A LARGE, SUSTAINED NA+-DEPENDENT AND VOLTAGE-DEPENDENT K+ CURRENT IN SPINAL NEURONS OF THE FROG EMBRYO

1. Neurons from the Xenopus embryo spinal cord were dissociated and conventional patch clamp techniques were used to record the whole-cell currents in the presence of tetrodotoxin (TTX). 2. The outward currents of the acutely isolated spinal neurons were rapidly reduced to about half their control value by substitution of extracellular Na+ with N-methyl-D-glucamine, lysine or choline. 3. The use of Li+ as a Na+ substitute partially reduced the outward currents. 4. The reversal potential of the Na+-sensitive current was close to the K+ equilibrium potential and could be altered by changing extracellular K+. The Na+-sensitive current was therefore a K+ current. 5. The Na+-sensitive K+ current was voltage dependent and activated in a sustained manner and appeared very similar to the delayed rectifier present in these neurons. 6. While the Na+-sensitive current increased with voltage as might be expected for an outward current, at very positive potentials it progressively decreased in amplitude. The voltage range over which this decrease was present moved closer to zero as the levels of intracellular Na+ were increased. The tail currents evoked by positive test potentials did not correspondingly decrease in amplitude, suggesting that channel block was rapidly relieved by stepping back to the holding potential. 7. Intracellular perfusion of the patch pipette with solutions containing varying amounts of Na+ (0-20 mm) showed that the K+ currents could be increased in a dose-dependent manner by raising intracellular Na+. The current had an EC50 for Na+ of 7.3 mm and a Hill coefficient of 4.6. 8. Single channel recordings from isolated inside-out patches revealed a channel that gated more frequently when the bathing levels of Na+ were elevated from 3 to 12 or 50 mm. Xenopus spinal cord neurons therefore possess a current that is not only voltage dependent but is also sensitive to internal Na+. 9. Xenopus spinal neurons possess a transient Na+ current (blocked by the inclusion of TTX) and a leak channel permeable to Na+. The inward leakage of Na+ appeared to provide the Na+ necessary for the gating of the Na+-dependent channel. 10. Blocking the Na+-K+ exchange pumps by removing extracellular K+, reduced the effect of removal of external Na+, suggesting that the Na+-K+ exchange pumps could be important in controlling the submembrane Na+. 11. A working model that depends on the actions of the Na+-K+ exchange pumps, inward leakage and diffusion from the pipette has been put forward to predict how the submembrane levels of Na+ vary with voltage, membrane permeability to Na+, Na+ in the pipette and extracellular Na+. 12. The Na+-dependent current itself has been provisionally modelled by a kinetic scheme that assumes binding of Na+ to the closed channel, voltage-dependent opening and a fast voltage-dependent blockage of the open channel by Na+. 13. The model for the Na+-dependent current predicts that this current could be modulated by the accumulation of Na+ resulting from the neural activity during swimming in the embryo.