6o MEMBRANE PERMEABILITY AND DRUG ACTION 



through the pores is much more important in the case 

 of the anions, and the anions to a first approximation 

 fall into group two. 



In the fifth group we collect together all those instan- 

 ces where substances, or groups of substances, do not 

 conform to groups one to four or with the transition 

 stages between the different groups. These are the in- 

 stances in which secretory activity plays a part in deter- 

 mining the passage of the molecules across a membrane. 

 In such cases, directly or indirectly, the cells concerned 

 are expending energy to promote the passage of mole- 

 cules across a membrane. Usually the permeability of 

 cells in such cases is a non-linear function of the con- 

 centration of the molecule concerned. As the concen- 

 tration of the molecules which are secreted increases in 

 the cell environment, the secretory mechanism tends 

 to become saturated, and the rate of secretion increases 

 much more slowly than does the concentration in the 

 environment. The existence of secretion is also often 

 revealed by a degree of anoxia of the tissues concerned, 

 or by the moderate use of enzyme poisons which inter- 

 fere with the metabolic processes which provide the 

 energy for secretion, or even in some cases possibly with 

 the mechanism of secretion itself. At present our knowl- 

 edge of secretion is much too slight for us to be able to 

 predict with any confidence what the quantitative effect 

 will be of a change in molecular structure upon the rate 

 of secretion. 



