Growth hormone (GH)-releasing peptide and GH releasing hormone stimulate GH release from subpopulations of somatotrophs in rats.
The synthetic hexapeptide GH-releasing peptide (His-D-Trp-Ala-Trp-D-Phe-Lys-NH2; GHRP-6) and GH releasing hormone (GHRH) are both potent stimulators of GH release in rats. Using reverse hemolytic plaque assay (RHPA), we have compared the effects of human GHRH and GHRP-6 on GH release from the dispersed individual cells of rat anterior pituitary. In a single RHPA, we quantified the percentage of plaque forming cells (% PFC) and their mean plaque area (MPA) after 30 min-incubation, and calculated a total secretion index (TSI) by multiplying % PFC and MPA. 10 nM GHRH and 100 nM GHRP-6 each caused a significant increase in % PFC (%) (GHRH 39.15, GHRP-6 29.4, vs vehicle 24.3, P < 0.01), MPA (x 10(-2) microns2) (GHRH 124.04, GHRP-6 94.80, vs vehicle 44.57, P < 0.01) and TSI (x 10(-2)) (GHRP-6 32.87, vs vehicle 10.84, P < 0.01). Simultaneous addition of both secretagogues caused a further increase in GH release (%PFC 46.4, MPA 142.55, TSI 69.82, P < 0.01 vs vehicle), although the effect was additive but not synergistic. Somatostatin analog, SMS201-995 (SMS) partially suppressed all parameters in GH secretion after stimulation by GHRH and/or GHRP-6. A double RHPA was then performed to test whether all somatotrophs respond equally to GHRH and GHRP-6 or some cells formed plaques only be either GHRH or GHRP-6. There were somatotrophs responsive to only GHRH (23.3% vs control 6.2%, P < 0.01), those responsive to only GHRP-6 (11.9% vs control 6.1%, P < 0.01), and those responsive to both GHRH and GHRP-6 (7.8% vs control 0.2%, P < 0.01). These results confirmed the previous findings that GHRP-6 and GHRH directly but independently stimulate GH release from the pituitary cells, and further suggest that presence of at least three functionally distinct somatotroph subpopulations concerning the responsiveness to GHRP-6 and GHRH in rats.
Mitani
M
M
Department of Medicine, Kobe University, School of Medicine, Japan.
Kaji
H
H
Abe
H
H
Chihara
K
K
D000818:Animals; D002478:Cells, Cultured; D013006:Growth Hormone / Q000378:metabolism*; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology*; D006462:Hemolytic Plaque Technique; D006801:Humans; D008297:Male; D009842:Oligopeptides / Q000494:pharmacology*; D010903:Pituitary Gland, Anterior / Q000166:cytology / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley
D009842:Oligopeptides; C041048:growth hormone releasing hexapeptide; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone