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Abstract

Current generated by the electrogenic Na⁺/K⁺ pump protein was determined in oocytes of Xenopus laevis as strophantidine-sensitive current measured under voltage clamp. Under conditions of reduced intracellular [Na⁺] and [ATP], both to values below 1 mM, and in extracellularly K⁺-free medium, the Na⁺/K⁺ pump seems to operate in a reversed mode pumping Na⁺ into the cell and K⁺ out of the cell. This is demonstrated by strophantidine-induced hyperpolarization of the membrane and inward-directed current mediated by the pump protein. In addition, strophantidine-sensitive uptake of ²²Na⁺ can be demonstrated under these conditions. The pump current decreases with membrane depolarization as expected for a pump cycle that involves inward movement of positive charges during Na⁺ translocation.