COMPARATIVE STUDY OF MEMBRANE PERMEABILITY 627 



intestine and Xa selective in the colon and frog skin. The inner membrane 

 is Xa and K selective in the colon and small intestine while it is K selective 

 in the frog skin. The active transport mechanism for Xa is located at both 

 outer and inner faces in the small intestine while it is located at the inner 

 membrane in the colon and the frog skin. 



Koefoed-Johnsen and Ussing [4] have demonstrated that in frog skin, 

 the active transport of Xa is in fact a forced exchange for K. This has not 

 yet been demonstrated in the intestinal epithelium. Such a picture 

 describes satisfactorily how the potential difference develops under a 

 wide varietv of conditions. Let us first consider the small intestine cell in 

 the presence of a non-penetrating anion (Fig. i, A). Xa diffuses into the 



Mucosal 



or 

 outside 



Serosal 



or 



inside 



OK 



K(7) Small 



intestine 



Colon 



Skin 



Fig. I. Scheniatic representation of the permeability characters of epithelial 

 cells from 



A = turtle small intestine 

 B = turtle colon 

 R = frog skin 



The oblique arrows indicate passive diffusion. AT is the mechanism of active 

 transport with a one-to-one exchange for K and Xa in frog skin ; this type of 

 exchange has not yet been demonstrated in intestinal cells. The level of the 

 chemical symbols designate the concentration levels of the cations. Explanations 

 in text. 



cell from the outside border and gives rise to a diffusion potential which 

 makes the cell positive in relation to the outside. K diffuses out in the 

 opposite direction. The magnitude of the potential difference across the 

 outer border is given bv the Goldman equation [21] : 





(I) 



