Molecular Aspects of Water Transport

Title: Molecular Aspects of Water Transport
Author: Harris, H. William
Publisher: Pediatric Nephrology
Date Published: May 01, 1992
Reference Number: 275
Due to its fundamental importance, the movement of water across cell membranes has been an active area of research for more than 100 years. This subject is central to consideration of normal water metabolism by terrestrial animals, as well as derangements in overall water balance that are frequently encountered by nephrologists in the care of their patients. The objective of this review is to discuss the most basic aspects of cell membrane water permeability and provide a framework for these data in the context of the care of pediatric patients with renal disease. While the water permeability of most cell membranes can be accounted for by the diffusion of water across the lipid bilayer, other cells, including the red blood cell and certain epithelial cells that line the proximal and collecting tubules of the kidney and the urinary bladder of amphibians, possess specialized water channels. Water channels are composed of specialized proteins that create aqueous pores across cell membrane. Currently, there are active research efforts to isolate and characterize water channel proteins from these cell types. Data concerning the distribution, permeability and function of these various water channels will greatly enhance our knowledge of how water is transported across cell membranes.

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)

Vertebrates have developed complex regulatory systems to maintain body water balance in the face of constant internal and external change. Since movement of water across cell membranes is one of the primary ways the body maintains water balance, such movement has been intensely studied for more than 100 years. In his review, Dr. William Harris discusses the most basic aspects of membrane water permeability.

Osmosis is the passage of a solvent (e.g., water) through a semipermeable membrane. The direction of the solvent moves from a less concentrated solution (i.e. a solution with relatively less particles per volume of solvent) to a more concentrated solution (i.e., a solution with more particles per volume of solvent). The semipermeable membrane acts as a sieve to keep some of the larger particles in the less concentrated solution from flowing into the more concentrated solution. The osmotic process stops when both solutions on either side of the membrane have an equal concentration of particles to solvent.

Semipermeable membranes surround most of the cells in the body, allowing water, amino acids, simple sugars and salts to pass through. It is generally believed that the layers of fat which constitute cell membranes partially control the degree of water permeability of that membrane. Some portions of cell membrane water permeability may be due to the flow of water through the watery interiors of ion transport proteins that span the fatty layers of the cell. However, all cell membranes do not have an equal degree of permeability.

Researchers noted that water can flow across some cell membranes in larger volumes than simple water and osmotic permeability could account for. They hypothesized the existence of water channels -- molecular structures that insert themselves into specific cell membranes in order to make them more water permeable than usual. Red blood cells, kidney proximal tubule cells, kidney collecting duct cells and other select epithelial cells (cells that comprise the epithelium -- the covering of internal and external surfaces of the body) all contain water channels.

Research strongly suggests the water channel in the blood is made up of a protein of a molecular mass of 28 kDA. This water channel is believed to be similar, but not identical, in form and function to the water channels found in kidney epithelial cells, which are also comprised of proteins.

Research indicates that the water channels in the kidney epithelial cells are contained in little sacs called vesicles. When the antidiuretic hormone (ADH) contacts the cell it starts a molecular sequence that prompts the water channel containing vesicles (WCV) to travel to and fuse with the apex of the cell membrane (the apical membrane). When the WCVs are fused with the membrane, the water channels insert themselves in the membrane, allowing water to flow through. When ADH removes itself from the cell, the water channel removes itself from the membrane and the WCV recycles back inside the cell.

Current research is focused on further identifying, purifying and cloning the proteins that comprise the water channels activated by ADH so that the structure and function of the water channels will be understood at the molecular level. This knowledge will contribute to the understanding of the most basic aspects of nephrology and physiology.