Growth hormone bioactivity and immunoactivity in tall children

In subjects with constitutional tall stature, both low and high GH response to stimulation tests have been observed when measured by commercial kits. To investigate the reason for these conflicting results, we evaluated growt h hormone (GH) secretion using different assays as well as GH-binding prote in and insulinlike growth factor-I (IGF-I) concentrations in tall children. Serum samples were collected from 22 prepubertal constitutionally tall chi ldren, aged 2.87-13.25 years, during two pharmacological tests to evaluate serum GH levels measured by both immunofluorometric assay (IFMA) and the Nb 2 celt bioassay, Serum IGF-I values were evaluated by RIA. Circulating low affinity (LA) and high affinity (HA) CH-binding proteins (GHBPs) were evalu ated by FPLC gel filtration. Considering the highest serum CH levels as mea sured by IFMA, the 22 tall subjects were divided into two groups: group A i ncluding 16 children with blunted serum GH peak levels (5.78+/-0.68 ng/ml) and group B including 6 subjects with normal serum GH peak values (15.73+/- 1.56 ng/ml). No differences were observed in serum GH peak levels as measur ed by the Nb2 cell bioassay between group A (14.77+/-1.54 ng/ml) and group B (16.03+/-1.96 ng/ml), and between both groups and 11 age-and sex-matched controls (12.25+/-1.19 ng/ml). In group A, the Nb2 celt bioassay/IFMA ratio of serum GH peak levels (0.29+/-0.08) was significantly higher (p<0.05) th an in group B (0.07+/-0.01). No differences were found in serum LA-GHBP and HA-GHBP as well as in IGF-I concentrations between the 16 patients of grou p A and the 6 of group B. Likewise, no difference in auxological parameters was found between the two groups. The biological activity of GH evaluated using the Nb2 cell bioassay is similar in tall children with a low GH respo nse as measured by IFMA in comparison with those with a normal CH response, and is in agreement with both the auxological data and serum IGF-I concent rations. (C) 1999, Editrice Kurtis.