Vessel size-dependent roles of endothelial nitric oxide synthases system. The contribution of EDRFs to endothelium-dependent vasodilatations markedly varies as a function of vessel size; endothelium-derived NO mainly mediates vasodilatation of relatively large, conduit vessels, while EDH factors that of resistance arteries. BH4, tetrahydrobiopterin; cGMP, cyclic GMP; Cu,Zn-SOD, copper-zinc superoxide dismutase; eNOS, endothelial nitric oxide synthase; H2O2, hydrogen peroxide; iNOS, inducible NOS; I/R, ischemia-reperfusion; KCa, calcium-activated potassium channel; nNOS, neuronal NOS; NO, nitric oxide; NOSs, nitric oxide synthases; VSMC, vascular smooth muscle cells.