Characterization of the cardiac Na+/K+ pump by development of a comprehensive and mechanistic model

A large amount of experimental data on the characteristics of the cardiac Na+/K+ pump have been accumulated, but it remains difficult to predict the quantitative contribution of the pump in an intact cell because most measurements have been made under non-physiological conditions. To extrapolate the experimental findings to intact cells, we have developed a comprehensive Na+/K+ pump model based on the thermodynamic framework (Smith and Crampin, 2004) of the Post–Albers reaction cycle combined with access channel mechanisms. The new model explains a variety of experimental results for the Na+/K+ pump current (INaK), including the dependency on the concentrations of Na+ and K+, the membrane potential and the free energy of ATP hydrolysis. The model demonstrates that both the apparent affinity and the slope of the substrate–INaK relationship measured experimentally are affected by the composition of ions in the extra- and intracellular solutions, indirectly through alteration in the probability distribution of individual enzyme intermediates. By considering the voltage dependence in the Na+- and K+-binding steps, the experimental voltage–INaK relationship could be reconstructed with application of experimental ionic compositions in the model, and the view of voltage-dependent K+ binding was supported. Re-evaluation of charge movements accompanying Na+ and K+ translocations gave a reasonable number for the site density of the Na+/K+ pump on the membrane. The new model is relevant for simulation of cellular functions under various interventions, such as depression of energy metabolism.