Regulation of GH-releasing hormone receptors.

SECRETION and gene expression of GH are controlled by hypothalamic GH-releasing hormone (GRH) which acts through a G protein-coupled receptor, GRH receptor, resulting in activation of cyclic AMP and A-kinase systems in the somatotropes [1]. Many humoral factors affect GH responses to GRH [1] and some of their effects may be caused by qualitative and quantitative changes in GRH receptors. Glucocorticoids are one of the most important factors modifying GH secretion and gene expression. Many lines of evidence indicated that glucocorticoids stimulate GH secretion and gene expression at least by their acute effects [25]. It has also been reported that dexamethasone (Dex) increases GRH binding sites in the rat pituitary membrane [6]. We therefore questioned whether Dex regulates GRH receptor gene expression, since most of the biological effects of glucocorticoids are attributed to their genomic actions on target genes in many types of cells [7]. To investigate GRH receptor gene expression at mRNA levels, a highly-sensitive method is required, because GRH receptor mRNA content is very low even in the anterior pituitary [8]. Conventional Northern blot analysis of total RNA in the pituitary tissue of a single rat fails to detect this lowabundant mRNA. Polymerase chain reaction (PCR) is extremely sensitive and quantitative PCR is useful for examining the regulation of GRH receptor mRNA levels. We previously established a competitive reverse transcription (RT)-PCR method [9, 10]. In the present study, we examined the