Coupled NaCl transport: cotransport or parallel ion exchange?

In recent years it has become apparent that at least part of the transcellular movements of Na and Cl are linked by a process or processes which couple the entry of Na to Cl across the apical cell membrane of the intestinal cell. In some tissues, eg, gallbladder and renal tubule, this coupled transport of Na and Cl may be the predominant electrolyte-transporting mechanism. Studies in rabbit ileal brush-border membrane vesicles present evidence for a coupled NaCl process that has the same ionic specificities and similar kinetics as the processes demonstrated by influx techniques across the apical membrane of the intact epithelium. However, the vesicles also exhibit Na:H and Cl:HCO3 exchange processes and the inhibitors thought to be specific for either the NaCl cotransport system (loop diuretics), for the Na:H exchange (high-dose amiloride), or for the Cl:HCO3 exchange (disulfonic stilbenes such as SITS or DIDS), do not prove to be so. It is possible that all three processes could be present in intestinal brush-border membranes. Furthermore, the techniques of isolating vesicles could uncouple or otherwise inhibit the coupled NaCl process. Nonetheless, the preponderance of evidence at this time indicates that parallel ion exchangers of Na:H and Cl:HCO3, as initially suggested by Turnberg et al [4], account for coupled NaCl transport in rabbit ileum. Additional studies will be necessary to determine if this is the only mechanism in the intestinal apical membrane and whether this conclusion applies to other tissues such as gallbladder and renal tubule.