sentence: Since its discovery in 1987 as the “ endothelial derived relaxation factor ” ( EDRF ) in peripheral blood vessels ( Palmer et al ., 1987 ) , three distinct isoforms of the NO-producing enzyme nitric oxide synthase ( NOS ; brain / neuronal NOS , inducible NOS , endothelial NOS ) have been described ( Alderton et al ., 2001 ; Garthwaite , 2008 ) .