THE CHARACTERIZATION OF NEW ENERGY DEPENDENT CATION TRANSPORT PROCESSES IN RED BLOOD CELLS

This paper is primarily concerned with the characterization of the processes associated with the active transport of Na and K as seen in the human red blood cell. In particular, the main objective of the work to be described is the demonstration that there are two distinct pump mechanisms, resident in the plasma membrane, each of which actively transports Na out of the cell. One of these pumps is already well established. It is the Na-K coupled pump which is inhibited by cardiac glycosides and utilizes directly ATP as its source of energy.la2 The second pump is new. It turns out that this second Na pump is insensitive to cardiac glycosides but is dependent upon the presence of Na in the external medium. These two pumps have entirely different properties, and our aim here in showing these distinctive features will necessarily indicate the reality of the second pump. In order to relate more clearly these distinctions in the various flux pathways, it will be helpful to review briefly the relevant features of Na and K transport in human red blood cells. I t is generally considered that three conceptually distinct membrane pathways are available for the movement of either Na or K. Thus, in the model as illustrated in FIGURE 1, ions can move by an active transport or pump pathway, by passive diffusion or leak pathways, and by exchange dift‘usion. Movement of an ion through a pump is uphill against its electrochemical potential gradient and is, therefore, energy-de~endent.~ The leak refers to movement of an ion downhill in the direction of its electrochemical gradient. Exchange diffusion is a special type of diffusion, not thought to require energy, and can be seen only with radioactive tracers.” Because red cells are very permeable to anions (chloride) relative to cations, it is necessary that the cell actively transport both Na and K. The pump, as presented in FIGURE 1 , is the well-known Na-K coupled pump, which operates to exchange specifically K outside for Na inside. On the other hand, exchange diffusion involves the exchange of Na for Na or K for K. No net movement of cations can occur by the mechanism of exchange diffusion. In this model, net movements can only occur via the pump or leak pathways. The point of departure from the scheme of kinetic pathways as presented here came with the realization that the phenomenon of Na exchange diffusion siniply does not occur in the human red cell. Rather, the entire second pump for Na is contained in the component labeled in FIGURE 1 as Na exchange diffusion. Evidence in support of this conclusion will be presented below. It is important to keep in mind the operational bases by which these various flux components can be identified and studied experimentally. Let us first make some general comments before showing some examples of these operational distinctions. The various fluxes can be estimated by measurement of the unidirectional movements of Na or K using radioactive tracers. In addition, net