Effects in vitro of New Growth Hormone Releasing Peptide (GHRP‐1) on Growth Hormone Secretion from Ovine Pituitary Cells in Primary Culture

Continuous perifusion of pituitary cells was used to study the effects of a newly synthesized GHRP (GHRP‐1 or KP 101) on growth hormone (GH) secretion from ovine pituitary cells and these have been compared to effects of growth hormone‐releasing factor (GRF) and the original growth hormone‐releasing peptide (GHRP‐6). GH was continuously released at a constant rate during perifusion and secretion was increased by KP 101, GHRP‐6 and GRF in a dose‐dependent manner. The half‐maximal effective dose of KP 101 and GHRP‐6 was 10⁻⁷ M, an order of magnitude higher than that for GRF. The maximal effects of KP 101 and GHRP‐6 were similar but significantly less than the maximal effect of GRF. Blockade of calcium channels with Cd²⁺ (2 mM) totally and reversibly abolished the releasing effects of all three peptides. Like GHRP‐6, the GH release induced by KP 101 was not affected by a GRF antagonist ([Ac‐Tyr¹, D‐Arg²]‐GRF 1–29, 1 μM) which significantly reduced the effect of GRF on GH release. For each peptide, the response to a second application (1 h after the first application) was lower than the first response. When GRF (or KP 101, GHRP‐6) was applied first and then KP 101 or GHRP‐6 (or GRF) given 1 h later, the second response was not attenuated. Only a small additive effect on the release of GH by GRF was obtained by the co‐administration of either KP 101 or GHRP‐6. This result was achieved with maximal doses of the peptides, but not with half‐maximal doses. These data indicate that KP 101 has similar potency and GH releasing properties to GHRP‐6 but both are less potent than GRF. There is no synergistic effect of the peptides with GRF and only a small additive effect of KP 101 or GHRP‐6 on GRF‐stimulated GH release from ovine pituitary cells in vitro. KP 101 stimulates GH release by a mechanism that involves a common step employed also by GRF and GHRP‐6, an increase in calcium influx. In addition, our data strongly suggest that KP 101 like GHRP‐6 do not act through the GRF receptor and that there is no cross‐desensitization the GRF elicited response.