Effect of a single injection of prolactin on the serum concentrations of thyroid hormones and corticosterone and liver monodeiodinase in the domestic fowl before and after hatching.

The influence of an intravenous injection of ovine prolactin on the liver monodeiodinase activity and serum concentrations of thyroid hormones and corticosterone was studied in Hisex embryos and chicks after hatching. An injection of 1 and 10 micrograms ovine prolactin into 18-day-old chick embryos increased serum concentrations of tri-iodothyronine (T3) five- and eightfold respectively after 2 h. At the same time serum concentrations of thyroxine (T4) and reverse T3 (rT3) were decreased in the chick embryo, but only with 10 micrograms prolactin. This was accompanied by a doubling of the liver monodeiodinase activity. Serum concentrations of corticosterone, however, were not influenced by the prolactin injections. In the 5-day-old chicken, serum concentrations of T3, rT3, T4 and liver T4-5'-monodeiodinase activity were not influenced by 1 or 10 micrograms prolactin. Serum concentrations of corticosterone after injection of 1 or 10 micrograms prolactin were doubled compared with controls. These results are compatible with a prolactin-induced shift from a T4-5-monodeiodination into a T4-5'-monodeiodination in the liver at the end of incubation. This effect, however, is not mediated through a prolactin-induced corticosterone release.