146 PHOSPHATES AND THERAPEUTIC AGENTS 



barbiturates such as dial and phenobarbitone, inactive upon 

 phosphorus/oxygen ratios in vitro are nevertheless effective as 

 anaesthetics at concentrations some 10 times less than those 

 necessary to produce an inhibition of oxygen uptake in the mito- 

 chondrial preparations. Other differences include the fact that 

 dinitrophenol is not a depressant and that anaesthesia is not 

 accompanied by fever and increased metabolism, both of which 

 normally follow the administration of an agent uncoupling oxidative 

 phosphorylation. At present it must be concluded that there is 

 little basis for attributing the action of depressants in vivo to the 

 inhibitions of phosphorylative mechanisms which have been 

 observed with particulate preparations in vitro. Closer correlations 

 have been found with intact tissues stimulated in vitro by electrical 

 impulses, and these are now described. 



Effects upon Stimulated Tissues 



The development of systems capable of responding biochemically 

 to applied electrical stimulus, in a manner analogous to the brain 

 in vivo (see p. 115), has led to a re-examination of the action of 

 depressants and anticonvulsants upon cerebral metabolism under 

 the new conditions. It is relevant to note that the metabolic rates 

 of the systems involved in the metabolism of glucose and the 

 uptake of oxygen in cerebral slices in vitro are normally below the 

 rates operating in vivo presumably because of the restraint imposed 

 by the low levels of phosphate acceptors and inorganic phosphate. 

 Removal of such a restraint by increasing the levels of those 

 phosphates raises the metabolic rate to that found in vivo. This 

 state is achieved when cerebral tissues are subjected to electrical 

 impulses. In such a condition a partial inhibition of a reaction is 

 more readily detectable by its overall effect. For example, a 

 reduction of half in the activity of an enzyme need not be detectable 

 in unstimulated tissue if the activity is not rate limiting. In 

 stimulated tissue a similar reduction could make the remaining 

 activity the rate limiting step and thus the inhibition would become 

 detectable. Such a case is the effect of 10"^ M iodoacetic acid upon 

 the oxygen uptake of cerebral slices for at this concentration the 

 electrically stimulated oxygen uptake was suppressed though no 

 effect was detectable upon the unstimulated oxygen uptake 

 (Heald, 1953). 



The effects of depressants upon oxygen uptake and lactic acid 



