OxyHb reverses Angelis Salt–mediated relaxation. Vessels were precontracted and tension measured as described in “Materials and methods.” Relaxation (indicated by a decrease in tension) was stimulated by addition of GSH (100 μM), followed by increasing concentrations of Angelis salt (AS) as indicated. OxyHb (10 μM) was added as indicated and effects on vessel tension monitored. A representative vessel tracing from 3 separate experiments is shown. Experiments were conducted in KH buffer, equilibrated with 95% O₂,5%CO₂,at 37°C.