The concentrations of progesterone, estrone and estradiol-17β have been measured by radioimmunoassay in serial samples of peripheral plasma from rhesus monkeys (Macaca mulatto) during the final third of normal pregnancy, and during the induction of abortion with prostaglandin F2α (PGF2α). In 3 out of 5 animals, the plasma progesterone concentration increased during the 5-10 days preceding spontaneous labor. In no animal did the progesterone concentration decrease before parturition. The concentration of estradiol-17β was greater than that of estrone in most plasma samples. The levels of both estrogens in maternal plasma increased within 5 days before parturition, but decreased rapidly after the fetus had been delivered. Daily maternal injections of PGF2α (10-15 mg/day) administered around day 150 of pregnancy reliably induced premature delivery. The concomitant steroid hormone changes were variable, although most monkeys showed a decline in the plasma progesterone and estrogen levels during the final 24 hr before expulsion of the fetus. The present results support the view that an increase in estrogen production may be an important component of the endocrine control of normal parturition. The pharmacological action of PGF2α in inducing abortion in the rhesus monkey appears to be different from the mechanisms which effect the spontaneous onset of labor.
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