Human myometrial cells in culture express specific binding sites for urinary trypsin inhibitor

Urinary trypsin inhibitor (UTI), which is present in amniotic fluid, prevents uterine contractility during pregnancy possibly via specific binding protein mechanisms. To test for the presence of UTI binding sites on the cell surface, we prepared cultured myometrial cells obtained at biopsy from 12 pregnant women and performed binding, competition, and cross-linking experiments using a specific radiolabelled UTI as a ligand. We report for the first time two classes of binding sites of differing affinities. Scatchard analysis at 4 degrees C, using radioiodinated UTI, revealed that UTI binds to 35 000 high affinity binding sites/cell (K(d) = 9.1x10(-9) mol/l) and 450 000 lower affinity binding sites/cell (K(d) = 3.5x10(-7) mol/l) in cultured myometrial cells. It appears to be the low affinity site that is internalized, and this has been identified as a protein of approximately 45 kDa by cross-linking and immunoaffinity labelling studies. Monoclonal antibodies against the NH(2)-terminal fragment of UTI abrogated specific binding of this protein to the cells. Treatment of the cells with hyaluronidase resulted in >80% inhibition of the [(125)I]-labelled UTI binding to the cells. These data show that the UTI binding site, which is hyaluronidase sensitive, is expressed on the surface of human uterine myometrial cells to accumulate the UTI molecule during pregnancy.