Tuesday, August 23, 2016

Piling on The Pressure

ENaC's (epithelial sodium channels) tasks do not end at birth.  It plays a vital role in regulating the amount of sodium in your blood and this, in turn, determines your blood pressure. If ENaC channels malfunction, your blood pressure can skyrocket, putting you at risk of a stroke.

Your kidneys are sophisticated organs that clean the blood, continuously filtering out toxins and waste products and flushing away excess water. Waste processing takes place in about a million individual units known as nephrons, where tufts of fine blood vessels, know as capillaries, are entwined with tiny tubules that act as urine-collecting devices.
Amazingly, the whole of your blood passes through the kidney twice every hour. The red blood cells and plasma proteins are retained in the capillary,  but the salts and water are forced out into the kidney tubule. Almost all of the sodium and much of the water that is filtered are subsequently reabsorbed as the fluid passes down the kidney tubules. What remains is stored in the bladder and excreted as urine.

ENaC channels in the membranes of the kidney tubule cells are responsible for reabsorption of sodium. As in the lung , sodium uptake is accompanied by water, which leads to an increase in blood volume and, because the circulation is a closed system, raises the blood pressure. A diet high in salt (sodium chloride) is bad for you because more sodium is taken up, which drags more water with it, increasing your blood volume and therefore your blood pressure.
Conversely, if blood sodium levels are low, insufficient water is retained by the body, leading to a fall in blood pressure. This is why it is important to ensure that you eat enough unrefined sea salt in a hot climate , where a lot of salt is lost through sweating.

Mutations in any of the three genes that make up the ENaC channel affect blood pressure.  Those that lead to increased ENaC activity cause a hereditary form of hypertension known as Liddle's disease, whereas those that reduce ENaC activity result in low blood pressure. The latter are particularly dangerous as they lead to a life-threatening salt-losing syndrome in newborns and infants.  Because sodium uptake is reduced, less water is reabsorbed, so that the child quickly becomes dehydrated and the blood concentration of other ions ( especially potassium ) becomes unbalanced.  The disease is fatal unless it is quickly recognised and treated.

Fortunately, mutations in ENaC are rare. However, it is thought that one reason for the greater incidence of high blood pressure and its attendant complications in black people than in Caucasians is because they have relatively common variants in their ENaC channels genes that predispose them to increased sodium uptake.
Why this is the case is uncertain, but one suggestion is that people living near the Sahara evolved very efficient mechanisms for absorbing salt as it was in such short supply. While this is advantage when salt is only rarely obtainable, it becomes a handicap in our present world where much processed food is very high in salt.

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