Fig. 6. Effect of (A) H₂S (as Na₂S), (B) the substrate for H₂S synthesis, L-cysteine (cysteine), and inhibitors of H₂S production, (C) amino-oxyacetate (AOA) and (D) hydroxylamine (HA), on oxygen consumption (_O2) by hagfish dorsal aortas. _O2 was stimulated by 10 µmol l^–1 H₂S and 10 mmol l^–1 cysteine and inhibited by 100 µmol l^–1 and 1 mmol l^–1 H₂S and by 10 mmol l^–1 AOA and 10 mmol l^–1 HA. Carbachol (100 µmol l^–1) pre-treatment (+CBC) did not affect _O2 at any [H₂S] compared to untreated (–CBC) vessels, although _O2 was significantly different between 10 µmol l^–1 and 100 µmol l^–1 H₂S in carbachol pretreated vessels. Mean + s.e.m.; N=7 (H₂S), 5 (cysteine), 4 (AOA), 4 (HA) groups of 4–6 vessels per group; ^*significantly different from respective control; significantly different from +CBC control.