C. ADRLAN M. HOGBEN 1 85 



nulrienl or serosal surface to the secretory or mucosal surface and the secretory 

 to nutrient (S -^ N) flux is the unidirectional movement in the opposite direc- 

 tion. The nutrient surface is the serosal surface and the nutrient solution is in 

 contact with the surface of the epithelium which is ordinarily exchanging with 

 plasma, while the secretory surface is the mucosal surface and the secretory 

 solution corresponds to the gastric lumen receiving gastric secretion. In this 

 particular series of experiments, Table i, the data are averages obtained from 

 periods of alternate exposure to 100% O2 and 5 % CO2, 95 % O2. This extraneous 

 feature modifies the rate of H+ secretion at the expense of the mucosal current 

 (46) without changing the chloride fluxes and is not germane to the present 

 inquiry. The species of frog and details of experimental technique differ from a 

 previous study which yielded comparable results (44, 48). 



There is an evident chloride flux asymmetry of appreciable magnitude. Before 

 relating this to the short-circuit current cognizance must be taken of spon- 

 taneous H"^ secretion with its positive charge transfer in the same direction as 

 the negative charge transfer of net chloride (ACl) transport. As the algebraic 

 sum of net ion transfer must equal the short circuit current a provisional equa- 

 tion for the short circuit current (I) could be written: 



AC1~ = H"*" + I (all in the same units) 



The equation is satisfied by experiment. Any other active transport systems 

 either are quantitatively insignificant or there are additional equal but opposite 

 transport systems. There are not many ions likely to contribute significantly 

 to the mucosal current. Sodium and potassium have already been excluded. 

 Endogenous lactate preferentially accumulates in the nutrient solution but 

 under conditions used to study ion transport, it is quantitatively insignificant 

 (47). Thus we may conclude that the source of the distinctive potential main- 

 tained across the gastric mucosa is a current of actively transported chloride 

 ions. This current is discharged through the internal shunt of the epithelium, 

 the conductance of all passive ions, to become manifest as the distinctive gastric 

 mucosal potential. Only a portion of the actively transported chloride ions con- 

 stitute a current of net charge transfer, the remainder match actively trans- 

 ported H"*" ions. 



The other striking feature of the chloride fluxes is an unexpectedly high secre- 

 tory to nutrient flux, counter to active transport. Comparison of this counter 

 flux with the membrane conductance allows analysis of the manner in which 

 chloride moves in this direction. It can be shown that if the back-flux were 

 solely a matter of passive diffusion, this flux would be less than that to be ex- 

 pected from the partial conductivity of the ion and still less than the membrane 

 conductance (48). In point of fact, the back-flux is more than twice as large 

 as the total membrane conductance. Therefore, part of the chloride moving 

 from the secretory to nutrient solution is non-conducting, indicating that it is 



