Interaction of External K, Na, and Cardioactive Steroids with the Na-K Pump of the Human Red Blood Cell

The interaction of extracellular Na (Na_o), K (K_o), and strophanthidin with the Na-K pump of the human red blood cell has been investigated. Inhibition by submaximal concentrations of strophanthidin rapidly reaches a level which does not increase further over a relatively long period of time. Under these circumstances, it is possible to apply a steady-state kinetic analysis to the interaction of Na_o, K_o, and strophanthidin with the pump. In Na-free solutions, strophanthidin increases the apparent K_1/2 of the pump for K_o, but does not change the form of the relation between the reciprocal of the active K influx (ⁱM_K^P–1) and the reciprocal of [K_o] ([K_o]^–1); the relation both in the presence and absence of strophanthidin is adequately described by a straight line. In solutions containing Na, strophanthidin changes the form of the curve describing the relation between ⁱM_K^P–1 vs. [K_o]^–1; the curve becomes more parabolic in solutions containing strophanthidin. The rate of ouabain binding to K-free cells has also been measured; in the absence of K, the rate of binding is unaffected by Na_o. The data are considered in terms of a simple kinetic model. The findings can be explained if it is supposed that at low external K the form of the pump combined with one Na_o is more likely to combine with strophanthidin than is the uncombined form of the pump. The uncombined form of the pump is more likely to combine with K even at very low K_o than with strophanthidin.