OZONE-REACTIVE ABSORPTION BY PULMONARY EPITHELIAL LINING FLUID CONSTITUENTS

Previous studies have suggested that the rate of inhaled Os absorption from the intrapulmonary gas phase is principally mediated by reaction-dependent mechanisms rather than by physical solubility, tissue diffusion, or blood flow (Postlethwait ef al., 1994, Toxicol. Appl. Pharmacol. 125, 77-89). The initial site of interaction between O-3 and the lung surface occurs at the gas/liquid interface of the epithelial lining fluid (ELF). Therefore, we investigated (a) whether reactive uptake by ELF constituents could account for pulmonary uptake and (b) whether selected constituents acted as O-3-specific absorption targets. Rat ELF was harvested by bronchoalveolar lavage. By injecting the same lavage fluid a second {(BALF)(2)} and/or third {(BALF)(3)} time into fresh lungs, a more concentrated form of ELF was obtained. Controlled quasi-steady-state exposures (O-3 in air; 30-min duration) of cell-free BALF and model substrates (reduced glutathione, GSH) were utilized. Results were based on temperature-specific fractional and normalized uptake rates (r). We observed the following: (1) Buffer pH substantially influenced O-3 absorption by GSH but by BALF only modestly. (2) Uptake displayed significant [BALF] and [GSH] dependence. (3) Fractional uptake decreased (BALF and GSH) with increasing [O-3] although absolute uptake increased. (4) Absorption demonstrated temperature dependence. Arrhenius plots {In(r) vs 1/T} were used to compute activation energies (E(a)) and Q(10). (BALF)(1) E(a) = 3387 cal/g mol with Q(10) = 1.20. GSH (1 mM) E(a) = 2240 with Q(10) = 1.13. (5) Increasing flow reduced fractional uptake in a nonlinear fashion. (6) Dialysis (1000-molecular-weight cutoff) reduced uptake by (BALF)(1) moderately (-30%). Sulfhydryl depletion produced minimal effect (-10%), while ascorbate depletion (-37%) and combined sulfhydryl and ascorbate depletion (-39%) were the most effective. Treatments produced lesser effects on (BALF)(3). We conclude that the pH, aqueous substrate, and temperature-dependence and the E(a) and Q(10) are consistent with reaction-dependent O-3 uptake by ELF. The analogous absorption characteristic between the ELF and intact lung (temperature, [O-3], contact time) suggests that the ELF represents the primary site for O-3-reactive absorption. Reduced sulfhydryls do not appear to substantially interact with inhaled O-3. Principal absorption targets may include ascorbate, phospholipids, and other moderate to large molecular weight constituents. (C) 1995 Academic Press, Inc.