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Dehydration reverses vasopressin antagonist-induced diuresis and aquaporin-2 downregulation in rats
D Marples1, B M Christensen, J Frokiaer
1Physiology Department, University of Leeds, Leeds LS2 9NQ, United Kingdom.
Abstract:
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 V2-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 were 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 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 V2-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 V2 receptors.
Insights
Vasopressin V2-receptor antagonist OPC-31260 modestly reduces aquaporin-2 (AQP2) expression in rats, independent of urine flow. Dehydration reversed this effect, suggesting partial V2 receptor mediation of AQP2 regulation.
Area of Science:
- Nephrology
- Molecular Biology
- Endocrinology
Background:
- Vasopressin regulates water reabsorption in the kidney by controlling aquaporin-2 (AQP2) expression and localization.
- Understanding AQP2 regulation is crucial for managing conditions like diabetes insipidus.
Purpose of the Study:
- To investigate the role of vasopressin and dehydration in regulating AQP2 expression in rat kidneys.
- To determine the effects of a V2-receptor antagonist (OPC-31260) on AQP2 levels and localization.
Main Methods:
- Rats were treated with OPC-31260 alone or with concurrent dehydration.
- AQP2 protein and mRNA levels were measured using Western and Northern blotting.
- AQP2 distribution was analyzed via immunocytochemistry and immunoelectron microscopy.
Main Results:
- OPC-31260 treatment increased urine output and decreased urine osmolality, reducing AQP2 protein and mRNA levels.
- AQP2 was localized to intracellular vesicles, indicating impaired plasma membrane delivery.
- Dehydration during OPC-31260 treatment restored AQP2 levels, suggesting a role for thirsting signals.
Conclusions:
- V2-receptor antagonism causes a modest decrease in AQP2 expression, not solely due to increased urine flow or medullary washout.
- Modulation of AQP2 expression is partly mediated by V2 receptors, but other pathways are also involved.
- These findings offer insights into the complex regulation of renal water transport.