Human placental lactogen inhibits growth without changing serum levels of IGF-1 in rats: an apparent specific action of the hormone.

Previous work in our laboratory has shown that the internal environment of rats has reduced growth-promoting activity during the second half of gestation and this condition is associated with resistance to the anabolic effects of GH. The placenta appears to be responsible for this condition but injections of estradiol plus progesterone into virgin females did not mimic it. Accordingly, it seemed worthwhile to test the effects of a placental lactogen (PL) for possible growth inhibitory effects. In the present study the effects of human (h)PL on skeletal growth in young female rats and on the growth of embryonic tissue transplants under their kidney capsules were investigated. Human (h) and bovine (b) GH, and ovine prolactin (oPRL) were also tested to determine whether the results obtained with hPL were specific. Twice daily subcutaneous injections of a high dose of hPL (10 mg/day), but not of oPRL (5 mg/day) for 7 days inhibited both host tail growth and tibial epiphyseal plate width, and growth of whole 10-day embryo transplants. Injections of hGH at 1 mg/day for 8 days significantly increased host skeletal growth and growth of 12-day embryonic head transplants; at the same dose, neither bGH nor oPRL affected growth of the embryonic heads or of the host tibial epiphyseal plate width, but the bGH increased host tail growth. By contrast, the 1 mg/day dose of hPL significantly reduced the host's tibial epiphyseal plate width, tail growth, and transplant growth; lower doses of hPL (10 and 100 micrograms/day) were also inhibitory.(ABSTRACT TRUNCATED AT 250 WORDS)