ALVEOLAR PERMEABILITY ENHANCEMENT BY OLEIC-ACID AND RELATED FATTY-ACIDS - EVIDENCE FOR A CALCIUM-DEPENDENT MECHANISM

Pulmonary exposure to oleic acid (OA) is associated with permeability alterations and cellular damage; however, the exact relationship betwe en these two effects has not been clearly established. Using cultured alveolar epithelial monolayers, we demonstrated that OA and some other fatty acids (less than or equal to 50 mu M) can induce permeability c hanges without detectable cellular damage. At higher concentrations, h owever, OA caused severe membrane damage and leakage to solute flux. T he permeability enhancing effect of OA was observed with both the para cellular marker H-3-mannitol and the lipophilic transcellular indicato r C-14-progesterone. While the effect of OA on transcellular permeabil ity may be attributed to its known effect on membrane fluidity, the pa racellular promoting effect of OA and its mechanism are not well estab lished. We postulated that OA may increase paracellular permeability t hrough a Ca2+-dependent tight junction mechanism. Using dual-excitatio n fluorescence microscopy, we demonstrated that OA can increase intrac ellular calcium, Ca2+(i), in a dose-dependent manner. This effect wa s transient at low OA concentrations (less than or equal to 50 mu M) b ut became more pronounced and sustained at higher concentrations. Free hydroxyl and unsaturated groups were required for this activation sin ce esterified OA (oleic methyl ester) and stearic acid (a saturated fa tty acid with equal chain length) had much reduced effects on both the Ca2+(i) and the permeability alterations. Degree of unsaturation wa s unimportant since linolenic acid (18:3), linoleic acid (18:2), and O A (18:1) had similar and comparable effects on the two parameters. Whe n the alveolar epithelium was bathed with Ca2+-free medium, OA failed to elevate Ca2+(i), suggesting that Ca2+ influx from the extracellul ar medium is responsible for the observed Ca2+(i) rise. This effect of OA was not due to nonspecific membrane damage since the monolayer m aintained its integrity and the Ca2+(i) returned to pretreatment lev els after an initial transient rise. Moreover, the permeability altera tion was fully reversible upon removal of OA. These results suggest th at the alveolar permeability may be reversibly enhanced by sublethal c oncentrations of oleic acid.