128 FACTORS AFFFXTING MFTABOLISM itl vitrO 



Stimulated by passing pulses through sterling silver electrodes for 

 10 min (the duration of the experiment) a lower radioactivity was 

 found in the acid-soluble phosphates and a fraction considered 

 to be phosphoprotein (Abood, 1954). The significance of these 

 results is difficult to assess. Comparison with the effects of pulses 

 in intact tissue is not possible since both the systems used and the 

 period of application of pulses are markedly different. Further, the 

 phosphoprotein showing response to pulses in the intact tissue is 

 not contained in the mitochondria (Heald, 1959). 



There is thus no really sound evidence supporting the view that 

 either electrical pulses support increased respiration or decrease 

 oxidative phosphorylation in cell-free systems and consequently 

 suggestions that such changes have a significance in cell-containing 

 tissues are open to considerable doubt. 



Effects of Potassium and Other Ions 



Amongst the normal environmental constituents of brain which 

 are capable of markedly affecting its metabolism, potassium salts 

 have long been known to be prominent. In the brain in vivo 

 potassium ions are present at a concentration of 92 mM (Hoagland 

 and Stone, 1948), but on slicing and placing in saline the quantities 

 rapidly fall to 10-20 mM. In the presence of oxygen and glucose, 

 levels of potassium ions gradually increase to reach 50 mM. Under 

 these conditions, potassium ions in the medium exchange with 

 those in the slice at 4-5% of the total/minute, the equilibrium 

 being complete within 30 min (Krebs et al, 1951; Terner ^/ «/., 

 1950; Korey, 1952). The quantities exchanged are approximately 

 180 jLtmoles/g hr-i (Davies and Krebs, 1952). Restoration, ex- 

 change and maintenance of adequate levels of potassium ions 

 requires a continuous expenditure of metabolic energy and it is 

 understandable that changes in the concentration of potassium 

 salts in the medium surrounding a slice should be accompanied by 

 changes in the general metabolism. 



When added to the medium, potassium salts increase oxygen 

 uptake and aerobic glycolysis the effect being maximal with 

 100 mM K"^", when oxygen uptake is almost doubled, but already 

 apparent at 20 mM (Ashford and Dixon, 1935), results which 

 have since been confirmed by numerous workers. The effect is 

 not specific to potassium salts, those of caesium and rubidium 

 promoting an equal response (Dickens and Greville, 1935). In 



