PH-DEPENDENCE OF CL HCO3 EXCHANGER IN THE RAT JEJUNAL ENTEROCYTE/

During bicarbonate absorption in rat jejunum, a Cl/HCO3 exchanger medi ates bicarbonate extrusion across the basolateral membrane of the ente rocyte. Previous studies demonstrated that anion antiport exhibits a p articular behaviour: its activity is positively affected by the presen ce of sodium, but the cation is not translocated by the carrier protei n. In view of the particular features of the jejunal Cl/HCO3 antiporte r, first we performed a pharmacological characterisation of the transp ort protein using various Cl channels blockers. Then, since it is well known that anion exchangers play a substantial role in cell pH regula tion, we investigated the possible involvement of jejunal basolateral Cl/HCO3 antiporter in intracellular pH maintenance. The sensitivity of the exchanger to pH was investigated by measuring Cl-36 uptake into b asolateral membrane vesicles either varying simultaneously intra- and extravesicular pH, or presetting at 7.4 external pH and varying only t he internal one. Experiments were performed both in the absence and in the presence of Na. In all the tested conditions, uptake peaked at pH of about 7.3-7.4 and then decreased, suggesting that the main functio n of Cl/HCO3 exchanger is related to HCO3 absorption rather than to in tracellular pH control. Since pH-regulating mechanisms counteracting a cidification are well known in the jejunal enterocyte, we investigated how it regulates pH after alkalinisation of the cytosol. We tested bo th basolateral and brush border membrane vesicles for the presence of a WH exchanger, but we could not give evidence for its presence by mea ns of Rb-86 uptake experiments. In conclusion, the jejunal enterocyte seems to lack a mechanism counteracting cellular alkalinisation: the m ain purpose of pH homeostasis might be to hinder acidification of the cytosol due to influx of protons and production of acid by the metabol ism. (C) 1998 Elsevier Science B.V. All rights reserved.