Cl-/base exchange and cellular pH regulation in enterocytes isolated from chick small intestine

Bicarbonate exporting transporters in the ovine ruminal epithelium

In order to stabilize the intraruminal pH, bicarbonate secretion by the ruminal epithelium seems to be an important prerequisite. The present study therefore focussed on the characterization of bicarbonate exporting systems in ruminal epithelial cells. Intracellular pH (pH(i)) was measured spectrofluorometrically in primary cultured ruminal epithelial cells loaded with the pH-sensitive fluorescent dye, 2,7-bis(carboxyethyl)-5(6')-carboxyfluorescein acetomethyl ester. Switching from CO2/HCO3- -buffered to HEPES-buffered solution caused a rapid intracellular alkalinization followed by a counter-regulation towards initial pH(i). The recovery of pH(i) was dependent upon extracellular chloride, but independent of extracellular sodium. Adding 500 microM H2DIDS significantly reduced the increase of pH(i). For further characterization of the bicarbonate exporting systems, we tested the ability to reverse the direction from HCO3- export to import in the absence of sodium and chloride. Under sodium and chloride-free conditions, counter-regulation after CO2-induced pH(i) decrease did not differ from pH(i) recovery in the presence of sodium and chloride. Existence of bicarbonate exporting systems in cultured ruminal epithelial cells and intact ruminal epithelium was verified by reverse transcription polymerase chain reaction (RT-PCR). Using RT-PCR and subsequent sequencing, expression of mRNA encoding for AE2, DRA and PAT1 could be found. Bicarbonate exporting systems could therefore be detected both on the functional and structural level.

HCO3(-)-dependent ion transport systems and intracellular pH regulation in colonocytes from the chick

The current study examines the presence of the Na+/HCO3- cotransporter and of the Cl-/HCO3- exchanger in chicken colonocytes and their role in cytosolic pH (pHi) homeostasis. pHi was measured with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) at 25 degreesC. Basal pHi was 7.16 in HEPES-buffered solutions and 7.06 in those buffered with HCO3-. Removal of external Cl- increased pHi and Cl- reinstatement brought the pHi towards resting values. These Cl--induced pHi changes were Na+-independent, inhibited by H2-DIDS and faster in the presence than in the absence of HCO3-. Cells recovered from alkaline loads by a mechanism that was Cl--dependent, Na+-independent and inhibited by H2-DIDS. This rate of Cl--dependent cell acidification decreased as the pHi decreased, with a Hill coefficient value close to 4. Removal of external Na+ decreased pHi and readdition of Na+ brought pHi towards the control values. The rate of the Na+-induced changes was not modified by the presence of HCO3- and was prevented by EIPA and unaffected by H2-DIDS. In the presence of EIPA cells partially recovered from a moderate acid load only when both Na+ and HCO3- were present. The EIPA resistant Na+- and bicarbonate-dependent pHi recovery was inhibited by H2-DIDS and occurred at equal rates in both Cl--containing and Cl--free solutions. It is concluded that in chicken colonocytes bathed in HCO3--buffered solutions, both the Na+/H+ exchanger and the Cl-/HCO3- exchanger participate in setting the resting pHi value. The latter transporter helps the cells to recover from alkaline loads and the first transporter, together with the Na+/HCO3- cotransporter, is involved in pHi recovery from an acid load.

Na+-H+ exchange and intracellular pH regulation in colonocytes from the chick

The involvement of Na(+)-H+ exchange in chicken colonocyte homeostasis was investigated. Colonocyte pH (pH(i)) was measured with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF). The proton ionophore FCCP reduced basal pH(i), indicating that cytosolic [H+] is not at electrochemical equilibrium across the membrane. External Na+ removal decreased pH(i) and subsequent addition of Na+ returns pH(i) towards its control value. The rate of pH(i) recovery from an acid load was Na(+)-dependent (K(m) for Na+, 24 mM) and inhibited by EIPA (IC50, 0.18 microM). The initial rate of Na(+)-dependent cell alkalization increased as the pH(i) decreased from 7.2 to 6.6 (Hill coefficient, 1.88). Radioisotope flux studies revealed that an outwardly directed proton gradient transiently stimulated Na+ uptake into BBMV isolated from the chick colon. EIPA and amiloride inhibited pH gradient-driven Na+ uptake (IC50 of 4 microM and 32 microM, respectively). The K(m) for Na+ of pH gradient-driven Na+ uptake was 6.8 mM. The Hill coefficient of the relationship between the initial rate of pH-driven Na+ uptake and the intravesicular pH was 0.70. It is concluded that a Na(+)-H+ exchanger is involved in pH(i) homeostasis in chicken colonocytes and that these cells possess at least two types of Na(+)-H+ antiporters with different sensitivity to EIPA and different kinetic parameters.

PKC activators stimulate H+ conductance in chicken enterocytes

Chicken enterocytes present a H+-conducting pathway involved in the recovery of intracellular pH (pHi) from an acid load. In the current study we have tested the effect of protein kinase C (PKC) activators on the rate of proton efflux through the H+-conducting pathway. The rate of proton efflux was increased by the addition of 1,2-dioctanoyl-rac-glycerol (DOG) or phorbol 12-myristate 13-acetate (PMA), but it was not affected by the addition of the inactive phorbol ester analogue, 4alpha-phorbol 12, 13-didecanoate. DOG stimulated the process in a dose-dependent manner with a half-maximal effect at 45 microM. Staurosporine and Zn2+ prevented the DOG-dependent increase in the rate of proton efflux. The rate of proton efflux was affected by the pH, and DOG shifted this relationship upward and to the right. These results suggest that the proton-conducting pathway is regulated by PKC.

Na+-HCO3(-) cotransporter and intracellular pH regulation in chicken enterocytes

The current studies examine the presence of the Na+-HCO3(-) cotransporter in chicken enterocytes and its role in cytosolic pH (pHi) regulation. The pH-sensitive dye 2',7'-bis(carboxyethyl)-5,6-carboxy-fluorescein (BCECF) was used to monitor pHi. Under resting conditions, pHi was 7.25 in solutions buffered with bis(2-hydroxyethyl)-1-piperazine ethanesulphonic acid (HEPES) and 7.17 in those buffered with HCO3(-). Removal of external Na+ decreased pHi and readdition of Na+ rapidly increased pHi towards the control values. These Na+-dependent changes were greater in HCO3(-)- than in HEPES-buffered solutions. In HCO3- - free solutions the Na+-dependent changes in pHi were prevented by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) and unaffected by 4,4'-diisothiocyanatostilbene disulphonic acid (H2-DIDS). In the presence of HCO3-, the Na+-induced changes in pHi were sensitive to both EIPA and H2-DIDS. In the presence of EIPA, cells partially recovered from a moderate acid load only when both Na+ and HCO3- were present. This pHi recovery, which was EIPA resistant, and dependent on Na+ and HCO3-, was inhibited by H2-DIDS and occurred at equal rates in both Cl--containing and Cl--free solutions. Kinetic analysis of the rate of HCO3- and Na+-dependent pHi recovery from an acid load as a function of the Na+ concentration revealed first-order kinetics with a Michaelis constant, Km, of 11 mmol/l Na+. It is concluded that in HCO3(-) buffered solutions both the Na+/H+ exchanger and the Na+-HCO3(-) cotransporter participate in setting the resting pHi in isolated chicken enterocytes and help the recovery from acid loads.

Proton conductance and intracellular pH recovery from an acid load in chicken enterocytes

1. Chicken enterocytes present a Na(+)-independent proton transport mechanism involved in pHi recovery from an acid load. In the current study the nature of this proton transport system is investigated. 2. The pHi of acid-loaded cells increased when transferred to Na(+)-free, pH 7.4 buffers, both at 6 and 65 mM extracellular potassium concentration. Addition of nigericin accelerated the rate of cell alkalinization. 3. When acid-loaded cells were transferred to a Na(+)-free, pH 6.5 buffer, the cells acidified further, regardless of the extracellular potassium concentration. The addition of nigericin increased the rate of acidification at 6 mM K+ but produced an alkalinization at 65 mM K+. 4. The rate of the Na(+)-independent regulatory cell alkalinization was inhibited by SCH 28080, DCCD, NBD-Cl, rotenone or Zn2+. Addition of valinomycin reversed the inhibition induced by SCH 28080, DCCD and NBD-Cl but not that induced by Zn2+ or rotenone. Zn2+ inhibition was abolished by the metal chelator DTPA. 5. Cytosolic acidification increased the rate of Na(+)-independent regulatory cell alkalinization. 6. The results suggest that the Na(+)-independent proton transport system is a Zn(2+)-sensitive proton-conducting pathway which is regulated by the cytosolic proton concentration.

Cytosolic pH regulation in chicken enterocytes: Na(+)-independent regulatory cell alkalinization

The mechanisms involved in intracellular pH (pHi) recovery from an acid load have been investigated in enterocytes isolated from chicken. Following an intracellular acidification, by abrupt withdrawal of NH4Cl, pHi alkalinized in the nominally absence of Na+ and bicarbonate. This Na(+)- and bicarbonate-independent (NBI) regulatory cell alkalinization became negligible when the pHi has reached a value of approx. 6.85. Addition of Na+ induced a rapid pHi recovery to control values. Rotenone, DCCD, vanadate, NBD-Cl, SCH 28080 and EIPA inhibited the NBI cell alkalinization, whereas bafilomycin A1, ouabain and H2-DIDS were without effect. Na(+)-dependent pHi recovery from an acid load was inhibited by EIPA and unaffected by SCH 28080 or DCCD. The rate of NBI cell alkalinization was a linear function of the electrochemical proton gradient. In high external K+ buffer plus valinomycin the line goes through the origin. Gramicidin accelerated the rate of NBI cell alkalinization, whereas it was slightly reduced by low external potassium. The results demonstrate that in intestinal epithelial cells exist at least two mechanisms for proton secretion: a Na(+)-H+ exchanger and a Na(+)- and bicarbonate-independent proton transport system. This latter mechanism appears to be a proton conductance pathway.

Kinetics of the chloride-anion exchanger of brush-border membrane vesicles isolated from chicken jejunum

The kinetic parameters of the apical Cl(-)-anion exchanger of chicken jejunum have been evaluated using brush-border membrane vesicles. SITS inhibited pH gradient-driven Cl- uptake in a dose-dependent manner with a IC50 of 700 microM. In the presence of a pH gradient (pH 7.7 inside, 5.5 outside), Cl- uptake tends to saturate with increasing external Cl- concentration. With 5 mM SITS the relationship between 36Cl- uptake and external Cl- concentration was linear. SITS-insensitive Cl- uptake may represent a diffusion component and has an apparent diffusion constant, Kd, of 0.068 +/- 0.002 nmol/mg protein per 15 s per mM. Eadie-Hofstee analysis of the data obtained by subtracting the SITS-insensitive component from the total uptake indicates a single transport system with a Km of 5 +/- 0.9 mM. A 50 mM outwardly directed bicarbonate gradient increased the Vmax from 3.63 +/- 0.33 to 6.40 +/- 0.54 nmol/mg protein per 15 s. The initial rate of H2-DIDS-sensitive Cl- uptake increased as the intravesicular pH increased, with a Hill coefficient greater than one. An intra- to extravesicular gradient of Cl-, I- and HCO3- stimulated Cl- uptake in the absence of a pH gradient.

Chloride transport in brush-border membrane vesicles from chick jejunum

This study sought to characterize the mechanism(s) of Cl- transport across brush-border membrane vesicles (BBMV) isolated from chick jejunum. An inwardly directed proton gradient stimulated chloride (36Cl-) uptake. This uptake was inhibited by SITS and H2-DIDS. pH-gradient-stimulated Cl- uptake was electroneutral, since it was only slightly decreased by voltage clamping the BBMV with K+ and valinomycin. An outwardly directed HCO3- gradient significantly increased chloride uptake in the presence of a pH gradient. pH-driven chloride uptake was reduced by the presence of several anions in the uptake buffer. The rank order of potency for inhibition of pH-driven Cl- uptake was Cl- > SCN- > HCO3- > I- > Glu- > HPO4(2-). In the absence of a pH gradient, chloride was less concentrated inside the vesicles than outside. Chloride uptake under these conditions was stimulated by a positive electrical potential inside the vesicles. This stimulation was inhibited by the addition of several anions outside the vesicles. The order of inhibitory potency was SCN- > I- > Cl- > HCO3- > Glu- > HPO4(2-). The results are consistent with the presence of a Cl-/base exchanger and a chloride conductance pathway in the brush-border membrane of chick small intestine.