CHAPTER III 



Membrane Permeability and Drug Action 



Introduction 



There have been many academic studies of the perme- 

 ability of natural membranes, but very few direct 

 studies on permeability to drugs. Consequently, most of 

 what can be said on this topic is based on permeability 

 to molecules which are not usually regarded as drugs. In 

 approaching this field we must distinguish between dif- 

 fusion and secretion. Diffusion is the movement of mol- 

 ecules produced by thermal agitation; it now has a 

 quantitative theory. Secretion is a process involving the 

 expenditure of energy by a living organism to move 

 molecules from one place to another: there" are no quan- 

 titative theories of secretion. 



The importance of permeability studies may be seen 

 from consideration of the sites at which a drug may act. 

 Even when some process such as absorption from the in- 

 testine is not involved, a drug always has to penetrate a 

 cell membrane unless its action is on the external sur- 

 face of a cell. In general, a drug which has penetrated 

 a cell membrane may combine with a receptor group and 

 also may be detoxicated more or less rapidly. For effi- 



