Growth-promoting properties of the internal milieu of pregnant and lactating rats.

The growth-promoting properties of the internal milieu of pregnant and lactating rats were investigated using transplanted whole rat embryos or fetal paws. When placed under the kidney capsule of intact nonpregnant hosts and incubated for 12 days, such transplants grow rapidly, and tissues differentiate normally. Thus they provide an accurate means of assessing the growth-promoting properties of the internal environment of host animals in different physiological states. Transplant growth during days 0-11 of pregnancy was similar to that observed in age-matched virgin control hosts during an equivalent 12-day period. However, growth of transplants was decreased by 40% in hosts during days 10-22 of pregnancy and by approximately 30% in hosts during days 1-13 or days 11-23 of lactation. Increase in tail length, which was used as an index of maternal skeletal growth, was reduced by 50% during the second half of pregnancy and the early and late periods of lactation compared with age-matched virgin females. No such inhibition was recorded during the first half of pregnancy. Compared with virgins, serum insulin-like growth factor I (IGF-I) levels were reduced by approximately 20% on days 6 and 10 of pregnancy, and by 63-66% during the second half of gestation. Serum IGF-I levels rose during lactation to reach prepregnancy levels by day 12, but a second decline occurred by day 18 postpartum.(ABSTRACT TRUNCATED AT 250 WORDS)