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

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.

Hydrochlorothiazide (HCTZ), 12 mg/24 hours, was administered during 7 days to Wistar rats by osmotic minipumps. 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-D_28K, Na⁺/Ca²⁺ exchanger (NCX1)], as well as the thiazide-sensitive Na⁺ Cl^- cotransporter (NCC) were determined by real-time quantitative polymerase chain reaction (PCR) and semiquantitative immunohistochemistry.

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-D_28K (53%± 6%), NCX1 (51%± 8%) and NCC (50%± 11%), regardless of ECV status or calciuresis. Immunohistochemistry revealed reduced TRPV5 (43%± 2%), calbindin-D_28K (59%± 1%) and NCC (56%± 4%) abundance. Furthermore, during HCTZ treatment, the subset of tubules coexpressing NCC and calbindin-D_28K was significantly reduced (43%± 5%) and a disturbed cellular localization of NCC was observed.

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.