The hypothalamic
peptide PRL-releasing
peptide (PrRP) has recently been cloned and identified as a
ligand of an orphan pituitary receptor that stimulates in vitro PRL secretion. PrRP also induces PRL release in rats in vivo, especially in normal cycling females. However, no information on the effects of PrRP in the human is available. To elucidate the role of PrRP in regulating human
anterior pituitary hormones, we used human PrRP-31 in primary cultures of human pituitary tissues, including fetal (20--27 weeks gestation) and normal adult pituitaries, as well as PRL- and GH-secreting
adenomas. PrRP increased PRL secretion from human fetal pituitary cultures in a dose-dependent manner by up to 35% (maximal effect achieved with 10 nM), whereas TRH was slightly more potent for PRL release. Coincubation with
estradiol resulted in enhanced fetal PRL response to PrRP, and GH release was only increased in the presence of
estradiol. Although PRL secretion from PRL-cell
adenomas was not affected by PrRP, PrRP induced PRL release from cultures of a GH-cell
adenoma that cosecreted PRL. PrRP enhanced GH release in several GH-secreting
adenomas studied by 25--27%, including GH stimulation in a mixed PRL-GH-cell
tumor. These results show for the first time direct in vitro effects of PrRP-31 on human pituitary cells. PrRP is less potent than TRH in releasing PRL from human fetal lactotrophs and is unable to release PRL from PRL-cell
adenomas in culture, but stimulated GH from several
somatotroph adenomas. Thus, PrRP may participate in regulating GH, in addition to PRL, in the human pituitary.