Dose dependency of the serum bio/immuno GH ratio in children during pharmacological secretion tests

Extrapituitary growth hormone

Pituitary somatotrophs secrete growth hormone (GH) into the bloodstream, to act as a hormone at receptor sites in most, if not all, tissues. These endocrine actions of circulating GH are abolished after pituitary ablation or hypophysectomy, indicating its pituitary source. GH gene expression is, however, not confined to the pituitary gland, as it occurs in neural, immune, reproductive, alimentary, and respiratory tissues and in the integumentary, muscular, skeletal, and cardiovascular systems, in which GH may act locally rather than as an endocrine. These actions are likely to be involved in the proliferation and differentiation of cells and tissues prior to the ontogeny of the pituitary gland. They are also likely to complement the endocrine actions of GH and are likely to maintain them after pituitary senescence and the somatopause. Autocrine or paracrine actions of GH are, however, sometimes mediated through different signaling mechanisms to those mediating its endocrine actions and these may promote oncogenesis. Extrapituitary GH may thus be of physiological and pathophysiological significance.

Is BaF3 bioassay useful to identify patients with bioinactive growth hormone?

We analyzed the ability of the BaF3 cell line bioassay to select patients with biologically inactive GH. We first evaluated the biological response of the Ba/F3-hGHR cells to rhGH additional doses from 10 to 5000 pg/ml. The concentration points corresponding to the linear part of the curve were selected. We then analyzed a group of sera, diluted like the standard, including the entire range of GH concentrations that can be analyzed by bioassay. The serum/standard area below the curve ratio was calculated. Serum GH immunoactivity determined by IMMULITE/GH bioactivity ratios was calculated. Our experimental data showed that GH-bioactivity/GH-immunoactivity ratios below 0.303 are indicative of a bioinactive GH molecule. This bioassay would recognize only extreme cases of GH bioinactivity, and it would not be a useful tool in the search for patients with altered forms of GH.

Variations in Biological and Immunological Activity of Growth Hormone during the Neonatal Period

BACKGROUND/AIMS

It was postulated that a high growth hormone (GH) bioactivity might explain the rapid growth rate of neonates. The aim of this study is to verify changes in serum GH biological potency (Bio-/Immuno-GH ratio) and their effects on serum growth factors during the first month of life in term and preterm babies.

METHODS

Blood samples were collected from 10 small-for-gestational-age preterm (SGAPT), 17 appropriate for gestational age preterm (AGAPT) and 26 AGA term (T) neonates on days 4, 15 and 30 of life to evaluate serum GH values measured by IFMA (IFMA-GH) and bioassay (Bio-GH), serum insulin-like growth factor-I (IGF-I) and IGF-binding protein-3 (IGFBP-3).

RESULTS

High serum Bio-GH values on the first few days of life correspond to high IFMA-GH values, suggesting full biological potency of circulating GH. Furthermore, IGF-I/IGFBP-3 molar ratio values in preterm babies were higher than in full-term infants.

CONCLUSIONS

These data confirmed the hypothesis that the higher growth velocity in the first month of life of preterm neonates is due to an increased bioavailability of IGF-I. A progressive maturation of the hypothalamic-pituitary-IGF-I axis without any alteration in the GH biological potency seems to underpin the increase of the growth factors early in life.

Differences in serum GH cut-off values for pharmacological tests of GH secretion depend on the serum GH method. Clinical validation from the growth velocity score during the first year of treatment

BACKGROUND

The serum GH cut-off value for pharmacological tests of GH secretion (PhT GH) depends on the type of test and also on the method used for determining serum GH. Cut-off serum GH values as different as 5-10 ng/ml, have been reported, and have been validated biochemically. We have used the growth velocity (GV)-standard deviation score (SDS) during the first year of treatment with rhGH to validate these cut-offs on a biological basis.

METHODS

Fifty pre-pubertal patients with short stature (height < or =-2 SDS and GV < or =-1.2 SDS) were studied. GH deficiency (GHD) was diagnosed in 39 patients, on the basis of clinical and auxological parameters and on the serum concentration of IGF-1, and non-GHD in the other 11 patients. Two PhT GH (arginine and clonidine) were carried out in the 50 patients. Serum GH was determined by two different methods: one detecting most of serum GH isoforms, named Total GH (HGH Bio-Tech, MAIA Clone), and another one, only detecting the 22 kDa GH, named 22K GH (GH-22K IFMA, Wallac).

RESULTS

Basal data: all patients with GHD and with non-GHD had maximal serum GH response (MaxR) values below and above the cut-off, respectively, for the serum Total GH and 22K GH. The mean 22K GH/Total GH ratio was similar to previous publications. Post-rhGH treatment data: the two groups improved their height SDS during the first year of treatment, particularly patients with GHD. A receiver-operator curve was used to define the best threshold for post-treatment GV-SDS that separates GHD from non-GHD patients. This value was 1.91 GV-SDS. A negative correlation between first year treatment GV-SDS and pre-treatment serum GH MaxR was found for the two assays (p < 0.001). Then, the best cut-off GV-SDS, previously calculated with the receiver-operator curve (1.91 SDS) was used to interpolate the corresponding serum GH values, as determined by the two methods. For Total GH, the value was 10.8 ng/ml, and for 22K GH, it was 5.4 ng/ml.

CONCLUSION

The cut-off values calculated by biological means to separate GHD from non-GHD were remarkably similar to those calculated biochemically (10.0 and 4.8 ng/ml, respectively) for Total and 22K GH. This is a biological validation for using different cut-off values, appropriate for each assay, to diagnose GHD.