The effects of growth hormone (GH)-releasing peptides on GH secretion in perifused pituitary cells of adult male rats.

The present study investigated the GH secretory activities of two distinctively different peptides: human pancreatic GH-releasing factor 44 (GRF-44) and a synthetic peptide His-DTrp-Ala-Trp-DPhe-Lys-NH2 (GHRP). GH secretion was studied in perifused dispersed anterior pituitary cells from male rats 24 or 48 h postdispersion. GRF-44 was 60 times more potent than GHRP and elicited linear increases in GH secretion between 0.3 and 30 ng (0.06-6 pmol), whereas the GHRP dose range was 3-300 ng (3.44-344 pmol). Hourly pulses of GHRP (30 ng) and GRF-44 (12.5 ng) stimulated consistent GH responses. An apparent priming effect was observed with GRF-44 at a dose of 3 ng. Continuous infusion of either peptide resulted in desensitization, with GH secretion being monophasic during GHRP infusion and biphasic during GRF-44 infusion. These results demonstrate that GRF-44 and GHRP, two peptides that have striking differences in their structure and size, stimulate in vitro GH secretion with remarkable similarity. However, the differences in potency, slope of the dose response, and pattern of GH secretion during continuous GRF-44 or GHRP infusion suggest that each stimulates in vitro GH secretion by slightly different mechanisms. GHRP or similar synthetic peptides may be useful probes to study GH secretory mechanisms.