Membrane current and intracellular sodium changes in a snail neurone during extrusion of injected sodium.

1. Sodium was injected into an identified snail neurone by passing current between two intracellular micro-electrodes, the membrane potential being recorded with a third micro-electrode.2. The injection of about 25 p-equiv Na, but not the injection of similar quantities of K or Li, caused a hyperpolarization of up to 20 mV. This response to Na injection was blocked by application of ouabain or removal of external K, indicating that it was due to the stimulation of an electrogenic pump.3. To measure the current produced by the sodium pump the output of a feed-back amplifier was fed into the cell via a fourth intracellular micro-electrode so as to keep the average membrane potential constant. The pump current, measured in this way, rose at a constant rate during, and declined exponentially after, an injection of Na, the decline having an average time constant of 4.4 min. The total charge transferred by the pump was between a third and a quarter of the charge passed to inject sodium.4. An intracellular Na-sensitive glass micro-electrode was used to follow changes in the concentration of intracellular Na ions. The results showed that both the pump current and the rate of Na extrusion were proportional to the concentration of intracellular Na ions above the normal level.5. It was concluded that about two thirds of the Na extruded was coupled to the active transport of other ions, probably to the uptake of K, the uncoupled third producing the electrogenic effect.