The Sodium-Potassium Exchange Pump: II. Analysis of Na+-Loaded Frog Sartorius Muscle

A model for the Na-K exchange pump was applied to data on Na⁺-loaded frog sartorius muscle, and was used to relate the rate of adenosine triphosphate (ATP) hydrolysis to the electrical properties of the cell membrane. Membrane hyperpolarization was considered to arise from an electrical current which was produced by the hydrolysis reaction coupled to ion movements, and which flowed across the membrane. The reaction rate, as calculated from hyperpolarization, agreed with direct measurements of ATP hydrolysis and with the rate estimated from Na⁺ tracer efflux studies. Although Na⁺ is actively extruded, the model showed that K⁺ is inwardly transported if the potassium permeability of the membrane is less than about 6.6 × 10^-6 cm/sec, as is suggested by resistance data. Calculations indicated that the reaction conductance L_rr was relatively constant when compared with the reaction rate and reaction free energy for large changes in internal and external ionic concentrations. Its value agreed with the value obtained from the dependence of Na⁺ tracer efflux on external K⁺. A set of experiments was suggested which would provide a more complete interpretation of the data.