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Is GHRH Receptor Essential to GHRP-2-Induced GH Secretion in Primary Cultured Rat Pituitary Cells?

Lab of Animal Molecular Physiology, Faculty of Agriculture, Shinshu University, Naganoken 399-4598, Japan; Endocrine Cell Biology Group (C.C.), Prince Henry’s Institute of Medical Research, Clayton 3168, Australia

Address all correspondence and requests for reprints to: Sang-gun Roh, Lab of Animal Molecular Physiology, Faculty of Agriculture, Shinshu University, Naganoken 399-4598, Japan. E-mail: sangroh{at}gipmc.shinshu-u.ac.jp

Abstract

It is still controversial in rat whether the stimulation of GH secretion by GH-releasing peptides (GHRP) requires both GHRP receptor (GHRP-R) and GH-releasing hormone receptor (GHRH-R). To clarify this issue, we have postulated that inhibition of GHS-R or GHRH-R gene transcription should block GHRP-2-induced GH secretion. Rat pituitary cells were incubated for 3 days in the presence or absence of antisense 18-mer phosphorothiate oligonucleotides (ONs) complementary to the codon region of GHS-R or GHRH-R mRNAs. A significant decrease in GHRH-R and GHS-R mRNA levels was found in corresponding antisense-treated cells compared with the control cells treated with sense ON. Treatment with antisense GHS-R ON reduced (but not abolished) GHRP-2-induced GH secretion although GHRH-induced GH secretion was not altered. GHRH-stimulated GH secretion was totally abolished by the treatment with antisense GHRH-R ON, whereas GHRP-2 induced GH secretion was not affected. Treatment of cells with both GHS-R and GHRH-2 ONs however completely inhibited GHRH and GHRP-2-stimulated GH secretion. These results suggest that GHRH-R is vital for GHRH-induced GH secretion but only partially involved in GHRP-2-stimulated GH secretion under the condition of down-regulation of GHS-R gene transcription.

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