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1 The Water and Salt Research Center, University of Aarhus, Aarhus, Denmark; Institute of Experimental Clinical Research, University of Aarhus, Aarhus, Denmark
2 The Water and Salt Research Center, University of Aarhus, Aarhus, Denmark; Institute of Anatomy, University of Aarhus, Aarhus, Denmark
3 Laboratory of Kidney and Electrolyte Metabolism, National Institutes of Health, Bethesda, MD, USA
4 The Water and Salt Research Center, University of Aarhus, Aarhus, Denmark; Institute of Experimental Clinical Research, University of Aarhus, Aarhus, Denmark; Department of Clinical Physiology, Aarhus University Hospital-Skejby, Aarhus, Denmark
* To whom correspondence should be addressed. E-mail: jf{at}iekf.au.dk.
The expression of aquaporin-2 (AQP2) is decreased in rats with bilateral ureteral obstruction (BUO) and unilateral ureteral obstruction (UUO). Therefore, the expression of additional renal aqauaporins (AQP1 - 4) and phosphorylated AQP2 (p-AQP2) known to play a role in urinary concentration were examined in a Wistar rat model with 24 hours of UUO. In the obstructed kidneys immunoblotting revealed a significant decrease in the expression of inner medullary AQP2 to 42 ± 4%, p-AQP2 to 23 ± 5%, AQP3 to 19 ± 6%, AQP4 to 11 ± 5 % and AQP1 to 64 ± 8% of sham levels. AQP1 expression located at the proximal tubule decreased to 74 ± 4% of sham levels, p<0.05. Immunocytochemistry confirmed the downregulation of AQP3, AQP4 and p-AQP2. In the contralateral non-obstructed kidney immunoblotting also revealed significant reductions of AQP1 in inner medulla, outer medulla and cortex, whereas expression of AQP2, -3, -4 and p-AQP2 were unchanged. Furthermore, we collected the urine from both obstructed and non-obstructed kidneys for 2 hours, respectively, after 24 h of UUO. Urine collection from the obstructed kidney during 2 hours after release of UUO revealed a significant reduction in urine osmolality and solute-free water reabsorption (TcH2O). Moreover, an increased urine production and TcH2O was observed from the contralateral kidneys. To examine whether vasopressin-independent mechanisms are involved in AQP2 regulation vasopressin-deficient Brattleboro (BB) rats with 24 h of UUO were examined. Immunoblotting revealed downregulation of AQP2, p-AQP2, AQP3, and AQP1 in the obstructed kidney and downregulation p-AQP2 and AQP1 in the non-obstructed kidney. In conclusion: 1) UUO is associated with severe downregulation of AQP2, AQP3, AQP4, and AQP1; thus all these AQPs may play important roles for the impaired urinary concentrating capacity in the obstructed kidney; 2) the reduced levels of AQP1 in the non-obstructed kidney may contribute to the compensatory increase of urine production, and 3) downregulation of AQPs in BB rats support the view that vasopressin-independent pathways may be involved in AQP2 and AQP3 regulation in the obstructed kidneys.
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