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Downregulation of vasopressin receptor and aquaporin 2 in the inner medullary collecting duct of chronic renal failure kidney : significance of angiotensin II and endothelin regulation Lee, Andrew B. H.

Abstract

In chronic renal failure (CRF), inability to concentrate urine is, in part, due to downregulation of aquaporin 2 (AQP2) in the renal collecting duct. In the present study; we investigated the interrelationship of endothelin (ET), vasopressin, and AQP2 in the inner medullary collecting duct (IMCD) of kidneys obtained from CRF (5/6 nephrectomized) and control rats (sham-operated). Two series of experiments were performed. In the first series, clearance studies showed that serum BUN increased in the CRF rats by 53.2% (from 23.27 ± 1.08 to 49.75 ± 3.13 mg%, p < 0.01). GFR decreased by 61.5% (from 0.89 ± 0.04 to 2.31 ± 0.09 ml/min, p < 0.01), and FEH2O increased by 81.0% (from 0.68 ± 0.05 to 3.57 ± 0.27 %, p < 0.01). ET-1 mRNA in the CRF rat IMCD increased by 56.3% (from 0.31 ± 0.04 to 0.71 ±0.10 amol/μg, p < 0.01). ETB mRNA decreased by 55.7% (from 14.00 ± 0.70 to 6.20 ± 0.43 amol/μg, p < 0.01). AQP2 mRNA decreased by 31.9% (from 13.90 ± 0.73 to 9.74 ± 0.83 amol/μg, p < 0.01). Changes of the ETB mRNA correlated with the changes in AQP2 mRNA, whereas changes in ET-1 mRNA inversely correlated with the changes in AQP2 mRNA. In the second series of experiment, an ACE inhibitor, enalapril, was administered to control and CRF rats. Animals were divided to 2 subgroups: group 1 consisted of control and CRF rats without any treatment; group 2 comprised of control and CRF rats treated with enalapril. Renal clearance studies showed no significant renal functional improvement after enalapril treatment. In CRF treated rats, serum BUN increased by 55.6% and GFR decreased by 58.0% compared to the control treated rats. In group one (without treatment), ET-1 mRNA increased by 69.9% (from 0.31 ± 0.03 to 1.03 ± 0. 26 amol/μg, p < 0.01). ETB mRNA decreased by 65.8% (from 15.50 + 0.38 to 5.30 ± 0.36 amol/μg, p < 0.01). V2R mRNA decreased by 50.9% (from 1350.13 ± 222.92 to 662.68 ± 162.47 amol/μg, p < 0.01). AQP2 mRNA decreased by 29.7% (from 13.45 ± 1.34 to 9.46 ± 0.60 amol/μg, p < 0.01) in the CRF rats. In group 2 (enalapril treated), ET-1 mRNA increased by 41.1% (from 0.33 ± 0.07 to 0.56 ± 0.05 amol/μg, p < 0.01). ETB decreased by 43.9% (from 16.18 ± 1.09 to 9.07 ± 0.53 amol/μg, p < 0.01). V2R decreased by 40.4% (from 1558.76 ± 129.64 to 929.40 ± 211.83 amol/μg, p < 0.01). AQP2 decreased by 12.5% (from 20.68 ± 0.72 to 18.10 ± 2.17 amol/μg, p < 0.01) in the CRF rats. In situ hybridization images confirmed these results. Compared to the comparable control rats, immunoblotting showed that V2R protein decreased by 52.4% and 42.9% in the untreated CRF and treated CRF rats, and AQP2 protein level decreased by 29.0% and 11.8% respectively. Although enalapril partially restored V2R and AQP2 mRNA level in the CRF rats, there was no significant change in the FEH2O after treatment. These data indicate that: 1. ET-1 mRNA increased, whereas ETB, V2R, and AQP2 mRNA decreased in the IMCD of CRF rats. 2. ACE inhibitor enalapril partially normalized ET-1, ETB, V2R, and AQP2 mRNA changes in the IMCD of the CRF rats. 3. Enalapril partially restored V2R and AQP2 protein levels in the IMCD of CRF rats. 4. Enalapril did not improve overall renal function in the CRF rats. 5. Enalapril partially corrected AQP2 mRNA levels, but failed to normalize FEH2O in the CRF rats, suggesting that collecting duct AQP2 level is partially responsible for the urine concentrating defect in this renal segment.

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