Fig. 6. Double immunofluorescence staining of the vacuolar-type proton pump (V-ATPase: A,C,E,G) and Na⁺/K⁺-ATPase (B,D,F,H) in the gill filaments of killifish adapted to freshwater with low-(A,B,G,H), mid- (C,D) and high- (E,F) NaCl. V-ATPase immunoreactivity was detected in Na⁺/K⁺-ATPase-immunoreactive mitochondria-rich (MR) cells in fish adapted to a low-NaCl environment (A,B), but the staining was extinguished when antibody was pre-absorbed with the antigen (G). V-ATPase-immunoreactivity was faint in higher NaCl environments (C,E). Scale bar, 50 µm.