Sodium Chloride Transport by Rabbit

The results of the present study indicate that NaC1 transport by in vitro rabbit gallbladder must be a consequence of a neutral coupled carriermediated mechanism that ultimately results in the active absorption of both ions; pure electrical coupling between the movements of Na and C1 can be excluded on the grounds of electrophysiologic considerations. Studies on the unidirectional influxes of Na and CI have localized the site of this coupled mechanism to the mucosal membranes. Studies on the intracellular ion concentrations and the intracellular electrical potential are consistent with the notion that (a) the coupled NaCI influx process results in the movement of C1 from the mucosal solution into the cell against an apparent electrochemical potential difference; (b) the energy for the uphill movement of CI is derived from the Na gradient across the mucosal membrane which is maintained by an active Na extrusion mechanism located at the basolateral membranes; and (¢) C1 exit from the cell across the basolateral membranes is directed down an electrochemical potential gradient and may be diffusional. Finally, as for the case of rabbit ileum, the coupled NaC1 influx process is inhibited by elevated intracellular levels of cyclic 3~,5r-adenosine monophosphate. A working model for transcellular and paracellular NaCI transport by in vitro rabbit gallbladder is proposed. I N T R O D U C T I O N The results of a number of in vitro studies have suggested that a neutral, coupled carrier mechanism is responsible for Na and C1 absorption by fish (I) and rabbit gallbladder (2-4). Support for this notion rests mainly on the observations that absorption of Na and C1 is not associated with a significant spontaneous transepithelial electrical potential difference (PD) when the mucosal and serosal solutions have identical compositions. In addition, replacement of either Na or C1 with nontransported ions abolishes net ion and fluid absorption without affecting the transepithelial PD. Thus, coupling THE JOURNAL OF GENERAL PHYSIOLOGY • VOLUME 65, 1975 • pages 769-795 769 on A ril 9, 2017 D ow nladed fom Published June 1, 1975