FACTORS AFFECTING METABOLISM in vitro 115 



phocreatine did not decrease immediately but after a lag period 

 of 2 sec (Fig. 12). During this period levels of adenosine tri- 

 phosphate fell at a rate of some 700 jitmoles/g hr"^ but rose again 

 when the breakdown of phosphocreatine commenced. Such a 

 result is in harmony with the concept of phosphocreatine main- 

 taining levels of adenosine triphosphate by means of creatine 

 phosphokinase. 



Breakdown of phosphocreatine was always accompanied by an 

 increase in the amount of inorganic orthophosphate present in the 

 tissue, the increase corresponding almost exactly in quantity to 

 the amount of phosphocreatine disappearing. However, the rate 

 of production, 800 />tmoles/g hr"\ was lower than the rate of break- 

 down of phosphocreatine (see below). Knowledge of the origin 

 of this extra inorganic phosphate is still slight. It could not be 

 derived directly from phosphocreatine for if phosphocreatine 

 breakdown was induced by suddenly increasing the level of 

 potassium salts in the medium to 90 mM (p. 130) levels of inorganic 

 phosphate did not increase until those of phosphocreatine had 

 fallen to about half the original level. The extra inorganic phos- 

 phate must also be firmly retained by the tissue for none passes 

 to the medium during prolonged electrical stimulation. Although 

 suggestions have been made that inorganic phosphate in tissues 

 does not exist in a free but in a bound form, direct evidence for 

 this is still lacking. At present it seems likely that the increase in 

 inorganic phosphate is ultimately due to the breakdown of some 

 phosphate derivative other than phosphocreatine. 



The rate of breakdown of phosphocreatine is 1200-1 400 /xmoles/g 

 hr~i, a rate which is the greatest yet measured for a metabolic 

 reaction involving phosphates in intact tissue, and one which 

 agrees well with values calculated from experiments in vivo (p. 56). 

 The rate gives no reason to suppose that the breakdown is the 

 result of electrical impulses inhibiting oxidative phosphorylation. 

 Under the conditions of the experiment the rate of oxygen uptake 

 for slices of guinea pig cerebral cortex was 55 /xmoles 02/g wet wt. 

 hr~^ (Heald, 1954). Thus if the phosphorus/oxygen ratio in the 

 intact slice is 3-0, the total requirement of the tissue for energy-rich 

 phosphate to maintain a steady state in the absence of pulses would 

 not exceed 330;Ltmoles/g wet wt. hr~^. Even if this rate was equal 

 to the turnover of phosphocreatine electrical pulses could not bring 

 about a rate of 1400/xmoles/g hr "^ merely by stopping oxidative 



