CENTRAL NERVOUS SYSTEM METABOLISM IN VITRO 



1837 



Potassium and Other Ions 



Increasing the concentration of potassium salts in 

 normal salines to o. 1 m causes respiration and aerobic 

 glycolysis to increase by about 100 per cent (9). 

 Substrates which support increased oxygen uptake 

 include glucose, fructose, lactate and pyruvate, but 

 not members of the tricarboxylic acid cycle (35, 1 15). 

 As with electrical pulses anaerobic glycolysis is de- 

 creased (35). The effect is not specific to the potas- 

 sium ion and is given, although to a markedly lesser 

 degree, by lithium, rubidium and caesium ions (35). 

 Sodium salts are ineffective (35, 36, 141). Although 

 the effect is most marked at a concentration of o. 1 m, 

 it is readily detectable at 0.02 m (9, 141 ). In addition 

 to increasing oxygen uptake and aerobic glycolysis, 

 potassium salts at 0.03 m increase free acetylcholine 

 production in cerebral slices (122). In higher con- 

 centrations, levels of creatine phosphate are de- 

 creased (73, 141). 



The mechanism of the effect is not clear. Potassium 

 ions are known to increase the rate of conversion of 

 phosphoenolpyruvate to pyruvate by accelerating the 

 phosphorylation of adenylic acid to adenosine 

 triphosphate (153). It has been suggested that in their 

 presence an increased quantit) of pyruvate is thus 

 made available for oxidation (115) and an increased 

 formation of adenosine triphosphate ensues (()_'). 

 These proposals are unlikely since pyruvate itself 

 does not yield maximal rates of oxygen uptake when 

 used as sole substrate, and levels of creatine phos- 

 phate, linked with those of adenosine triphosphate, 

 are decreased by o.l m potassium salts. It seems more 

 reasonable to suppose that increased potassium s.ihs 

 act by depolarization of the neuronal membrane 

 (129) in a manner analogous to that suggested an- 

 electrical pulses. The increased metabolism is thus 

 a reflection of the increased energy expenditure in- 

 volved in restoration of the more normal state. Effects 

 of potassium, and other ions considered here, upon 

 individual enzyme systems or on metabolism of 

 particulate preparations are outside the scope of this 

 article, but it is of interest to note in passing that 

 oxygen uptake of cerebral mitochondria is not 

 affected by concentrations of potassium chloride 

 ranging from 0.02 mM to 50.0 mM (158). 



Changes of metabolism of tissue slices with other 

 ions have been noted. Thus decreasing the levels of 

 calcium salts in the medium, increases the oxygen 

 uptake (103) but decreases anaerobic glycolysis 

 (167). On the other hand increasing the calcium 

 level to 0.082 m had no effect upon respiration, 



aerobic glycolysis or levels of creatine phosphate 

 (141). In the absence of sodium ions oxygen uptake 

 and aerobic glycolysis are decreased, together with 

 levels of creatine phosphate (141). In such media 

 slices show no response to electrical pulses. Ammonium 

 salts at 0.03 m increase oxygen uptake and aerobic 

 glycolysis, and decrease levels of creatine phosphate 

 (141, 218) and rates of phospholipid metabolism 

 (180), analogous to the effect of potassium. However, 

 the ammonium effect is detectable at 0.3 mM, some 

 hundred times lower than that of potassium required 

 for the same effects. At levels of 0.05 M ammonium 

 salts, acetylcholine production is inhibited (122) 

 Little effect was detected at lower levels. 



Metabolit Inhibitors 



Elucidation of metabolic pathways or the mecha- 

 nisms involved in a given reaction has been greatly 

 assisted in disintegrated preparations by the use of 

 agents inhibiting one, or at the most two, particular 

 enzyme systems to a marked degree while having 

 litde effecl upon others. As a result, study of the 

 cHcets upon cerebral metabolism of inhibitors with a 

 known point of action lias been carried out by many 

 workers in attempts to correlate such effects with 

 those of known therapeutic agents. Of the inhibitors 

 used to demonstrate tin- participation of energy- 

 yielding processes in metabolism of the central nerv- 

 ous svstcm, probably the best known .ne fluoride, 

 iodoacelic acid, malonic acid, cyanide, azide and 

 2,4-dinitrophenol. 



With cerebral tissue slices in the presence of in- 

 creasing concentrations of inhibitors such as fluoride 

 and iodoacetate, which primarily block the conver- 

 sion of glucose to pyruvate, both respiration and 



glycolysis decrease. Inhibitors, such .is malonale 01 

 cyanide which act .11 points in the conversion of 

 pyruvate to carbon dioxide and water, decrease 

 oxygen uptake and increase glycolysis (72, 192). 

 Dinitrophcnol, which prevents the permanent esterili- 

 cation of adenyl derivatives to adenosine triphosphate 

 and in consequence maintains high concentrations 

 of inorganic phosphate and phosphate acceptors 

 within the tissue, increases both oxygen uptake and 

 glycolysis (141). Azide, which has a similar effect upon 

 oxidative phosphorylation in liver preparations (117), 

 also inhibits the cytochrome system (90) and de- 

 creases levels of energy-rich phosphate, aerobic 

 glycolysis and oxygen uptake (99). 



Systems sensitive to agents such as the above might 

 be expected to be most easily detectable under con- 



