114 



FACTORS AFFECTING METABOLISM in Vttro 



phosphocreatine in relation to the applied potential was noted 

 above (Fig. 11). Similar partial decreases are also related to the 

 duration of the impulse. Thus with a condenser discharge of 

 10 V peak potential and a duration of 0-4-0-8 msec, phospho- 

 creatine decreased to 0-8 /zmoles/g wet wt. in 5 sec, a decrease 

 which did not occur when the duration of the pulse was reduced to 

 0- 1 msec. Since these experiments were carried out with slices in 



2 4 6 8 10 



Period of stimulation (sec) 



Fig. 12. Changes in phosphocreatine (PCr), inorganic phosphate 



(P) and adenosine triphosphate (ATP) in slices of guinea pig 



cerebral cortex during the passage of electrical pulses. Data from 



Heald (1954). 



oxygenated saline depletion of phosphocreatine cannot be attri- 

 buted to a partial anoxia accompanying electrical excitation and 

 thus represents a direct response of the tissue phosphates to the 

 stimulus. Results such as these are important in understanding the 

 nature and extent of the stimulus required to evoke different 

 degrees of metabolic response. They represent an interesting 

 counterpart to those situations in vivo in which the application 

 of different stimuli result in a breakdown of phosphocreatine to 

 different degrees. Following the application of the pulses phos- 



