Differential role of ·NO in GSNO-, SNOoxyHb-, and SNOmetHb-mediated vessel relaxation. Vessels were precontracted and tension was measured as described in “Materials and methods.” Relaxation was stimulated by addition of GSH (100 μM), followed by addition of either (A) GSNO (50 nM), (B) SNOoxyHb (1 μM heme, 250 nM SNO), or (C) SNOmetHb (1 μM heme, 250 nM SNO). Once relaxation exceeded 50% of maximal tension, oxyHb (10 μM) was added to scavenge ·NO and effects on vessel tension monitored. Panels A-C show representative traces. All experiments were conducted in KH buffer, equilibrated with 95% O₂,5%CO₂,at 37°C.