R. D. KEYNES 



prevented potassium absorption, and soon made it inexcitable. Excitability could 

 again be restored, immediately but not for long, by washing the outside of the axon 

 in fresh sea water still containing dinitrophenol. These observations suggest that, at 

 least in cephalopod axons, the primary function of nerve metabolism is to provide 

 energy for the recovery processes which are responsible for the absorption of potas- 

 sium and extrusion of sodium after activity, and that energy-yielding metabolic 

 mechanisms do not intervene directly in the generation of the action potential. 



The problem has been studied in more detail with the help of radioactive isotopes, 

 and with intracellular microelectrodes. In squid and Sepia axons loaded with 24 Na 

 there is a continual outward movement of the isotope through the cell membrane, 

 which apparently results from the operation of an active transport mechanism. 

 Blocking of metabolism with dinitrophenol, cyanide or azide, results in a gradual 

 reduction of the sodium efflux to about one-twentieth of its initial value, and the 

 efflux can later be restored by washing the inhibitor away (Hodgkin and Keynes, 

 1953a, 1954a). This inhibition of the sodium pump has been observed under a wide 

 variety of experimental conditions; it occurs whatever method is used to introduce 

 24 Na into the axon, and is very little affected by changes in the external medium — 

 the effect even persists in an axon soaked in an isotonic dextrose solution containing 

 almost no salts. Somewhat to our surprise, we have also found that the potassium 

 influx is cut down by inhibitors to about one-seventh of its resting value. This con- 

 flicts with the earlier view (see Keynes, 1951) that the fluxes of K+ ions moving across 

 the membrane are wholly passive, but fits with other recent evidence suggesting that 

 the active transport mechanism works by an inward potassium transfer more or less 

 tightly coupled to the sodium extrusion. Thus in cephalopod axons (Hodgkin and 

 Keynes, 1953^) abolition of the potassium influx by removing all the external potas- 

 sium results in a reversible decrease of the sodium efflux. The interaction of sodium 

 and potassium fluxes cannot be mediated through the usual effect of potassium con- 

 centration on the resting membrane potential, since we have found (Hodgkin and 

 Keynes, 1954^) that the sodium efflux in Sepia axons is not altered perceptibly by 

 quite large polarizations of the membrane, so that there must be some more specific 

 form of coupling between them. It is tempting to suggest that such coupled ionic 

 pumps may be quite widespread, although the only supporting evidence available 

 at present is that a similar effect of external potassium on sodium efflux has been 

 observed both in erythrocytes (Harris and Maizels, 1951) and in frog muscle (Keynes, 



1954); 

 This type of coupled pump would be neutral in that it would transfer no net 



charge across the membrane. The evidence just considered is therefore consistent 



with the further observation that in a squid axon, poisoning with dinitrophenol only 



causes a slow decline in the resting and action potentials (Hodgkin and Keynes, 



1954a), as would be expected if under these conditions the intracellular potassium 



content is falling, and sodium is rising, faster than in an untreated axon. We have 



also confirmed with 24 Na that the rapid sodium movements during activity are 



almost unaltered by dinitrophenol, when at the same time the resting sodium efflux 



has been brought to a standstill. In cephalopod axons it seems clear that there can 



be no very direct connexion between the mechanisms involved in conduction and in 



recovery, since each can go on working when the other is put out of action (an 



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