The Journal of Membrane Biology

, Volume 121, Issue 1, pp 23–36

Voltage dependence of current through the Na,K-exchange pump ofRana oocytes

  • Michael M. Wu
  • Mortimer M. Civan

DOI: 10.1007/BF01870648

Cite this article as:
Wu, M.M. & Civan, M.M. J. Membrain Biol. (1991) 121: 23. doi:10.1007/BF01870648


We have studied current (IStr) through the Na, K pump in amphibian oocytes under conditions designed to minimize parallel undesired currents. Specifically,IStr was measured as the strophanthidin-sensitive current in the presence of Ba2−, Cd2+ and gluconate (in place of external Cl). In addition,IStr was studied only after the difference currents from successive applications and washouts of strophanthidin (Str) were reproducible. The dose-response relationship to Str in four oocytes displayed a meanK0.5 of 0.4 μm, with 2–5 μm producing 84–93% pump' block. From baseline data with 12 Na+-preloaded oocytes, voltage clamped in the range [−170, +50 mV] with and without 2–5 μm Str, the averageIStr depended directly onVm up to a plateau at 0 mV with interpolated zero current at −165 mV. In three oocytes, lowering the external [Na+] markedly decreased the voltage sensitivity ofIp, while producing only a small change in the maximal outwardIStr. In contrast, decreasing the external [K+] from 25 to 2.5mm reducedIStr at 0 mV without substantially affecting its voltage dependence. At K+ concentrations of ≤1mm, both the absolute value ofIStr at 0 mV and the slope conductance were reduced. In eight oocytes, the activation of the averagedIStr by [K+]o over the voltage interval [−30, +30 mV] was well fit by the Hill equation, with K=1.7±0.4mm andnH (the minimum number of K+ binding sites) =1.7±0.4. The results unequivocally establish that the cardiotonic-sensitive current ofRana oocytes displays only a positive slope conductance for [K+]o>1mm. There is therefore no need to postulate more than one voltage-sensitive step in the cycling of the Na, K pump under physiologic conditions. The effects of varying external Na+ and K+ are consistent with results obtained in other tissues and may reflect an ion-well effect.

Key Words

amphibian oocyteNa, K-exchange pumpstrophanthidinsodiumpotassiumvoltage sensitivity

Copyright information

© Springer-Verlag New York Inc. 1991

Authors and Affiliations

  • Michael M. Wu
    • 1
  • Mortimer M. Civan
    • 1
    • 2
    • 3
  1. 1.Graduate Group of BioengineeringUniversity of PennsylvaniaPhiladelphia
  2. 2.Department of PhysiologyUniversity of PennsylvaniaPhiladelphia
  3. 3.Department of MedicinesUniversity of PennsylvaniaPhiladelphia