Basal and fatty-acid-modulated G-protein function was studied in 1–3-day-pre-term, fetal guinea-pig, type II (fATII) pneumocyte apical membrane. Unstimulated (tonic) high-affinity GTPase activity (measured as [γ-32P]GTP hydrolysis rate) was high and 77% pertussis toxin-insensitive. Alteration of this activity was used as a marker of G-protein regulation. Arachidonic acid (AA) showed a dose-dependent (IC50 = 48±8 μM) inhibition of activity at concentrations significantly below critical micellar concentrations; this effect was mimicked by other polyunsaturated fatty acids (IC50 for linoleic acid = 47±2 μM; IC50 for oleic acid= 106±11 μM). 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 attenuated by blocking eicosanoid metabolism with inhibitors of 5′-lipoxygenase, cyclo-oxygenase and P-450. In order to explore further the mechanism of AA–G-protein interaction, the effect of AA on the time course and equilibrium binding of [35S]GTP[S] to apical membrane was studied. Consistent with our GTPase assay data, AA inhibited binding with an IC50 value of 71±1 μM; stearic acid did not mimic this effect. This is the first report of unsaturated-fatty-acid-specific modulation of lung G-protein function: since AA also up-regulates perinatal lung alveolar Na+ transport, we suggest this lipid/G-protein switch helps maintain pulmonary fluid homoeostasis around birth.
Present address: Department of Physiology, Worsley Medical and Dental Building, University of Leeds, Leeds LS2 9JT, U.K.