Effect of growth hormone-releasing peptide 1-6 on GH secretion-stimulated by GHRH and pyridostigmine in lambs.

The GHRP-6 seems to act at a pituitary site, activating different intracellular messenger pathways from those utilized by GHRH, and at the hypothalamic level where receptors for GHRP-6 have been demonstrated. This study examines the effect of GHRP-6 on GH secretion in vitro and in vivo. Lamb adenohypophysial cell cultures were subjected to a challenge with 1) 10 nM GHRH-1-29; 2) 1 microM GHRP-6; and 3) 10 nM GHRH plus 1 microM GHRP-6. Both peptides released GH, GHRP-6 being less potent than GHRH, and the GH response to GHRH and GHRP-6 not being synergistic, without statistically significant differences between GHRH+GHRP-6 and GHRH alone. In in vivo studies, six lambs received 15 microg/kg GHRH (1-29) or 15 microg/kg GHRH plus 10 microg/kg GHRP-6 and six other lambs received 100 microg/kg GHRP-6 or 3 mg/kg pyridostigmine plus 100 microg/kg GHRP-6. The results have shown that the combination of GHRH plus GHRP-6, at low doses, causes higher GH peak (p < 0.05) and higher GH area under curve (p < 0.05) with respect to administration of GHRH alone. The administration of GHRP-6 plus pyridostigmine produced in a stronger GH response to GHRP-6 (100 microg/kg) in the amplitude of the GH peak (p < 0.001) and in the area under the GH response curve (p < 0.001). The complementary interactions of GHRP-6 with GHRH or pyridostigmine in releasing GH seem to indicate independent actions of these compounds. These results suggest that GHRP-6 potentiates the GH secretion stimulated by GHRH at the hypothalamic level, in these animals.