No abstract

The kidneys play a crucial role in maintaining calcium (Ca2+) and magnesium (Mg2+) homeostasis by regulating these minerals' reabsorption. In the thick ascending limb of Henle's loop (TAL), Ca2+ and Mg2+ are reabsorbed through the tight junctions by a shared paracellular pathway formed by claudin-16 and claudin-19. Hypercalcemia activates the Ca2+-sensing receptor (CaSR) in the TAL, causing upregulation of the pore-blocking claudin-14 (CLDN14) that reduces Ca2+ and Mg2+ reabsorption from this segment. Additionally, a high Mg2+ diet is known to increase both urinary Mg2+ and Ca2+ excretion. Since Mg2+ may also activate the CaSR, we aimed to investigate whether CaSR-dependent increases in CLDN14 expression also regulate urinary Mg2+ excretion in response to hypermagnesemia. Here we show that a Mg2+-enriched diet increased urinary Mg2+ and Ca2+ excretion in mice, however this occurred without detectable changes in renal CLDN14 expression. The administration of a high Mg2+ diet to Cldn14-/- mice did not cause more pronounced hypermagnesemia nor significantly alter urinary Mg2+ excretion. Finally, in vitro evaluation of CaSR-driven Cldn14 promoter activity in response to increasing Mg2+ concentrations revealed that Cldn14 expression only increases at supraphysiological extracellular Mg2+ levels. Together, these results suggest that CLDN14 is not involved in regulating extracellular Mg2+ balance following high dietary Mg2+ intake.

Background Treatment with the aminoglycoside antibiotic gentamicin can be associated with severe adverse effects, including renal calcium wasting. The underlying mechanism is unknown but it has been proposed to involve activation of the Ca2+-sensing receptor (CaSR) in the thick ascending limb, which would increase expression of claudin-14 (CLDN14) and limit Ca2+ reabsorption. However, no direct evidence for this hypothesis has been presented. Methods We studied the effect of gentamicin in vivo using mouse models with impaired Ca2+ reabsorption in the proximal tubule and the thick ascending limb. We used a Cldn14 promoter luciferase-reporter assay to study CaSR activation and investigated the effect of gentamicin on activity of the distal nephron Ca2+ channel transient potential receptor vanilloid 5 (TPRV5), as determined by patch-clamp in HEK293 cells. Results Gentamicin increased urinary Ca2+ excretion in wild-type mice following acute and chronic administration. This calciuretic effect was unaltered in mice with genetic CaSR overactivation and was present in furosemide-treated animals, whereas the calciuretic effect in Cldn14-/-mice and mice with impaired proximal tubular Ca2+ reabsorption (claudin-2 [CLDN2]-deficient Cldn2-/- mice) was equivalent to that of wild-type mice. In vitro, gentamicin failed to activate the CaSR. In contrast, patch-clamp analysis revealed that gentamicin strongly inhibited rabbit and human TRPV5 activity and that chronic gentamicin administration downregulated distal nephron Ca2+ transporters. Conclusions Gentamicin does not cause hypercalciuria via activation of the CaSR-CLDN14 pathway or by interfering with proximal tubular CLDN2-dependent Ca2+ reabsorption. Instead, gentamicin blocks distal Ca2+ reabsorption by direct inhibition of the Ca2+ channel TRPV5. These findings offer new insights into calcium wasting in patients treated with gentamicin.

Active reabsorption of magnesium (Mg2+) in the distal convoluted tubule (DCT) of the kidney is crucial for maintaining Mg2+ homeostasis. Impaired activity of the Na+‐Cl−‐cotransporter (NCC) has been associated with hypermagnesiuria and hypomagnesemia, while increased activity of NCC, as observed in patients with Gordon syndrome, is not associated with alterations in Mg2+ balance. To further elucidate the possible interrelationship between NCC activity and renal Mg2+ handling, plasma Mg2+ levels and urinary excretion of sodium (Na+) and Mg2+ were measured in a mouse model of Gordon syndrome. In this model, DCT1‐specific expression of a constitutively active mutant form of the NCC‐phosphorylating kinase, SPAK (CA‐SPAK), increases NCC activity and hydrochlorothiazide (HCTZ)‐sensitive Na+ reabsorption. These mice were normomagnesemic and HCTZ administration comparably reduced plasma Mg2+ levels in CA‐SPAK mice and control littermates. As inferred by the initial response to HCTZ, CA‐SPAK mice exhibited greater NCC‐dependent Na+ reabsorption together with decreased Mg2+ reabsorption, compared to controls. Following prolonged HCTZ administration (4 days), CA‐SPAK mice exhibited higher urinary Mg2+ excretion, while urinary Na+ excretion decreased to levels observed in control animals. Surprisingly, CA‐SPAK mice had unaltered renal expression of Trpm6, encoding the Mg2+‐permeable channel TRPM6, or other magnesiotropic genes. In conclusion, CA‐SPAK mice exhibit normomagnesemia, despite increased NCC activity and Na+ reabsorption. Thus, Mg2+ reabsorption is not coupled to increased thiazide‐sensitive Na+ reabsorption, suggesting a similar process explains normomagnesemia in Gordon syndrome. Further research is required to unravel the molecular underpinnings of this phenomenon and the more pronounced Mg2+ excretion after prolonged HCTZ administration.