Effects of growth hormone releasing hormone on insulin action and insulin secretion in a hypopituitary patient evaluated by the clamp technique

Growth hormone treatment in adolescent males with idiopathic short stature: changes in body composition, protein, fat, and glucose metabolism

CONTEXT

Cross-sectional observations show an inverse relationship between pubertal increase in GH and insulin sensitivity, suggesting that pubertal insulin resistance may be mediated by GH.

OBJECTIVE

Our objective was to assess longitudinally the effects of short-term GH supplementation in adolescent males with non-GH-deficient idiopathic short stature (ISS) on body composition, substrate metabolism, and insulin sensitivity. Children with ISS were studied to simulate the pubertal increase in GH secretion.

PARTICIPANTS AND SETTING

Eight males with ISS (10.8-16.5 yr) were recruited from pediatric endocrinology clinics at an academic medical center.

STUDY DESIGN

Participants were evaluated in the General Clinical Research Center before and after 4 months of GH supplementation (0.3 mg/kg.wk). Body composition was assessed with dual-energy x-ray absorptiometry. Whole-body glucose, protein, and fat turnover were measured using stable isotopes. In vivo insulin action was assessed during a 3-h hyperinsulinemic (40 mU/m(2).min) euglycemic clamp.

RESULTS

GH supplementation led to 1) increase in hepatic glucose production and fasting insulin levels, 2) increase in lean body mass and decrease in fat mass, and 3) improvement in cardiovascular lipid risk profile. Plasma IGF-I levels correlated positively with insulin levels.

CONCLUSIONS

Four months of GH supplementation in adolescent males with ISS is associated with significant body composition changes and hepatic insulin resistance.

Longitudinal study of physiologic insulin resistance and metabolic changes of puberty

Cross-sectional studies have shown that 1) adolescents are insulin resistant compared with prepubertal children and adults, 2) pubertal insulin resistance is likely mediated by growth hormone (GH), and 3) pubertal insulin resistance is associated with increased fat oxidation and decreased glucose oxidation. The aim of this study was to assess the validity of these cross-sectional observations by performing a longitudinal study in normal children during the prepubertal and pubertal periods. Nine healthy, normal weight, prepubertal children underwent hyperinsulinemic-euglycemic and hyperglycemic clamp studies for evaluation of insulin sensitivity and insulin secretion. Children had repeat evaluations during puberty. Consistent with cross-sectional observations, this longitudinal study demonstrated that during puberty: 1) insulin sensitivity decreased by approximately 50%, 2) the decrease in insulin sensitivity was compensated by a doubling in insulin secretion, and 3) the decrease in insulin sensitivity was independent of changes in percentage of body fat. Puberty was associated with increased total body lipolysis and decreased glucose oxidation. A novel observation is the demonstration of approximately 50% decrease in adiponectin levels at the pubertal time point. These metabolic changes are proposed to be partially mediated by increased GH secretion and are consistent with the Randle cycle of competition between glucose and fat oxidation.