Endothelium-derived relaxing factor
Endothelium-derived relaxing factor (EDRF) is produced and released by the endothelium to promote smooth muscle relaxation. The best-characterized is nitric oxide (NO). However, in a paper published in 2008 Science Reports, investigators identified H2S (Hydrogen Disulfide) as another EDRF in addition to classical NO. H2S KO mice showed diminished blood pressure control. 
It is released in response to a variety of chemical and physical stimuli. It causes the smooth muscle in the vessel wall to relax.
Shortly after his death in May of 2009, Furchgott's website at SUNY Downstate Medical Center continued to state that his group was "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.
- H2S as a physiologic vasorelaxant: hypertension in mice with deletion of cystathionine gama lyase. 2008. Science
- "Robert Furchgott". Archived from the original on July 3, 2009. Retrieved 2015-03-16.
- Chowdhary S, Townend JN (April 2001). "Nitric oxide and hypertension: not just an endothelium derived relaxing factor!". J Hum Hypertens 15 (4): 219–27. doi:10.1038/sj.jhh.1001165. PMID 11319669.