Basolateral Cl/HCO3 exchange in rat jejunum: the effect of sodium

Basolateral membrane vesicles isolated from rat jejunum were used to characterize a Cl/HCO3 exchange mechanism previously evidenced. Cl uptake experiments provided no evidence for Cl/OH countertransport, confirming anyhow the presence of Cl/HCO3 antiport, which was inhibited by 2 mM furosemide and unaffected by 2 mM amiloride. An outwardly directed Na gradient stimulated Cl uptake and this effect was increased if Na was present at both vesicle surfaces. To investigate the mechanism of coupling between Na and the transport protein, we performed Na uptake experiments. Na uptake was unaffected by cis-bicarbonate and trans-Cl gradients; the reversal of anion gradients was still ineffective. Similar results were obtained when a pH difference across the membrane vesicles was imposed. This study seems to suggest that Na is not transported by the Cl/HCO3 exchanger and that another mode of Na dependence must be taken into account.