Fig. 4. The effect of sodium nitroprusside (SNP; 0.5 mmol l^–1) on in situ Na⁺/K⁺-ATPase activity (measured as ouabain-sensitive Rb⁺ uptake) in kidney (A) and gill (B) tissue incubated in the presence or absence of the nitric oxide scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO; 1 mmol l^–1). Samples were pre-incubated for 10 min before the Rb⁺ uptake assay. Different letters indicate significant difference (P<0.05). Values are means ± S.E.M. (N=8).