Kinetic evaluation of the Na‐K pump reaction mechanism.

1. The ouabain‐sensitive K influx was measured at varying external K concentrations ([K]o) and at several fixed internal Na concentrations ([Na]c). The cells were nominally K‐free and the solutions Na‐free. Both the apparent maximal velocity (VM) and the apparent Michaelis constant for K (KK) increased as Nac increased. The ratio app. VM/app. KK increased with increasing Nac. 2. The ouabain‐sensitive Cs influx was measured at varying external Cs concentrations and at several fixed Nac in K‐free cells and Na‐free solutions. Both app. VM and app. Kcs increased as Nac increased and the ratio app. VM/app. Kcs increased with increasing Nac. 3. The data were evaluated in terms of ping‐pong model and a simultaneous model for the pump reaction mechanism. The simultaneous model described the data adequately and the ping‐pong models did not. 4. The K influx was measured at varying external K concentrations in solutions containing Na and at a low and high Nac; the cells contained K. The relation between the pump rate and the external K concentration was sigmoid. A Hill equation was fitted to the data. KK was higher in the high Nac cells, but the Hill coefficient (n) was not altered as Nac increased. 5. The K influx was measured at varying internal Na concentrations and two fixed external K concentrations; the cells contained K. The relation between the pump rate and Nac was sigmoid. When a Hill equation was fitted to the data, it was found that KNac was higher at the high external K concentration, but n was the same at both K concentrations.