PITUITARY LACTOGENIC HORMONE CONCENTRATION DURING PREGNANCY IN THE RAT1

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Abstract
A study was made in rats to ascertain whether the growth and secretory phases of the mammary gland during pregnancy were associated with variation in concentration of pituitary lactogenic hormone. Pituitary lactogenic hormone levels were determined by the common pigeon micro method of assay. Pituitary weight was low during most of pregnancy, but increased markedly a short time before parturition (day 21). Pituitary lactogenic hormone increased during the first two-thirds of pregnancy during which time there was growth, but little secretory activity of the mammary gland. Peak concentration (.051 I.U./ mg pituitary) was reached on day 16. As the secretory activity of the mammary gland progressively increased during the last one-third of pregnancy, pituitary lactogenic hormone levels decreased markedly reaching the lowest point (.004 I.U./mg) on day 21. The significance of this is discussed. The influence of 1, 3.6, 15 and /[mu]g/ day estradiol benzoate (E.B.) for 5 days upon pituitary weight and lactogenic hormone concentration was studied in mature female rats. Greatest pituitary enlargement occurred with 15 pg/day E.B. while pituitary lactogenic hormone increased in concentration as the log-dose of E.B. The level resulting from 1 /[mu]g/day E.B. (.054 I. U./mg) closely approximated the highest level found during pregnancy.