Endothelium-derived relaxing factor

Endothelium-derived relaxing factor (EDRF) is produced and released by the endothelium to promote smooth muscle relaxation. The most well-characterized is nitric oxide (NO). Some sources equate EDRF and nitric oxide.

It is released in response to a variety of chemical and physical stimuli. It causes the smooth muscle in the vessel wall to relax.

EDRF was discovered and characterized by Robert F. Furchgott, a winner of the Nobel Prize in Medicine in 1998 with his co-researchers Louis J. Ignarro and Ferid Murad.

According to Furchgott's website at SUNY Downstate Medical Center, "...we are investigating whether the endothelium-derived relaxing factor (EDRF) is simply nitric oxide or a mixture of substances".

Although there is strong evidence that nitric oxide elicits vasodilation, there is some evidence tying this effect to neuronal rather than endothelial reactions.

Terminology
According to some sources, EDRF refers not only to nitric oxide but also to other substances. Endothelium actually produces a number of endothelial-derived relaxing factors such as:
 * prostacyclin (PGI2)
 * Endothelium-derived hyperpolarizing factor, or EDHF (There is some evidence to suggest that EDHF is potassium.)
 * Heme proteins from blood, which can also stimulate the inducible isoform of Heme oxygenase (HO-1, also known as heat shock protein 32), which produces Carbon monoxide, which can relax smooth muscle.

Older candidate molecules include nitroxyl and hydroxylamine.

The endothelium also produces a number of vasoconstricting agents, and there is a balance of constricting and relaxing signals.