DISCUSSION ON ANIMAL TISSUE RESPIRATION 273 



piratory quotient as slices, and are similarly affected by additions of 

 various substrates and by variations in the ionic content of the med- 

 ium. An exception to this rule is that the respiration of slices, unlike 

 that of suspensions, is less well maintained in isotonic sucrose than in 

 isotonic saline solution. When isotonic suspensions are used, air may 

 be used in reaction flasks instead of oxygen. This is an advantage, 

 since it has been found that, while the respiration rates in the presence 

 of air and in the presence of oxygen are identical for 90 minutes, the 

 rate in the presence of oxygen falls off much more rapidly there- 

 after. A similar slowly appearing toxic effect of oxygen can also be 

 demonstrated with slices, but with slices the presence of oxygen is 

 necessary, since otherwise the initial respiration rate is limited by 

 inadequate diffusion of oxygen into the tissue. 



After homogenizing fresh brain in isotonic medium, the rate of 

 respiration falls off very rapidly at first, thereafter less rapidly. Slices 

 and isotonic suspensions prepared from slices do not show the initial 

 very rapid decrease in rate. This suggests that a specially labile part of 

 the respiratory activity can be observed with suspensions of fresh 

 tissue, but is lost during the slow process of preparing for experi- 

 ments on slices. 



Salts induce specific as well as osmotic effects. Isotonic solutions of 

 sodium chloride, nitrate, sulfate, and especially phosphate increase 

 the initial respiration rate appreciably when added to suspensions in 

 isotonic sucrose. Bicarbonate in physiological concentration has no 

 special effect. Calcium and magnesium ions have inhibitory effects 

 on the respiration but cause better maintenance of rate. Potassium 

 in high concentrations is also inhibitory. The initial inhibitory ef- 

 fect of magnesium in the concentration found in serum is fairly 

 small, and this ion is known to take part in reactions of carbohydrate 

 metabolism. For studies of brain respiration, tissue homogenized in 

 isotonic sodium chloride medium containing magnesium is therefore 

 recommended. 



However, respiratory activities measured on suspensions or slices 

 in any given medium cannot be considered to represent the true 

 physiological activity of brain tissue in vivo until more is known of 

 the effective concentrations of ions in the cells' immediate environ- 

 ment in vivo and until more is known of the differences that may 

 exist between brain in its normal physiologically active condition 

 and the tissue which has been subjected to the abnormal injuries 

 and stimuli of in vitro work. 



The marked difference between tissue homogenized in isotonic 



