain water or in PTH KO and DKO mice receiving 1.5 and 0.75% Ca, respectively, in their drinking water. Values are means ± SE (n = 8/genotype). *P < 0.05 for comparison of Mg-loaded values with the respective control values; #P < 0.05 for comparison of WT with PTH KO values; **P < 0.05 for comparison of PTH KO with DKO values.

Fig. 7.

A: effects of Mg loading on serum calcitonin concentration in WT, PTH KO, and DKO mice receiving plain drinking water. B: effect of 7 days of Mg supplementation on serum calcitonin in WT mice receiving plain water or in PTH KO and DKO mice receiving 1.5 and 0.75% Ca, respectively, in their drinking water. Values are means ± SE (n = 8/genotype). *P < 0.05 for comparison of Mg-loaded values with the respective control values; #P < 0.05 for comparison of WT with PTH KO values; **P < 0.05 for comparison of PTH KO with DKO values.

Fig. 8.

A: effects of Mg loading on serum 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] concentration in WT, PTH KO, and DKO mice receiving plain drinking water. B: effect of 7 days of Mg supplementation on serum 1,25(OH)₂D₃ in WT mice receiving plain water or in PTH KO and DKO mice receiving 1.5 and 0.75% Ca, respectively, in their drinking water. Values are means ± SE (n = 8/genotype). *P < 0.05 for comparison of Mg-loaded values with the respective control values; #P < 0.05 for comparison of WT with PTH KO values; **P < 0.05 for comparison of PTH KO with DKO values.

Fig. 9.

Changes in serum PTH in WT mice supplemented with 100 mM Mg in the drinking water. Results are means ± SE (n = 8). *P < 0.05 for comparison of the Mg-loaded value with the respective control value.