Assessment of growth hormone insulin like growth factor-I axis in Down’s syndrome

As GH therapy has been reported to increase growth velocity in children with Down’s syndrome (DS), we studied the GH-IGF-I axis in some DS patients affected by growth retardation without serious congenital malformation, malnutrition or pathological thyroid or adrenal function. IGF-I and IGF-II were evaluated in 39 patients in basal conditions. The patients were subsequently divided into two groups with respect to the IGF-I basal value: Group 1 (GR 1) consisting of patients with abnormally low basal IGF-I concentration as compared to age matched control subjects, group 2 (GR 2) consisting of patients with IGF-I in the normal range. In 6 GR 1 patients and 12 GR 2 patients we evaluated GH and IGF-I concentrations after stimulation with arginine (0.5 g/kg bw), and recombinant GH (4 IU im). In the same patients, GH radioreceptor assay and serum GH-binding protein were evaluated. In all patients IGF-II proved normal (534±23 ng/ml; mean±SE), while IGF-I was pathological in 36% of subjects. The cause of the defective IGF-I secretion in these patients does not seem to depend on an impaired GH axis, as no significant difference in arginine-stimulated GH peak values was seen between GR 1 (29.6±5.3 ng/ml) and GR 2 (15.1±2.24 ng/ml). IGF-I concentration evaluated 12, 24, and 48 h after arginine stimulation was significantly increased only in GR 2 patients (peak value: 0.95±0.1, p=0.0003 vs baseline; GR 1: 0.34±0.05 U/ml). By contrast, administration of GH produced a rise in IGF-I in both groups; though the mean peak value in GR 1 (0.63±0.09 U/ml; p=0.0005 vs baseline) was lower than that in GR 2 (1.07±0.1), the size of the increase in relation to the baseline values was similar in both groups (GR 1: 173±29%; GR 2: 87±16%). In agreement with these findings we have indirectly proved the presence of GH receptor in the GR 1 patients, and demonstrated their normal levels by evaluating the GH-binding protein (DS: 21.5±6.8% Bound/Total; controls: 23.6±11.8%). On the other hand, the evaluation of the GH RRA/IR-MA ratio revealed a presence of a reduced bioactivity of endogenous GH in GR 1 (0.81±0.1; GR 2: 1.05±0.06), and particularly in two patients (0.41 and 0.61 respectively). In conclusion, our data suggest that impaired growth in some patients affected by Down’s syndrome can be accounted for by a GH molecule that is immunoreactive but not bioactive.