Mayeen Mohammed, of Santiago High School, Corona, California, asks :-

What is congenital diabetes insipidus?

Kirk Hamilton, a kidney physiologist at Otago University, responded.

First, diabetes insipidus is a problem with the kidneys (the organs that help filter the blood) and it is not the sugar disorder known as diabetes mellitus. In fact, an individual with diabetes insipidus passes a lot of urine throughout the day and night. Therefore, it is no surprise that this individual has an excessive thirst. Diabetes insipidus can be caused by a number of problems either with the brain or within the kidney, itself.

There are different kinds of diabetes insipidus. Congential diabetes insipidus is the type in which a special protein (a receptor called the vasopressin receptor type 2) is not properly formed or transported to a specific membrane of distal tubule cells of the nephron (the smallest function unit of the kidney). Therefore, when vasopressin (a hormone produced in the brain) is carried by the blood to the distal tubule cells, it is unable to bind with the receptor, or it binds but the receptor does not work properly, or the receptor has never reached the membrane of these cells. All three problems result from various mutations of the vasopressin receptor protein.

Normally for you or I, vasopressin will bind to the vasopressin receptor on the kidney tubule cell. This leads to a series of cellular events that results in water channels (aquaporin-2) being transported out to the membrane of the cell that faces (lines) the tubule. Then, the water channels will reabsorb water from the urine, and that water is ultimately transported back to the blood. Over an entire day, we only urinate about one litre of fluid from our body.

So, someone with congential diabetes insipidus is unable to reabsorb the normal amount of water from the urine and thus, excretes a very large volume of urine each day, as high as 20 litres a day. Nonetheless, these individuals can live a long and healthy life, but are always mindful of the need of water!

For further questions check out the website for the Diabetes Insipidus Foundation (http://www.diabetesinsipidus.org).