Differential regulation of GHRH-receptor and GHS-receptor expression by long-term in vitro treatment of ovine pituitary cells with GHRP-2 and GHRH

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.