Na/K pump model with Na-dependent inactivation from E1 states. (A) Cartoon of Na/K pump function with electrogenic steps marked by lightning bolts. Binding sites open to the cytoplasmic side in E1 states and to the extracellular side in E2 states. Translocation of three Na outwardly with ATP hydrolysis and 2K inwardly with dephosphorylation are irreversible reactions under the assumption that cytoplasmic ADP and Pi are negligible. Release of the first Na ion moves 0.75 charges through membrane field, binding/release reactions of the other ions within E2 states moves 0.3 charges, and occlusion of Na from E1 states moves 0.25 charges. Inactivation occurs from all E1 states and recovery has the same Na and voltage dependence as the occlusion of cytoplasmic Na. (B) State diagram used to model Na/K pump function. The normal cycle occurs clockwise. Kinetically defined states are numbered from 0 to 7, and their ion occupancy is indicated. Reaction rates are designated by the numbering of states. E1 states bind 3Na and 2K instantaneously. Reaction rates K12 and K71 are assumed to be zero.