Intracellular sodium activity and the sodium pump in snail neurones

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Abstract
1. Recessed‐tip Na+‐sensitive micro‐electrodes were used to measure [Na+]i continuously in snail neurones for experiments lasting up to several hours. The average resting [Na+]i in twenty‐two cells was 3·6 m M.2. Inhibition of the Na pump by ouabain caused [Na+]i to increase at an average rate of 0·54 m‐mole/min. This corresponds to a passive influx of Na quantitatively similar to that observed in squid axons.3. Changing external K over the range 1‐8 m M had little effect on [Na+]i, but K‐free or 0·25 m M‐K Ringer caused a rise in [Na+]i.4. Increasing membrane potential by up to 90 mV caused an increased influx of Na, but did not inhibit the pump.5. Reducing external Na caused a decrease in [Na+]i but did not affect the pump rate at a given [Na+]i. The pump rate at low [Na+]i was proportional to [Na+]i minus a threshold value of about 1 m M.6. The Na pump appeared still to be electrogenic at subnormal rates of activity.7. It is concluded that, given sufficient external K, the rate of the Na pump depends principally on [Na+]i. Changes in external Na or membrane potential appear to affect the pump only indirectly, by changing the Na influx and thus [Na+]i.