The synergistic effects of His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 on growth hormone (GH)-releasing factor-stimulated GH release and intracellular adenosine 3',5'-monophosphate accumulation in rat primary pituitary cell culture.
His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP-6) stimulated GH release from rat primary pituitary cells in a time- and dose-dependent manner. Stimulation was observed after a 15-min, but not a 4-h, incubation. The concentrations of GHRP-6 required for half-maximal and maximal stimulation were 7 x 10(-9) and 10(-7) M, respectively. GH release induced by GHRP-6 was not affected by the addition of either naloxone or the GRF antagonist [N-Ac-Tyr1,D-Arg2]GRF-(1-29)-NH2. The latter inhibited GRF-stimulated GH release by shifting the dose-response curve to the right. His-D-Trp-D-Lys-Trp-D-Phe-Lys-NH2, an analog of GHRP-6, inhibited GH release stimulated by GHRP-6 without affecting that induced by GRF. When present together at maximal concentrations, GHRP-6 and GRF produced a synergistic effect on GH release. GHRP-6 had no effect on intracellular cAMP levels, whereas GRF increased intracellular cAMP concentrations by 3-fold. Combined treatment of pituitary cells with GRF and GHRP-6 resulted in a potentiation of the GRF-induced increase in cAMP levels. Basal GH release was reduced by 30% after pretreatment with GHRP-6 (10(-7) M) for 1 h. Pretreatment with GHRP-6 also decreased the subsequent response to GHRP-6, but not GRF. In contrast, pretreatment with GRF for 1 h had no effect on the subsequent action of GHRP-6 or GRF on GH release. The desensitization induced by GHRP-6 was completely reversed within 1 h after removal of the peptide. Results from this study indicate that GHRP-6 and GRF stimulated GH release from somatotrophs via different receptors and through discrete mechanisms.
Cheng
K
K
Merck, Sharp, and Dohme Research Laboratories, Department of Growth Biochemistry and Physiology, Rahway, New Jersey 07065.
Chan
W W
WW
Barreto
A
A
Convey
E M
EM
Smith
R G
RG
D000818:Animals; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000378:metabolism*; D004357:Drug Synergism; D013006:Growth Hormone / Q000378:metabolism*; D013007:Growth Hormone-Releasing Hormone / Q000031:analogs & derivatives / Q000494:pharmacology*; D007700:Kinetics; D008297:Male; D009270:Naloxone / Q000494:pharmacology; D009842:Oligopeptides / Q000494:pharmacology*; D010446:Peptide Fragments / Q000494:pharmacology; D010902:Pituitary Gland / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D017337:Sermorelin / Q000031:analogs & derivatives*
D009842:Oligopeptides; D010446:Peptide Fragments; D009270:Naloxone; C041048:growth hormone releasing hexapeptide; D017337:Sermorelin; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; C058677:somatotropin releasing hormone (1-29)amide, N-acetyl-tyrosyl(1)-arginyl(2)-; D000242:Cyclic AMP