Partial reaction schemes for the Na⁺ transport limb of the Na⁺/K⁺-ATPase cycle. Scheme 1 was introduced by Holmgren and Rakowski (2006) to describe the dependence of Na⁺/K⁺-ATPase presteady-state currents on intracellular [Na⁺]. At saturating intracellular concentrations of Na⁺ and ATP, the intracellular Na⁺ binding (and phosphorylation) is at rapid equilibrium, and the subsequent slow occlusion step kinetically isolates the intracellular Na⁺ binding step from the more rapid extracellular Na⁺ release steps. Scheme 2 refers to Vasilyev et al. (2004), who provided a concept for the mainly H⁺-driven inward currents of the Na⁺/K⁺-ATPase observed in Na⁺- and K⁺-free extracellular solutions. At low pH_ext, the energy barrier for equilibration of occluded Na⁺ ions with the intracellular space is reduced so that the occluded state can be reached from the extracellular as well as the intracellular solution. See Appendix for details.