COMPARATIVE STUDY OF MEMBRANE PERMEABILITY 623 



(h) while the potential difference is around 30 mV across the gastric 

 mucosa, the caecum and the colon, it is only a few m\' across the 

 epithelium of the small intestine. 



These observations raise two important points : what is the origin of the 

 potential difference recorded and why is the potential difference across the 

 small intestine so low ? We have therefore measured the fluxes of Na in the 

 small intestine and in the colon of Testudo hermanni G. F. Gmelin, using 

 the double labelling and the short-circuit current techniques [3]. We have 

 chosen the turtle because in this species it is quite easy to strip the muscle 

 layers from the epithelium [2]. 



TABLE II 



Influx and Outflux of Na across the Isolated Epithelium of the Small 

 Intestine and the Colon in the Turtle Testudo hermanni G. F. Gmelin 



The epithelium is bathed with physiological saline on both sides. Results 

 obtained when 2-4,dinitrophenol (DXP) is applied are also given. The fluxes, the 

 net flux and the short-circuit current are expressed in mcoul. cm"-.H^^. C = con- 

 trol. DNP concentration: o-i m^L Experimental periods: i hr. 



Table II gives the results obtained. In this Table influx means flux 

 from mucosal to serosal side while outflux means the flux in the opposite 

 direction. 



It is clear from the results given that there is an active transport of Na 

 from the mucosal to the serosal side in both small intestine and colon. The 

 values of the short-circuit current are always smaller than the correspond- 

 ing values of net flux. This means that a cation must be transported from 

 the serosal to the mucosal side or that an anion is transported in the 

 opposite direction. The flux values are higher in the small intestine than 

 in the colon. Since the DNP inhibits the influx in the colon while it 

 inhibits, at least partly, both influx and outflux in the small intestine, this 

 could mean that part of the outflux, in the latter case, is due to active 

 transport. 



