Na+/H+ exchange mechanism in the basolateral membrane of the rat enterocyte

Interaction between Na ions and the Cl/HCO3 exchanger in basolateral membranes from rat jejunum

The jejunal basolateral Cl/HCO3 exchanger is modulated by two Na-dependent regulatory sites located on the inner and outer membrane surfaces. The aim of this work was to focus on the interaction between the anion exchanger and intracellular or extracellular sodium. Uptake studies, performed using basolateral membrane vesicles, provided kinetic parameters as a function of outside or inside Na concentration. The intracellular Na-sensitive modifier site seems to be primarily involved in the modulation of the Cl/HCO3 exchanger.

Bicarbonate and chloride transport across rat ileal basolateral membrane

The mechanisms of HCO3- and Cl- transport across basolateral membranes from rat ileum were investigated in isolated vesicles by means of uptake experiments. Neither Cl-/HCO3- exchanger nor Na(+)-(HCO3-)n cotransport seem to be present in ileal basolateral membranes. Moreover Cl- uptake is unaffected by cis Na+ and/or K+ gradients, indicating the absence of Na(+)-Cl-, K(+)-Cl- and Na(+)-K(+)-2Cl- symport activity. An electrically conductive pathway seems to be responsible for both HCO3- and Cl- fluxes. Evidence is also given for the presence of a Na+/H+ exchanger at the basolateral pole of ileal enterocytes.

Cl/HCO3 exchange in the basolateral membrane domain of rat jejunal enterocyte

Basolateral membrane vesicles isolated from rat jejunal enterocyte and well purified from brush border contamination were tested to examine Cl and HCO3 movements. Uptake experiments provided no evidence for a coupling between Na and HCO3 fluxes; K-HCO3 and K-Cl cotransports also could be excluded. Transport studies revealed the presence of a Cl/HCO3 exchanger accepting other anions and inhibitable by the disulfonic stilbenes SITS and DIDS. We can exclude that the evidenced HCO3-dependent Cl uptake is due to brush border contamination, since in jejunal brush border membranes this mechanism, if present, has a very low transport rate. Besides the Cl/HCO3 antiporter, a Cl-conductive pathway seems to exist in jejunal basolateral membranes.

Na(+)-H+ exchange activity and cellular pH regulation in enterocytes isolated from chick small intestine

Intracellular pH (pHi) and Na+/H+ exchange activity have been examined in isolated chicken enterocytes using pH sensitive fluorescence dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF), and in a nominally bicarbonate-free buffer. Under resting conditions the pHi (7.18) was higher than that observed in the presence of the proton ionophore FCCP (6.98), indicating that [H+] is below the value predicted for electrochemical equilibrium across the plasma membrane, i.e., pHi is regulated. Removal of extracellular Na+ lowered pHi by 0.28 units and subsequent addition of 80 mM Na+ rapidly increased pHi towards the control value. The acidification induced by Na(+)-removal was prevented by 1 mM amiloride. After an intracellular acidification by exposure to 30 mM NH4Cl during 5 min, the pHi decreased from approx. 7.18 to approx. 6.86. Subsequent alkalinization of cells back to control pHi was observed after addition of Na+ or Li+ but not TEA+. Na(+)-dependent recovery of pHi after an acid-load was unaffected by valinomycin, and was 82% reduced by 1 mM amiloride. The inhibitory action of amiloride was abolished by 10 microM monensin. The initial rate of pHi recovery from an acid-load following exposure to Na+ exhibited simple saturation kinetics, with an apparent Km of 12.5 mM Na+ and maximum velocity of alkalinization of approx 0.2 pH units.min-1. The rate of pHi recovery was inversely proportional to pHi. The 'set point' for the exchanger is approx. 7.35. It is concluded that in chicken enterocytes the Na+/H+ exchange system is not quiescent at resting pHi and, thus, contributes to the maintenance of a steady-state pHi at neutral or slightly alkaline levels.

Characterization of basolateral membrane Na/H antiport in rat jejunum

Na uptake studies were performed in order to examine the activity of a Na/H exchanger in basolateral membrane vesicles isolated from rat jejunum. Experiments were carried out under voltage-clamped conditions in order to avoid electrodiffusional ionic movements. 1 mM Na uptake was found to be enhanced by an outward proton gradient and its initial rate was further increased by the presence of monensin or nigericin. The pH gradient-driven Na uptake was inhibited by 2 mM amiloride and unaffected by 0.1 mM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. The initial rate of the proton gradient-induced Na uptake was saturable with respect to external Na, with a Km of 13.6 +/- 1.4 mM and a Vmax of 35.4 +/- 2.2 nmol/mg protein per min. Li competed with Na for the exchange process, whereas K, Rb, Cs, tetramethylammonium had no effect. We conclude that rat jejunal basolateral membrane contains a Na/H exchanger whose properties are similar to those of the antiporter identified in the brush-border membrane.