Water Channels in Cell Membranes
| Title: | Water Channels in Cell Membranes |
|---|---|
| Author: | Verkman, Alan S. |
| Publisher: | Annual Review of Physiology |
| Date Published: | January 01, 1992 |
| Reference Number: | 210 |
N/A
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)
Researchers have discovered several characteristics associated with water channels:
- The osmotic water permeability coefficient (Pf) is high. That is, the amount of water that transports across the cell membrane due to osmotic forces is relatively high.
- The ratio of osmotic to diffusional water permeability (Pf/Pd) is greater than unity. This means more water flows through the cell membrane by means of the water channels than by diffusing across the cell membrane.
- The energy required to activate water transport across the cell membrane is low.
- Water channel function is inhibited in the presence of mercury compounds.
One class of water channels, such as those found in mammalian kidney collecting ducts and toad urinary bladder, is responsive to the antidiuretic hormone, vasopressin (VP). Researchers propose that the following cycle occurs in the presence of VP. When VP is present, the water channels, which sit in little sacs called vesicles, are transported in those vesicles from their holding area within the cell to the apex of the cell membrane (the apical membrane). Once there, the vesicles fuse with the membrane and the water channels are inserted in the membrane. When VP absents itself from the cell, the vesicles retrieve the water channel from the membrane and shuttle back to their holding area within the cell. The cell membrane then returns to its normal degree of permeability. This model of the cycle of water channel insertion (exocytosis) and retrieval (endocytosis) is known as the Shuttle Hypothesis.
Verkman, et al., provided the first direct evidence to confirm that a VP-induced shuttle process of water channel-bearing vesicles does occur in the kidney collecting duct. Their work, along with the research of others, also showed that these water channels are highly selective for water. That is, they primarily allow only water to channel through them, excluding larger protons and other solutes found in body water.
Researchers concluded that the shuttling of water channel vesicles was an efficient and selective process. When they looked into what happened to the water channels inside the vesicles once they were retrieved from the membrane, they found that, unlike other retrieval processes, the water channels did not acidify and get digested by enzymes. Instead, they remained intact and were recycled to the cell membrane when signaled to go there by VP.
Though researchers have a body of evidence strongly pointing to the existence of water channels, they as yet do not have a precise picture of what they look like. However, several distinct proteins have been identified by their molecular weight as constituents of water channels. The structural and functional characterization of these proteins will be more completely understood after they have been cloned and expressed in laboratory cell cultures. Fortunately, these proteins have been found to be suitable for expression cloning.