Fig. 9. The effect of 12 h exposure to 1% CO₂ in air (CO₂), 1% CO₂ in air with 10 mmol l^–1 Hepes buffered-water (CO₂+Hepes), and 100% air with 10 mmol l^–1 Hepes-buffered water (Air+Hepes) on the mRNA expression of carbonic anhydrase (CA2), H⁺-ATPase, NHE2 and Na⁺/K⁺-ATPase -1a (NKA) in the gills of rainbow trout. Control fish were exposed to 100% air for 12 h in unbuffered water. Expression data were normalized to ng total RNA concentration with the control value set to one. Significant differences from the control are indicated by an asterisk (P<0.05). Data are means ± s.e.m. (N=6).