The effects of membrane potential on active and passive sodium transport in Xenopus oocytes

1. The effects of membrane potential on the Na+-K+ pump were studied by measuring membrane current and 22Na+ efflux in voltage-clamped Xenopus oocytes. The effects of inhibiting the Na+-K+ pump with strophanthidin were examined. 2. Strophanthidin produced an inward shift of membrane current which reversed on removal of the drug. In control oocytes the magnitude of this current was not significantly affected by changing membrane potential over the range -20 to -160 mV. 3. In another series of experiments the intracellular Na+ concentration ([Na+]i) was elevated either by overnight Na+-K+ pump inhibition (strophanthidin or exposure to K+-free solutions) or by loading with nystatin. This Na+-loading increased the magnitude of the strophanthidin-sensitive current. The ratio of strophanthidin-sensitive 22Na+ efflux:strophanthidin-sensitive current was consistent with that expected from a 3Na+-2K+ exchange. 4. When [Na+]i was elevated the strophanthidin-sensitive current was sensitive to changes of membrane potential. Hyperpolarization from -20 to -80 mV decreased the current to 60% of control. It is suggested that the current is not sensitive to membrane potential at normal [Na+]i because the over-all reaction is rate limited by the availability of intracellular Na+. 5. The application of strophanthidin decreased the rate of 22Na+ efflux. Both the strophanthidin-insensitive and the strophanthidin-sensitive components of efflux were sensitive to changes of membrane potential. The strophanthidin-insensitive component was not greatly affected by hyperpolarization from -40 to -160 mV but was increased by depolarization to +40 mV. 6. In Na+-loaded oocytes, the strophanthidin-sensitive component of 22Na+ efflux was inhibited by hyperpolarization negative from -40 mV. Hyperpolarization from -40 to -160 mV decreased the efflux by 54 +/- 5%. Over the limited range of potentials for which a comparison could be made, the effects on 22Na+ efflux were somewhat less than on the electrogenic Na+-K+ pump current. On average there was no significant effect of depolarizing from 0 to +40 mV. However, in some experiments a clear inhibition of the efflux was observed. If the oocytes were not Na+ loaded there was no significant effect of membrane potential on the strophanthidin-sensitive Na+ efflux. 7. These results show that the effects of membrane potential on the net reaction of the Na+-K+ pump (as measured by the electrogenic current) result partly from an inhibition of the forward mode of operation. However, there is also evidence to suggest a contribution from stimulation of the reverse reaction.