Intracellular signaling mechanisms mediating ghrelin-stimulated growth hormone release in somatotropes.
Ghrelin is a newly discovered peptide that binds the receptor for GH secretagogues (GHS-R). The presence of both ghrelin and GHS-Rs in the hypothalamic-pituitary system, together with the ability of ghrelin to increase GH release, suggests a hypophysiotropic role for this peptide. To ascertain the intracellular mechanisms mediating the action of ghrelin in somatotropes, we evaluated ghrelin-induced GH release from pig pituitary cells both under basal conditions and after specific blockade of key steps of cAMP-, inositol phosphate-, and Ca2+-dependent signaling routes. Ghrelin stimulated GH release at concentrations ranging from 10-10 to 10-6 m. Its effects were comparable with those exerted by GHRH or the GHS L-163,255. Combined treatment with ghrelin and GHRH or L-163,255 did not cause further increases in GH release, whereas somatostatin abolished the effect of ghrelin. Blockade of phospholipase C or protein kinase C inhibited ghrelin-induced GH secretion, suggesting a requisite role for this route in ghrelin action. Unexpectedly, inhibition of either adenylate cyclase or protein kinase A also suppressed ghrelin-induced GH release. In addition, ghrelin stimulated cAMP production and also had an additive effect with GHRH on cAMP accumulation. Ghrelin also increased free intracellular Ca2+ levels in somatotropes. Moreover, ghrelin-induced GH release was entirely dependent on extracellular Ca2+ influx through L-type voltage-sensitive channels. These results indicate that ghrelin exerts a direct stimulatory action on porcine GH release that is not additive with that of GHRH and requires the contribution of a multiple, complex set of interdependent intracellular signaling pathways.
Malagón
María M
MM
Department of Cell Biology, University of Córdoba, Córdoba, Spain. bc1mapom@uco.es.
Luque
Raul M
RM
Ruiz-Guerrero
Ernesto
E
Rodríguez-Pacheco
Francisca
F
García-Navarro
Socorro
S
Casanueva
Felipe F
FF
Gracia-Navarro
Francisco
F
Castaño
Justo P
JP
D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000378:metabolism; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004347:Drug Interactions; D005260:Female; D054439:Ghrelin; D013006:Growth Hormone / Q000378:metabolism*; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology; D036361:Peptide Hormones / Q000494:pharmacology*; D010902:Pituitary Gland / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D011493:Protein Kinase C / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects* / Q000502:physiology*; D013004:Somatostatin / Q000494:pharmacology; D034421:Sus scrofa; D010738:Type C Phospholipases / Q000378:metabolism
D054439:Ghrelin; D036361:Peptide Hormones; D013004:Somatostatin; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; D010738:Type C Phospholipases; D000262:Adenylyl Cyclases; D002118:Calcium