Fig. 2. The effect of pre-incubation time with sodium nitroprusside (SNP) on in situ Na⁺/K⁺-ATPase activity (measured as ouabain-sensitive Rb⁺ uptake) in kidney (A) and gill (B) tissue. The samples were pre-incubated with SNP (1 mmol l^–1) for the time indicated before the Rb⁺ uptake assay. Different letters indicate significant difference (P<0.05). Values are means ± S.E.M. (N=8).