Self‐inhibition of chloride transport in ehrlich ascites tumor cells

Previous studies have shown that mediated Cl − transport which occurs by at least two processes (Cl − ‐dependent cation cotransport and Cl − self‐exchange) becomes progressively inhibited when extracellular Cl − exceeds about 60 mM (Hoffmann et al., 1979). To account for this type of kinetic behavior, that is, self‐inhibition, an anion transport system possessing two sites, a high affinity transport site and a lower affinity modifier site is suggested (Dalmark, 1976). In the present experiments we have attempted to determine which of the mediated transport pathways is susceptible to self‐inhibition by studying the dependence of the steady state Cl − flux on the extracellular Cl − concentration and how DIDS, an inhibitor of Cl − self‐exchange, and H + affect this relationship. Addition of DIDS to Ehrlich cells results in inhibition of Cl − transport at every Cl − concentration tested (40–150 mM). Moreover, the Cl − flux/Cl − concentration relationship no longer exhibits self‐inhibition, suggesting that this phenomenon is a characteristic of the Cl − self‐exchanger rather than of the Cl − ‐dependent cation cotransport system. Lowering the extracellular pH (pHo) from 7.35 to 5.30 stimulates Cl − transport by a process that saturates with respect to [H + ]. Half‐maximal stimulation occurs at pHo 6.34. A comparison of the kinetic parameters, K s and J max , calculated from the ascending limb of the Cl − flux/Cl − concentration curve at pHo 7.30 to those at pHo 5.50 show that the values for K s are almost identical (23.6 mM and 21.3 mM, respectively), while the values for J max [(22.2 mEq/Kg dry wt). min] differ by only 15%. This finding along with the observation that DIDS completely blocks H + stimulation of Cl − transport is compatible with the suggestion that H + interact at the modifer site of the Cl − self‐exchanger and thereby prevents self‐inhibition.