1832 



11 AMJIIin IK Ul- I'llYSICH 1 >(.\ 



m.i ri ii'in mi ii.oi;y ill 



latcd that if the potassium in the slice is all in the 

 intracellular space, then, under such conditions, up- 

 take of potassium does not involve an increase in 

 intracellular potassium concentration. 



Although glutamate increases the level of potassium 

 in the slices, K.orey (96) found that it did not promote 

 an increased rate of exchange of radioactive potas- 

 sium between cat cerebral slices and the surrounding 

 medium. Under conditions where potassium accumu- 

 lation is optimal, this exchange involves some 3.5 to 

 4 per cent of the total tissue potassium per min. (104). 

 It has generally been assumed in such calculations 

 thai all the potassium of cerebral slices is readily 

 exchangeable, but some contrary evidence exists. 

 Thus it has been calculated (89) from the data of 

 Krebs et al. (104) that in adult cerebral tissue a pro- 

 portion of the total potassium has a low rate of turn- 

 oxer as compared with the remainder. Such slowly 

 exchangeable potassium may be bound to phospho- 

 lipids (-,o) for this fraction is absent in the infant un- 

 myelinated rat brain (89). It has also been shown 

 (193) thai pari of the potassium of homogenates of 

 cerebral tissues does not pass through an ultratilter 

 whereas the cerebral sodium salts are readily filtered. 



Carbohydrate 



( arbohydrate metabolism is of outstanding; impor- 

 tance to the central nervous system. For this reason 

 ii has been frequently and adequately reviewed, and 

 the function of the present section is solely to quote 

 such accounts. Individual enzyme processes of gly- 

 lic and oxidative sequences, studied specifically in 

 central nervous system preparations, have been de- 

 tailed from the point of view of their quantitative 

 requirements for substrates and cofactors and, especi- 

 ally, rates oi reactii 36, 171). Respiration and 



glycolysis .is organized sequences have been described 

 1 ) ■;, 88, 136, -'-'J 1, including methods of study, normal 

 values in many mammalian species, substrates, 

 aerobic and anaerobic processes. Factors conditioning 

 the balance between aerobic and anaerobic processes 

 have been appraised quantitatively 1 1 |6). I he varia- 

 tion in level of carbohydrate metabolism with change 

 in 1 1 1 1 1 ( tional activity has been quantitatively ex- 

 pressed and mechanisms of the adjustment appraised 

 1 ;6 I he development oi processes ol carbohydrate 

 metabolism in the growing nervous system has been 



reviewed ( r;b, ZI5) bolh with respeel tO 1iKl1vid11.1l 



enzyme reactions I also to the over-. ill processes in 



cell-containing tissues M.mv apposite comparisons 



have been made between the carbohydrate metabol- 

 ism of the brain in vivo and in vitro (45, 80, 136). 



Amino Acids and Proteins 



Although cerebral tissues in vivo are well known to 

 contain almost all the known amino acids in a freely 

 extractable form (220) or combined as protein (16, 

 17m, with the exception of glutamic acid and com- 

 pounds related mctabolically little is known of cere- 

 bral amino acid metabolism in vitro and still less of 

 protein metabolism. Slices of rat or guinea-pig 

 cerebral tissue in a phosphate or bicarbonate saline 

 were unable to oxidize any of 13 naturally occurring 

 amino acids other than L ( + ) glutamate (iOI, 102, 

 -Mb) when these were added singly to the tissue 

 preparation. With human cerebral tissue and amino 

 acid mixtures, the situation appears to be more com- 

 plex. Thus a mixture of amino acids containing 

 glutamate failed to maintain the respiration of 

 cerebral cortical slices over a period of 3 hr., although 

 glutamate at a level identical with that in the mixture 

 prevented such a decrease (45). The initial oxygen 

 uptake with the amino acid mixture was similar to 

 that obtained with pyruvate as the substrate. It was 

 suggested that either the oxidation of glutamate in a 

 mixture is modified by the presence of other amino 

 acids or is not solely responsible for the oxygen uptake 

 of slices respiring in such a mixture 



Failure to metabolize certain amino acids may be 

 partly due to permeability barriers, for rat cerebral 

 tissue homogenates oxidativcly decarboxylate or 

 dcaminate 1. -valine (51 I, m. -alanine (55), ni -phenyla- 

 lanine, 1. -tryptophane, D-histidine, L-arginine and 

 I. -lysine (37). Metabolism ceased when the atmosphere 

 of oxygen was replaced bv nitrogen. The carbon of 

 glycine is metabolized by at least two routes Thus 

 the carboxyl carbon was metabolized largely via 

 decarboxylation while the methyl carbon was incor- 

 porated largely into the residue remaining after ex- 

 traction of lipids ,md compounds soluble in trichloro- 

 acetic acid. This residue was considered to be protein 

 (ill) bul would also contain nucleic acids, nucleo- 

 prolein and probably proteolipid. The decarboxylase 

 svsiem was associated with particulate material and 

 was not operative in acelonc powders. This is in con- 



n.ist to the alanine decarboxylase system which was 



equally active in both ho genates and acetone 



powders I 36 



ll is probable that in vitro at least, a major soun e oJ 

 the carbon in certain of the free cerebral amino acids 

 derives from glucose. With r.H cerebral cortex slices 



