In sheep pituitary cells, tiny amounts of GHRP‑2 together with low‑dose GHRH boosted growth‑hormone release more than larger doses. Low doses also changed the cells' receptor levels, making them more responsive to future GHRH or ghrelin signals. High doses stopped the benefit and even lowered receptor expression.

GH secretion is regulated by GHRH and somatostatin via actions on their specific receptors in pituitary somatotropes. Ghrelin and synthetic analogs, GHRPs, also stimulate GH release via GHS-receptors (GHS-R). To examine the long-term effect of GHRH and/or GHRP on somatotropes, primary cultured ovine somatotropes were treated with GHRH (10(-9) and 10(-8) M) and GHRP-2 (10(-8) and 10(-7) M) for up to 2 d. After treatment, culture medium was collected for GH assay, and total RNA was extracted for RT-PCR analysis. To evaluate cell cultures used in this report, somatotrope-enriched pituitary cells were challenged by 6 h GHRH and dexamethasone (DEX) treatment. As expected, GHRH significantly decreased, whereas DEX increased, the levels of GHRHR mRNA. Combined low doses of GHRH (10(-9) M) and GHRP-2 (10(-8) M) treatment for 24 h increased accumulated GH secretion, significantly more than that induced by high doses of GHRH (10(-8) M) and GHRP-2 (10(-7) M). While levels of GHRH-R mRNA increased, GHS-R mRNA levels were decreased by low doses of GHRH and GHRP-2 for 24 h. High doses of GHRH and/or GHRP-2 for 2 d did not increase GH secretion in the second day of treatment and reduced the level of GHRH-R mRNA. High doses of GHRP-2 treatment decreased the levels of both GHRH-R and GHS-R mRNA. Low doses of GHRH and/or GHRP-2 for 2 d increased the level of GHS-R mRNA without changing GHRH-R mRNA levels. Such treatment also increased ghrelin- (10(-9) M) or ghrelin/GHRH (10(-9) M)-induced GH secretion. These results suggest that low doses of GHRP-2 and GHRH prime somatotropes for stimulation by GHRH and ghrelin.