430 



9. INHIBITION IN CELLS AND TISSUES 



tion than Na"^, with the exception of the kidney. For this reason K"^ salts 

 should be used as the major source of cation in most work on isolated en- 

 zymes. The following ionic concentrations might be considered as approach- 

 ing a reasonable intracellular environment: 135 mM K"^, 20 milf Na"*", 20 

 mM Mg++, 1 mM Ca+^, 20 mM HCO3", 10 mil inorganic phosphate, and a 

 pH of 6.7-6.8. The difficulty is in providing sufficient anion to balance the 

 required cation concentrations, since in the cell this anion deficit is sup- 

 plied by various organic substances. It is likely that it is generally safer 

 to use KCl rather than high concentrations of phosphate, since most en- 

 zymes are not affected markedly by Cl~ concentration. The excellent re- 

 view of Caldwell (1956) may be consulted for the intracellular pH's of the 

 individual tissues but it is clear that most mammalian tissues have a pH 

 slightly below neutrality when determined by the best techniques. Or- 

 ganic ions make up a major fraction of the total in most cells; in frog muscle 

 the values have been given as amino acids 8.8 mM, phosphocreatine 35.2 

 mM, carnosine 14.7 mM, creatine 7.4 mM, lactate 3.9 mM, ATP 4.0 mM, 

 hexose monophosphates 2.5 mM (Conway, 1957), and there are many other 



Table 9-1 



Sodium and Potassium Concentrations in Vertebrate Tissues " 



" Relatively less data are available for the other physiological ions: Mg++ — 14 raM 

 in frog muscle, 15 mM in rat ventricle, 36 mM in rat liver, 11 mM in rat erythrocytes; 

 Cl~ = 1.5 mM in frog muscle, 8 milf in rat ventricle, 27 mM in toad nerve, 18 mM 

 in rat liver, 82 mM in rat erythrocytes; HCO7 = 10 mM in rat muscle, 14 mM in 

 rat ventricle, 20 mM in rat liver, 28 mM in rat erythrocytes, 12.4 mM in frog muscle; 

 phosphate = 7.3 milf in frog muscle; total phosphates = 143 mM in rat muscle, 100 

 mM in rat ventricle, 123 mM in rat liver. The values have been summarized from nu- 

 merous references and represent only approximate concentrations, which may vary 

 rather widely with the age or the state of the animal. 



