Electrogenic K + transport by the Na + –K + pump in rat cardiac ventricular myocytes

1 The involvement of electrogenic reaction steps in K + transport by the Na + ,K + ‐ATPase was determined in rat cardiac ventricular myocytes using whole‐cell patch clamp techniques. 2 Under K + –K + exchange conditions and in the presence of extracellular K + or Tl + at concentrations that stimulated submaximal levels of steady‐state Na + ,K + ‐ATPase activity, ouabain‐sensitive transient currents were observed during (‘on’) and after (‘off’) step changes in membrane potential ( V m ). 3 The quantity of charge moved during the transient currents depended, in a saturable manner, on the magnitude of the voltage step. Maximal ouabain‐sensitive ‘on’ and ‘off’ charges were calculated to be 9.6 ± 0.9 and 9.1 ± 0.4 fC pF −1 ( n = 4 ), respectively, with an effective valency of 0.48 ± 0.07 ( n = 7 ). 4 Kinetics of the transient currents were independent of V m and Tl o + at positive potentials, but became more rapid at increasingly negative V m values in an ion concentration‐dependent fashion. 5 These data demonstrate that electrogenic steps participate in K + transport by the Na + ,K + ‐ATPase and that the electrogenic step is extracellular ion binding. 6 The temperature‐and V m ‐dependent properties of transient charge movements were compared under K + –K + and Na + –Na + exchange conditions. The data suggest that extracellular K + and Na + binding occur at different sites in the enzyme or to different enzyme conformations. The sum of the effective valencies, 1.14 ± 0.12, demonstrates that the electrogenicity of extracellular ion binding can explain the V m dependence of ion transport by the Na + ,K + ‐ATPase.