116 FACTORS AFFECTING METABOLISM in Vttro 



phosphorylation from proceeding. There is good reason to beheve 

 that the normal turnover rate of phosphocreatine in cerebral tissues 

 in vitro is about 150 jitmoles/g hr~^ making the rate of breakdown in 

 response to pulses almost ten times greater. The rate of breakdown 

 also exceeds the maximal rate of phosphorylation required for the 

 metabolism of glucose when proceeding at its greatest rate in 

 cerebral tissues both in vivo and in vitro. It was found (Mcllwain 

 and Tresize, 1956) that pulses, applied in the system described 

 above, increased glucose metabolism, as judged by the rate of 

 lactic acid production, to 200-220 /xmoles/g hr " ^ The maximal 

 rate of glucose uptake in cat brain stimulated in vivo with electrical 

 impulses can be calculated to be 370jLtmoles/g hr~i (Klein and 

 Olsen, 1947) which accords well with the value of 390/xmoles 

 glucose/g hr~^ observed as the maximal rate of hexokinase activity 

 in dispersions of rat brain (Long, 1951). Thus, even assuming 

 that all the hexose monophosphate formed was immediately 

 rephosphorylated to form fructose-1 : 6-diphosphate the maxi- 

 mum requirements for energy-rich phosphate would be unlikely to 

 exceed 780^moles/g wet wt. hr~^. More probably the rate is 

 about 400jLtmoles/g hr~^ These considerations suggested that a 

 major part of the energy-rich phosphate released in response to 

 pulses was not used to phosphorylate glucose but was metabolized 

 by some other pathway. 



In experiments designed to follow this pathway slices of guinea 

 pig cerebral cortex were incubated in a saline containing glucose 

 for a preliminary period to allow metabolism to reach a steady 

 state. Radioactive phosphate was then added to the medium and 

 metabolism allowed to proceed for 3 min, at the end of which 

 period the specific radioactivity of phosphocreatine and the 

 y-phosphorus of adenosine triphosphate were in equilibrium 

 (Fig. 13). The distribution of radioactivity in the other acid- 

 soluble phosphates was not uniform, the time period being deliber- 

 rately chosen to obtain this situation. Upon removal of the tissues 

 from the saline and replacing in fresh saline not containing 

 radioactive phosphate, the specific radioactivities of the acid- 

 soluble phosphates decreased partly owing to a loss of radioactivity 

 to the saline and also to the redistribution of radioactivity into the 

 other fractions. Electrical pulses applied for no more than 10 sec 

 during this phase produced a rapid and marked decrease in the 

 specific radioactivity of inorganic phosphate, phosphocreatine and 



