Dexamethasone increases growth hormone (GH)-releasing hormone (GRH) receptor mRNA levels in cultured rat anterior pituitary cells.

To examine the effects of glucocorticoid (GC) on growth hormone (GH)-releasing hormone (GRH) receptor gene expression, a highly-sensitive and quantitative reverse-transcribed polymerase chain reaction (RT-PCR) method was used in this study. Rat anterior pituitary cells were isolated and cultured for 4 days. The cultured cells were treated with dexamethasone for 2, 6, and 24 h. GRH receptor mRNA levels were determined by competitive RT-PCR using a recombinant RNA as the competitor. Dexamethasone significantly increased GRH receptor mRNA levels at 5 nM after 6- and 24 h-incubations, and the maximal effect was found at 25 nM. The GC receptor-specific antagonist, RU 38486 completely eliminated the dexamethasone-induced enhancement of GRH receptor mRNA levels. Dexamethasone did not alter the mRNA levels of beta-actin and prolactin at 5 nM for 24 h, whereas GH mRNA levels were significantly increased by the same treatment. The GH response to GRH was significantly enhanced by the 24-h incubation with 5 nM dexamethasone. These findings suggest that GC stimulates GRH receptor gene expression through the ligand-activated GC receptors in the rat somatotrophs. The direct effects of GC on the GRH receptor gene could explain the enhancement of GRH-induced GH secretion.