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Abstract: TH-PO340

Insulin-Regulated Aminopeptidase Is Required for Appropriate Dilution of Urine After Acute Hypotonic Stress

Session Information

Category: Fluid‚ Electrolyte‚ and Acid-Base Disorders

  • 1001 Fluid‚ Electrolyte‚ and Acid-Base Disorders: Basic


  • Zuchowski, Yvonne, Vanderbilt University Medical Center, Nashville, Tennessee, United States
  • Terker, Andrew S., Vanderbilt University Medical Center, Nashville, Tennessee, United States
  • Harris, Raymond C., Vanderbilt University Medical Center, Nashville, Tennessee, United States
  • Arroyo Ornelas, Juan Pablo, Vanderbilt University Medical Center, Nashville, Tennessee, United States

Vasopressin is a peptide hormone that regulates water balance and acts on the type 2 vasopressin receptor (V2R) to stimulate water reabsorption in the kidney via the water channel aquaporin-2 (AQP2). The insulin-regulated aminopeptidase (IRAP) is a zinc-dependent aminopeptidase that degrades vasopressin. However, the impact of IRAP KO on water balance is unknown.


Adult (8-12 week) IRAP wildtype (WT) and KO male mice were used for experimentation (4-6/group). Immunofluorescence and immunoblotting were performed on kidneys at baseline to localize IRAP and assess AQP2 protein amount. Urine and plasma electrolytes were measured before and after water load (1ml sterile water/IP) or 24hr water restriction. Urine osmolality was measured in IRAP KO before and after administration of the V2R antagonist, OPC 31260 (10mg/kg/IP).


IRAP was expressed in the glomerulus, the loop of Henle, and the collecting tubules. At baseline, IRAP KO mice had increased urine osmolality (1995 vs 781 p<0.01 mOsm/L) and elevated copeptin (110 vs 53 p<0.05 pg/ml), but total AQP2 protein expression (1.10 vs. 1.00 p=NS) was not different between WT and KO mice. After 24hr water restriction, urine osmolality increased in both WT (781 to 3626 mOsm/L p<0.0001) and KO mice (1995 to 4200 mOsm/L p<0.001), but there were no differences between groups. However, after acute water load, urine osmolality in WT mice decreased (987 vs 380 mOsm/L p<0.05) but not in IRAP KO mice (3280 vs 2801 mOsm/L p=NS). Treatment with OPC31260 decreased urine osmolality in IRAP KO mice (221 vs 1067 p<0.05). Additionally, serum sodium levels after water load were lower in IRAP KO compared to WT (127 vs 133 mEq/L p<0.05).


At baseline, IRAP KO mice have higher urine tonicity and higher copeptin than WT mice. Urine osmolality is similar between groups after 24hr water restriction suggesting normal concentrating ability. However, IRAP KOs are unable to dilute their urine in response to an acute water load. Treatment with a V2R antagonist decreased urine osmolality in IRAP KO, suggesting that persistent V2R activation contributes to the baseline elevated urine osmolality in IRAP KO mice.


  • NIDDK Support