Alveolar Permeability Enhancement by Oleic Acid and Related Fatty Acids: Evidence for a Calcium-Dependent Mechanism

Abstract

Pulmonary exposure to oleic acid (OA) is associated with permeability alterations and cellular damage; however, the exact relationship between these two effects has not been clearly established. Using cultured alveolar epithelial monolayers, we demonstrated that OA and some other fatty acids (≤50 µM) can induce permeability changes without detectable cellular damage. At higher concentrations, however, OA caused severe membrane damage and leakage to solute flux. The permeability enhancing effect of OA was observed with both the paracellular marker ³H-mannitol and the lipophilic transcellular indicator ¹⁴C-progesterone. While the effect of OA on transcellular permeability may be attributed to its known effect on membrane fluidity, the paracellular promoting effect of OA and its mechanism are not well established. We postulated that OA may increase paracellular permeability through a Ca²⁺-dependent tight junction mechanism. Using dual-excitation fluorescence microscopy, we demonstrated that OA can increase intracellular calcium, [Ca²⁺]_i , in a dose-dependent manner. This effect was transient at low OA concentrations (≤50 µM) but became more pronounced and sustained at higher concentrations. Free hydroxyl and unsaturated groups were required for this activation since esterified OA (oleic methyl ester) and stearic acid (a saturated fatty acid with equal chain length) had much reduced effects on both the [Ca²⁺]_i and the permeability alterations. Degree of unsaturation was unimportant since linolenic acid (18:3), linoleic acid (18:2), and OA (18:1) had similar and comparable effects on the two parameters. When the alveolar epithelium was bathed with Ca²⁺ -free medium, OA failed to elevate [Ca²⁺ ]_i , suggesting that Ca²⁺ influx from the extracellular medium is responsible for the observed [Ca²⁺]_i rise. This effect of OA was not due to nonspecific membrane damage since the monolayer maintained its integrity and the [Ca²⁺]_ireturned to pretreatment levels after an initial transient rise. Moreover, the permeability alteration was fully reversible upon removal of OA. These results suggest that the alveolar permeability may be reversibly enhanced by sublethal concentrations of oleic acid.