Inhibition of Na,K-ATPase activity by sodium azide (a), SNP (b) or SNAP (c). Cells were incubated with or without (control) test compounds for 20 min, then homogenized and Na,K-ATPase activity in control and treated samples was measured under identical conditions. The results are the mean±s.e.m. of data from six independent experiments (a and c), or 12–18 experiments (b). Statistical comparisons were made by analysis of variance followed by the Bonferroni post hoc multiple comparison tests. *,**and ***indicate significant differences from the control value at P<0.05, P<0.01 and P<0.01, respectively.