Low aquaporin-2 excretion in the nephrotic syndrome: an escape from the vasopressin regulating effect.

Purpose: Experimental studies suggest that the nephrotic syndrome is associated with "vasopressin escape", characterized by low aquaporin-2 (AQP2) expression in the collecting duct despite high vasopressin secretion. We investigated this phenomenon in patients with the nephrotic syndrome.

Patients and methods: We recruited 47 patients with proteinuric kidney disease who were distributed into the following four groups: 1) nephrotic syndrome with kidney dysfunction (n=10); 2) nephrotic syndrome with normal kidney function (n=16); 3) partial remission of nephrotic syndrome (n=10); and 4) minimal proteinuria (n=11). Nine healthy volunteers comprised a control group. Serum copeptin level (as a marker of vasopressin secretion) and urinary AQP2 were measured using ELISA.

Results: Nephrotic syndrome was associated with a significant increase in serum copeptin levels compared with those in the other groups (all P <0.05). In patients with nephrotic syndrome and a partial remission of nephrotic syndrome combined, there was more than a ten-fold decrease in the median urinary AQP2 excretion (0.03 ng/mL) compared with healthy volunteers (0.41 ng/mL; P <0.001) and more than a five-fold decrease compared with patients with minimal proteinuria (0.21 ng/mL; P <0.05). Unlike copeptin levels, the median urinary AQP2 excretion in patients with minimal proteinuria also decreased but less significantly than in those with nephrotic syndrome. There was a negative correlation between the urinary AQP2 excretion and daily proteinuria (R=-0.41; P =0.005).

Conclusion: Our clinical study was the first to demonstrate low AQP2 excretion in nephrotic syndrome that may indicate an escape from the vasopressin regulating effect.