Angiotensin II AT1 Receptor Blockade Decreases Vasopressin-Induced Water Reabsorption and AQP2 Levels in NaCl-Restricted Rats

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Title: Angiotensin II AT1 Receptor Blockade Decreases Vasopressin-Induced Water Reabsorption and AQP2 Levels in NaCl-Restricted Rats
Authors: Kwon, Tae-Hwan; Nielsen, Jakob; Knepper, Mark; Frokiaer, Jorgen; Nielsen, Soren
Publisher: American Journal of Physiology: Renal Physiology
Date Published: April 01, 2005
Reference Number: 685
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Vasopressin and ANG II, which are known to play a major role in renal water and sodium reabsorption, are mainly coupled to the cAMP/PKA and phosphoinositide pathways, respectively. There is evidence for cross talk between these intracellular signaling pathways. We therefore hypothesized that vasopressin-induced water reabsorption could be attenuated by ANG II AT1 receptor blockade in rats. To address this, three protocols were used: 1) DDAVP treatment (20 ng/h sc for 7 days, n = 8); 2) DDAVP (20 ng/h sc for 7 days) and candesartan (1 mg.kg-1.day-1 sc for 7 days) cotreatment (n = 8); and 3) vehicle infusion as the control (n = 8). All rats were maintained on a NaCl-deficient diet (0.1 meq Na+.200 g body wt-1.day-1) during the experiment. DDAVP treatment alone resulted in a significant decrease in urine output (3.1 ± 0.2 ml/day) compared with controls (11.5 ± 2.2 ml/day, P < 0.05), whereas the urine output was significantly increased in response to DDAVP and candesartan cotreatment (9.8 ± 1.0 ml/day, P < 0.05). Consistent with this, rats cotreated with DDAVP and candesartan demonstrated decreased urine osmolality (1,319 ± 172 mosmol/kgH2O) compared with rats treated with DDAVP alone (3,476 ± 182 mosmol/kgH2O, P < 0.05). Semiquantitative immunoblotting revealed significantly decreased expression of medullary aquaporin-2 (AQP2) and AQP2 phosphorylated in the PKA phosphorylation consensus site Ser-256 (p-AQP2) in response to DDAVP and candesartan cotreatment compared with DDAVP treatment alone. In addition, cortical and medullary AQP1 was also downregulated. Fractional sodium excretion (FENa) and plasma potassium levels were markedly increased, and the expressions of the cortical type 3 Na+/H+ exchanger (NHE3), thiazide-sensitive Na-Cl cotransporter (NCC), and Na-K-ATPase were significantly decreased in response to DDAVP and candesartan cotreatment. Moreover, medullary type 1 bumetanide-sensitive Na-K-2Cl cotransporter expression showed a marked gel mobility shift from 160 to approximately 180 kDa corresponding to enhanced glycosylation, whereas expression was unchanged. In conclusion, ANG II AT1 receptor blockade in DDAVP-treated rats was associated with decreased urine concentration and decreased AQP2 and AQP1 expression. Moreover, FENa was increased in parallel with decreased expression of NHE3, NCC, and Na-K-ATPase. These results suggest that ANG II AT1 receptor activation plays a significant role in regulating aquaporin and sodium transporter expression and modulating urine concentration in vivo.

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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 hormones angiotensin II (ANG II) and vasopressin (AVP) both play important roles in the kidney’s ability to concentrate urine, balance body water, and conserve water and sodium (NA). AVP, when it links with its vasopressin-2 receptor (V2R) initiates this molecular sequence: adenylyl cyclase is activated, which elevates cyclic adenosine monophosphate (cAMP) levels. cAMP activates protein kinase A (PKA), which links a phosphate group to a specific point of the aquaporin-2 (AQP2) water channel protein, as well as to points on several sodium transporters. AQP2 then moves from the kidney collecting duct principal cell interior to its membrane where it allows water to enter the cell.

In addition, when AVP links with V2R, intracellular calcium is mobilized in a part of the kidney called the inner medullary collecting duct (IMCD). AVP also increases the intracellular level of cAMP in the thin ascending loop (TAL), a section of the kidney called the loop of Henle, and increases the rate of active sodium chloride (NaCl) absorption in the TAL. All these effects help the kidney to reabsorb body water.

ANG II, when it binds with its AT1 receptor, regulates blood flow in the kidneys, the kidney’s glomular filtration rate (GFR), and its secretion of the hormone, aldosterone, which promotes the retention of sodium and water. ANG II also promotes a host of other effects throughout the kidney such as regulating molecular sequences for intracellular calcium, PKC activity, and transport of urea and sodium. All this indicates that ANG II plays an important role in the kidney’s ability to concentrate urine.

Interestingly, a comparison of the actions of AVP and ANG II suggest that there is an intermingling among some of the molecular sequences initiated by these hormones. This cross talk led Tae-Hwan Kwon, et al., to hypothesize that AVP and ANG II may have a synergetic effect on the kidney’s ability to concentrate urine.

To discover if this is so, the researchers devised the following experiment. They took three groups of eight rats and fed each a low salt diet in order to elevate their blood level of ANG II. They left one group as a control, treated another group with DDAVP, a synthetic analogue of AVP. (The combination of DDAVP and a low sodium diet ensures there will be high levels of ANG II in the rats’ blood.) The researchers treated the last group with DDAVP and candesartan, a pharmaceutical that prevents ANG II from binding with its receptor, AT1. If ANG II can’t bind with AT1, then it cannot initiate the molecular sequence in the kidney that it normally does.

The researchers reasoned that if ANG II couldn’t bind with AT1, it would result in an increased urine output and more dilute urine among the rats treated with both DDAVP and candesartan. The team also looked to see if this group of rats had fewer AQP2s and kidney sodium transporters than either the control group or the DDAVP treated group.

The research team found that the DDAVP/candesartan group significantly increased their urine output; their urine was more dilute; there was significantly decreased levels of AQP2 in the IMCD; and significantly decreased levels of AQP2s that were phosphorylated (a condition necessary for the AQP2 to get to the cell membrane). Also, there was a decrease in sodium transporters in the kidney.

These experimental results suggest that ANG II binding with AT1 plays a significant role in regulating the numbers of AQP2s and sodium transporters in the kidney. Thus, it plays an important role in the kidney’s ability to concentrate urine.