The growth hormone (GH) response to GH-releasing hormone (GHRH) is strongly inhibited by previous administration of recombinant human GH (rhGH), likely as a consequence of a somatostatin-mediated GH negative autofeedback. Hexarelin (HEX), a synthetic hexapeptide belonging to the GH-releasing peptide (GHRP) family, possesses a GH-releasing activity greater than that of GHRH both in animals and in man. The mechanism of action of GHRPs is yet to be completely clarified, although concomitant actions at the pituitary and hypothalamic level have been hypothesized. To further clarify the mechanisms of action underlying the GH-releasing activity of HEX, in six normal young volunteers we studied the effects of rhGH (2 U intravenously [IV]) on the GH response either to GHRH (2 microg/kg IV) or to HEX (2 microg/kg IV) alone or combined with GHRH and/or pyridostigmine ([PD], 120 mg orally). The GH-releasing effect of HEX was higher than that of GHRH (area under the curve [AUC], 2,200.8 +/- 256.9 v 792.2 +/- 117.6 microg/L/h, P < .001), whereas combined administration of the two substances induced a true synergistic effect, with GH release after HEX plus GHRH (4,259.2 +/- 308.0 microg/L/h) being higher (P < .02) than the arithmetic sum of the GH increases induced by each compound separately administered. After rhGH administration, the GH-releasing effect of HEX was blunted (1,468.9 +/- 193.7 microg/L/h, P < .04; inhibition of 32.1%), whereas that of GHRH was nearly abolished (102.0 +/- 7.8 microg/L/h, P < .02; inhibition of 86.1%). The GH response to combined administration of HEX and GHRH was also blunted by the previous rhGH bolus (3,070.6 +/- 481.8 microg/L/h, P < .02; inhibition of 26.7%). PD did not modify the GH-releasing effect of HEX either alone (2,456.8 +/- 317.5 microg/L/h) or combined with GHRH (4,009.1 +/- 360.8 microg/L/h). rhGH was again able to blunt the GH response to HEX combined with PD (1,619.3 +/- 237.9 microg/L/h, P < .02), but failed to modify the GH response to HEX combined with GHRH and PD (4,548.4 +/- 698.0 microg/L/h). In conclusion, these results demonstrate that rhGH administration only blunts the GH-releasing activity of HEX, but abolishes that of GHRH. The blunting effect of rhGH on the GH response to HEX is probably mediated by a concomitant reduction in the activity of GHRH-secreting neurons and an increase of somatostatinergic tone.