Anion exchange in type II pneumocytes freshly isolated from adult guinea-pig lung

Basolateral Cl- uptake mechanisms in Xenopus laevis lung epithelium

A thin liquid layer covers the lungs of air-breathing vertebrates. Active ion transport processes via the pulmonary epithelial cells regulate the maintenance of this layer. This study focuses on basolateral Cl(-) uptake mechanisms in native lungs of Xenopus laevis and the involvement of the Na(+)/K(+)/2 Cl(-) cotransporter (NKCC) and HCO(3)(-)/Cl(-) anion exchanger (AE), in particular. Western blot analysis and immunofluorescence staining revealed the expression of the NKCC protein in the Xenopus lung. Ussing chamber experiments demonstrated that the NKCC inhibitors (bumetanide and furosemide) were ineffective at blocking the cotransporter under basal conditions, as well as under pharmacologically stimulated Cl(-)-secreting conditions (forskolin and chlorzoxazone application). However, functional evidence for the NKCC was detected by generating a transepithelial Cl(-) gradient. Further, we were interested in the involvement of the HCO(3)(-)/Cl(-) anion exchanger to transepithelial ion transport processes. Basolateral application of DIDS, an inhibitor of the AE, resulted in a significantly decreased the short-circuit current (I(SC)). The effect of DIDS was diminished by acetazolamide and reduced by increased external HCO(3)(-) concentrations. Cl(-) secretion induced by forskolin was decreased by DIDS, but this effect was abolished in the presence of HCO(3)(-). These experiments indicate that the AE at least partially contributes to Cl(-) secretion. Taken together, our data show that in Xenopus lung epithelia, the AE, rather than the NKCC, is involved in basolateral Cl(-) uptake, which contrasts with the common model for Cl(-) secretion in pulmonary epithelia.

Superoxide-dependent iron uptake: a new role for anion exchange protein 2

Lung cells import iron across the plasma membrane as ferrous (Fe2+) ion by incompletely understood mechanisms. We tested the hypothesis that human bronchial epithelial (HBE) cells import non-transferrin-bound iron (NTBI) using superoxide-dependent ferri-reductase activity involving anion exchange protein 2 (AE2) and extracellular bicarbonate (HCO3-). HBE cells that constitutively express AE2 mRNA by reverse transcriptase-polymerase chain reaction and AE2 protein by Western analysis avidly transported NTBI after exposure to either Fe2+ or Fe3+, but reduction of Fe3+ to Fe2+ was first required. The ability of HBE cells to reduce Fe3+ and transport Fe2+ was inhibited by active extracellular superoxide dismutase (SOD). Similarly, HBE cells that overexpress Cu,Zn SOD after adenoviral infection with AdSOD1 showed diminished iron uptake. The role of AE2 in iron uptake was indicated by three lines of evidence: (i) lack of both iron reduction and iron transport in bicarbonate-free buffer at controlled pH, (ii) failure of HBE cells treated with stilbene AE inhibitors to reduce Fe3+ or transport iron, and (iii) inhibition of iron uptake in HBE cells by inhibition of AE2 protein expression with antisense oligonucleotides. We thus disclose a novel ferri-reductase mechanism of NTBI uptake by human lung cells that employs superoxide exchange for HCO3- by AE2 protein in the plasma membrane.

Hypothesis: do voltage-gated H(+) channels in alveolar epithelial cells contribute to CO(2) elimination by the lung?

Although alveolar epithelial cells were the first mammalian cells in which voltage-gated H(+) currents were recorded, no specific function has yet been proposed. Here we consider whether H(+) channels contribute to one of the main functions of the lung: CO(2) elimination. This idea builds on several observations: 1) some cell membranes have low CO(2) permeability, 2) carbonic anhydrase is present in alveolar epithelium and contributes to CO(2) extrusion by facilitating diffusion, 3) the transepithelial potential difference favors selective activation of H(+) channels in apical membranes, and 4) the properties of H(+) channels are ideally suited to the proposed role. H(+) channels open only when the electrochemical gradient for H(+) is outward, imparting directionality to the diffusion process. Unlike previous facilitated diffusion models, HCO(-)(3) and H(+) recombine to form CO(2) in the alveolar subphase. Rough quantitative considerations indicate that the proposed mechanism is plausible and indicate a significant capacity for CO(2) elimination by the lung by this route. Fully activated alveolar H(+) channels extrude acid equivalents at three times the resting rate of CO(2) production.

Respiratory acidosis in carbonic anhydrase II-deficient mice

To investigate the role of carbonic anhydrase (CA) II on pulmonary CO2 exchange, we analyzed arterial blood gases from CA II-deficient and normal control mice. CA II-deficient mice had a low arterial blood pH (7.18 +/- 0.06) and HCO3- concentration ([HCO3-]; 17.5 +/- 1.9 meq/l) and a high Pco2 (47.4 +/- 5.3 mmHg), consistent with mixed respiratory and metabolic acidosis. To eliminate the influence of metabolic acidosis on arterial blood gases, NaHCO3 (4 mmol/kg body weight) was given intraperitoneally, and arterial blood gases were analyzed 4 h later. Normal mice had a small increase in pH and were able to maintain Pco2 and [HCO3-]. The metabolic acidosis in CA II-deficient mice was corrected ([HCO3-], 22.9 +/- 2.4 meq/l), and respiratory acidosis became more profound (Pco2, 50.4 +/- 2.4 mmHg). These results indicate that CA II-deficient mice have a partial respiratory compensation for metabolic acidosis. We conclude that CA II-deficient mice have a mixed respiratory and metabolic acidosis. It is most likely that CO2 retention in these animals is due to CA II deficiency in both red blood cells and type II pneumocytes.

NO3--induced pH changes in mammalian cells. Evidence for an NO3--H+ cotransporter

The effect of NO3- on intracellular pH (pHi) was assessed microfluorimetrically in mammalian cells in culture. In cells of human, hamster, and murine origin addition of extracellular NO3- induced an intracellular acidification. This acidification was eliminated when the cytosolic pH was clamped using ionophores or by perfusing the cytosol with highly buffered solutions using patch-pipettes, ruling out spectroscopic artifacts. The NO3-- induced pH change was not due to modulation of Na+/H+ exchange, since it was also observed in Na+/H+ antiport-deficient mutants. Though NO3- is known to inhibit vacuolar-type (V) H+-ATPases, this effect was not responsible for the acidification since it persisted in the presence of the potent V-ATPase inhibitor bafilomycin A1. NO3-/HCO3- exchange as the underlying mechanism was ruled out because acidification occurred despite nominal removal of HCO3-, despite inhibition of the anion exchanger with disulfonic stilbenes and in HEK 293 cells, which seemingly lack anion exchangers (Lee, B. S., R.B. Gunn, and R.R. Kopito. 1991. J. Biol. Chem. 266:11448- 11454). Accumulation of intracellular NO3-, measured by the Greiss method after reduction to NO2-, indicated that the anion is translocated into the cells along with the movement of acid equivalents. The simplest model to explain these observations is the cotransport of NO3- with H+ (or the equivalent counter-transport of NO3- for OH-). The transporter appears to be bi-directional, operating in the forward as well as reverse directions. A rough estimate of the fluxes of NO3- and acid equivalents suggests a one-to-one stoichiometry. Accordingly, the rate of transport was unaffected by sizable changes in transmembrane potential. The cytosolic acidification was a saturable function of the extracellular concentration of NO3- and was accentuated by acidification of the extracellular space. The putative NO3--H+ cotransport was inhibited markedly by ethacrynic acid and by alpha-cyano-4-hydroxycinnamate, but only marginally by 4, 4'-diisothiocyanostilbene-2,2' disulfonate or by p-chloromercuribenzene sulfonate. The transporter responsible for NO3--induced pH changes in mammalian cells may be related, though not identical, to the NO3--H+ cotransporter described in Arabidopsis and Aspergillus. The mammalian cotransporter may be important in eliminating the products of NO metabolism, particularly in cells that generate vast amounts of this messenger. By cotransporting NO3- with H+ the cells would additionally eliminate acid equivalents from activated cells that are metabolizing actively, without added energetic investment and with minimal disruption of the transmembrane potential, inasmuch as the cotransporter is likely electroneutral.

ABC1, an ATP binding cassette transporter required for phagocytosis of apoptotic cells, generates a regulated anion flux after expression in Xenopus laevis oocytes

The ATP binding cassette transporter ABC1 is a 220-kDa glycoprotein expressed by macrophages and required for engulfment of cells undergoing programmed cell death. Since members of this family of proteins such as P-glycoprotein and cystic fibrosis transmembrane conductance regulator share the ability to transport anions, we have investigated the transport capability of ABC1 expressed in Xenopus oocytes using iodide efflux and voltage-clamp techniques. We report here that ABC1 generates an anion flux sensitive to glibenclamide, sulfobromophthalein, and blockers of anion transporters. The anion flux generated by ABC1 is up-regulated by orthovanadate, cAMP, protein kinase A, and okadaic acid. In other ABC transporters, mutating the conserved lysine in the nucleotide binding folds was found to severely reduce or abolish hydrolysis of ATP, which in turn altered the activity of the transporter. In ABC1, replacement of the conserved lysine 1892 in the Walker A motif of the second nucleotide binding fold increased the basal ionic flux, did not alter the pharmacological inhibitory profile, but abolished the response to orthovanadate and cAMP agonists. Therefore, we conclude that ABC1 is a cAMP-dependent and sulfonylurea-sensitive anion transporter.

Na(+)-dependent and Na(+)-independent systems of choline transport by plasma membrane vesicles of A549 cell line

Membrane vesicles of A549 lung cells accumulate choline by two pathways: the Na(+)-independent uphill uptake of choline [Michaelis-Menten constant (Km) approximately 44 microM; steady-state gradient approximately 45 at 5 microM external choline] is dependent on a transmembrane H+ gradient, is relatively insensitive to hemicholinium-3, is amiloride sensitive, and is abolished by valinomycin plus carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP). The Na(+)-dependent active choline uptake (Km approximately 4 microM, inhibitor constant for hemicholinium-3 approximately 0.1 microM), is specific for Na+, is amiloride and FCCP sensitive, and is electrogenic: the overshoot using K(+)-loaded vesicles and NaCl gradient was increased by valinomycin. The time of the overshoot peak, T was approximately 90 s in a NaSCN medium (or in presence of other lipid-soluble anions), a value close to that for alpha-aminoisobutyrate as substrate (T = approximately 1.5 min). T was lengthened in NaCl medium to approximately 10 min, and the overshoot was abolished by impermeant anions. External Cl- is not required for the choline uptake: valinomycin produced an overshoot in the presence of only impermeant anions, with T approximately 90 s. Most of the above properties are shared by the high-affinity Na(+)-dependent choline transport in synaptosomes. The characteristics of the Na(+)-dependent choline uptake by membrane vesicles of A549 cells are consistent with an electrogenic choline(+)-Na+ cotransport, with the rate-limiting anion (e.g., Cl-) influx balancing the positive charges transferred into the vesicles. The data are also consistent with an involvement of an amiloride-sensitive choline+/H+ antiport (or choline(+)-OH- symport) in the low- and high-affinity choline uptake pathways.