628 E. SCHOFFENIELS 



where Em — Ec is the potential difference between the mucosal solution and 

 the intracellular fluid, P is the coefficients of relative permeability for Na 

 and K of the outer (w) membrane. The other symbols have their usual 

 meaning. 



It is evident from this equation that P'^ and P'^.^ being equal as well as 

 the ratio of concentration for Na and K, there will be no potential difference 

 across the cell membrane. The potential at the inner border is : 



The total potential difference across the epithelium is then 



E = {Em-Ec) + {Ec-Es) 



E will depend not only on the relative magnitude of the concentration 

 ratios but also on the relative values of the P's. This is well demonstrated 

 in Table III. Equations (i) and (2) show also why the results of experi- 

 ments J and K (Table III) are not in contradiction with the conclusion 

 that both mucosal and serosal faces of the small intestine are permeable to 

 K. It is indeed evident that a decrease in the extracellular K concentration 

 will lead to a smaller overall change in potential than an increase. 



The situation is more complex if we consider the behaviour of CI. 

 Since in most cells, the CI distribution is generally thought to be entirely 

 passive, we would have to assume that in the specific case of the intestinal 

 cell, the cellular CI concentration is equal to that in the extracellular fluid. 

 But low intracellular CI concentration could nevertheless be found if the 

 cell possesses a mechanism of active extrusion for this anion located at the 

 inner border. This is certainly the case since we have found (Table II) 

 that the short circuit current is smaller than the net flux of Na. More 

 direct evidence may be found in the results of Durbin et al. [5], showing 

 that in the rat small intestine CI is actively transported from the mucosal 

 to the serosal side (see also [6]). This question will be settled as soon as we 

 have not only measured the flux of CI with tracers, but also determined 

 with micro-electrodes the exact magnitude of (Etn — Ec) and {Ec — Es). 



As far as the colon is concerned, the total potential difference may also 

 be related to the sum of two potential differences arising at the outer and 

 inner borders of the cell. Since the spontaneous potential difference 

 increases if SO4 replaces CI, we have to introduce, in the Goldman 

 equation [22], the concentration ratio for CI. Thus 



E = {Em-Ec) + {Ec-Es) 

 and 



^^ P^A^^\n + P'MC\) Pk(K). + Pk(Na). + P^i(Ca 

 zF ''P?^a(Na), + PS(Cl). Pk{Kl + PUA^^l + Pa{C\\- ^^^ 

 Equation (3) is in agreement with the results given in Table III. 



