(A) Effects of MTSET on four Na,K-ATPase cysteine mutants: conformation-dependent effects. Means ± SEM of K+-induced currents at −50 mV in oocytes expressing the cysteine mutants or WT α subunit. The currents, induced by adding 10 mM external K+ to a 100 mM external Na+ solution, measured after MTSET exposure are expressed after normalizing for the current measured before exposure to MTSET. MTSET was perfused for 2 min at a concentration of 250 μM, either with 10 mM external K+, to shift the equilibrium toward the E1 conformation (black bars), or in a Na+-free and K+-free solution (gray bars), to promote the E2P conformation of the Na,K-ATPase. Between 9 and 10 measurements were performed for each condition. Error bars represent the SEM. The effect of MTSET was statistically significant (P < 0.01) for each mutant in the E1 conformation condition versus WT. *, P < 0.05; **, P < 0.01, for the comparison between the E1 and E2 conformation for the same mutant. (B) Effects of MTSET at low concentrations. For each mutant, the effect of a concentration that resulted in approximately half of the maximum effect was tested in the presence of Na+ and K+. *, P < 0.05 for the comparison between the E1 and E2 conformation for the same mutant.