Binding of fluorescent phosphatidylcholine in amniotic fluid.

Fetal lung maturity can be predicted by the technique of fluorescence polarization using fluorescent phosphatidylcholine instead of diphenylhexatriene. To further elucidate the mechanism of this assay, we used high-speed centrifugation to isolate amniotic fluid lamellar bodies. Each of 75 amniotic fluid samples was separated into two fractions: a lamellar body pellet and a lamellar body-free supernatant fluid. Amniotic fluid and these fractions were assessed in a fluorescence polarization assay using a fluorescent phosphatidylcholine. Regardless of the maturity of the fetal lung, the lamellar body fractions had low polarization values (0.127-0.216), whereas the lamellar body-free fractions had high polarization values (0.266-0.344). Compared with the polarization of amniotic fluid, the fluorescence intensity of the lamellar body fractions had a strong inverse correlation (r = -0.871). The polarization of lamellar body pellets was not linearly related to the polarization of amniotic fluids. These findings do not support the theory that this fluorescence polarization assay measures the microviscosity of surfactant lipids. Instead, we propose that this assay indicates the quantity of surfactant relative to the quantity of nonsurfactant receptors of fluorescent phosphatidylcholine in amniotic fluid.