Bilateral Ureteral Obstruction Downregulates Expression of Vasopressin-Sensitive AQP-2 Water Channel in Rat Kidney
| Title: | Bilateral Ureteral Obstruction Downregulates Expression of Vasopressin-Sensitive AQP-2 Water Channel in Rat Kidney |
|---|---|
| Authors: | Marples, David; Knepper, Mark; Nielsen, Soren; Frokiaer, Jorgen |
| Publisher: | American Journal of Physiology |
| Date Published: | April 01, 1996 |
| Reference Number: | 194 |
Polyuria after release of bilateral ureteral obstruction (BUO) is frequently seen in patients with urological disorders. In this study, we examined the effect of BUO and release of BUO on the expression of the vasopressin-regulated water channel aquaporin-2 (AQP-2) in rat kidney. Ureters were obstructed for 24 h in all experiments, and BUO was either not released or released for 24 or 48 h or 7 days. Each group of experimental rats were matched with sham-operated controls. One kidney was used for membrane fractionation and immunoblotting, whereas the contralateral was fixed for immunocytochemistry. Immunoblotting demonstrated a significant reduction in AQP-2 expression in inner medullar during 24 h of BUO to 26 +/- 8% (P < 0.001). Release of BUO was associated with immediate onset of a predominant osmotic-dependent polyuria. Forty-eight hours after release of BUO, the reduction in AQP-2 expression persisted (19 +/- 8%, P < 0.001), concurrent with a marked nonosmotic postobstructive polyuria, as determined by a significant reduction in free-water clearance (-50 +/- 7 vs. -85 +/- 10 microliters.min-1.kg-1, P < 0.05). Immunofluorescence and immunoelectron microscopy confirmed the reduced levels of AQP-2 in collecting duct principal cells. Seven days after release, the renal excretion of water and electrolytes had almost normalized. However, the downregulation of AQP-2 was not partly reversed (49 +/- 14%, P < 0.001), and, consistent with this, the urinary concentrating capacity was significantly reduced 7 days after release to a 18-h period of thirst. This strongly suggests that the persistent downregulation of AQP-2 is the cause of the slow recovery in concentration capacity. In conclusion, BUO and release of BUO were associated with a marked reduction in expression of AQP-2, coincident with the development and maintenance of postobstructive polyuria. Thus reduced AQP-2 levels may represent an important factor in the slow recovery from postobstructive diuresis.
This translation by the NDI Foundation is to assist the lay reader. To provide a clear, accessible interpretation of the original article, we eliminated or simplified some technical detail and complicated scientific language. We concentrated our translation on those aspects of the article dealing directly with NDI. The NDI Foundation thanks the researchers for their work toward understanding and more effectively treating this disorder.
© Copyright NDI Foundation 2007 (JC)
After the obstruction to the ureters is released through healing, surgery or some other means, it is normal for the patient to temporarily experience post-obstructive diuresis (POD): the chronic passage of large volumes of dilute urine. Dilute urine is urine with a high water to particle (e.g. electrolytes) ratio.
The mechanisms involved in POD after BUO release are not fully understood, though it is clear that the proximal and distal tubule (the beginning and the end, respectively, of the little tube running from the glomerulus to the kidney collecting duct) show reduced ability to reabsorb sodium and water. It is also known that the collecting duct's ability to reabsorb water is impaired in POD, as indicated by the increase in the net amount of solute-free water moving from the blood to the urine.
Research has recently confirmed that water-transporting proteins called aquaporin-2s (AQP2s), located in the kidney collecting ducts, play a large role in allowing water to be reabsorbed through the collecting duct. When AQP2s are defective, as in the case of autosomal recessive nephrogenic diabetes insipidus (NDI), the water cannot be reabsorbed through the collecting duct and the NDI patient urinates excessive amounts of dilute urine.
Frokiaer, et al., emphasized the importance of examining whether reduced expression of AQP2 is a common feature in a variety of disorders associated with NDI, including POD. In their present study, the authors examined what effect BUO, and the subsequent correcting of BUO, had on the amount of AQP2s found in the rats' kidneys. To do this, they selected a number of experimental and control rats. They obstructed the experimental rats' ureters for a 24-hour period. One group of the experimental rats did not get its BUO released. All the rest did after 24 hours. Then they had their urine output and chemistry followed: one group for 24 hours, one group for 48 hours, and one group for seven days. All the rats had their kidney tissue removed and examined for the presence and location of AQP2.
The authors found that the number of AQP2s was reduced in the experimental rats. This showed that a 24-hour period of BUO can significantly reduce AQP2 expression. When the BUO was released (i.e. when the researchers removed the binding that was obstructing the rat ureters) there was an immediate onset of POD, the passage of large amounts of dilute urine. This was associated with the reduced number of AQP2s in the kidneys. The low numbers persisted 24 hours and 48 hours after the BUOs were released. This finding demonstrated a significant water reabsorption defect in the kidney collecting ducts. Since AQP2s play a major role in allowing water reabsorption to occur, the fact that water reabsorption is not as efficient when there are less AQP2s is not surprising.
The symptoms of POD persisted for six days, normalizing only on day seven. At seven days after the release of BUO there still was a decreased number of AQP2s in the kidney tissue, though not as great as at either 24 or 48 hours. This strongly suggests that reduced levels of AQP2 are associated with reduced ability to concentrate urine, and that persistent down-regulation of AQP2 is the cause of the slow recovery of the rats' urine concentrating capacity.
By running a series of sophisticated immunoassays, the authors were able to confirm their findings of reduced AQP2 expression in the kidney tissue, specifically the kidney inner medullary portion, as a response to BUO. The authors noted that though AQP2 levels remained lower than the controls after seven days, the experimental rats still showed a return to normal urine concentrating ability at day seven. The authors explained that the AQP2s that were present in the experimental group at this time were distributed in the apex of the collecting duct cells, the location where AQP2s must be if water is to be reabsorbed through the collecting duct. So even though there were fewer AQP2s, they managed to be in the right spot for urine concentration and water reabsorption to occur.
Another interesting suggestion stimulated by this study and supported by additional studies by the authors is that VP is not the only thing that controls AQP2 expression and distribution. This was exemplified by several things:
- In BUO, AQP2 levels are reduced although VP levels are elevated.
- AQP2s are able to get to the apex of the cell membrane even though they were in reduced numbers.
- In a previous study, the authors found that dehydrating rats by withholding water for 12 hours raised the AQP2 levels.