Study of oxytocin receptor: II. Oxytocin and prostaglandin F2.ALPHA. receptors in human myometria and amnion-decidua complex during pregnancy and labor.

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Abstract
Oxytocin receptors (OXT-R) and prostaglandin F2.alpha. receptors (PFG2.alpha.-R) in human myometrium, amnion and decidua during pregnancy and at parturition were examined in an effort to clarify their role in the initiation and maintenance of uterine contractions. The number of binding sites for OXT in myometria showed an increase as gestation advanced (1st trimester vs. at term; 205 .+-. 90 vs. 671 .+-. 98 fmol/mg protein, N = 5, P < 0.01), and a rapid decrease following the onset of labor (254 .+-. 60 fmol/mg protein, N = 5, P < 0.02). The number of PGF2.alpha.-R, remained unchanged throughout pregnancy and in labor. This myometrial PGF2.alpha. binding capacity was .apprx. 1/20-1/30 that of the OXT binding, while binding affinity was almost equal. The OXT-R both in amnion and decidua, which was 1/6-1/7 that in myometrium, showed no significant changes throughout pregnancy or after the onset of labor. Binding affinity for each tissue was almost the same and appeared to increase towards term, but no statistical significance was detected. The presence of OXT-R and PGF2.alpha.-R in the 3 functionally distinct entities of pregnant human uterus, myometrium, amnion, and decidua is thus confirmed. Among the components, the OCT binding increased only in the myometrium during pregnancy, suggesting this tissue specially responds to OXT. There was a constant binding in myometria for PGF2.alpha.. In in vitro incubation of the tissue with PGF2.alpha., the predominant change observed was an increase in OXT-R binding affinity. This change was observed in all these specimens at term before labor. Since human uterine contractions can be induced effectively with PGF2.alpha. throughout pregnancy while induction by OXT is only feasible near term, these observations seem to explain the differences in efficacy of the 2 uterotonic compounds at different stages of pregnancy.