The levels of endothelin-1 (ET-1) and the characteristics of endothelin (ET) receptors were investigated in the amnion, chorion laeve, decidua vera and placenta by using a specific radioimmunoassay for ET-1 and a saturation binding assay for ET. ET receptor gene expression in these tissues was also examined by Northern blot analysis. The levels of ET-1-like immunoreactivity (ET-1-LI) in the human amnion, chorion laeve, decidua vera and placenta obtained at elective Caesarean section before labour onset at term (mean +/- s.e.m.) were 1260 +/- 380 (n = 6), 3740 +/- 980 (n = 4), 4550 +/- 780 (n = 4) and 2450 +/- 470 (n = 4) pg g-1 wet weight, respectively. The levels of ET-1-LI in the tissues obtained after spontaneous vaginal deliveries at term did not differ from these. Gel-permeation chromatography and reverse-phase high-performance liquid chromatography revealed that the ET-1-LI in these tissues was mainly composed of ET-1. Scatchard analysis of the saturation binding assay for [125I]-labelled ET-1 and [125I]-labelled ET-3 indicated that high concentrations of both ET-A and ET-B subtypes of receptor were present in membrane fractions from the chorion laeve, decidua vera and placenta. However, in membrane fractions from the amnion, no ET receptors could be detected. These results were confirmed by Northern blot analysis using human ET-A and ET-B receptor cDNA probes. Taken together, these results suggest that the amnion is not the site of action of amniotic ET, and that ET may be involved in the regulation of functions of the chorion laeve or decidua vera.
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