Peripheral blood and ovarian levels of sex steroids in the lactating rat.

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Abstract
Changes in progesterone (P), 20.alpha.-dihydroprogesterone (20.alpha.-OHP), testosterone (T) and estradiol-17.beta. (E2) in peripheral blood, in corpora lutea (CL) of pregnancy and lactation and non-luteal ovarian tissue (NLO), and serum levels of LH [luteinizing hormone], FSH and prolactin (PRL) were determined by radioimmunoassay in lactating rats nursing 8 pups. CL of pregnancy and lactation and NLO were removed at various days after post-partum ovulation (day 0 of lactation = day of parturition). Serum E2 remained low until day 10 of lactation followed by a slight increase from Days 12 to 21. Serum T was unchanged throughout lactation. Low levels of T and E2 in NLO were observed until day 12, followed by a gradual increase on day 15 and onward. These changes correlated with low levels of serum LH from days 2 to 12 and a return to basal levels on days 15 to 21. Throughout lactation, serum FSH remained at basal levels corresponding to diestrous values during the estrous cycle. High levels of serum PRL were observed from days 4 to 10. Serum levels of P reached maximal values between days 8 and 10 and decreased after day 12. Changes in serum 20.alpha.-OHP were inversely related to serum P throughout lactation. The CL of pregnancy contained large amounts of 20.alpha.-OHP and small amounts of P on days 1 and 2 followed by an abrupt decline until day 21. A marked increase in content and concentration of P in the CL of lactation occurred between days 6 and 12 followed by a gradual decline by day 21. Content and concentration of 20.alpha.-OHP in the CL of lactation increased progressively from day 1 to 21. The CL of pregnancy secrete a large amount of 20.alpha.-OHP and a small amount of P during the early stages of lactation. Thereafter, the CL of lactation markedly increase their ability to secrete P. High levels of P and PRL during the first half of lactation along with strong suckling stimuli may be involved in the suppression of follicular development, presumably by lowering basal levels of serum LH.