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1. Fifty to ninety per cent of the Na efflux from axons of Loligo forbesi is inhibited by ouabain. The properties of the ouabain-sensitive component of the Na efflux are different from those of the ouabain-insensitive component.

2. In unpoisoned axons with an average Na content of 75 m-mole/kg axoplasm the bulk of the ouabain-sensitive Na efflux is dependent on external K.

3. In the presence of 460 mM Na in the external medium, raising the external K concentration from 0 to 100 mM increases the ouabain-sensitive Na efflux along a sigmoid curve which shows signs of saturating at high K concentrations.

4. The curve relating ouabain-sensitive K influx to external K concentration is similar in shape to that for the ouabain-sensitive Na efflux. At all K concentrations examined the ouabain-sensitive K influx was less than the ouabain-sensitive Na efflux.

5. Potassium-free sea water acts rapidly in reducing the Na efflux. There is no appreciable difference between the rates of action of K-free sea water on the Na pump and Na-free sea water on the action potential.

6. Caesium and Rb can replace external K in activating the ouabain-sensitive Na efflux. Both the affinity and maximum rate of the Na efflux mechanism are lower when Cs replaces K as the activating cation.

7. Isosmotic replacement of external Na by either choline or dextrose, but not Li, increases the affinity of the ouabain-sensitive Na efflux mechanism for external K without appreciably affecting the maximum rate of pumping. External Li behaves like external Na and exerts an inhibitory action on the Na efflux.

8. There is a large ouabain-sensitive Na efflux into K-free choline or dextrose sea waters. Addition of either Na or Li to the external medium reduces this efflux along a section of a rectangular hyperbola. The properties of this efflux suggest that there is a residual K concentration of up to 2 mM immediately external to the pumping sites in the axolemma.

9. Over the range of internal Na concentrations studied (16-140 m-mole/kg axoplasm) the ouabain-sensitive Na efflux increased linearly with Na concentration.

10. Tetrodotoxin (10^-6 g/ml.) reduces the Na influx by about half, but does not affect the ouabain-sensitive Na efflux.

11. Isobutanol (1% v/v) reversibly decreases both the ouabain-sensitive and ouabain-insensitive components of the Na efflux.

12. Application of 2 mM cyanide to axons immersed in K-free sea water produces a transient rise in the Na efflux. This rise is not seen if ouabain is included in the sea water. The rise in efflux occurs at a time when the axons are partially poisoned and contain adenosine triphosphate (ATP) but no arginine phosphate (ArgP). A similar, but maintained rise can be obtained after application of dinitrophenol (DNP) at pH 8·0. The increased Na efflux in these partially poisoned axons is also inhibited by ouabain.

13. Under conditions of partial-poisoning by alkaline DNP, there is a ouabain-sensitive Na influx from K-free sea water. The ouabain-sensitive Na influx is of similar size to the ouabain-sensitive Na efflux. These results show that in partially-poisoned axons immersed in K-free sea water intracellular Na exchanges with extracellular Na in a one-for-one manner by a ouabain-sensitive route. External Li cannot replace external Na in maintaining this process.

14. Axons partially poisoned with alkaline DNP are not insensitive to external K. In the absence of external Na their response to external K is essentially the same as that seen in unpoisoned axons.

15. Possible mechanisms are discussed for the appearance of Na-Na exchange in partially poisoned axons.

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