title: Increased expression of renal TRPM6 compensates for Mg(2+) wasting during furosemide treatment.
abstract: BACKGROUND
  Furosemide is a loop diuretic, which blocks the Na(+), K(+), 2Cl(-) cotransporter (NKCC2) in the thick ascending limb of Henle (TAL). By diminishing sodium (Na(+)) reabsorption, loop diuretics reduce the lumen-positive transepithelial voltage and consequently diminish paracellular transport of magnesium (Mg(2+)) and calcium (Ca(2+)) in TAL. Indeed, furosemide promotes urinary Mg(2+) excretion; however, it is unclear whether this leads, especially during prolonged treatment, to hypomagnesaemia. The aim of the present study was, therefore, to determine the effect of chronic furosemide application on renal Mg(2+) handling in mice.
  
  
  METHODS
  Two groups of 10 mice received an osmotic minipump subcutaneously for 7 days with vehicle or 30 mg/kg/day furosemide. Serum and urine electrolyte concentrations were determined. Next, renal mRNA levels of the epithelial Mg(2+) channel (TRPM6), the Na(+), Cl(-) cotransporter (NCC), the epithelial Ca(2+) channel (TRPV5), the cytosolic Ca(2+)-binding protein calbindin-D28K, as well parvalbumin (PV), claudin-7 (CLDN7) and claudin-8 (CLDN8), the epithelial Na(+) channel (ENaC) and the Na(+)-H(+) exchanger 3 (NHE3) were determined by real-time quantitative polymerase chain reaction. Renal protein levels of NCC, TRPV5, calbindin-D28K and ENaC were also measured using semi-quantitative immunohistochemistry and immunoblotting.
  
  
  RESULTS
  The mice chronically treated with 30 mg/kg/day furosemide displayed a significant polyuria (2.1 ± 0.3 and 1.3 ± 0.2 mL/24 h, furosemide versus control respectively, P < 0.05). Furosemide treatment resulted in increased serum concentrations of Na(+) [158 ± 3 (treated) and 147 ± 1 mmol/L (control), P < 0.01], whereas serum K(+), Ca(2+) and Mg(2+) values were not significantly altered in mice treated with furosemide. Urinary excretion of Na(+), K(+), Ca(2+) and Mg(2+) was not affected by chronic furosemide treatment. The present study shows specific renal upregulation of TRPM6, NCC, TRPV5 and calbindin-D28K.
  
  
  CONCLUSIONS
  During chronic furosemide treatment, enhanced active reabsorption of Mg(2+) via the epithelial channel TRPM6 in DCT compensates for the reduced reabsorption of Mg(2+) in TAL.
authors.affiliation: Department of Physiology , Radboud University Nijmegen Medical Centre , Nijmegen , the Netherlands.
authors.forename: Annelies A
authors.initials: AA
authors.lastname: van Angelen
authors.affiliation: Department of Physiology , Radboud University Nijmegen Medical Centre , Nijmegen , the Netherlands.
authors.forename: AnneMiete W
authors.initials: AW
authors.lastname: van der Kemp
authors.affiliation: Department of Physiology , Radboud University Nijmegen Medical Centre , Nijmegen , the Netherlands.
authors.forename: Joost G
authors.initials: JG
authors.lastname: Hoenderop
authors.affiliation: Department of Physiology , Radboud University Nijmegen Medical Centre , Nijmegen , the Netherlands.
authors.forename: René J
authors.initials: RJ
authors.lastname: Bindels
keywords: DCT; TRPM6; furosemide; mouse; polyuria