626 E. SCHOFFENIELS 



(10) In sulphate saline, a decrease in Na concentration in the serosal 

 fluid increases the potential difference in the colon and in the small 

 intestine. An increase in K concentration in the mucosal solution increases 

 the potential difference across the small intestine while it does not modify 

 the potential difference in the colon (expt. L). 



(11) In sulphate saline a decrease in Na concentration in the musocal 

 solution produces an inversion of the potential difference in the small 

 intestine, while it decreases the potential difference in the colon. An 

 increase in K concentration decreases then the potential difference in both 

 tissues (expt. M). 



The above observations suggest the following conclusions. Since the 

 replacement of CI by SO4 in the solutions bathing both sides of the colon 

 produces an increase in potential difference, it is clear that this epithelium 

 is relatively impermeable* to SO4. The epithelium of the small intestine is 

 equally permeable (or impermeable) to CI and SO4 since the replacement 

 of CI by SO4 has no effect on the potential difference. This result could also 

 be explained if one postulates that the epithelium, although impermeable 

 to SO4, is permeable to K and Na. This possibility is most likely to be 

 correct since the other results demonstrate that the epithelium is permeable 

 to Na and K. Moreover, using ^'S as tracer, we have been able to show 

 that the permeability coefficient for SO4 in frog skin, turtle colon and 

 intestine are of the same order of magnitude (unpublished results). Never- 

 theless the fact that the replacement of CI by SO4 does not much affect the 

 magnitude of the potential difference when the Na concentration is 

 modified, seems to suggest that the passive permeability of the small 

 intestine to CI is low. 



The small spontaneous potential difference existing across the small 

 intestine is related to the concentrations of Na and K in the mucosal and 

 serosal solutions : it is thus clear that both mucosal and serosal sides of the 

 epithelium are permeable to these ions. On the other hand, in the colon the 

 mucosal side is permeable to Na, but impermeable to K, while the serosal 

 side is permeable to both ions. The conclusions may be summarized in the 

 following scheme (Fig. i). 



This is a schematic representation for the permeability characters of 

 the cells forming the turtle intestine epithelium (Fig. i, A and B). The 

 permeability characters of the frog skin are also given, for comparison 

 (Fig. I, C). The outer membrane is in contact with the mucosal or outside 

 solution while the inner membrane is in contact with the serosal or inside 

 solution. 



The outward facing membrane is Na and K selective in the small 



* Impermeability must be considered in terms of relativity or in statistical 

 terms of probability. Given a highly sensitive method, any substance can be shown 

 to cross a membrane. 



