EFFECTS ON PERMEABILITY AND ACTIVE TRANSPORT 183 



confine our attention to distinguishing between the first two possibilities. 

 The following methods may be suggested, although in certain instances 

 other approaches may be applicable. The use of radioactively labeled sub- 

 stances, especially in studies of ions, can provide information on influx 

 and efflux rates independently of total or over-all changes, but a good deal 

 of caution is necessary in their interpretation. It is best if such studies can 

 be done under a variety of conditions. For example, if the active transport 

 is purely aerobic, anoxia will abolish it; the rate of efflux under these con- 

 ditions should be compared with that in the presence of iodoacetate. If the 

 rate of loss is much greater with iodoacetate, one can conclude that some 

 membrane action has been exerted. It is also occasionally possible to com- 

 l)arc the effects of the inhibitor on the accumulation of different substances 

 which are chemically similar, some of these preferably not being actively 

 transported, since a change in permeability will generally be reflected in 

 the movements of all these substances, whereas an inhibition of active trans- 

 port will affect some more than others. The variation of the inhibition with 

 1)H may provide useful information; if the site of action is within the cell, 

 the inhibition will be greater at lower pH's where penetration is better, but 

 if it is on the membrane the inhibition may be greater at higher pH's. 

 However, one is not certain that the pertinent metabolic systems are within 

 the cell, and the surface effects could be on the transport systems them- 

 selves. 



If the action is shown to be on active transport, the site of inhibition 

 may be on either the exergonic or endergonic phase. The simplest and most 

 used method for distinguishing these is to attempt to establish whether 

 there is a correlation between metabolic and transport inhibitions. In the 

 case of iodoacetate, inhibition of glycolysis and transport may be simultan- 

 eously determined, but since there is no necessary quantitative relationship 

 here, as previously discussed, this is usually very unsubstantial evidence. 

 Overcoming of the inhibition by pyruvate is better evidence for a block of 

 the EM pathway. Such correlations will be discussed in the following sec- 

 tions; only under exceptional conditions do they provide valid proof for 

 a site of action. In this connection, analyses for intracellular ATP and 

 creatine-P can be of some value. A fall in these high-energy substances 

 implies an inhibition of exergonic reactions, while a lack of change or an 

 increase suggests an action on the transport systems themselves. 



Two particularly ingenious and useful localization techniques were pro- 

 posed by Davenport et al. (1955) in connection with their work on the inhi- 

 bition of gastric secretion. The first procedure involves the determination 

 of the inhibitions of both exergonic and endergonic processes (e.g., lactate 

 formation and HCl secretion) under a variety of conditions. If there is a 

 consistent correlation between these inhibitions under all conditions, it is 

 likely that the action is on the exergonic phase, whereas if the correlation 



