Effect of GHRH and GHRP-2 treatment in vitro on GH secretion and levels of GH, pituitary transcription factor-1, GHRH-receptor, GH-secretagogue-receptor and somatostatin receptor mRNAs in ovine pituitary cells

Growth hormone (GH)-releasing hormone (GHRH) and GH-releasing peptides (GHRPs) stimulate the release of GH through their specific receptors on somatotropes. Combined GHRH and GHRP administration causes a synergistic GH release in vivo by an unknown mechanism. The current study focuses on the direct action of GHRH and GHRP on several molecular targets in somatotropes.To clarify the mechanism of action, ovine somatotropes were used to measure the expression of mRNAs encoding for GH, pituitary transcription factor-1 (Pit-1), GH-secretagogue receptor (GHS-R), GHRH-R, somatostatin receptor subtypes (sst-1 and sst-2) and GH release after GHRH and GHRP-2 treatment for 0.5, 1, 1.5 and 2 h.GHRH (10 nM), GHRP-2 (100 nM) and combined GHRH-GHRP-2 increased the levels of GH mRNA and GH release from 0.5 to 2 h in a time-dependent manner. The levels of Pit-1, GHRH-R and GHS-R mRNA were increased after 0.5 h treatment of cells with GHRH and GHRP-2. The levels of sst-1 but not sst-2 mRNA were significantly increased after 0.5 and 1 h of GHRH treatment. In contrast, both sst-1 and sst-2 mRNA expression was inhibited after 0.5-2 h of GHRP treatment.These data demonstrate a direct in vitro modification of ovine somatotropes by GHRH and GHRP-2 resulting in altered GHRH-R, GHS-R, Pit-1, sst-1, sst-2 and GH gene expression; this may underlie the regulatory action of GHRH and GHRP-2 on GH secretion.