PKA failed to increase Na/K pump current at 25 mM Na⁺_i. (A) Na/K pump current was activated by application of ATP in 50 mM and then 25 mM Na⁺_i. After the current reached steady state, PKA was applied at 25 mM Na⁺_i without effect during 2 min. Subsequent application of 50 mM Na⁺_i minimally increased the current. (B) Bar graph summarizing results from six patches (four at 50 mM Na⁺_i) in which similar maneuvers were performed.