DIRECT MODULATION OF G-PROTEINS BY POLYUNSATURATED FATTY-ACIDS - A NOVEL EICOSANOID-INDEPENDENT REGULATORY MECHANISM IN THE DEVELOPING LUNG

Basal and fatty-acid-modulated G-grotein function was studied in 1-3-d ay-pre-term, fetal guinea-pig, type II (fATII) pneumocyte apical membr ane. Unstimulated (tonic) high-affinity GTPase activity (measured as [ gamma-P-32]GTP hydrolysis rate) was high and 77 % pertussis toxin-inse nsitive. Alteration of this activity was used as a marker of G-protein regulation. Arachidonic acid (Atl) showed a dose-dependent (IC50 = 48 +/- 8 mu M) inhibition of activity at concentrations significantly be low critical micellar concentrations; this effect was mimicked by othe r polyunsaturated fatty acids (IC50 for linoleic acid = 47 +/- 2 mu M; IC50 for oleic acid = 106 +/- 11 mu M). Saturated fatty acids showed no effect. The effect of AA on ouabain-insensitive ATPases in the same preparation was significantly lower, suggesting a specificity of the GTPase modulation effect. AA modulation of GTPase activity was not att enuated by blocking eicosanoid metabolism with inhibitors of 5'-lipoxy genase, cyclo-oxygenase and P-450. In order to explore further the mec hanism of AA-G-protein interaction, the effect of AA on the time cours e and equilibrium binding of [S-35]GTP[S] to apical membrane was studi ed. Consistent with our GTPase assay data, AA inhibited binding with a n IC50 value of 71 +/- 1 mu M; stearic acid did not mimic this effect. This is the first report of unsaturated-fatty-acid-specific modulatio n of lung G-protein function: since AA also up-regulates perinatal lun g alveolar Na+ transport, we suggest this lipid/G-protein switch helps maintain pulmonary fluid homoeostasis around birth.