|The medical significance of the water channels
During the past ten years, water channels have developed into a highly topical research field. The aquaporins have proved to be a large protein family. They exist in bacteria, plants and animals. In the human body alone, at least eleven different variants have been found.
The function of these proteins has now been mapped in bacteria and in plants and animals, with focus on their physiological role. In humans, the water channels play an important role in, among other organs, the kidneys.
The kidney is an ingenious apparatus for removing substances the body wishes to dispose of. In its windings (termed glomeruli), which function as a sieve, water, ions and other small molecules leave the blood as ‘primary’ urine. Over 24 hours, about 170 litres of primary urine is produced. Most of this is reabsorbed with a series of cunning mechanisms so that finally about one litre of urine a day leaves the body.
From the glomeruli, primary urine is passed on through a winding tube where about 70% of the water is reabsorbed to the blood by the aquaporin AQP1. At the end of the tube, another 10% of water is reabsorbed with a similar aquaporin, AQP2. Apart from this, sodium, potassium and chloride ions are also reabsorbed into the blood. Antidiuretic hormone (vasopressin) stimulates the transport of AQP2 to cell membranes in the tube walls and hence increases the water resorption from the urine. People with a deficiency of this hormone might be affected by the disease diabetes insipidus with a daily urine output of 10-15 litres.
Membrane channels are a precondition for all living matter. For this reason, increased understanding of their function constitutes an important basis for understanding many disease states. Dehydration of various types, and sensitivity to heat, are connected with the efficancy of the aquaporins. The European heat waves of recent years, for example, resulted in many deaths where the cause has sometimes been connected to problems in maintaining the body-fluid balance. In these processes the aquaporins are of crucial importance.
Disturbances in ion channel function can lead to serious diseases of the nervous system as well as the muscles, e.g. the heart. This makes the ion channels important drug targets for the pharmaceutical industry.