Fig. 5. Evidence for H₂S involvement in trout gill chemoreceptors. (A) Injection of 5 µmol H₂S into the buccal cavity of an unanesthetized 600 g trout previously implanted with a pressure cannula in, and flow probe around, the bulbus arteriosus produces a bradycardia within 5 s that mimics a hypoxic bradycardia. P_va, ventral aortic pressure; CO cardiac output. (B) Heart rate following intrabuccal injection of a 1 ml bolus of H₂S in intact trout (black bars; N=13–15) or trout with either the first (Gill 1; white bars; N=7) or second (Gill 2; stippled bars; N=6) pair of gills removed. Dashed lines indicate mean ± s.e.m. heart rate of all fish prior to H₂S. H₂S produced a dose-dependent bradycardia in control trout that was similar to that produced in trout with the second pair of gills removed. Removal of the first pair of gills decreased H₂S sensitivity. Significantly different from pre-H₂S; ^*significantly different from intact trout after H₂S injection. The bradycardia at 5 and 10 mmol l^–1 is significantly (^*) attenuated in trout with the first pair of gills removed relative to the two other groups. Adapted from Olson (Olson et al., 2008b), with permission.