Binding of platelet-activating factor to oviductal membranes during early pregnancy in the rabbit

The present study explores the ability of rabbit oviductal membranes to bind tritiated platelet-activating factor [³H]PAF on days 3 and 6 of pregnancy. Under optimal conditions (25°C, 120 min) equilibrium saturation analysis revealed only one class of binding sites, characterized by K(d)s(nM), 80.03 ± 11.60 and 11.17 ± 7.09 and B(max)s, (pmol/mg protein), 5.25 ± 2.23 and 1.08 ± 0.22 (N = 3, mean ± SEM) for ampullar membranes on days 3 and 6, respectively. The corresponding values for isthmic membranes were K(d)s, 86.56 ± 12.01 and 52.43 ± 30.49 and B(max)s, 9.41 ± 0.67 and 2.88 ± 1.96 for days 3 and 6, respectively. Significant differences between days 3 and 6 were observed only in the binding affinities for the ampullar membranes and the binding capacities for the isthmic binding sites. [³H]PAF binding was inhibited in the following order of decreasing potency: lyso-PAF > PAF C18:0 > U66985 > PAF C16:0 for day 3 ampullar membranes; and lyso-PAF C16:0 > PAF C18:0 > U66985 > PAF C16:0 for day 6 ampullar membranes. These studies show the existence of specific oviductal membrane PAF binding sites, the binding parameters of which may be related to the stage of pregnancy, rather than to the spatial location along the oviduct. The relative proportion of endosalpinx to myosalpinx between the ampulla and isthmus may have masked inherent differences and account for the relatively low affinity binding. The physiological significance of oviductal membrane PAF binding is yet to be established.