FACTORS AFFECTING METABOLISM in Vttro 133 



salts (Pappius and Elliot, 1956) and to the absence of a satisfactory 

 method of determining the specific radioactivity of the intracellular 

 inorganic phosphate. 



Changes in levels of phosphocreatine and the incorporation of 

 radioactive phosphate into different phosphorylated compounds 

 are brought about by ions other than potassium. The omission of 

 calcium or magnesium ions, or the addition of ammonium ions at 

 10 mMto the medium increased respiration and aerobic glycolysis 

 and decreased levels of phosphocreatine (Mcllwain, 1952; 

 Mcllwain and Gore, 1952). With similar changes in the ionic 

 composition of the medium the incorporation of radioactive phos- 

 phate into phospholipids, nucleic acids, phosphoproteins and 

 residual organic phosphorus was markedly decreased (Findlay 

 et al, 1954). 



A basis for these changes is perhaps more readily suggested for 

 potassium salts than for the others. It is widely held that part of 

 the energy expended by nervous tissue is involved in the perfor- 

 mance of osmotic work, extruding sodium and maintaining a high 

 concentration of potassium ions within the cell. An increase in 

 the concentration of potassium ions outside the cell might therefore 

 be expected to increase the energy expenditure necessary tn main- 

 tain the ionic balance. This in turn would decrease the amount o: 

 energy- rich phosphate available for other purposes. This view is 

 certainly an oversimplification of mechanisms which also involve 

 sodium and calcium ions. Nevertheless the technique described 

 provides a direct method of relating changes in metabolism to 

 changes in ionic balances by varying the concentrations of the ions 

 themselves. 



Acetylcholine 



An effect of acetylcholine upon the metabolism of the phospho- 

 lipids considered to be related to part of its action in vivo has been 

 the subject of several papers by Hokin and Hokin. In the presence 

 of eserine, acetylcholine at 10" ^-10-^ M increased the incorpora- 

 tion of radioactive phosphate into the phospholipids of cerebral 

 slices by 50-100%, the increase being greatest in the lecithins and 

 cephalins (Hokin and Hokin, 1955«). The effect was barely 

 detectable within 1 hr but after 3 hr incubation in oxygenated 

 saline measurable changes were obtained. Further fractionation 

 of the phospholipids showed that the greatest change took place 



