Intracellular sodium activity and the sodium pump in snail neurones

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. © 1972 The Physiological Society