Intracellular sodium activity and the sodium pump in snail neurones

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 mM.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 mM had little effect on Na(+)(i), but K-free or 0.25 mM-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 mM.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).