Downregulation of Aquaporin-2 Parallels Changes in Renal Water Excretion in Unilateral Ureteral Obstruction

Title: Downregulation of Aquaporin-2 Parallels Changes in Renal Water Excretion in Unilateral Ureteral Obstruction
Authors: Frokiaer, Jorgen; Christensen, Birgitte Monster; Marples, David; Djurhuus, Jens; Jensen, Uffe B.; Knepper, Mark; Nielsen, Soren
Publisher: American Journal of Physiology
Date Published: August 01, 1997
Reference Number: 196
In bilateral ureteral obstruction, both aquaporin-2 (AQP2) levels and urinary concentrating capacity are markedly reduced. However, the mechanisms involved in AQP2 downregulation are unknown. In rats with unilateral ureteral obstruction (UUO) the relative role of intrarenal and systemic factors can be evaluated. Semiquantitative immunoblotting revealed a marked decrease in AQP2 in obstructed kidneys to 23 +/- 7% (n = 9) of sham levels. This downregulation persisted 24 h after release of UUO. Furthermore, there was a significant but less extensive downregulation of AQP2 in the nonobstructed kidneys to 75 +/- 7% (n = 9) of sham levels. Consistent with impairment of collecting duct water reabsorption, free water clearance was greatly elevated in the obstructed kidneys (-2 +/- 1, determined immediately after release) and only moderately elevated in nonobstructed kidneys (-44 +/- 5 compared with sham-operated controls (-59 +/- 3 Also AQP2 mRNA levels were reduced in obstructed kidneys. Immunocytochemistry confirmed the marked decrease in AQP2 expression in obstructed kidneys. In nonobstructed kidneys, AQP2 was predominantly found in intracellular vesicles, which together with the reduced expression and elevated free water clearance strongly suggests a role of AQP2 in the observed compensatory diuresis from nonobstructed kidneys. The much lower AQP2 protein and mRNA levels in obstructed vs. nonobstructed kidneys are consistent with intrarenal factors playing a major role for downregulation of AQP2.

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)

The importance of the water-transporting protein, aquaporin-2 (AQP2), in helping the kidney maintain body water balance is now well established. Located in the principal cells of the kidney collecting duct, AQP2 waits in small sacs called vesicles just beneath the cells' apical membrane. When signaled by the antidiuretic hormone, vasopressin (VP), AQP2 is shuttled to the apical membrane where it is inserted. The inserted AQP2 acts as a water channel through which the kidney may reabsorb water from the collecting duct into its inner tissues.

It is becoming increasingly established that water reabsorption through the collecting duct is regulated by short- and long-term mechanisms involving VP and AQP2. In the short-term, VP signals AQP2 to insert itself into the cell membrane to make it more water permeable. In the long-term, the number of AQP2s generally available is significantly dependent on the level of VP circulating in the blood.

Other variables such as lithium treatment, low blood potassium, and water retention are associated with changes in the number of AQP2s. These variables can affect the long-term regulation of AQP2, causing a decrease in their numbers which can lead to water balance disorders such as acquired nephrogenic diabetes insipidus (NDI). Obstruction of the ureter (the single fibromuscular tube running from each kidney to the bladder which conveys urine from the kidney to the bladder) also causes down regulation of AQP2s leading to NDI.

In this report, Frokiaer, et al., report on their experiment to determine if the blockage of one of the two ureters, or unilateral ureteral obstruction (UUO), results in any changes in the number and distribution of AQP2s in the kidney collecting duct. If so, the authors wanted to know if the changes were correlated with changes in the volume of urine voided.

In previous research, the authors had shown that obstructing both ureters (bilateral ureteral obstruction - BUO) caused a significant reduction in the number and distribution of AQP2s. The reduction, caused by both systemic changes and changes in the kidney, persisted for seven days after the obstruction was released. It was accompanied by the chronic passage of large volumes of dilute urine. The authors wanted to know if the changes in AQP2 number and distribution (if any) induced by UUO would be caused by systemic changes, local changes in the kidney, or a combination of both.

The authors experimented on rats, binding one of each rat's ureters for a 24-hour period. One group didn't have their obstruction released; another group had theirs released after 24 hours; another group had theirs released after 24 hours and had their urine collected for a two-hour period after the release of the obstruction. There was a control group of rats as well. All the groups had their urine collected during the 24-hour obstruction period. And all had their kidney tissues analyzed.

Frokiaer, et al., found that UUO markedly reduced the number of AQP2s found in the collecting duct cells of the obstructed kidney, and the fewer AQP2s were distributed in the apical membrane of the collecting duct cells. The reduction in number remained 24 hours after the obstruction was released. UUO did not change the rats' total urine output or water intake. The authors speculated that the nonobstructed kidney compensated for the obstructed kidney by doubling its urine output.

Examining the unobstructed kidney revealed that there was a less marked but significant reduction in the number of AQP2s. The AQP2s that were expressed were located more within the cell than in the cell membrane, indicating that the AQP2s were not acting to make the cell membranes more water permeable. That these two factors impaired the collecting duct water reabsorption in the unobstructed kidney was evidenced by the increase in the net amount of solute-free water moving from the blood to the urine. (The level of free-water clearance in the obstructed kidney was even greater.) The reduction in AQP2 numbers was accompanied by less efficient water reabsorption and the inability to concentrate urine. So, the urine voided from the nonobstructed kidney was dilute.

The authors determined that factors within the kidney largely accounted for the downregulation of AQP2s in UUO, though systemic factors could have played a supporting role, as indicated by the reduced AQP2 in the nonobstructed kidney and the continuing downregulation of AQP2 24 hours after obstruction release. Local factors could involve changes in prostaglandin E2 production, changes in intracellular calcium and other changes in the phosphoinositide pathway. In addition, the rate at which AQP2s are synthesized and degraded, both local factors, could be affected by UUO.