Decreased expression of aquaporin 2 in the collecting duct of mice lacking the vasopressin V1a receptor

Vasopressin V1a receptor null (V1aR(-/-)) mice recently showed incomplete urinary concentration due to higher urine volume during control and water diuresis (euhydration), but showed normal response during dehydration (Aoyagi et al., Am J Physiol 295: F100-7, 2008). Water balance, plasma vasopressin, plasma and urine osmolality, and aquaporin 2 (AQP2) expression in the kidney of wild-type (WT) and V1aR(-/-) mice were therefore further examined using improved methods of urine collection (urinary bladder urine). V1aR(-/-) mice demonstrated a lower urine osmolality (3,360 +/- A 138 vs. 3,610 +/- A 47 mOsm/kgH(2)O) and a higher plasma osmolality (354.3 +/- A 1.3 vs. 342.5 +/- A 1.5 mOsm/kgH(2)O) after dehydration for 24 h compared to WT mice (P < 0.05). In contrast, the plasma vasopressin concentration was significantly (P < 0.001) higher in the V1aR(-/-) mice (48.8 +/- A 4.8 vs. 22.1 +/- A 2.4 pg/ml). On the other hand, although the AQP2 protein expression in the kidney was increased after dehydration, the basal (control) and dehydration-induced AQP2 protein levels were significantly lower in V1aR(-/-) mice compared to WT mice (by Western blotting). Staining by an anti-AQP2 antibody in the luminal membrane of the collecting ducts was increased in both V1aR(-/-) and WT mice after dehydration, but was relatively weaker in the V1aR(-/-) mice (by immunohistochemistry). Moreover, urinary excretion of AQP2 protein, an index of the luminal AQP2 expression, was significantly (P < 0.05) lower in the V1aR(-/-) mice. V1aR signaling may be fundamentally important for the expression of AQP2 in the collecting ducts during control conditions and dehydration.