Sequential administration of arginine and arginine plus GHRH to test somatotroph function in short children

The hormonal diagnosis of GH deficiency in childhood is conventionally based on the GH response to at least two provocative stimuli. Among these, arginine (ARG) has long been considered a classical, centrally mediated stimulus of GH secretion. ARG is also able to potentiate the GH response to GHRH, likely inhibiting hypothalamic somatostatin; this combined test is one of the most potent to explore the maximal secretory capacity of somatotroph cells. Based on these premises, we verified whether the sequential administration of ARG and ARG+GHRH could be feasible as single step provocative test to evaluate the GH releasable pool in short children. To this goal, 48 normal short children (35 M and 13 F, 12.0+/-0.4 yr, PS 1: 255 II-IV: 23) underwent a test with ARG (0.5 g/kg i.v. from 0 to +30 min) followed by a coadministration of ARG (from +120 to 150 min) plus GHRH (1 microg/kg i.v. at +120 min). ARG alone elicited a clear GH response (mean peak vs baseline: 12.1+/-1.7 vs 2.0+/-0.4 microg/l, p<0.001, Cmax range 12-51.0 microg/l). Following this GH rise, the hormonal levels at +120 min approached to baseline levels (4.2+/-0.8 microg/l) but then showed marked response to the coadministration of ARG+GHRH. The GH peak following ARG+GHRH (mean peak: 47.8+/-3.3 microg/l, p<0.001; Cmax 22.4-150.0 microg/l) was clearly higher (p<0.001) than that recorded after ARG alone. The GH responses to both ARG and ARG+GHRH were independent of gender, puberty, height velocity, body mass index (BMI) and IGF-I levels. Nine normal short children (16%) had GH peaks lower than 7 microg/l after ARG alone, while none showed GH peak below 20 microg/l after ARG+GHRH. Thus, ARG alone is a good stimulus of GH secretion but false positive responses frequently occur in normal short children. ARG+GHRH is a more potent stimulus giving no false positive responses even after previous challenge with ARG alone. Testing with sequential administration of ARG and ARG+GHRH may allow the single step evaluation of the somatotroph response to central and pituitary stimuli in short children.