The study shows that the peptide GHRP-2 (and its sibling GHRP-6) can directly make human fetal pituitary cells release growth hormone, though it’s less powerful than the natural hormone GHRH. When both peptides are used together, they add up their effects but don’t boost each other beyond that. The peptide doesn’t change other hormones like ACTH or prolactin and can counteract the suppressive actions of IGF‑1 or somatostatin.

The synthetic hexapeptide GH-releasing peptide (GHRP) stimulates a dose-dependent release of GH in humans in vivo and in animals both in vitro and in vivo via a specific receptor in the hypothalamus and pituitary. To determine the action of GHRP in the human fetal pituitary, reverse transcription-PCR was performed, and GHRP receptor messenger ribonucleic acid expression was detected in fetal pituitaries of 18- and 31-week gestation. Therefore, primary human fetal pituitary cultures (second and third trimesters) were treated with GHRP-6. GHRP-6 dose-dependently increased GH secretion from human fetal pituitary cultures by up to 80% (P < 0.001; maximal effect achieved with 100 nmol/L), whereas GHRH (10 nmol/L) stimulated GH by up to 120% (P < 0.001). However, GHRP together with GHRH was additive (up to 2.8-fold GH induction) with no evidence of further positive synergy. In contrast to GHRH, GHRP-6 did not alter human ACTH and PRL levels. A treatment time of 2 h was required for maximal GH stimulation. GHRP-6 reversed suppressed GH levels in cultures cotreated with either insulin-like growth factor I (P < 0.0001) or somatostatin (P < 0.05). GHRP-6 and GHRP-2 (100 nmol/L) had similar effects in stimulating human GH release. These results show a direct in vitro action of GHRP on human pituitary cells. GHRP is less potent than GHRH in directly releasing GH from human somatotrophs, and only additive effects of these two peptides occur at the level of the human fetal pituitary.