INTERACTIONS OF GALANIN AND ARGININE ON GROWTH-HORMONE, PROLACTIN, AND INSULIN-SECRETION IN MAN

Galanin (GAL), a 29 amino acid neuropeptide, is known to increase both basal and growth hormone-releasing hormone (GHRH)-induced growth hormone (GH) secretion while not significantly increasing prolactin (PRL) secretion in man. GAL is also endowed with an inhibiting effect on glucose-stimulated insulin release in animals, but not in man. We studied the effect of GAL (80 pmol/kg/min infused over 60 minutes) on the arginine-(ARG, 30 g infused over 30 minutes) stimulated GH, PRL, insulin, and C-peptide secretion in eight healthy volunteers (age, 20 to 30 years). GAL induced an increase of GH (GAL v saline, area under curve [AUC], mean ± SEM: 316.5 ± 73.9 v 93.2 ± 20.9 μg/L/h, P < .05), but failed to modify both PRL and insulin secretion. GAL enhanced the ARG-induced stimulation of both GH (1,634.1 ± 293.1 v 566.9 ± 144.0 μg/L/h, P < .02) and PRL secretion (1,541.9 ± 248.8 v 1,023.8 ± 158.7 μg/L/h, P < .02). On the contrary, GAL blunted the ARG-stimulated insulin (816.3 ± 87.7 v 1,322.7 ± 240.9 mU/L/h, P < .05), as well as C-peptide secretion (105.1 ± 9.8 v 132.8 ± 17.3 μg/L/h, P < .02). ARG administration induced a transient increase of glucose levels (P < .01 v baseline) followed by a significant decrease (P < .05 v baseline). This latter effect was prevented by the coadministration of GAL. In conclusion, these results show that in man GAL potentiates the GH response to ARG, suggesting that these drugs act at the hypothalamic level, at least in part, via different mechanisms. The coadministration of GAL and ARG induces a marked increase of PRL secretion, although the mechanisms underlying this effect are unclear. The blunting effect of GAL on the ARG-induced insulin and C-peptide secretion suggests that this neuropeptide may also modulate β-cell secretion in man. © 1992.