Physiological role of human placental growth hormone.

Placental growth hormone (PGH) is the product of the GH-V gene specifically expressed in the syncytiotrophoblast layer of the human placenta. PGH differs from pituitary growth hormone by 13 amino acids. It has high somatogenic and low lactogenic activities. Assays of PGH by specific monoclonal antibodies reveal that in the maternal circulation from 15-20 weeks up to term, PGH gradually replaces pituitary growth hormone which becomes undetectable. It is secreted by the placenta in a non-pulsatile manner. This continuous secretion appears to have important implications for physiological adjustment to gestation and especially in the control of maternal IGF1 levels. PGH secretion is inhibited by glucose in vitro and in vivo, and is significantly decreased in the maternal circulation in cases of pregnancies with intrauterine growth retardation. PGH does not appear to have a direct effect on fetal growth, as this hormone is not detectable in the fetal circulation. However the physiological role of PGH might also include a direct influence on placental development via an autocrine or paracrine mechanism as suggested by the presence of specific GH receptors in this tissue.