How does water get into every cell in the body?

The reason water is so important to your health, is because it is everywhere in your body. At one point one of the biggest questions in medicine, was how is water able to pass through cell membranes, yet other even smaller molecules could not.

The existence of channels mediating the flow of water and small solutes through biological tissues such as the wall of the urinary bladder or even across the membrane of individual cells was postulated as early as in the midnineteenth century (Brücke, 1843; Ostwald, 1890; Pfeffer, 1877). In the late 1950s, it was found that water is rapidly transported through the red blood cell membrane via water-selective channels that exclude ions and other solutes (Sidel and Solomon, 1957). Studies of water transport in various organisms and tissues over the next 30 years suggested that water channels have a narrow selectivity filter that prevents proton (H3O+) flow while maintaining a very high permeation rate for H2O (up to 109 molecules per second); but even as late as 1987 nobody had been able to identify a water channel protein (Finkelstein, 1987) and the very concept of water-specific channels was still controversial.

The elusive water channels were finally discovered by Peter Agre. In the mid 1980s, Agre was studying Rh blood group antigens from the red cell membrane. In 1988, he isolated a new 28 kDa membrane protein of unknown function, CHIP28, from both red cells and renal tubules (Denker et al., 1988). After obtaining an N-terminal peptide sequence (Smith and Agre, 1991) and then the whole cDNA sequence (Preston and Agre, 1991) of CHIP28, he realized that this might be the long-sought-after water channel.

Shortly thereafter, Agre proved this conclusively by demonstrating that expression of CHIP28 in Xenopus oocytes made the cells swell rapidly when placed in a hypo-osmotic medium (Preston et al., 1992), Fig. 1. The same phenomenon was observed when purified CHIP28 was reconstituted into liposomes (Zeidel et al., 1992). In both cases, swelling was inhibited by Hg2+, a treatment known to block water transport across the red cell membrane.

The discovery of CHIP28 (now called aquaporin 1 or AQP1) was a decisive moment in the study of cell water channels. Aquaporin-like proteins have since been found throughout the living world; in humans alone, there are at least 11 different aquaporin-like proteins, many of which have been linked to various diseases (Agre et al., 2002; Schrier and Cadnapaphornchai, 2003). Plants have an even higher number of aquaporins, with no less than 35 different versions found in the model plant Arabidopsis thaliana (Javot and Maurel, 2002).

The physiological importance of the aquaporins is perhaps most conspicuous in the kidney, where some 150-200 liters of water need to be resorbed from the primary urine each day. This is made possible mainly by the AQP1 and AQP2 aquaporins. AQP1 is expressed in the proximal tubules and the descending vasa recta, while AQP2 is expressed in the collecting duct. The expression of AQP2 at the plasma membrane is regulated by vasopressin, and decreased or increased AQP2 levels have been associated with nephrogenic diabetes insipidus as well as with several conditions associated with fluid retention such as congestive heart failure (King and Yasui, 2002).

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