Enhanced vasoactivity of SNOHb at low oxygen tensions. The degree of Hb deoxygenation that occurs in vessel baths perfused with 95% N₂, 5% CO₂ was determined by measurement of the oxygen tensions (see “Materials and methods”) and of the oxygen-binding curve to SNOHb at 37°C; V indicates fractional saturation (A). Panel B shows the visible spectra of SNOoxyHb (3 μM heme, 0.4SNO per tetramer) obtained from vessel baths perfused with 95% N₂,5%CO₂ (—), SNOoxyHb subsequently exposed to air (---), and after addition of sodium dithionite (....). Panel C shows concentration-dependent relaxation responses to SNOmetHb (1 SNO per Hb tetramer) in the presence of GSH (100 μM) under low oxygen (•) or high oxygen (▪) conditions. Fold changes in EC₅₀ for relaxation were determined as described in “Materials and methods” and shown in Table 1.