GHRP-6 induces a biphasic calcium response in rat pituitary somatotrophs.
The mechanism of action of His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP-6), a synthetic peptide which specifically induces the secretion of growth hormone (GH) in rat somatotrophs, is still poorly understood. We have studied the effects of GHRP-6 on the cytosolic free calcium concentration ([Ca2+]i) of somatotrophs in primary culture. [Ca2+]i was monitored in individual somatotrophs by dual emission microspectrofluorimetry, using Indo-1 as the intracellular fluorescent Ca2+ probe. A short application of GHRP-6 (10(-5) M, 10 s) induced a biphasic Ca2+ response in most cells (44%), which consisted in a rapid and large rise in [Ca2+]i followed by sustained oscillations. This response is dose dependent in a range of concentrations from 10(-10) to 10(-5) M. The first phase of the GHRP-6 response persisted in the absence of Ca2+ in the extracellular medium, whereas the second phase was inhibited. The application of Ca2+ channel blockers like cadmium chloride (200 microM) or PN-200-110 (200 nM) also prevented the second phase. Conversely, when the cells were pretreated with thapsigargin (TG) (100 nM), the first phase of the GHRP-6 Ca2+ response was abolished, whereas the second phase alone was preserved. When the cells were depleted in PKC by incubation with 10(-6) M PMA for 24 h, the second phase of the GHRP-6 response was inhibited, and only the first phase was maintained. These results were corroborated by using phloretin, a PKC inhibitor. These data show that GHRP-6 induces a biphasic elevation of the [Ca2+]i in rat somatotrophs. The first phase is probably due to mobilization of the intracellular Ca2+ stores, whereas the second phase is a PKC-dependent process.
Bresson-Bépoldin
L
L
Laboratoire de Neurophysiologie, CNRS URA 1200, Université de Bordeaux II, France.
Dufy-Barbe
L
L
D000595:Amino Acid Sequence; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D005260:Female; D005455:Fluorescent Antibody Technique; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000494:pharmacology*; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000378:metabolism*; D011493:Protein Kinase C / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology
D009842:Oligopeptides; C041048:growth hormone releasing hexapeptide; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate; D002118:Calcium