INDUCIBLE NITRIC-OXIDE SYNTHASE IS PRESENT IN THE RAT PLACENTA AT THEFETAL-MATERNAL INTERFACE AND DECREASES PRIOR TO LABOR

The aim of this study was to investigate the expression and distributi on patterns of the inducible isoform of nitric oxide synthase (iNOS) i n rat placenta during gestation and term labour. The expression of iNO S isoform was assessed by sodium dodecyl sulphate-polyacrylamide gel e lectrophoresis (SDS-PAGE) and immunoblotting with monoclonal antibodie s. Two specific bands were detected corresponding to 135 and 124 kDa i n all placenta samples. The upper band (135 kDa) was identified as iNO S due to its correspondence with the band obtained with mouse macropha ges (positive control). Compared with its concentrations on day 16, iN OS decreased steadily toward the end of gestation to similar to 37% on day 20, 20% on day 22 before labour and 12% during labour (P < 0.01). The lower band (124 kDa) drastically increased (to almost double) fro m day 16 to day 18 but returned to initial values on day 22, during de livery. Immunohistochemical staining of placentae at day 16 and 22 usi ng rabbit polyclonal anti-iNOS antibody revealed labelling specificall y concentrated in the trophospongial cell layer, at the fetal-maternal interface. The most conspicuous [NOS staining was associated with isl ands of cells referred to as vacuolated 'glycogen cells). Staining was greatly decreased during labour. The changes in placental iNOS expres sion suggest a 'paracrine' role for NO in regulating uterine contracti lity, blood flow and immunosuppression required for pregnancy maintena nce. NO withdrawal al term may also be involved in the initiation of l abour.