The study shows that GHRP‑2 directly triggers growth hormone release from cow pituitary cells and that its effect adds up with the natural hormone‑releasing factor (GHRF). The hormone boost depends on calcium entering the cells and involves protein‑kinase C and cAMP signaling pathways. Blocking calcium channels, the GHRF receptor, or adding somatostatin reduces the effect.

We conducted this study to investigate the mechanisms of action of growth hormone-releasing peptide-2 (D-Ala-D-beta Nal-Ala-Trp-D-Phe-Lys-NH2; GHRP-2) in bovine anterior pituitary primary cell culture. Doses of GHRP-2 from 10(-13) to 10(-7) M) increased (P < .05) GH secretion. The GHRP-2 (10(-7) M) and GH-releasing factor (GRF; 10(-7) M) administered together had an additive effect on the release of GH (P < .05). Somatostatin (1 microM) decreased GH secretion in response to GHRP-2 and(or) GRF (P < .05). Secretion of GH in response to GHRP-2 was blocked (P < .01) by a GRF receptor antagonist (.1 microM). Nifedipine (10 microM), a voltage-dependent Ca2+ channel blocker, inhibited (P < .01) GHRP-2-stimulated GH release. The GH release in response to GHRP-2 and 4 beta-phorbol-12-myristate-13-acetate (10(-7) M), a protein kinase C activator, was additive (P < .01). Forskolin (30 microM), a cAMP elevating agent, further stimulated (P < .01) the GH release in response to GHRP-2. Bovine GH concentrations in culture media were assayed by indirect competitive enzyme immunoassay. These results showed that GHRP-2 1) stimulates GH secretion from bovine pituitary cells, 2) may partially act via GRF receptor, 3) has GH secretion activity caused by Ca2+ influx via Ca2+ channels, and 4) may increase GH secretion via protein kinase C and cAMP pathways.