Relationship between Ovarian Progestin Secretion and Corpora Lutea Function in Pregnat Rats

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Abstract
Progesterone and 20α-hydroxypregn-4-en-3-one (20α-OH-P) secretory rates of rat ovaries were measured during pregnancy and pseudopregnancy. In the pregnant rats, 20α-hydroxysteroid dehydrogenase (20α-OH-SDH) activity of the ovaries, wet weights of corpora lutea and their concentrations of cholesterol, progesterone and 20α-OH-P were also determined. Progesterone secretion increased but fluctuated during pregnancy: a small rise on Day 4 was followed by a fall on Day 7, and a marked rise on Days 13 to 15 was followed by a great fall on Days 20 to 23 (the day sperm was found in vaginal smears was designated as Day 1). 20α-OH-P secretion decreased throughout pregnancy, although a small rise appeared around Day 15. The second peak of progesterone secretion and the small peak of 20α-OH-P occurred in coincidence with the growth of corpora lutea which had remained fairly constant by Day 10. After the cessation of corpora lutea growth on Day 16, both progestin secretions turned to decrease. Cholesterol concentration in corpora lutea, which remained relatively low during the first half of pregnancy but gradually increased with a gentle slope by Day 17, suddenly doubled the value on Day 18 when progesterone secretion kept decreasing. Progesterone secretion continued to decrease until the day of parturition (Days 22 to 23). 20α-OH-P began to increase on Day 22 in accordance with the increase of 20α-OH-SDH activity in corpora lutea, which had never appeared before Day 20. After parturition, progesterone secretion began to increase on the third day, conversely with a rapid drop in 20α-OH-P secretion in lactating rats. Corpora lutea progesterone, which showed the highest value on Days 5 to 8 of pregnancy followed by a gradual decrease to the lowest on Day 23, did not appear to play a special part in the regulation of progesterone secretion. In pseudopregnancy, progesterone secretion increased beginning 3 to the maximum on Day 7 and declined thereafter to Day 14, while 20α-OH-P secretion decreased during the same period of time, but the secretory rate was generally higher than that in pregnancy, so that a higher ratio of progesterone to total progestin (progesterone plus 20α-OH-P) was obtained in pregnancy than in pseudopregnancy.

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