20 METABOLISM IN THE NORMAL FUNCTIONAL STATE 



situation in anaesthetized rats injected intracranially, i.e. into the 

 ventricular fluid, showed that the acid-hydrolysable phosphorus of 

 adenosine triphosphate incorporated radioactivity at the greatest 

 rate followed by phosphocreatine. Of the total radioactivity of the 

 sample some 30% was present as adenosine triphosphate within 

 2 min of the injection. Within 30 min, the radioactivities of the 

 acid-hydrolysable phosphorus of adenosine triphosphate and of 

 phosphocreatine were equal to each other and to that of inorganic 

 phosphate. Calculations (Lindberg and Ernster, 1950) based 

 partly on the assumption that none of the injected phosphate was 

 lost to the blood, showed that the phosphorus of phosphocreatine 

 and adenosine triphosphate was renewed at the rate of 40 /xg 

 phosphorus/g wet wt. hr^^ which corresponds to a rate of 77-78 

 /xmoles phosphorus/g wet wt. hr~^. 



This value is presumably a measure of oxidative phosphoryla- 

 tion in the brain in vivo and consequently is far too low. In vitro, 

 slices of rat cerebral cortex metabolizing glucose in suitable salines, 

 consume oxygen at the rate of 70 /xmoles Og/g wet wt. hr "i. Assum- 

 ing a phosphorus/oxygen ratio of three (see Ochoa and Stern, 

 1952; Mcllwain, 1959) this rate corresponds to a rate of renewal 

 of the phosphorus of adenosine triphosphate of 420 /xmoles 

 phosphorus/g wet wt. hr~i. There is good evidence to show that 

 the rates of oxygen consumption of the intact brain in vivo are 

 double those found with slices in vitro (Mcllwain, 1956, 1959). 

 Thus the figure for the rate of renewal of the phosphorus of 

 adenosine triphosphate is more probably 840 /xmoles phosphorus/g 

 wet wt./hr. Anaesthesia is known to decrease the rate of oxygen 

 consumption of the brain, but even so the rate of phosphorus 

 renewal calculated by Lindberg and Ernster is probably no more 

 than a tenth of the actual rate. A similar discrepancy between 

 experimentally determined rates of renewal and those expected 

 from rates of oxygen uptake has been noted by Vladimirov and 

 Rubel (1957) in determinations of the renewal of phosphorus in 

 hexose monophosphates from rat brain. Measurements were made 

 upon rats injected subcutaneously with radioactive phosphorus. 

 Thus the route of entry of phosphate to the brain differed from 

 that in the experiments of Lindberg and Ernster. It was calculated 

 that the phosphorus of the hexose monophosphate was renewed at 

 a rate of 8-4 mg phosphorus/ 100 g tissue hr~^, which corresponds 

 to about 3-0/xmoles/g wet wt. hr~i. This value is considerably 



