Mechanisms of Action of Growth Hormone-Releasing Peptide-2 in Bovine Pituitary Cells

We conducted this study to investigate the mechanisms of action of growth hormone-releasing peptide-2 (D-Ala-D-βNal-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 μM) 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 μM). Nifedipine (10 μM), a voltage-dependent Ca ²⁺ channel blocker, inhibited (P < .01) GHRP-2-stimulated GH release. The GH release in response to GHRP-2 and 4β-phorbol-12-myristate-13-acetate (10^-7 M), a protein kinase C activator, was additive (P < .01). Forskolin (30 μM), 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 enzymeimmunoassay. 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.