METABOLISM IN THE NORMAL FUNCTIONAL STATE 21 



below the glucose consumption of human brain of about 17-20 

 /xmoles/g wet wt. hr~i (Mcllwain, 1959) and even lower than that 

 in the dog (Himwich and Fazekas, 1937) which can be calculated 

 to be 36 /xmoles/g wet wt. hr"^. 



Reasons for such differences are probably threefold and include, 



(a) the unequal rate of penetration of phosphorus to all parts of 

 the brain 



(b) the possibility of localization of chemically identical sub- 

 stances with different rates of metabolism 



(c) the methods of calculation of renewal rates. 



As noted above, radioactive phosphate entering the brain, follow- 

 ing either intracisternal or intravenous injection does not penetrate 

 to all parts of the brain equally rapidly. In such circumstances 

 samples taken from the whole brain shortly after injection will 

 contain mixtures of phosphates of higher and lower radioactivities. 

 Direct evidence relating to this has been provided by an interesting 

 study of Baranov (1957). Rats, subcutaneously injected with 

 radioactive phosphate, were frozen in liquid air 17 hr later and the 

 brain removed and slowly fixed in an acetone-alcohol mixture at 

 0°. It was shown that this procedure did not result in a loss of 

 phosphocreatine or adenosine triphosphate which were found in 

 quantities of 3-0)Ltmoles/g wet v^l;. cortical tissue, both before and 

 after fixation. Transverse sections of the cerebellum, taken at 

 0-45 mm intervals were found to contain similar quantities of the 

 phosphates but the specific radioactivities decreased as the depth 

 from the surface of the cerebellum to the cut section increased. 

 Differences of this type make the ** equilibrium point " such as 

 used by Lindberg and Ernster of doubtful value for calculations of 

 renewal rates. 



The presence of chemically identical substances with different 

 rates of metabolism has so far only been hinted at in the brain. 

 Such hints derive indirectly from the recognized differences exist- 

 ing in the oxygen consumption and enzymic activities of different 

 parts of the brain and of the differing nerve cell bodies (Bollard 

 and Mcllwain, 1957; Lowry, 1957). Whether such differences are 

 also reflected in rates of metabolism of radioactive phosphate is 

 not known. Since oxygen uptake and phosphorylation are normally 

 interdependent phenomena, regions with a lowered rate of oxygen 

 consumption might reasonably be expected to have a lowered rate 



