Coupled sodium-chloride transport by rabbit ileal brush-border membrane vesicles

Uptake of Na and Cl into brush-border membrane vesicles isolated from rabbit ileal epithelial cells was investigated with a rapid filtration technique using ²²Na and ³⁶Cl as tracers. The rank order of anion dependence for Na uptake in the absence of anion gradients was Cl ≃ SNC > NO₃ > gluconate. The sequence of cation specificity for Cl uptake was Na ≃ Li > K > choline. The transport of Na and Cl were both inhibited by harmaline, furosemide, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid, and 4,4'-diisothiocyano-2,2'-stilbene disulfonic acid. Carrier mediation of Cl-stimulated Na transport was suggested by the competition for ²²Na uptake with increasing concentrations of unlabeled Na in the presence of Cl but not when gluconate was the counterion. Chloride-dependent Na uptake had an apparent K(m) of 4.5 mM and a V(max) of 20 mmol.mg prot^-1.15 s^-1. Na-H exchange and Cl-OH (or HCO₃) exchange were also demonstrated in these vesicles. These findings confirm the presence of an electrically neutral carrier-mediated, Na-Cl-coupled transport process in the apical cell membrane of rabbit ileal epithelial cells. The nature of the coupling of Na to Cl transport, i.e., NaCl symport or a process that combines Na-H antiport with Cl-OH (or HCO₃) antiport, remains to be determined.