Polarity of alveolar epithelial cell acid-base permeability

Abstract

We investigated acid-base permeability properties of electrically resistive monolayers of alveolar epithelial cells (AEC) grown in primary culture. AEC monolayers were grown on tissue culture-treated polycarbonate filters. Filters were mounted in a partitioned cuvette containing two fluid compartments (apical and basolateral) separated by the adherent monolayer, cells were loaded with the pH-sensitive dye 2′,7′-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, and intracellular pH was determined. Monolayers in HCO[Formula: see text]-free Na+ buffer (140 mM Na+, 6 mM HEPES, pH 7.4) maintained a transepithelial pH gradient between the two fluid compartments over 30 min. Replacement of apical fluid by acidic (6.4) or basic (8.0) buffer resulted in minimal changes in intracellular pH. Replacement of basolateral fluid by acidic or basic buffer resulted in transmembrane proton fluxes and intracellular acidification or alkalinization. Intracellular alkalinization was blocked ≥80% by 100 μM dimethylamiloride, an inhibitor of Na+/H+exchange, whereas acidification was not affected by a series of acid/base transport inhibitors. Additional experiments in which AEC monolayers were grown in the presence of acidic (6.4) or basic (8.0) medium revealed differential effects on bioelectric properties depending on whether extracellular pH was altered in apical or basolateral fluid compartments bathing the cells. Acid exposure reduced (and base exposure increased) short-circuit current from the basolateral side; apical exposure did not affect short-circuit current in either case. We conclude that AEC monolayers are relatively impermeable to transepithelial acid/base fluxes, primarily because of impermeability of intercellular junctions and of the apical, rather than basolateral, cell membrane. The principal basolateral acid exit pathway observed under these experimental conditions is Na+/H+ exchange, whereas proton uptake into cells occurs across the basolateral cell membrane by a different, undetermined mechanism. These results are consistent with the ability of the alveolar epithelium to maintain an apical-to-basolateral (air space-to-blood) pH gradient in situ.

Impact of buffering hypercapnic acidosis on cell wounding in ventilator-injured rat lungs

“…Bicarbonate buffer is widely used in a number of clinical conditions, but, because it acts as a CO 2 donor, it has the potential to further reduce intracellular pH because of the free diffusion of CO 2 across cell membranes, including those of alveolar epithelial cells (9). THAM, on the other hand, acts as a proton acceptor and is subsequently renally excreted (2), avoiding local production of CO 2 and therefore having little additive effect on intracellular acid-base balance.…”

hPepT1 mediates bacterial tripeptide fMLP uptake in human monocytes

“…Impermeability to acid of apical cell membrane has already been described for alveolar epithelial cells. 28 This difference of permeability to H þ in polarized and nonpolarized cells could be due to the fact that membrane proteins are segregated into two different membrane domains in polarized cells, therefore the communications/interactions between transporters/ exchangers/channels and others is different from those in nonpolarized cells. In addition, polarized cells have tight junctions that are known to be impermeable to H þ 28 and which also account for the high transepithelial electrical resistance (TER) observed for well-differentiated/polarized cells.…”

Hypercapnic Acidosis Impairs Plasma Membrane Wound Resealing in Ventilator-injured Lungs

“…However, bicarbonate buffers are CO 2 donors and transiently lower intracellular pH. For example, the apical membranes of polarized alveolar pneumocytes are impermeable to transepithelial acid-base fluxes, but they are permeable to CO 2 (54). To the extent to which membrane repair processes could be dependent on intracellular pH and not extracellular pH, the infusion of bicarbonate buffers may well be counterproductive.…”