132 FACTORS AFFECTING METABOLISM in vittO 



salts at 29 mM (Findlay et ai, 1954). On the other hand, Tsukada 

 et al. (1958) found that potassium salts at 100 mM markedly 

 increased the incorporation of radioactive phosphate into the 

 phosphoprotein fraction and into the phosphorus derived from the 

 sum of phosphoprotein and residual organic phosphorus, called 

 *' protein bound " phosphorus. The results are not necessarily 

 incompatible. Thus, differences exist between the media used 

 (bicarbonate buffered saline by Findlay et al., glycylglycine- 

 buffered saline by Tsukada et al.) the period of incubation and the 

 relative concentrations of sodium and potassium ions in the 

 medium. With high concentrations of potassium ions respiration, 

 initially increased, gradually falls and after 4 hr is little more than 

 the normal respiration (Dickens and Greville, 1935), reasons which 

 governed the choice of 90 min incubation by Tsukada et al. Even 

 here the stimulated respiration had still decreased markedly from 

 its initial value. Relative concentrations of sodium and potassium 

 ions appear to be the most important factor. In the experiments 

 of Findlay et al. levels of sodium ions were decreased when the 

 potassium ions were increased, the final level of sodium ions being 

 in the region where respiratory response to potassium ions is not 

 observed. Tsukada et al. added potassium salts to the medium 

 without altering the concentration of sodium ions. In both sets of 

 experiments the use of intermediate concentrations led to essen- 

 tially similar results. Thus, potassium ions at 29-50 mM with 

 a corresponding decrease in levels of sodium ions decreased the 

 incorporation of radioactive phosphate into the fractions measured, 

 as did the presence of 5 mM potassium ions in the absence of 

 sodium ions. At 26-30 mM, potassium salts deplete levels of 

 phosphocreatine in cerebral slices. 



Lowered rates of incorporation are not apparently explicable in 

 terms of changes in the specific radioactivity of the inorganic 

 phosphate either in the medium or in the slice (Findlay et al., 

 1954). Thus in the presence of 29 mM potassium salts, the specific 

 radioactivity of inorganic phosphate in the slice was little changed 

 from that in slices incubated in a medium containing 5T mM 

 potassium salts, yet the specific activities of the individual phos- 

 phates was decreased (Table 20, Expt. 2). However it must be 

 remembered that situations such as this present diflficulties of 

 interpretation owing to changes in the intracellular space during 

 metabolism in media containing varying quantities of potassium 



