Hexarelin, a potent GHRP analogue: interactions with GHRH and clonidine in young and aged dogs.

The GH-releasing activity of hexarelin was evaluated in unanesthetized young and aged dogs. Hexarelin (15.6-250 micrograms/kg, i.v.) significantly stimulated GH secretion in a dose-dependent fashion in dogs of both age groups. The ability of hexarelin to potentiate the GH-releasing effect of GHRH (2 micrograms/kg, i.v) and clonidine (4 micrograms/kg, i.v.) was then tested. Hexarelin strikingly potentiated the effect of GHRH both in young and aged dogs, whereas it potentiated the effect of clonidine in young dogs only. Because clonidine acts on the hypothalamus to release GHRH, its failure to synergize with hexarelin in aged dogs is likely due to an age-related impairment of GHRH-secreting neurons.