EFFECTS ON PERMEABILITY AND ACTIVE TRANSPORT 181 



iodoacetate brings about an increase in the ratio from 0.60 to 0.67. This 

 might be due to an inhibition of the utilization of the amino acids since it 

 was shown that incorporation into proteins is reduced, over the experimen- 

 tal period the utilization being depressed more than the ujitake. 



Equilibrium and Nonequilibrium States 



In studying the effects of inhibitors on transport and accumulation, 

 usually two ty]ies of experiment are performed. The cells in an equilibrium 

 state with a fully developed concentration gradient of the substance are 

 treated with the inhibitor, and the loss of the substance from the cells is 

 determined. The other type of experiment involves depleting the cells of 

 the substance by incubation in abnormal media or storing at low tempera- 

 ture, and then measuring the reaccumulation of the substance when the 

 cells are restored to normal conditions. From what has been said above 

 it is clear that the results may be somewhat different in each case. In the 

 first type of experiment, the rate of loss will depend not only on how much 

 the pump is inhibited but perhaps even more on the permeability of the 

 membrane to the substance; it is obvious that if the permeability is zero, 

 there will be no loss even though the pump is completely stopped. In the 

 second type of experiment, the inhibition of the accumulation may depend 

 on the permeability, but to a lesser degree, and some inhibition will always 

 be seen. The second method is more sensitive and useful than the former, 

 but is often less ]:)hysiological in that when an inhibitor is applied to nor- 

 mal tissue it is presumably initially in an equilibrium state. 



Another factor upon which the response of the equilibrium state depends 

 is the degree of cellular activity, particularly when one is concerned with 

 ions. For example, a resting nerve with high internal K+ and low internal 

 Na+, and with relatively low permeabilities for these ions, may retain the 

 normal ionic distribution and membrane potential for long periods even 

 though the active transport is blocked. However, if the nerve is stimulated 

 and suffers a series of depolarizations, each resulting in a small loss of K+ 

 and gain of Na+, the concentration gradients will be reduced and the po- 

 tential will fall. This is one important basis for the relationship between 

 degree of inhibition and functional activity of the tissue. 



A final factor is the rate at which the active transport system is operating. 

 In a resting cell at equilibrium the pumps may be working at a low level, 

 whereas in states deviating from equilibrium the pumps will be operating 

 more actively. It is quite possible that one type of exergonic metabolism 

 may be sufficient for the resting state, but that other metabolic energy 

 sources must be tapped when the pumps are very active. Thus the response 

 to an inhibitor may depend on the degree of deviation from an equilibrium 

 condition, not only because of different rates of exergonic metabolism but 

 because of possibly different pathways involved. 



