Fig. 1. Hypoxia (100% N₂) and H₂S (1 mmol l^–1; as total sulfide, H₂S and HS^–) produce identical contractions in lamprey dorsal aorta (DA; A), relaxations in norepinephrine (NE, 10^–6 mol l^–1) pre-contracted rat thoracic aorta (TA; B), and tri-phasic contraction–relaxation–contraction (1, 2, 3) in norepinephrine pre-contracted rat pulmonary artery (PA; C). Air indicates return to normoxia; w, wash. In U-46619 (thromboxane A₂ mimetic, 10^–7 mol l^–1) pre-contracted bovine pulmonary artery (PA; D) H₂S produces relaxation at low concentrations and contractions at higher concentrations. The left panel shows an individual trace, and in the right panel the two phases are separated into individual dose–response curves (N=8); stippled rectangle indicates range of reported [H₂S] (measured in mol l^–1) in rat plasma. Horizontal time bar in all figures is 10 min, vertical tension scale is 0.5 g. Adapted from Olson et al. (Olson et al., 2006) with permission.