Effect of growth hormone-releasing peptide-2 (GHRP-2) and GH-releasing hormone (GHRH) on the the cAMP levels and GH release from cultured acromegalic tumours.
There is a difference between the sheep and rat somatotrophs in the response to growth hormone-releasing peptide-2 (GHRP-2), which raises the question of what the response may be in human somatotrophs. In the present study, cells were obtained from seven human acromegalic tumours and the effects of GHRP-2 were studied. Cells were dissociated and kept in primary culture for 1-3 weeks before experimentation. Application of GHRP-2 for 30 min induced a significant increase in GH secretion from the cultured cells from all seven tumours whereas human GH-releasing hormone (hGHRH) at a dose of 10 nM induced a significant GH release in only four of seven tumours. The intracellular levels of cAMP in all seven tumours were significantly increased by both 10 nM GHRP-2 and GHRH, but the response to GHRH was significantly higher than the response to GHRP-2. The adenylyl cyclase inhibitor, MDL 12330A, blocked the effect of GHRH and GHRP-2 on intracellular cAMP levels, whereas the Ca2+ channel blocker Co2+ (0.5 mM) did not attenuate the cAMP response. For the tumours in which GH secretion was increased by GHRH and GHRP-2, the cAMP antagonist Rp-cAMP blocked the GH response to GHRH but not to GHRP-2. When a protein kinase A (PKA) inhibitor (H89) was applied, GHRH stimulated GH release was blocked, but cAMP accumulation was not affected. The response to GHRP-2 was not altered by H89. Calphostin C [a protein kinase C (PKC) inhibitor] reduced the effect of GHRP-2 on the secretion of GH but did not affect the response to GHRH. Both GHRH and GHRP-2 increased the intracellular Ca2+ concentration in a concentration-dependent manner. We conclude that (1) GHRH increases GH secretion from human GH tumours via the cAMP pathway whereas GHRP-2 increases GH secretion mainly via the PKC pathway; (2) GHRH increases cAMP (without GH release) in a subset of tumours whereas GHRP-2 increases cAMP levels (slightly) and GH secretion in all tumours; and (3) GHRP-2 and GHRH do not act on the same receptor on human somatotrophs derived from acromegalic tumours.
Prince Henry's Institute of Medical Research, Clayton, Victoria, Australia.
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Chen
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Pullar
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Loneragan
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Zhang
I J
IJ
Clarke
D000172:Acromegaly / Q000378:metabolism* / Q000473:pathology; D000067956:Adenylyl Cyclase Inhibitors; D002121:Calcium Channel Blockers / Q000494:pharmacology; D000242:Cyclic AMP / Q000037:antagonists & inhibitors / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D007987:Gonadotropin-Releasing Hormone / Q000494:pharmacology*; D013006:Growth Hormone / Q000378:metabolism*; D006728:Hormones / Q000494:pharmacology*; D006801:Humans; D009842:Oligopeptides / Q000494:pharmacology*; D010911:Pituitary Neoplasms / Q000378:metabolism* / Q000473:pathology; D047428:Protein Kinase Inhibitors
D000067956:Adenylyl Cyclase Inhibitors; D002121:Calcium Channel Blockers; D004791:Enzyme Inhibitors; D006728:Hormones; D009842:Oligopeptides; D047428:Protein Kinase Inhibitors; D007987:Gonadotropin-Releasing Hormone; D013006:Growth Hormone; D000242:Cyclic AMP; C091874:growth hormone-releasing peptide-2