Expression and homologous regulation of GH secretagogue receptor mRNA in rat adrenal gland.
OBJECTIVE
GH secretagogues (GHSs) elicit a variety of biological effects in several endocrine and non-endocrine target tIssues, including activation of the hypothalamic-pituitary-adrenal axis. The latter is mainly carried out through a central hypothalamic action; yet the possibility of additional effects directly at the adrenal level cannot be ruled out. The aims of this study were to evaluate the expression and homologous regulation of the GHS-receptor (GHS-R) gene in rat adrenal and to assess the effects of synthetic (GH releasing peptide-6 - GHRP-6) and natural (ghrelin) ligands of GHS-R upon basal and ACTH-stimulated corticosterone secretion in vitro.


METHODS
Analysis of adrenal expression of target mRNAs (GHS-R, GHS-R1a, ghrelin, and several steroidogenic factors) was conducted by means of primer-specific, semi-quantitative RT-PCR. Evaluation of corticosterone secretion by incubated adrenal tIssue was carried out by specific RIA.


RESULTS
RT-PCR analysis demonstrated expression of the GHS-R gene, but not of the gene encoding the cognate ligand ghrelin, in rat adrenal. Moreover, expression of the mRNA coding for the type 1a GHS-R (GHS-R1a), i.e. the biologically active receptor form, was demonstrated. The adrenal expression of the GHS-R message appeared under the regulation of homologous signals in vitro, as short-term incubation of adrenal samples in serum-free medium induced a significant increase in GHS-R mRNA levels that was inhibited by exposure to different doses of GHRP-6 (10(-9)-10(-5) mol/l) or ghrelin (10(-7) mol/l). Notably, an opposite pattern of homologous regulation of GHS-R gene expression was observed at the pituitary. Finally, short-term stimulation with increasing concentrations of GHRP-6 (10(-9)-10(-5) mol/l) or ghrelin (10(-7) mol/l) failed to alter basal and ACTH-stimulated corticosterone secretion in vitro, neither did it modify ACTH-stimulated mRNA expression levels of several upstream elements in the steroidogenic route: the steroidogenic acute regulatory (StAR) protein, and the enzymes P450 cholesterol side-chain cleavage (P450scc) and 3beta-hydroxysteroid dehydrogenase (3beta-HSD).


CONCLUSIONS
Our study provides novel evidence for the expression and homologous regulation of the GHS-R gene in rat adrenal. However, our results cast doubts on the possibility of direct adrenal actions of ligands of the GHS-R in the regulation of corticosterone secretion in the rat.
Barreiro
M L
ML
Department of Cell Biology, Physiology and Immunology, Section of Physiology, Faculty of Medicine, University of Cordoba, Avda Menendez Pidal s/n, 14004 Cordoba, Spain.
Pinilla
L
L
Aguilar
E
E
Tena-Sempere
M
M
D000311:Adrenal Glands / Q000378:metabolism* / Q000502:physiology; D000818:Animals; D015870:Gene Expression; D013006:Growth Hormone / Q000378:metabolism; D008297:Male; D012333:RNA, Messenger / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D011956:Receptors, Cell Surface / Q000235:genetics*; D043562:Receptors, G-Protein-Coupled; D054440:Receptors, Ghrelin; D020133:Reverse Transcriptase Polymerase Chain Reaction
D012333:RNA, Messenger; D011956:Receptors, Cell Surface; D043562:Receptors, G-Protein-Coupled; D054440:Receptors, Ghrelin; D013006:Growth Hormone