Evidence for Homology Between Ovine and Human Placental Lactogens

The biological and immunological properties of highly purified ovine placental lactogen (oPL) and human placental lactogen (hPL) have been compared. Like hPL and ovine prolactin (oPR). oPL stimulates lactation in vivo and in vitro and binds to prolactin receptors from mammary tissue. Like hPL and ovine growth hormone (oGH), oPL binds to growth hormone receptors from liver. OPL of equivalent activity to hPL in the prolactin receptor assay is approximately 20 times more active than hPL (but 5 times less active than oGH) in the growth hormone receptor assay. Immunologically, oPL cross-reacts with oGH but not with hPL. The striking homologies between oPL, and hPL indicate that sheep is an excellent animal model in which to study placental lactogen physiology.