Reversal of the sodium pump in human red cells.

A study has been made of some chemical reactions and ion movements related to the sodium pump in the membrane of the human red cell. Part One describes chemical studies with erythrocyte ghosts to test whether incorporation of radioactive orthophosphate (32Pi) into adenosine triphosphate (ATP) was associated with concentration gradients of sodium and potassium across the membrane. Some incorporation was always found irrespective of the ionic distribution. Additional labelling depended on the ionic composition of ghosts and medium, and was prevented by ouabain. There was no ouabain-sensitive labelling with a gradient of only either sodium or potassium. The results show that a part of the incorporation of orthophosphate into ATP appeared to occur by reversal of the sodium pump. In Part Two, movements of sodium and potassium down concentration gradients are described. A part of both sodium entry and potassium loss was sensitive to ouabain. Further, ouabain-sensitive potassium efflux was dependent on external sodium, and ouabain-sensitive sodium influx was abolished when potassium was added to the external medium. In the absence of external potassium, cells lost potassium and gained sodium in a way partly sensitive to ouabain. A modified operation of the sodium pump seems to bring about exchange diffusion of sodium as well as a coupled movement of sodium entry and potassium loss. The energetics of ion movements in relation to reactions of ATP were studied in Part Three. The number of ions moving downhill via the pump per mole orthophosphate incorporated into ATP was the same as the stoichiometry for active transport, indicating the same efficiency for ATP splitting and ATP labelling. The general conclusion is that the reactions of ATP are closely linked to the migrations of ions across the membrane.