The mechanism of initiation of labor in human.

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Our previous study that dopamine has a stumilatory effect on prostaglandin production by human decidual tissue supports the hypothesis that amniotic fluid dopamine is related to initiation or maitenance of human parturition. In the present study, we demonstrated the presence of dopamine DA-1 receptor using [^3H]SCH23390, a dopamine DA-1 receptor ligand. The equilibrium dissociation constant (Kd) was 1.87 nM, and the maximum binding capacity (Bmax) was 79.0 fmol/mg protein. The competitive experiments using agents known to interact with DA-1 receptors and agents known to interact with DA-2 receptors and beta-adrenergic receptors dmonstrated that high-affinity binding sites for [^3H]SCH23390 was DA-1 receptor. Phosholipase A_2 (PLA_2) is considered to be the rate-limiting step in PG production. Recently, several isoforms of PLA_2 have been detected. Cytosolic 85 kD isoforms, named cytosolic PLA_2 (cPLA_2) is considered to release arachidonic acid for synthesis of PG via receptor stimulation. In the present study, PLA_2 in cytosol fraction of human amnion and decidua was characterized sn-2 fatty acid specificity and gel-filtration chromatography. The ratio of arachidonic acid to oleic acid release was 2.12 in amnion and 2.06 in decidua. The gel-filtration chromatography demonstrated that the molecular mass was about 85 kD.These results indicated this enzyme corresponds to the cPLA_2, The changes of cPLA_2 activity in amnion and decidua was studied during labor. At vaginal delivery, the activity in decidua was significantly lower than before the onset of labor. From these results, we suggest that the prostaglandin production in human decidua via dopamine receptors may be involved in initiation of labor.