124 FACTORS AFFECTING METABOLISM in vitVO 



have been discussed (cf. Rydon, 1958; Aldridge, 1956) but whether 

 they apply to phosphoproteins of the type considered above is not 

 known. 



A system such as that of Fig. 14 is necessary to any chemical 

 linkage of labile phosphates to the functional events in the tissue, 

 and at present the phosphoprotein fraction appears to occupy a 

 prominant place. Whether this is the only substance of high 

 molecular weight involved in this manner, and the mechanism by 

 which the energy is used, are subjects for further work. 



Resynthesis of Phosphocreatine after Electrical Stimulus 



In vivo, in the period following the administration of electro- 

 shock, the phosphocreatine of rat brain was resynthesized at a rate 

 of 240 jLtmoles/g wet wt. hr~^ (Dawson and Richter, 1950). Rates 

 similar to this have been found in slices of guinea pig cerebral 

 cortex also following brief electrical stimulus. After applying 

 pulses for 7 sec, a period sufficient to reduce the levels of phos- 

 phocreatine, the phosphate was resynthesized to the normal level 

 at a rate of 150 jitmoles/g hr~^ (Fig. 15). At the same time the levels 

 of inorganic phosphate decreased. Here, as in other experiments 

 with tissue slices, the supply of oxygen could not be a limiting 

 factor and it seems probable that 150/xmoles/g hr~^ represents the 

 normal rate of synthesis of phosphocreatine. Experiments with 

 radioactive phosphate (p. 117) have supported the view that phos- 

 phocreatine is resynthesized by adenosine triphosphate. Never- 

 theless such resynthesis appears to involve other factors which 

 are not understood. Thus the continued passage of electrical 

 pulses for 20 min increased the time required for resynthesis from 

 17 sec to 90 sec (Fig. 15), the latter process having a lag period of 

 10 sec before any resynthesis was detectable. The factors involved 

 do not appear to include fumarate, found by Mcllwain and Gore 

 (1953) to restore the oxygen uptake and phosphocreatine levels 

 depleted by a similar technique, nor do they include additional 

 creatine, for inclusion of both of these in the medium at 20 n\M 

 failed to increase the rate of phosphocreatine resynthesis following 

 depletion by prolonged electrical stimulus. 



So far, direct measurements of the oxygen uptake by slices 

 during the short periods studied, have not been reported. It 

 seems probable (Mcllwain, 1952) that changes in oxygen uptake in 

 response to pulses are prompt and cease within 30 sec or less of 



