Resistance of the pulmonary epithelium to movement of buffer ions

Abstract

. We have used fluorescent pH indicators and a trifurcated optical bundle to determine whether the apical surfaces are less permeable to ionized buffers than the membranes that separate the vasculature from the tissues in intact rat lungs. In the first set of experiments, the air spaces were filled with perfusate containing FITC-dextran (mol wt 60,000) or 2Ј,7Ј-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Air space pH fell progressively from 7.4 to 6.61 Ϯ 0.03 (mean Ϯ SE, n ϭ 11, air space buffers at 10 mM). Perfusion for 2 min with 2 mM NH 4Cl increased air space pH by 0.142 Ϯ 0.019 unit, without a subsequent acidic overshoot. Infusions of NaHCO 3 and sodium acetate reduced pH without a subsequent alkaline overshoot. In the second set of experiments, cellular pH was monitored in air-filled lungs after perfusion with BCECF-AM. Injections of NH 4Cl caused a biphasic response, with initial alkalinization of the cellular compartment followed by acidification after the NH 4Cl was washed from the lungs. Subsequent return of pH to normal was slowed by infusions of 1.0 mM dimethyl amiloride. These studies suggest that lung cells are protected from air space acidification by the impermeability of the apical membranes to buffer ions and that the cells extrude excess H ϩ through basolateral Na ϩ /H ϩ exchangers.air space acidification; intracellular pH; ammonium; bicarbonate; acetate RELATIVELY LITTLE IS KNOWN about acid-base balance across the membranes that separate the blood and the small amount of fluid that lines the air spaces. Before birth, the fluid in the air spaces is typically acidic (pH 6.27) in fetal lambs (1). Utilizing microelectrodes, Nielson et al. (12) found that the pH of the alveolar surface liquid of rat alveoli averages 6.92. They suggested that the relatively low pH of this fluid might play a role in protein activity, surfactant properties, and macrophage function. Kyle et al. (10) reported that the pH of airway surface fluid of the ferret trachea averaged 6.85. They proposed that the low pH of the airways might influence ciliary activity and the interaction of bacteria with the airway mucosa. Using an in vivo fluorescent technique, Jayaraman et al. (8) obtained a pH of 6.95 in the surface fluid of tracheas in anesthetized mice. Lubman and Crandall (11) reviewed a variety of mechanisms that could acidify the air spaces. Joseph et al. (9) used a fluorescent pH sensitive dye, 2Ј,7Ј-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF), to conduct studies of intracellular pH on alveolar epithelial cell monolayers. These monolayers were bathed with a nonbicarbonate solution buffered with 6 mM HEPES. Replacement of apical fluid with acidic (pH 6.4) or basic (pH 8.0) solutions had little effect on intracellular pH. In contrast, changes in basolateral fluid pH caused rapid responses in intracellular pH. Intracellular alkalinization was blocked Ն80% by dimethyl amiloride, an inhibitor of the Na ϩ /H ϩ exchanger. No measurements were provided for the movement of specific buffers such as HCO 3 Ϫ acros...

Voltage-Gated Proton Channels: Molecular Biology, Physiology, and Pathophysiology of the HVFamily

“…It is well known that carbonic anhydrase facilitates CO 2 diffusion (177, 191, 192, 426) and does so specifically in intact lung (138, 149, 206, 257, 258, 298, 479, 485). However, hH V 1-mediated H + flux is unlikely to be a quantitatively significant contributor to this process under normal conditions because of the absence of carbonic anhydrase in the apical subphase liquid (100, 138, 139, 480). …”

Targeting Inhaled Therapy beyond the Lungs

“…Historically, the influence of molecular weight on the transport of macromolecules was studied using fluorescent-labeled dextrans [61,62] or other polymers [63]. Clearance has been shown to be linearly and reciprocally related to molecular weight regardless of the species [64,65].…”

Voltage-gated proton channels

“…The main drawback of this mechanism is that it requires spontaneous recombination of H + and HCO 3 − in the alveolar subphase (liquid lining the alveolar air space) and because this fluid lacks carbonic anhydrase [163, 164], the reformation of CO 2 would occur slowly. Although indirect evidence opposing this mechanism has been published [165–167], a definitive test has not been reported.…”