Most exhaled water is produced as gaseous water vapor, which can be collected in cooled condensers. The presence of nonvolatile solutes in these condensates suggests that droplets of respiratory fluid (RF) have also been collected. However, calculation of RF solute concentrations from condensates requires estimation of the dilution of RF droplets by water vapor. We used condensate electrolyte concentrations to calculate the dilution of RF droplets in condensates from 20 normal subjects. The total ionic concentration (conductivity) was 497 plus minus 68 (mean plus minus SEM) muM. Of this, 229 plus minus 43 muM was NH(4)(+), but little NH(4)(+) was collected from subjects with tracheostomies, indicating oral formation. The Na+ concentration in condensate ([Na+](cond)) averaged 242 plus minus 43 muM. Large variations in [Na(+)](cond) correlated well with variations of K+ in condensate ([K+](cond)) and Cl-) in condensate ([Cl-](cond)), and were attributed to differences in respiratory droplet dilution. Dividing condensate values of ([Na+] + [K+] ) by those of plasma indicated that RF represented between 0.01% and 2.00% of condensate volumes. Calculated values for Na+, K+, Cl-, lactate, and protein in RF were [Na+](RF) = 91 +/- 8 mM, [K+](RF) = 60 +/- 11 mM, [Cl-](RF) = 102 +/- 17 mM, [lactate](RF) = 44 +/- 17 mM, and [protein](RF) = 7.63 +/- 1.82 g/dl, respectively.

Exhaled breath condensates have been widely used to detect inflammatory mediators in the fluid that covers airway surfaces of patients with inflammatory lung disorders. This approach is much less invasive than bronchoalveolar lavage, but respiratory droplets are markedly diluted by large and variable amounts of water vapor. We estimated the dilution of respiratory droplets by comparing concentrations of nonvolatile, reference indicators (total nonvolatile cations, urea or conductivity) in 18 normal subjects with normal plasma concentrations by assuming similar concentrations in the respiratory fluid and plasma. The volatile cation, NH4+ (most of which is delivered as NH3 gas from the mouth), represented 93 +/- 3% (SEM) of the condensate cations. More than 99% of the NH4+ was removed by lyophilization, making it possible to use conductivity to estimate total nonvolatile ionic concentrations and facilitating analysis of urea. Conductivity was significantly correlated with electrolyte and urea concentrations. Estimates of dilution based on total cations, conductivity, and urea were not significantly different (cations: 20,472 +/- 2,516; conductivity: 21,019 +/- 2,427; and urea: 18,818 +/- 2,402). These observations suggest that the conductivity of lyophilized samples can be used as an inexpensive, simple, and reliable method for estimating dilution of nonvolatile, hydrophilic mediators in condensates.

. 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...