Temporal Relationship Between Secretory Patterns of Gonadotropins, Estrogens, Progestins, and Prostaglandin-F in Periparturient Rats

Abstract

To study changes in the secretory patterns of eight different hormones in relation to the day of parturition, blood samples were obtained sequentially from the ovarian vein, the uterine vein, and the posterior vena cava from each of 36 rats under chloral hydrate anesthesia. The blood sampling, which lasted for half an hour, was done during pregnancy from Day 18 to term and during the first 8 days after parturition. For each day, four rats were used. Plasma from the ovarian vein blood was analyzed for estradiol, estrone, progesterone, and 2Oα-dihydroprogesterone (20α-OH-P), that from the uterine vein for prostaglandin (PG) F, and that from the vena cava for LH, FSH, and prolactin by respective radioimmunoassays and the concentration of various hormones correlated with Day 22 when parturition occurred. The progesterone concentration dropped significantly on Day 19, but no significant rise in PGF was observed at this time. The 20α-OH-P secretion remained low until Day 20 and showed a striking rise on Day 21 which was maintained until Day 3 postpartum. The estradiol level rose significantly first on Day 21, i.e., some 48–72 h after a drop in progesterone, but estrone secretion showed no marked variation. The rise in estradiol secretion coincided with a significant increase in the PGF level and was followed by a surge in LH on Day 22. The FSH level showed a tendency for decline while prolactin concentration rose at approximately the same time as estradiol. It is suggested that the luteolytic action of PGF is unlikely to contribute to a reduction in the progesterone secretion on Day 19 and, since the withdrawal of “progesterone block” is believed to be one of the first steps in the initiation of parturition, it is unlikely that PGs provide a trigger for this process. It is likely that PGs participate in the later stages of parturition by expelling fetuses through their oxytocic property. The data suggest that, in some respects, hormonal interrelationship preceding parturition may be similar to that found on the day of proestrus in the cycle.