M. Tamaki et al., DEXAMETHASONE INCREASES GROWTH-HORMONE (GH)-RELEASING HORMONE (GRH) RECEPTOR MESSENGER-RNA LEVELS IN CULTURED RAT ANTERIOR-PITUITARY-CELLS, Journal of neuroendocrinology, 8(6), 1996, pp. 475-480
To examine the effects of glucocorticoid (Gd) on growth hormone (GH)-r
eleasing hormone (GRH) receptor gene expression, a highly-sensitive an
d quantitative reverse-transcribed polymerase chain reaction (RT-PGR)
method was used in this study. Rat anterior pituitary cells were isola
ted and cultured for 4 days. The cultured cells were treated with dexa
methasone for 2, 6, and 24 h. GRH receptor mRNA levels were determined
by competitive RT-PCR using a recombinant RNA as the competitor. Dexa
methasone significantly increased GRH receptor mRNA levels at 5 nM aft
er 6- and 24 h-incubations, and the maximal effect was found at 25 nM.
The GC receptor-specific antagonist, RU 38486 completely eliminated t
he dexamethasone-induced enhancement of GRH receptor mRNA levels. Dexa
methasone 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 sugges
t that GC stimulates GRH receptor gene expression through the ligand-a
ctivated GC receptors in the rat somatotrophs. The direct effects of G
C on the GRH receptor gene could explain the enhancement of GRH-induce
d GH secretion.