Differential effects of deltorphin on arginine and galanin-induced growth hormone secretion in healthy man

Recently we demonstrated the inhibitory action on Growth Hormone (GH) secretion of an opioid heptapeptide, deltorphin (DT), that is highly selective in binding δ-opioid receptors. To investigate the possible mechanism leading to the decrease in GH secretion by specific activation of δ-opioidergic pathway in man, we compared, in normal subjects, the effect of DT on GH secretion responses to two different GH secretagogues, namely arginine (ARG) and galanin (GAL). DT completely blunted the GH response to ARG, whereas it attenuated the GH response to GAL, but not at a statistically significant level. We suggest that the specific activation of δ-opioid receptors in man may exert an inhibitory influence on GH secretion principally by modulating endogenous hypothalamic somatostatin (SRIH) release.