EFFECTS ON THE ENDOGENOUS RESPIRATION 183 



The inhibition of rabbit ova respiration at various times postcoitum does 

 not change significantly (Fridhandler et al., 1957), while the inhibition of 

 trematode respiration decreases with time from excystment (Vernberg and 

 Hunter, 1960). When 20 raM malonate is added to rat ventricle slices 1 hr 

 after slicing, the inhibition of the respiration is only 25%, whereas initially 

 the inhibition is near 50% (Webb et al., 1949), and this relationship holds 

 for all malonate concentrations up to 100 vaM. We have seen that in the 

 presence of malonate the inhibition has been replaced by stimulation at 

 1 hr. Therefore the metabolism changes differently during the action of 

 malonate and the maxima in the time curves cannot be explained by 

 inherent alterations of the metabolic pattern. All of these results point to 

 the importance of considering the time factor in studies of malonate inhi- 

 bition. 



Another apparently important factor is the ion and buffer composition of 

 the medium, although no thorough studies have been done and the mecha- 

 nisms are not understood. Many years ago Annau (1935) observed that the 

 inhibition of the respiration of both rabbit liver and kidney slices is less 

 in Ringer than in phosphate medium. The results were quite variable but 

 on the average the inhibition is 30% in phosphate and 15-20% in Ringer 

 medium. Unfortunately, Annau did not state what form of malonate was 

 used nor did he mention pH control, so the results are perhaps unreliable. 

 The presence of bicarbonate abolishes the inhibition by malonate in ox 

 retina homogenates (Burgess et al., 1960), and it is probable that inhibition 

 in intact cells would also vary with the bicarbonate concentration. Bicarbon- 

 ate can, of course, facilitate the formation of oxalacetate through carboxyla- 

 tion reactions. When Ca++, Mg++, or K"*" is removed from the medium, the 

 inhibition of rat brain slice respiration by 10 mM malonate is not altered, 

 but in the presence of fumarate the inhibition becomes progressively greater 

 as these ions are successively removed (Greville, 1936). The addition of 

 Ca+"'" to nematode minces increases both the respiratory inhibition and the 

 inhibition of succinate oxidation (Massey and Rogers, 1950). The sensitivity 

 of chicory root slice respiration to malonate is markedly affected by K+ and 

 Li+ (Laties, 1959 b). Slices incubated with 50 milf K+ are inhibited more 

 and with 50 mM Li+ inhibited less than the fresh slices. It is also interesting 

 that increase in CO2 tension results in progressive disappearance of the mal- 

 onate inhibition, whereas increase in Og tension augments the malonate- 

 sensitive fraction of the respiration. It is thus clear that the medium can 

 play an important role in the response to malonate. Much work has been 

 done in quite nonphysiological media and the results are thus difficult to 

 apply to the actions of malonate in situ. Much more effort should be di- 

 rected at creating approximately physiological conditions. 



The functional activity of the tissue determines the level and type of 

 respiration, and therefore is often a major factor in the sensitivity to mal- 



