The GH-releasing effect of hexarelin, a synthetic hexapeptide, in newborns is lower than in young adults

The aim of the present study was to verify the GH-releasing effect of Hexarelin, a synthetic hexapeptide, in newborns who are known to have GH hypersecretion likely due to hyperactivity of GHRH-secreting neurons while somatostatinergic activity seems not fully operative. We studied in 6 newborns (NB, 2.5 ± 2.1 days), 12 prepubertal children (PC, 9.8 ± 0.45 yr) and 12 young adults (YA, 28.2 ± 0.2 yr) the GH response to Hexarelin (HEX, 2 μg/kg i.v.) compared to that observed after GHRH (1 μg/kg i.v.) in 6 NB (4.2 ± 0.4 days), 12 PC (9.9 ± 0.6 yr) and 12 YA (31.0 ± 1.3 yr). GH levels were assayed basally and 30 and 60 min after drug administration. In NB, mean (± SEM) basal GH levels were higher while IGF-I levels were lower than those recorded in PC and YA (GH: 34.8 ± 1.9 vs 2.8 ± 0.4 vs 1.4 ± 0.4 μg/l, p <0.0006; IGF-I: 36.3 ± 1.9 vs 152.0 ± 11.5 vs 175.8 ± 15.3 μg/l, p <0.0007); in the last two groups GH and IGF-I levels were similar. The mean delta GH peak after HEX in NB (32.8 ± 4.7 μg/l) was similar to that in PC (34.6 ± 4.3 μg/l) and lower (p <0.01) than that in YA (56.2 ± 7.4 μg/l). Delta GH peak after GHRH in NB (60.1 ± 1.5) was higher than those in PC and YA (20.8 ± 4.8 and 22.8 ± 3.4 μg/l) (p <0.005 and <0.002, respectively). In NB, the GH response to HEX was lower (p<0.005) than to GHRH while in PC and YA the somatotrope response to HEX was higher (p <0.03 and 0.0004, respectively) than to GHRH. TheSe data demonstrate that the GH-releasing effect of Hexarelin undergoes age-dependent variation being lower in newborns than in young adults, opposite to that observed after GHRH administration. The evidence that Hexarelin releases less GH than GHRH in newborns but not in prepubertal children and in young adults makes unlikely the hypothesis that the GH-releasing effect of this hexapeptide is mediated via endogenous GHRH release.