Ghrelin-induced growth hormone release from isolated rat anterior pituitary cells depends on intracellullar and extracellular Ca2+ sources.
Ghrelin, a novel growth hormone (GH)-releasing peptide, was isolated from the rat stomach as an endogenous ligand to the growth hormone secretagogues receptor. It is known that ghrelin stimulates the release of GH from the rat anterior pituitary gland, but the intracellular signal cascade in somatotrophs has not yet been clarified. In this study, using an isolated cell perifusion system, we examined whether ghrelin- and growth hormone-releasing hormone (GHRH)-induced GH secretion from rat pituitary cells depends on intra- and extracellular Ca(2+) and voltage-gated Ca(2+) channels. For this purpose, we first measured ghrelin- or GHRH-stimulated GH concentration following treatment with reduced extracellular Ca(2+) and/or thapsigargin, an endoplasmic reticulum Ca(2+) ATPase inhibitor. Reductions in the extracellular Ca(2+) concentration to 0.25 mM and to 0 mM resulted in decreases in ghrelin-stimulated GH secretion to 81% and 39% and decreases in GHRH-induced GH secretion to 83% and 13%, respectively, compared to the levels in the case of 2.5 mM Ca(2+) concentration, suggesting that extracellular Ca(2+) is essential for both ghrelin- and GHRH-induced GH secretion. Pretreatment with thapsigargin resulted in a reduction in ghrelin-induced GH secretion to approximately 60% of the control level, but GHRH treatment had not effect on the GH secretion. Moreover, preincubation with thapsigargin and 0 mM extracellular Ca(2+) concentration resulted in significant inhibition of GHRH- and ghrelin-induced GH secretion. Subsequently, to determine whether ghrelin-stimulated GH secretion was induced through voltage-gated Ca(2+) channels, we measured the ghrelin-stimulated GH concentration following treatment with nifedipine, an L-type Ca(2+) channel inhibitor, and found that the amount of GH secretion was reduced to 44% of the control level. Furthermore, by replacement of extracellular Na(2+) in the medium with N-methyl-D(-)-glucamine, an impermeable molecule, GH secretion was reduced to 47%. In this study, we demonstrated that the GH-stimulatory effect of ghrelin, unlike that of GHRH, is achieved through both intracellular and extracellular Ca(2+) sources and that ghrelin-induced extracellular Ca(2+) influx involves an L-type voltage-gated Ca(2+) channel and Na(+) influx.
Yamazaki
M
M
Department of Regulation Biology, Faculty of Science, Saitama University, 255 Shimo-ohkubo, Sakura-ku, Saitama 338-8570, Japan.
Kobayashi
H
H
Tanaka
T
T
Kangawa
K
K
Inoue
K
K
Sakai
T
T
D000818:Animals; D019540:Area Under Curve; D002118:Calcium / Q000008:administration & dosage / Q000494:pharmacology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D054439:Ghrelin; D013006:Growth Hormone / Q000378:metabolism*; D066298:In Vitro Techniques; D008297:Male; D009543:Nifedipine / Q000494:pharmacology; D036361:Peptide Hormones / Q000494:pharmacology*; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D011863:Radioimmunoassay; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000494:pharmacology
D002121:Calcium Channel Blockers; D054439:Ghrelin; D036361:Peptide Hormones; D013006:Growth Hormone; D012964:Sodium; D009543:Nifedipine; D002118:Calcium