Technical Note: A Radioimmunoassay for Porcine Intrauterine Folate Binding Protein1

A RIA was developed for porcine intrauterine folate binding protein (FBP). Displacement of [125I]FBP caused by increasing dilutions of uterine flushings collected from either d-15 pregnant or nonpregnant gilts or media from culture of endometrial tissue from d-15 pregnant or nonpregnant gilts was parallel to the displacement caused by the standard curve. Addition of known amounts of purified allantoic fluid FBP to dilutions of either intrauterine flushings or endometrial culture medium were measured accurately with the RIA. To test specificity, 2-mL samples of uterine flushings collected from d-15 pregnant and nonpregnant gilts were preincubated with 10 mCi of [3H]folic acid and then chromatographed using Sephadex G-100 (Sigma Chemical Co., St. Louis, MO). The fractions were subsequently assayed for radioactivity by liquid scintillation spectrophotometry and for FBP by RIA. The [3H]folic acid and FBP peaks coincided, indicating that the RIA is specific for FBP. Uterine flushings were collected on d 10, 11, 12, 13, 14, and 15 of the cycle or pregnancy from 1) White crossbred, 2) progesterone-treated White crossbred (200 mg of progesterone at 48 and 72 h after estrus), and 3) Meishan gilts and assayed for FBP. Total FBP increased 140-fold from d 10 to 15, and the pattern of change across day did not differ between pregnant and nonpregnant gilts. Progesterone treatment increased intrauterine FBP content on d 10 and 11. No difference in FBP concentrations was detected between White crossbred and Meishan gilts. These results indicate that the RIA for FBP is valid, allowing measurement of this protein in uterine flushings and endometrial culture medium. The onset of FBP secretion by the uterus between d 10 and 15 of the cycle or pregnancy is influenced by the timing of onset of progesterone influence in a manner similar to the endometrial proteins uteroferrin and retinol binding protein. In contrast to these endometrial proteins, FBP concentrations are similar in Meishan and White crossbred gilts.