Naloxone enhances the increase in plasma growth hormone induced by alpha 2-adrenergic stimulation in healthy males.

There is evidence that the alpha 2-adrenergic agonist clonidine interacts with the opioid system. In the present investigations, the effect of naloxone on the increase in plasma GH induced by clonidine and the more specific alpha 2-agonist guanfacine was studied in man. In a single-blind study, five healthy males received in randomized order either the preservatives in the naloxone preparation (control) or naloxone at two different doses (10 or 100 micrograms/kg) followed by an infusion of either diluted preservatives or naloxone (5 or 50 micrograms X kg-1 X h-1, respectively). Fifteen min after the bolus dose, clonidine (3 micrograms/kg) or guanfacine (15 micrograms/kg) was infused over 10 min in a single-blind order. Both clonidine and guanfacine induced an increase in plasma GH (P less than 0.05). Pre-treatment with naloxone at the higher dosage resulted in an enhanced (P less than 0.05) GH response to clonidine and guanfacine, respectively, whereas the lower dosage of naloxone was without effect. The increase in plasma GH did not correlate with basal mean arterial blood pressure, nor with the changes in mean arterial blood pressure induced by clonidine or guanfacine. These results indicate that the increase in plasma GH induced by alpha 2-adrenergic stimulation in normotensive subjects involves opioid receptors with moderate sensitivity to naloxone.