CLUES FROM ASSOCIATED EVENTS 



way the sodium-ion efflux was largely eliminated. The cyanide in- 

 fusion rapidly depleted the levels of ATP and arginine phosphate. 

 Subsequent infusion of either ATP or arginine phosphate, or of phos- 

 phoenolpyruvate or ADP, restored the sodium-ion efflux. Creatine 

 phosphate was without effect, presumably because this animal species 

 has no suitable kinase (Figure 26). ATP or arginine phosphate were 

 ineffective when applied to the exterior of the axon. The action 

 of these infusions was transient and could be demonstrated re- 

 peatedly at hourly intervals. The associations between the fluxes 

 of alkali metal ions and the flux of sugars and amino acids, plus 

 the mutual dependence of these transports on either respiration or 

 glycolysis, leads to the view that high-energy phosphate also ener- 

 gizes these transports. At the same time, considerations of the 

 energy expenditure for amino acid transport make it unlikely that 

 ATP cleavage is linked directly to the transfer of the amino acid 

 molecule (Heinz and Patlak, 1960). 



In a preliminary note, Jarnefelt (1961) reports that a micro- 

 somal preparation from brain fixes four or five times as much sodium 

 ion if ATP is added. The report does not permit a conclusion as to 

 whether this uptake by the organelle is a passive penetration, or a 

 binding, or what. Tosteson (1960) has shown that the genetically 

 potassium-low red blood cells of certain strains of sheep are rela- 

 tively low in such ATPase activity, compared with homologous 

 potassium-high erythrocytes. 



If ATP serves as the energy source for transport, the total 

 transport machinery must a priori appear as an ATP-splitting proc- 

 ess. We should not suppose that a single, simple ATP-splitting en- 

 zyme molecule accomplishes alkali metal transport. Urea synthesis 

 or muscle contraction may present a useful analogy to remind 

 us of the complexity that we may expect to find where an ATP- 

 splitting system performs work. The ATPase activity could for 

 example represent the sum of the kinase and the phosphatase re- 

 quired by Hokins' phosphatidic acid cycle; or it could be con- 

 siderably more complex. 



Although transport has a vectorial component, we have seen 

 earlier in this presentation that the energy may or may not actually 

 serve to propel the solute physically against the concentration gradi- 

 ent; several of the published schemes propose that the physical 

 migration in a final analysis occurs by diffusion. If, however, trans- 

 port is produced by the application of a translocational force of a 



83 



