Endothelin

Endothelins are 21-amino acid vasoconstricting peptides produced primarily in the endothelium having a key role in vascular homeostasis. Among the strongest vasoconstrictors known, endothelins are implicated in vascular diseases of several organ systems, including the heart, general circulation and brain.

Isoforms
There are three isoforms with varying regions of expression and two key receptor types, ETA and ETB.
 * ETA receptors are found in the smooth muscle tissue of blood vessels, and binding of endothelin to ETA increases vasoconstriction (contraction of the blood vessel walls) and the retention of sodium. These lead to increased blood pressure.
 * ETB is primarily located on the endothelial cells that line the interior of the blood vessels. When endothelin binds to ETB receptors, this leads to increased natriuresis and diuresis (the production and elimination of urine) and the release of nitric oxide (also called "NO" or endothelium-derived relaxing factor), all mechanisms that lower the blood pressure.
 * Both types of ET receptor are found in the nervous system where they may mediate neurotransmission and vascular functions.

Brain and nerves
Widely distributed in the body, receptors for endothelin are present in blood vessels and cells of the brain, choroid plexus and peripheral nerves. When applied directly to the brain of rats in picomolar quantities as an experimental model of stroke, endothelin-1 caused severe metabolic stimulation and seizures with substantial decreases in blood flow to the same brain regions, both effects mediated by calcium channels. A similar strong vasoconstrictor action of endothelin-1 was demonstrated in a peripheral neuropathy model in rats.

Balance
In a healthy individual, a delicate balance between vasoconstriction and vasodilation is maintained by endothelin, calcitonin and other vasoconstrictors on the one hand and nitric oxide, prostacyclin and other vasodilators on the other.

Overproduction of endothelin in the lungs may cause pulmonary hypertension, which can sometimes be treated by the use of an endothelin receptor antagonist, such as bosentan or sitaxsentan. The latter drug selectively blocks endothelin A receptors, decreasing the vasoconstrictive actions and allowing for increased beneficial effects of endothelin B stimulation, such as nitric oxide production. The precise effects of endothelin B receptor activation depends on the type of cells involved.