Fig. 8. Effects of hypoxia and quinidine on ventilation frequency in developing zebrafish. (A) Larvae 3–9 d.p.f. were subjected to control solution (Cont; P_O2=150 mmHg; filled bars) or hypoxia (Hox; P_O2=25 mmHg; open bars) in a continuously perfused chamber. Larvae responded to the hypoxic challenge with an increase in ventilation frequency (min^–1; mean ± S.E.M.) as early as 3 d.p.f. This response increased to a maximum at 7 d.p.f. Asterisks denote a significant increase in ventilation frequency compared to control at each developmental stage (P<0.05; Student's t-test). In addition, the ventilatory response to hypoxia was significantly greater at 7 d.p.f. compared to the response at previous stages (P<0.001; ANOVA–Bonferroni test). Sample sizes are indicated in the figure. (B) Larvae 3–10 d.p.f. were exposed to control solution (Cont; filled bars) or 1 mmol l^–1 quinidine (Quid; hatched bars) in the same perfusion system as in A. Ventilation frequency (mean ± S.E.M.) in 3 d.p.f. larvae was not significantly affected by the application of quinidine, but was quinidine-sensitive at 7 and 10 d.p.f. Asterisks indicate a significant difference from control at each developmental stage (P<0.05; ANOVA–Bonferroni test). Sample sizes are indicated in the figure.