Increased expression of renal TRPM6 compensates for Mg2 wasting during furosemide treatment

Abstract

BackgroundFurosemide 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 (Mg2) and calcium (Ca2) in TAL. Indeed, furosemide promotes urinary Mg2 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 Mg2 handling in mice.MethodsTwo 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 Mg2 channel (TRPM6), the Na+, Cl- cotransporter (NCC), the epithelial Ca2 channel (TRPV5), the cytosolic Ca2-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.ResultsThe 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+, Ca2 and Mg2 values were not significantly altered in mice treated with furosemide. Urinary excretion of Na+, K+, Ca2 and Mg2 was not affected by chronic furosemide treatment. The present study shows specific renal upregulation of TRPM6, NCC, TRPV5 and calbindin-D28K.ConclusionsDuring chronic furosemide treatment, enhanced active reabsorption of Mg2 via the epithelial channel TRPM6 in DCT compensates for the reduced reabsorption of Mg2 in TAL. © 2012 The Author.