Dehydration Reverses Vasopressin Antagonist-Induced Diuresis and Aquaporin-2 Downregulation in Rats
| Title: | Dehydration Reverses Vasopressin Antagonist-Induced Diuresis and Aquaporin-2 Downregulation in Rats |
|---|---|
| Authors: | Nielsen, Soren; Christensen, Birgitte Monster; Frokiaer, Jorgen; Marples, David; Knepper, Mark |
| Publisher: | American Journal of Physiology |
| Date Published: | September 01, 1998 |
| Reference Number: | 192 |
To examine the involvement of vasopressin and dehydration in the regulation of aquaporin-2 (AQP2) expression in rat kidney, we investigated the effects of treatment for 60 h with the V2-receptorantagonist OPC-31260 (OPC), alone and in conjunction with dehydration for the last 12 h. Changes in AQP2 protein and mRNA expression in kidney inner medulla were determined by Western and Northern blotting, and AQP2 distribution was analyzed by immunocytochemistry and immunoelectron microscopy. Treatment with OPC increased urine output fourfold, with a reciprocal decrease in urine osmolality. AQP2 expression decreased to 52 +/- 11% of control levels (n = 12, P < 0.05), and AQP2 was found predominantly in intracellular vesicles in collecting duct principal cells. This is consistent with efficient blockade of the vasopressin-induced AQP2 delivery to the plasma membrane and with the observed increased diuresis. Consistent with this, AQP2 mRNA levels were also reduced in response to prolonged OPC treatment (30 +/- 10% of control levels, n = 9). Five days of treatment with furosemide, despite producing even greater polyuria than OPC, was not associated with downregulation of AQP2 levels, demonstrating that AQP2 downregulation is not secondary to increased urine flow rate or loss of medullary hypertonicity. During 12-h thirsting in the continued presence of OPC, urine output dropped dramatically, to levels not significantly different from that seen in (nonthirsted) control animals. In parallel with this, AQP2 levels rose to control levels. Control experiments confirmed continued effective receptor blockade. These results indicate that the V2-receptor antagonist causes a modest decrease in AQP2 expression that is not a consequence of increased urine flow rate or washout of medullary hypertonicity. However, this decrease is much less marked than that seen in some forms of acquired nephrogenic diabetes insipidus. In conjunction with the effects of thirsting, this suggests that modulation of AQP2 expression is mediated partly, but not exclusively, via V2 receptors.
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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.
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It is known that VP plays a role in regulating the expression of AQP2s. What is not known is the extent of that regulation. Is VP the sole regulator of AQP2 expression? And does this apply to both short-term expression and long-term expression? (Short-term expression corresponds to the immediate increase and distribution of AQP2 during the water reabsorption and urine concentrating process. Long-term regulation determines the general level of AQP2s in the body.) Or are there other biologic mechanisms that co-regulate AQP2 expression?
To answer these questions, Marples, et al., designed the following experiment. They:
- selected an experimental group and a control group of rats,
- treated the experimental group with OPC-31260 (OPC) for 60 hours. (OPC is an agent that blocks the action of VP by binding with V2Rs. If the V2Rs are occupied by OPCs, VP cannot bind with them to signal AQP2s. A test was built into the experimental design to ensure that OPC's ability to reduce AQP2's expression was a direct result of OPC's binding with V2R.),
- dehydrated both groups of rats for a 24-hour period and for the last 12 hours of the 60-hour OPC treatment,
- measured the rats urine output and osmolality during the course of the OPC treatment period (Osmolality is the ratio of osmotically active particles in the urine to the solvent part of the urine. The ratio can indicate if the urine is concentrated or dilute.), and
- analyzed the rats' kidney tissue after the treatment period to determine what amount of AQP2 was in the collecting duct cells and in what parts of the cells it was distributed.
The authors found that during the OPC treatment, the experimental rats' urine output increased fourfold, and the urine produced was dilute. An analysis of the experimental rats' tissues showed that the AQP2s were found mainly inside the kidney collecting duct cells. This indicated that they could not get to the cell membranes to make them more water permeable because VP could not bind with V2Rs to signal AQP2s. This showed that VP does regulate AQP2s.
When mammals are in a dehydrated state, the amount of VP circulating through the system increases. During 12 hours of the 60-hour OPC treatment period, the rats were made dehydrated by restricting them from any liquid. During this period their urine production dropped significantly and their AQP2 levels rose. This indicated that something else in addition to VP must co-regulate the expression and distribution of AQP2s, because even though VP was being blocked from binding with V2Rs, AQP2s were still able to get to the cell membranes in enough numbers to reduce the urine output.
The above findings relate to the short-term regulation of the expression of AQP2 when it is expressed in response to immediate needs for the kidney to reabsorb water and concentrate urine. The authors note the long-term regulation modulation of collecting duct cell permeability, and hence, long-term regulation of AQP2 expression, also involves VP.
But a number of studies, including the authors', demonstrate that changes in long-term AQP2 expression can occur independently of changes in VP activity. For example, one of the authors' experiments involved lithium-treated rats (in specific doses, lithium markedly reduces the number of AQP2s and causes excretion of large volumes of urine). The rats were treated to maintain high circulating levels of dDAVP (a synthetic analog of VP which acts as VP does). The dDAVP was able to reduce the excessive urination somewhat, but without increasing AQP2 levels significantly. However, the AQP2s that were expressed were able to reach the cell membrane. When the rats were made dehydrated, they produced far more AQP2. However, the AQP2 produced could not reach the cell membranes and the rats did not reduce their excessive urination significantly. This suggests that a signal other than VP must be involved in the regulation of AQP2.
The results of their current experiment suggests to the authors that dehydration provides a powerful stimulus for increased AQP2 expression, both short- and long-term, that is likely to be completely or partly independent of VP.