Fig. 3. The effects of hypoxia (N₂) and H₂S are mutually competitive. (A) Individually, N₂ and H₂S produce similar contraction of lamprey dorsal aorta (DA; normalized to 80 mmol l^–1 KCl contraction=100%) while in the presence of N₂, H₂S (3x 10^–4 mol l^–1) relaxes and in the presence of H₂S, N₂ contractions are significantly (P0.05; N=8 vessels) reduced. (B) In norepinephrine (NE; 10^–6 mol l^–1) pre-contracted rat thoracic aortas initial exposure to hypoxia (N₂; top trace) or H₂S (3x10^–4 mol l^–1; bottom trace) produces a typical relaxation, whereas subsequent application of either H₂S (top) or hypoxia (bottom) results in either a slight contraction or no response. (C) In U-46619 (10^–6 mol l^–1)-contracted bovine pulmonary arteries, 3x10^–4 mol l^–1 H₂S relaxes a pre-existing N₂ contraction and N₂ relaxes a pre-existing H₂S contraction. H₂S is lost from continuously aerated baths in C after which normal hypoxic contractions are restored. Values are means ± s.e.m., N=8 vessels; horizontal and vertical scale bars in B and C = 10 min and 0.5 g, respectively.