Dehydration reverses vasopressin antagonist-induced diuresis and aquaporin-2 downregulation in rats

To examine the involvement of vasopressin and dehydration in the regulation of aquaporin-2 (AQP2) expression in rat kidney we investigated the effects of treatment for 60 h with the specific V(2)-receptor antagonist OPC-31260 (OPC), alone and in conjunction with dehydration for the last 12 h. Changes in AQP2 protein and mRNA expression in kidney inner medulla were determined by Western and Northern blotting, and AQP2 distribution was analyzed by immunocytochemistry and immunoelectron microscopy. Treatment with OPC increased urine output fourfold, with a reciprocal decrease in urine osmolality. AQP2 expression decreased to 52 +/- 11% of control levels (n = 12, P < 0.05), and AQP2 was found predominantly in intracellular vesicles in collecting duct principal cells. This is consistent with efficient blockade of the vasopressin-induced AQP2 delivery to the plasma membrane and with the observed increased diuresis. Consistent with this, AQP2 mRNA levels mere also reduced in response to prolonged OPC treatment (30 +/- 10% of control levels, n = 9). Five days of treatment with furosemide, despite producing even greater polyuria than OPC, was not associated with downregulation of AQP2 levels, demonstrating that AQP2 do downregulation is not secondary to increased urine flow rate or loss of medullary hypertonicity. During 12-h thirsting in the continued presence of OPC, urine output dropped dramatically, to levels not significantly different from that seen in (nonthirsted) control animals. In parallel with this, AQP2 levels rose to control levels. Control experiments confirmed continued effective receptor blockade. These results indicate that the V(2)-receptor antagonist causes a modest decrease in AQP2 expression that is not a consequence of increased urine flow rate or washout of medullary hypertonicity. However, this decrease is much less marked than that seen in some forms of acquired nephrogenic diabetes insipidus. In conjunction with the effects of thirsting, this suggests that modulation of AQP2 expression is mediated partly but not exclusively, via V(2) receptors.