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

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.

Osmolality and solute composition are strong regulators of AQP2 expression in renal principal cells

“…Nuclear abundance and overall expression of TonEBP are rapidly decreased by hypotonic challenge (27,38). As reported for AQP2 (24,37), dehydration increases the sodium-myo-inositol cotransporter mRNA expression in rat renal medulla, whereas water loading decreases it. This response is presumably due to alterations in the amount of TonEBP present in the nucleus (4).…”

“…Decreased AQP2 expression without changes in AVP or cAMP levels was observed in fasting and protein-deprived rats and humans (1,35). Water deprivation increased AQP2 expression in rats chronically given V 2 R antagonists (24), indicating that the effect of water deprivation on AQP2 expression is not solely due to AVP-elicited cAMP accumulation. Water loading decreased AQP2 expression, despite chronic administration of the V 2 Rspecific agonist desmopressin (8).…”

“…Altered TonEBP activity, due to altered medullary tonicity, might be responsible for AVP/cAMP-independent changes in AQP2 expression observed in water loading and thirsting (8,24). The increase in AQP2 expression from kidney cortex to inner medulla (29) is also well explained by an increasing activity of TonEBP from kidney cortex to medulla.…”

“…Increased AQP2 expression in rats is induced by water deprivation, despite chronic administration of V 2 R antagonists (24,30), possibly as a consequence of increased medullary osmolality derived from urea and NaCl. We therefore tested whether cAMP-stimulated AQP2 expression is altered by increased medium osmolality derived from elevated NaCl, urea, or sorbitol concentration.…”

“…In rats the downregulation of AQP2 expression by V 2 R antagonist treatment was reversed by subjecting the animals to thirst (24). In addition, senescent (30-moold) rats exhibited a significantly decreased AQP2 expression and papillary osmolality compared with younger (10-mo-old) animals, while papillary cAMP remained unaffected (32).…”