74 GENERAL METABOLISM in vitro 



requires that the nutrients normally obtained from the blood be 

 replaced by suitable salines. Of the many available, the bicar- 

 bonate- or phosphate-buffered slices of KrebsandHenseleit(1932), 

 based upon the content of inorganic ions in the serum, have been 

 the most widely used. Variations of these have been proposed by 

 Krebs (1950). Many workers modify the proportions of calcium 

 salts and replace the buffers by glycylglycine or 2-amino-2- 

 (hydroxymethyl)-l : 3-propanediol according to the particular 

 requirements of the experiment. In doing so it is to be remembered 

 that the metabolism of the tissue can be markedly altered by small 

 changes in the ionic composition of the medium (see for example 

 Mcllwain and Gore, 1952) and the effects of such changes in 

 relation to the phenomena being studied need to be carefully 

 assessed. Under suitable conditions slices of cerebral cortex 

 floating freely in saline, will respire at a steady rate for 2-3 hr. 



Other experimental arrangements with tissue slices include 

 provision for excitation by electrical currents. Here the tissue is 

 held between a grid of wire (Ayres and Mcllwain, 1953) which 

 does not prevent the oxygenation of the tissue but permits the 

 application of alternating electrical potentials. Such wire grids 

 may be constructed to fit into standard manometric flasks, to 

 enable small quantities of bathing fluid to be used (Rodnight and 

 Mcllwain, 1954), or to permit the rapid transfer of the tissue from 

 an incubating saline to an fixing agent such as trichloracetic acid 

 within 0-2-0-3 sec (Heald and Mcllwain, 1956). This latter 

 arrangement has proved particularly useful in the study of changes 

 occurring over periods of a few seconds or minutes. Choice of 

 arrangement depends upon the problem being studied. Examples 

 of the use of each are given throughout the following chapters. 



Tissue Dispersions 



Tissues can be ground, either by mashing on a warm, moist 

 plate with a spatula, to provide a brei, or more completely in a 

 glass tube with a fitting pestle of glass or plastic to yield dispersions 

 loosely termed **homogenates". The brei is now little used being 

 somewhat indeterminate in character and containing intact cells in 

 addition to disrupted material. Dispersion destroys the cell 

 structure. Nevertheless, the preparations obtained can exhibit 

 many of the important metabolic activities of the intact tissue such 

 as glucose oxidation, lactic acid production and oxidative phos- 



