ABSTRACT

In a retrospective analysis, we have compared the response of serum GH concentration to insulin-induced hypoglycaemia in 148 short prepubertal children (114 males, 34 females) aged between 3·9 and 11·9 years with the growth rate of the individual to determine 'cut-off' values for the diagnosis of GH insufficiency.

Sixty-three children grew with a height velocity standard deviation score (SDS) greater than −0·8 (group 1), which represents the growth velocity of children progressing along or closely parallel to the third height centile. Eighty-five children had a height velocity SDS of less than −0·8 (group 2). Median peak serum GH concentration responses to insulin-induced hypoglycaemia were 19·9 mU/l (range 1·5–54·4) in group 1 and 9·9 mU/l (range 0·7–46·2) in group 2 (Mann–Whitney; P < 0·001).

Using growth rate as the determinant of normality, the efficiency, sensitivity and specificity of the insulin-induced hypoglycaemia test were calculated using different serum GH concentration cut-off values to diagnose GH insufficiency. In our (Hybritech) assay, a cut-off value of 13·5 mU/l provided optimal performance in terms of efficiency (66%), sensitivity (64%) and specificity (70%).

The response of serum GH concentration to insulin-induced hypoglycaemia in short children growing at different growth rates was continuous. Each laboratory measuring serum GH concentrations needs to construct its own 'normal' cut-off value.

Journal of Endocrinology (1992) 133, 447–450

Abstract

The effects of a recombinant human GH-binding protein (rhGHBP; amino acids 1–238) on GH stimulation of rat Nb2 lymphoma cells were examined with an eluted stain assay system (ESTA). This precise bioassay utilizes the colorimetric reduction by stimulated Nb2 cells of a yellow tetrazolium salt (3-[4,5-dimethyl-thiazol-2-yl]-2,5-diphenyl-tetrazolium bromide) to a purple formazan as its end-point. The use of a lactogenic bioassay allowed the investigation of hGHBP specificity for human GH (hGH) as opposed to prolactin. rhGHBP inhibited pituitary hGH bioactivity in a dose-dependent manner. No significant inhibition of prolactin or ACTH bioactivity occurred. It was confirmed that recombinant 20 kDa hGH also stimulated the Nb2 cells and that its relative potency was ∼ 10% of that of pituitary-derived hGH. Stimulation by 20 kDa hGH was also inhibited by rhGHBP. The highly quantitative ESTA system demonstrated that the binding protein inhibited in a competitive manner. hGH activation of the Nb2 cells did not appear to be governed by a Michaelian first-order reaction. As might then be anticipated, the concentration of rhGHBP required for 50% inhibition of GH bioactivity (IC₅₀) changed with agonist concentrations for both 20 kDa and 22 kDa hGH. However, with equimolar concentrations of these two isohormones, the IC₅₀ of the binding protein was virtually identical. Potentiation of hGH bioactivity in vivo by low concentrations of hGHBP has been reported but was not observed in our in vitro system when tested over a wide range of binding protein concentrations.

In conclusion, the ESTA bioassay system permitted a detailed characterization of the inhibition of hGH bioactivity by rhGHBP. The hormonal specificity confirms earlier radioligand binding studies, except that we found that the 20 kDa hGH variant interacts with the rhGHBP.

Journal of Endocrinology (1994) 140, 445–453