Aim: 

The purpose of this study was to investigate whether the polyuria observed in hypercalciuric subjects may involve disregulation of the water channel aquaporin 2 (AQP2) through activation of the extracellular Calcium Sensing Receptor (CaSR) located in the apical membrane of renal collecting duct principal cells.

Methods: 

Urine osmolarity and AQP2 excretion were measured in urine samples obtained from hypercalciuric patients before and after dDAVP (1 deamino-8D arginino-vasopressin) administration. AQP2 excretion in urine samples was measured by Enzyme-Linked immunosorbent assay (ELISA). AQP2 trafficking to the plasma membrane was evaluated by cell surface biotynilation, immunofluorescence and immunoblotting in mouse collecting duct renal cells MCD4. Moreover, cAMP, RhoA and intracellular calcium levels in response to CaSR agonists were measured.

Results: 

In hypercalciuric paients, AQP2 excretion was significantly higher than in normocalciurics. In addition in hypercalciuric patients, AQP2 excretion and urinary osmolarity did not significantly increase after dDAVP administration compared to normocalciurics. Consistent with these results cell surface expression of AQP2 in unstimulated MCD4 cells exposed to CaRS agonists was higher than in control cells and did not increase significantly in response to FK. Moreover CaSR activation reduced the rise in cAMP in response to FK and increased RhoA activity.

Conclusion: 

These data would support the hypothesis that increased urinary calcium levels inhibits AQP2 targeting in response to vasopressin in humans possibly through activation of CaSR signaling.