Fate of Antidiuretic Hormone Water Channel Proteins after Retrieval from Apical Membrane
|Title:||Fate of Antidiuretic Hormone Water Channel Proteins after Retrieval from Apical Membrane|
|Authors:||Zeidel, Mark L.; Hammond, Timothy G.; Wade, James B.; Tucker, Julia; Harris, H. William|
|Publisher:||American Journal of Physiology|
|Date Published:||September 01, 1993|
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)
When the ADH removes itself from the cells, the WCVs take themselves out of the apical membranes. They move into pits on the cell surface that are coated with a protein called clathrin. These clathrin-coated pits, with the WCVs inside them, descend back inside the cell. When the pits, still containing the WCVs, move inside the cells, they lose their clathrin coating. At this point they are called endosomes.
Zeidel, et al., wanted to know the fate of the membrane proteins, including the WCs, as they sat inside their WCVs inside the endosomes. Using toad urinary bladder cells, the authors were able to track and harvest the WCVs and examine the membrane proteins contained within them. They harvested the endosomes from the bladder cells at different times after ADH was withdrawn from the cell: 10 minutes, 30 minutes and 60 minutes.
The authors found that the protein components within the WCVs were similar in the endosomes harvested at all three time periods. The authors were also able to determine that the fluid phase markers they used to track the membrane protein fragments always stayed with the membrane proteins during the entire time span of their testing period.
Next, the authors tested the membrane proteins contained within the WCVs to see if they could still increase apical cell water permeability. If they could, it would indicate that the WCs remain functional after they reenter the cell and perhaps they can be used by the cell again when ADH once again binds with a receptor on the cell's surface. In other words, it could indicate that the WCs stay intact and functional and therefore could be used by the cell repeatedly to increase its water permeability.
Zeidel, et al., found that the WCVs in the endosomes harvested at 10 and 30 minutes still contained functional WCs. This was determined because they still increased the cell water permeability, they required little energy to do so, and they remained sensitive to mercury compounds. The endosomes harvested at 60 minutes did not contain functional water channels.
There are two possible ways to explain why the WCs are inactive 60 minutes after they recycle back inside the cell. It may be that the endosomes somehow can separate the WC from the fluid phase marker that researchers used to track the WC, but the authors test results determined that this was not the case. The other explanation is that between 30 and 60 minutes after the WCs recycle from the apical membrane back into the cell they are inactivated. The authors feel this is a much more likely scenario.