Arginine counteracts the inhibitory effect of recombinant human insulin-like growth factor I on the somatotroph responsiveness to growth hormone-releasing hormone in humans

Insulin-like growth factor I (IGF-I) exerts a negative feedback effect on GH secretion via either direct actions at the pituitary level or indirect ones at the hypothalamic level, through stimulation of somatostatin (SS) and/or inhibition of GHRH release. In fact, recombinant human IGF-I (rhIGF-I) in humans inhibits spontaneous GH secretion as well as the GH response to GHRH and even more to GH/GH-releasing peptides, whose main action is on the hypothalamus, antagonizing SS and enhancing GHRH activity. The aim of the present study was to further clarify in humans the mechanisms underlying IGF-I-induced inhibition of somatotroph secretion. In six normal young volunteers (all women; mean +/- SEM: age, 28.3 +/- 1.2 yr; body mass index, 21.3 +/- 1.2 kg/m(2)) we studied the GH response to GHRH (1 mu g/kg, iv, at 0 min), both alone and combined with arginine (ARG; 0.5 g/kg, iv, from 0-30 min), which probably acts via inhibition of hypothalamic SS release, after pretreatment with rhIGF-I (20 mu g/kg, se, at -180 min) or placebo. rhIGF-I increased circulating IGF-I levels (peak at -60 us. -180 min: 54.9 +/- 3.9 vs. 35.9 +/- 3.3 mmol/L; Pt 0.05) to a reproducible extent, and these levels remained stable and within the normal range until 90 min. The mean GH concentration over 3 h (from -180 to 0 min) before ARG and/or GHRH was not modified by placebo or rhIGF-I. After placebo, the GH response to GHRH (peak, 23.6 + 2.9 mu g/L) was strikingly enhanced (P < 0.05) by ARG coadministration (69.6 +/- 9.9 mu g/L). rhIGF-I blunted the GH response to GHRH (13.1 +/- 4.5 mu g/gL; P < 0.05), whereas that to GHRH plus ARG was not modified (59.5 +/- 8.9 mu g/L), although it occurred with some delay. Mean glucose and insulin concentrations were not modified by either placebo or rhIGF-I. In conclusion, ARG counteracts the inhibitory effect of rhIGF-I on somatotroph responsiveness to GHRH in humans. These findings suggest that the acute inhibitory effect of rhIGF-I on the GH response to GHRH takes place on the hypothalamus, possibly via enhancement of SS release, and that ARG overrides this action.