Growth hormone therapy in normal short children induces a transitory decrease in plasma growth hormone releasing hormone levels and in human growth hormone responsiveness to exogenous growth hormone releasing hormone

Suppression and recovery of GH secretion after GH injection in non-GH deficient short children

OBJECTIVE

To identify the effect of exogenous GH on endogenous GH secretion in 48 non-GH deficient short children participating in a placebo-controlled trial of GH therapy on final adult height.

DESIGN

Night GH secretion (mean of levels every 20 min from 20.00 to 08.00 h) was evaluated at baseline, 6 months before starting placebo or GH (somatotropin, 0.222 mg/kg/ week, divided into 3 doses each week). At 6 months after starting injections, blood samples for GH were obtained hourly for 24 h after an injection, and every 20 min on each of the next two nights (with no additional placebo or GH injection).

RESULTS

IGF-I levels in the treatment group were elevated at 12 and 24 h but not at 36 h compared to the placebo group. Mean GH levels in the placebo group did not vary significantly among the four sampling periods. In the treatment group, the mean serum GH rose to a supraphysiological peak at an average time of 4 h after injection. Subsequently, mean GH level was significantly suppressed compared to placebo on the second night following GH injection, but returned to normal by the third night.

CONCLUSION

After 6 months of a thrice weekly GH treatment regimen in non-GH deficient short children, endogenous GH secretion was reduced from 24 to 36 h after injection compared to placebo and returned to control levels by 48 to 60 h after injection.

Effect of chronic treatment with biosynthetic growth hormone (GH) on the GH response to double GH-releasing hormone administration in children with short stature

Growth hormone (GH) exerts a negative feedback on its own secretion through direct and indirect mechanisms. Normal children, unlike adults, display consecutive GH responses after repeated GH-releasing hormone (GHRH) administration. In this study, we investigated the effects of long-term GH administration on this peculiar secretory pattern. Eight children with severe short stature and impaired GH responses to suprapituitary provocative stimuli associated with normal GH responsiveness to GHRH, underwent, while on chronic treatment with biosynthetic GH and on the 10th day following drug withdrawal, a double bolus GHRH test (two GHRH injections of 1 microg/kg b.w. given as i.v. boluses two hours apart). During GH therapy the GH response to the first GHRH bolus appeared to be reduced, though not significantly so, compared to that observed at diagnosis; after treatment withdrawal, this response returned quite similar to pretreatment. Both during and after treatment, the GH response to the second GHRH bolus was comparable for magnitude to that evoked by the first application. In conclusion, even prolonged treatments with rhGH do not induce persisting alterations of the physiological mechanisms subserving GH regulation.

Three year IGF-I treatment of children with Laron syndrome

Nine prepubertal children with Laron syndrome (6 males, 3 females) aged 0.5 to 14.6 years were treated by daily subcutaneous injections of IGF-I in doses of 150-200 micrograms/kg. All patients completed at least one year of treatment; six completed two years and five three years. During the first year, a significant increase in linear growth velocity, from a mean +/- SD of 4.7 +/- 1.3 to 8.2 +/- 0.8 cm/yr (p < 0.0001), was registered. In the second year the growth velocity was lower, but still significantly higher than before treatment. Bone maturation advanced proportionally with chronological age. A reduction in subcutaneous fat tissue was observed despite the body weight increase. There was no aggravation of the characteristic hypoglycemic episodes; on the contrary, there was a better tolerance to fasting. Significant increases in serum alkaline phosphatase, phosphorus and procollagens were registered throughout the study. In conclusion, IGF-I provides an effective replacement treatment for IGF-I-deficient children, mimicking most effects ascribed to growth hormone.

Galanin infusion restores the blunted GH responses to GHRH administration during GH treatment in children with constitutional growth delay

GH responses to GHRH (in basal conditions), GHRH (after six months of recombinant GH therapy) and GHRH plus galanin administration (after GH therapy) were determined in six children with constitutional growth delay (CGD). Before treatment, GHRH administration caused a clear rise of GH levels (mean GH peak: 38.0 +/- 7.2 ng/ml); mean GH net incremental area under the curve/120 min (GH nAUC) was 2631.0 +/- 519.8 ng/ml/120 min. During GH treatment, both height velocity and IGF-I levels significantly increased from 4.3 +/- 0.1 cm/yr to 9.4 +/- 0.9 cm/yr (p < 0.001) and from 218.7 +/- 12.3 ng/ml to 328.0 +/- 28.2 ng/ml (p < 0.001), respectively. After 6 months of GH therapy, mean GH peak (21.7 +/- 2.2 ng/ml) and mean GH nAUC (847.7 +/- 109.8 ng/ml/120 min) after GHRH administration were significantly lower (p < 0.05) than in basal conditions. Galanin infusion (10 micrograms/kg/bw) was able to completely restore the reduced GH responsiveness to GHRH administration. In fact, mean GH peak and mean GH nAUC after GHRH plus galanin were similar to those recorded after basal GHRH administration (GH peak: 37.5 +/- 4.0 ng/ml; GH nAUC: 2279.8 +/- 430.5 ng/ml/120 min). In conclusion, this study shows that the reduction of pituitary responsiveness to GHRH administration during GH treatment is not dependent on the depletion of the promptly GH releasable pool, since somatotroph refractoriness to GHRH can be overriden by the concomitant administration of galanin.