Fig. 2. Comparison of the relaxation efficacy of 1 mmol l^-1 H₂S (as Na₂S) and hypoxia (N₂) on 10 µmol l^-1 carbachol (CARB)-, 80 mmol l^-1 KCl- and 80 mmol l^-1 potassium acetate (KAc)-contracted trout urinary bladders (all N=8) and the effects of H₂S application on a pre-existing N₂ exposure and of N₂ application on a pre-existing H₂S exposure in pre-contracted bladders (all N=4). H₂S (as NaHS) relaxation of CARB-contracted vessels is also shown. Values are means ± s.e.m. H₂S was significantly less efficacious on KCl-contracted bladders than bladders pre-contracted with either CARB or KAc (). Bladders relaxed with either H₂S or N₂ did not respond to the other (N₂ or H₂S, respectively), irrespective of the pre-contractile agonist (^*). There was no difference in the relaxation produced by Na₂S and NaHS.