256 H. A. KREBS VOL. 4 (1950) 



show that if the medium is shaken with respiring tissues which produce CO2 continuously 

 pjj remains about 7.3. When the medium is allowed to stand for long periods or shaken 

 without tissues pn rises. 



Comparative measurements have shown in many cases^^' ^® that tissues kept in 

 these types of media respire at about the same rate as serum or saline serum substitutes 

 containing Ca and bicarbonate in physiological concentrations. 



A medium of the type A is prepared by omitting CaClj from medium I and replacing 

 18 parts of the NaHCOg solution by an isotonic phosphate buffer. Mix 



83 parts of 0.9% NaCl 

 4 parts of 1.15% KCl 

 I part of 2.11% KH2PO4 

 I part of 3.82% MgS04.7H20 



3 parts of 1.3% NaHCOg 



18 parts of Na-phosphate buffer (100 parts of o.i M Na2HP04 (1.78% Na2HP04.2 H2O) 

 and 25 parts of o.i M NaH2P04 (1.38% NaH2P04.H20)) 



4 parts of 0.16 M Na-pyruvate (or L-lactate) 

 7 parts of O.I M Na-fumarate 



4 parts of 0.16 M Na-L-glutamate 



5 parts of 0.3 M (5.4%) glucose 



In this calcium-free medium the concentrations of Na, K, Mg, CI and SO4 approxi- 

 mate to those of serum; the concentration of phosphate is about 20 times higher, and 

 that of HCO3 about 10 times lower, than the physiological values. 



Saline low in phosphate, bicarbonate, and CO^ {Medium III). Many previous obser- 

 vations indicate that calcium ions can influence the rate of respiration^'^' ^> ^^' ^''. It is 

 therefore useful to have a medium which, like the synthetic serum substance, contains 

 Ca in physiological concentrations but can, at the same time, be used in manometric 

 experiments where CO2 is being absorbed by alkali. The medium suggested differs from 

 medium II, apart from the inclusion of Ca, by a lower phosphate concentration and 

 therefore lowering buffering capacity. These differences are necessitated by the limited 

 solubility of Ca-phosphates. Mix 



95 parts of 0.9% NaCl 

 4 parts of 1.15% KCl 

 3 parts of 0.1 1 M CaCla 

 I part of 2.11% KH2PO4 

 I part of 3.82% MgS04.7H20 

 3 parts of 1.3% NaHCOg 



3 parts of Na-phosphate buffer (as described for medium II) 



4 parts of 0.16 M Na-pyruvate 

 7 parts of 0.1 M Na-fumarate 



4 parts of 0.16 M Na-L-glutamate 



5 parts of 0.3 M (5.4%) glucose 



O2 pressure 



In order to safeguard saturation of tissue slices with O, it is generally necessary to 

 have an O2 pressure of one atmosphere in the cup. It is known®^' ^^' ^^ that O2 of this 

 pressure has a poisoning effect on some of the oxidative enzymes. As these effects are 

 References p. 26y-26g. 



