Evidence for a role of protein kinase-C in His-D-Trp-Ala-Trp-D-Phe-Lys-NH2-induced growth hormone release from rat primary pituitary cells

Endocrinology. 1991 Dec;129(6):3337-42. doi: 10.1210/endo-129-6-3337.


We have recently reported that His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP-6) synergizes with GH-releasing factor (GRF) to increase GH release and cAMP accumulation in rat pituitary cells in vitro. This study was undertaken to further investigate the mechanism of action of GHRP-6 on GH release, particularly the involvement of protein kinase-C. Forskolin (10(-5) M), A23187 (10(-6) M), and phorbol 12-myristate 13-acetate (PMA; 10(-7) M) all stimulated GH release. However, only PMA can mimic the synergistic effects of GHRP-6 on GRF-stimulated GH release and intracellular cAMP accumulation. 4 alpha-Phorbol 12,13-didecanoate, an inactive phorbol ester, was unable to stimulate GH release or potentiate the effect of GRF. Extracellularly added phospholipase-C not only stimulated GH release in a dose-dependent manner, but also potentiated GRF-induced GH release. Phloretin, a protein kinase-C inhibitor, in a concentration range of 10-250 microM had very little or no effect on basal and GRF-stimulated GH release, but markedly inhibited the stimulatory effects induced by either PMA or GHRP-6. Incubation of rat pituitary cells with 10(-6) M PMA for 24 h completely down-regulated protein kinase-C, since such PMA-pretreated cells did not release GH in response to a second dose of PMA. The protein kinase-C-depleted cells had an attenuated GHRP-6 response, but they responded normally to GRF. Moreover, the synergistic effects of GHRP-6 and GRF on GH release and cAMP accumulation were also greatly inhibited by protein kinase-C down-regulation. These data suggest that the effects of GHRP-6 on GH release, either alone or together with GRF, are at least partially mediated via the activation of protein kinase-C.

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Calcimycin / pharmacology
  • Colforsin / pharmacology
  • Cyclic AMP / metabolism
  • Drug Synergism
  • Growth Hormone / metabolism*
  • Growth Hormone-Releasing Hormone / pharmacology
  • Male
  • Molecular Sequence Data
  • Oligopeptides / pharmacology*
  • Phloretin / pharmacology
  • Pituitary Gland / drug effects
  • Pituitary Gland / metabolism*
  • Protein Kinase C / antagonists & inhibitors
  • Protein Kinase C / metabolism*
  • Rats
  • Rats, Inbred Strains
  • Tetradecanoylphorbol Acetate / pharmacology


  • Oligopeptides
  • Colforsin
  • Calcimycin
  • growth hormone releasing hexapeptide
  • Growth Hormone
  • Growth Hormone-Releasing Hormone
  • Cyclic AMP
  • Protein Kinase C
  • Tetradecanoylphorbol Acetate
  • Phloretin