Thiazide-induced hypocalciuria is accompanied by a decreased expression of Ca2+ transport proteins in kidney

Abstract

Introduction. Thiazide diuretics have the unique characteristic of increasing renal Na+ excretion, while decreasing Ca2+ excretion. However, the molecular mechanism responsible for this thiazide-induced hypocalciuria remains unclear. The present study investigates the effect of thiazides on the expression of the proteins involved in active Ca2+ transport as well as the role of extracellular volume (ECV) status. Methods. Hydrochlorothiazide (HCTZ), 12 mg/24 hours, was administered during 7 days to Wistar rats by osmotic mini-pumps. In addition, ECV contraction was either prevented by Na+ repletion or induced by a low-salt diet. Expression levels of the proteins involved in active Ca2+ transport [i.e., epithelial Ca2+ channel (TRPV5/ECaC1), calbindin-D28K, Na+/Ca2+ exchanger (NCX1)], as well as the thiazide-sensitive Na+ Cl- co-transporter (NCC) were determined by real-time quantitative polymerase chain reaction (PCR) and semiquantitative immunohistochemistry. Results. HCTZ significantly reduced urinary Ca2+ excretion (22% ± 5% relative to controls). Hematocrit was significantly increased, confirming ECV contraction. In addition, Na+ depletion virtually abolished Ca2+ excretion (8% ± 1%), while Na+ repletion during HCTZ treatment prevented both ECV contraction and hypocalciuria. HCTZ significantly decreased mRNA expression of TRPV5 (71% ± 6%), calbindin-D28K (53% ± 6%), NCX1 (51% ± 8%) and NCC (50% ± 11%), regardless of ECV status or calciuresis. Immunohistochemistry revealed reduced TRPV5 (43% ± 2%), calbindin-D28K (59% ± 1%) and NCC (56% ± 4%) abundance. Furthermore, during HCTZ treatment, the subset of tubules coexpressing NCC and calbindin-D28K was significantly reduced (43% ± 5%) and a disturbed cellular localization of NCC was observed. Conclusion. These data suggest that ECV contraction is a critical determinant of the thiazide-induced hypocalciuria, which is accompanied by a decreased expression of Ca2+ transport proteins.