The heme redox state modulates SNOHb-mediated vasorelaxation. SNOHb with defined oxidation states and differing extents of S-nitrosation were added to isolated rat thoracic aorta and effects on tension determined as described in “Materials and methods.” (A) Representative vessel tension traces and demonstration of the ability of SNOoxyHb (1.4SNO per tetramer) to mediate relaxation responses in the presence of glutathione (GSH). Phenylephrine and L-NAME were added to vessel baths as indicated to induce a contraction. Once a steady tension had developed, GSH (100 μM) was added, followed by different Hb derivatives (1 μM in heme final concentration) as indicated. Addition of SNOoxyHb alone, GSH alone, or oxyHb + GSH had no effect on vessel tone, whereas SNOoxyHb + GSH stimulated vasodilatation. (B) Comparison of the dose response for relaxation induced by SNOoxyHb (2SNO per Hb tetramer, ▪) and SNOmetHb (1 SNO per Hb tetramer, •) expressed as the concentration of S-nitrosothiol (SNO) added. The range of heme concentrations over which a relaxation response was observed was approximately 10 to 300 nM for both SNOoxyHb and SNOmetHb. (C) The EC₅₀s (SNO concentration at which SNOoxyHb and SNOmetHb stimulate 50% of the maximal relaxation response). Values represent means ± SEM (n = 8) and include data from SNOHb preparations in which the concentration of SNO varied from 0.5 to 2 SNO per Hb tetramer, *P < .03 versus SNOoxyHb group. Both panels B and C demonstrate that SNOmetHb is a more efficient vasodilator compared with SNOoxyHb. (D) The UV-Vis spectra of SNOoxyHb and SNOmetHb taken from the vessel chambers after relaxation had been stimulated. For SNOoxyHb, CO gas was bubbled through the solution (forming HbCO) and for SNOmetHb, potassium cyanide (50 μM) was added (forming cyanometHb) and spectra measured as shown. Experiments were performed on 8 different vessel preparations in KH buffer equilibrated with 95% O₂,5%CO₂, at 37°C.