Platelet activating factor (PAF), a phospholipid with diverse biological functions, is implicated in persistent pulmonary hypertension of the newborn (PPHN), where high circulating plasma PAF level has been measured. The role of PAF and its receptors in pulmonary vascular remodeling associated with chronic hypoxia has not been investigated. We studied the effect of chronic hypoxia on PAF receptor (PAF-r) binding, PAF-r expression, and PAF metabolism in lung membranes of HAH fetal lambs compared to control fetal lambs. To study PAF-r binding, lung membrane proteins (100 μg/mL) were incubated at 4°C for 24 h in a 30 mM Tris buffer, pH 7.2, containing 0.25% BSA and 3 H-acetyl-C16-PAF (3 H-PAF). Receptor-bound PAF was extracted and quantified. PAF receptor protein expression was studied by immunohistochemistry. At low concentration of 3 H-PAF (0.05 nM), PAF-r binding (fmol/mg protein) in controls was 395 ± 38, which increased to 736 ± 32 at 0.3 nM of 3 H-PAF. In HAH lungs, corresponding values were 555 ± 46 and 892 ± 43 at low and high PAF concentrations respectively. Binding in HAH lamb lungs was 20-40% higher than in controls lungs. Immunohistochemical examination revealed greater PAF receptor protein expression in HAH lungs compared to controls. To study PAF catabolism, lung homogenates (100 μg/mL) of HAH lungs and controls were incubated for 15 min with 1.5 μM of 3 H-PAF at 37°C. PAF acetylhydrolase (PAH-ah) activity was quantified PAF-ah activity in HAH lungs (0.024 μmol/min/mg protein) was not different than activity in controls (0.022 μmol/mg protein/min). Our data show that exposure of fetuses to high-altitude hypoxia results in up-regulation of PAF-r without altering PAF catabolism. These findings suggest that increased PAF-r protein expression and increased PAF binding in these animals may predispose them to PPHN.
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