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Hormones of the Kidney, Skin, and Heart


1. Kidney

The human kidney secretes three hormones:

Renin

One of the functions of the kidney is to monitor blood pressure and take corrective action if it should drop. The kidney does this by secreting the proteolytic enzyme renin.

Erythropoietin (EPO)

Erythropoietin is a glycoprotein. It acts on the bone marrow to increase the production of red blood cells. Stimuli such as bleeding or moving to high altitudes (where oxygen is scarcer) trigger the release of EPO.

People with failing kidneys can be kept alive by dialysis. But dialysis only cleanses the blood of wastes. Without a source of EPO, these patients suffer from anemia.

Now, thanks to recombinant DNA technology, recombinant human EPO is available to treat these patients. Some of the drugs used to treat AIDS, zidovudine (AZT) for example, cause anemia as a side effect. Recombinant EPO helps AIDS patients cope with this one of the many problems that the disease creates.

Because EPO increases the hematocrit, it enables more oxygen to flow to the skeletal muscles. Some distance runners (and cyclers) have used recombinant EPO to enhance their performance. Although recombinant EPO has exactly the same sequence of amino acids as the natural hormone, the sugars attached by the cells used in the pharmaceutical industry differ from those attached by the cells of the human kidney. This difference can be detected by a test of the athlete's urine.

Recently it has been found that EPO is also synthesized in the brain when oxygen becomes scarce there (e.g., following a stroke), and helps protect neurons from damage. Perhaps recombinant human EPO will turn out to be useful for stroke victims as well.

Calcitriol

Calcitriol is 1,25[OH]2 Vitamin D3, the active form of vitamin D. It is derived from Calciferol in the blood is converted into the active vitamin in two steps:

Calcitriol Action

Calcitriol acts on the cells of the intestine to promote the absorption of calcium from the diet.

Calcitriol diffuses into cells and, if they contain receptors for it (intestine cells do), it binds to the receptor molecules. The receptor-ligand complex now can bind to its response element:

5' AGGTCAnnnAGGTCA 3'

This sequence of nucleotides (n can be any nucleotide) is found in the promoters of genes that are turned on by calcitriol. Once the hormone-receptor complex is bound to its response element, other transcription factors are recruited to the promoter and transcription of the gene(s) begins.

Deficiency disorders

Insufficient calcitriol prevents normal deposition of calcium in bone.

The most common causes are inadequate amounts of the vitamin in the diet or insufficient exposure to the sun.

However, some rare inherited cases turn out to be caused by inheriting two mutant genes for the kidney enzyme that converts 25[OH] vitamin D3 into calcitriol.


2. Skin

Although called a vitamin, calciferol and its products fully qualify as hormones because they are


3. Heart

Natriuretic Peptides

In response to a rise in blood pressure, the heart releases two peptides: Both hormones lower blood pressure by The latter two effects reduce the reabsorption of water by the kidneys. So the volume of urine increases as does the amount of sodium excreted in it.

These effects give ANP and BNP their name (natrium = sodium; uresis = urinate). The net effect of these actions is to reduce blood pressure by reducing the volume of blood in the circulatory system.

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17 August 2001