Fig. 2. H₂S has two dose-dependent effects on pre-contracted (10^–7 mol l^–1 U-46619) bovine pulmonary arteries. H₂S appears to produce a dose-dependent relaxation between 10^–8 and 10^–5 mol l^–1, whereas above 10^–5 mol l^–1 it produces a dose-dependent constriction. The EC₅₀ for relaxation (5.5±1.8x10^–7 mol l^–1) is significantly (P0.05) different from the EC₅₀ for contraction (3.7±1.5x10^–4 mol l^–1). (A) Single trace of cumulative doses; arrowheads indicate log[H₂S] (in mol l^–1) and time of addition of U-46619; W=wash. (B) Relaxation (filled circles); contraction (open squares; values extrapolated where curves overlap 10^–5 mol l^–1). Values are means ± s.e.m. of 8 vessels. Stippled rectangle indicates range of H₂S reported in the rat plasma (see text for details).