Anna Skottner

Biological characterization of purified native 20-kDa human growth hormone

Because of the propensity of the 20-kDa variant of human growth hormone (GH) to aggregate with itself and with 22-kDa human GH, it has been difficult to prepare monomeric 20-kDa GH in highly purified form. This has been a major complicating factor in determining whether 20-kDa GH has a biological activity profile distinct from that of 22-kDa GH. In the present study, native 20-kDa GH was isolated from a human GH dimer concentrate and purified by a procedure that included column electrophoresis in agarose suspension as a final separation step. This procedure yielded highly purified monomeric 20-kDa GH, which was contaminated to an extent of less than 1% with 22-kDa GH, and which exhibited only a small degree of dimerization upon storage. The native 20-kDa GH was quite active in stimulating growth in hypophysectomized rats, when growth was assessed by body weight gain, longitudinal bone growth, the stimulation of sulfation of cartilage, and the elevation of serum IGF-1 level. However, in all of these growth assays, the 20-kDa GH was somewhat less active than the native 22-kDa GH to which it was compared; e.g., in the body weight gain and longitudinal bone growth assays, it had an estimated potency of 0.6 relative to the 22-kDa GH. The 20-kDa GH exhibited substantial diabetogenic activity when tested for the ability to raise fasting blood glucose concentration and to impair glucose tolerance in ob/ob mice. Also, the native 20-kDa GH had significant in vitro insulin-like activity, although its potency was approximately 20% that of the native 22-kDa GH to which it was compared. Thus, the biological activity profile of native 20-kDa GH differs from that of 22-kDa GH primarily in that insulin-like activity is markedly attenuated.

Differential Effects of IGF-I and Insulin on Glucoregulation and Fat Metabolism in Depancreatized Dogs

The effects of equipotent glucose-lowering doses of insulinlike growth factor I (IGF-I) and insulin on tracer-determined glucose kinetics and several metabolites were compared in 14 experiments (7 in each group) in fasted, totally depancreatized dogs. This model prevented variations in insulin secretion induced by IGF-I and permitted evaluation of the effects of IGF-I on extrapancreatic glucagon. Steady-state moderate hyperglycemia (9.9 ± 0.2 mM) was maintained by a subbasal intraportal infusion of insulin (1.29 ± 0.17 pmol · kg−1 · min−1). This was continued throughout the experiment, allowing evaluation of IGF-I effects on insulin clearance. Human recombinant IGF-I or insulin was given intravenously as a primed infusion for 90 min, followed by a 50-min recovery period. The dose of IGF-I was a 2.6-nmol/kg bolus plus 57.4 pmol · kg−1 · min−1. The insulin dose required to induce the same plasma glucose decline as IGF-I (44 ± 6 vs. 43 ± 5%, NS) was 9–12 times lower (0.06-nmol/kg bolus + 6.4 ± 0.6 pmol · kg−1 · min−1). However, the mechanism of this decline differed with IGF-I and insulin; glucose production was much less suppressed (25 ± 9 vs. 42 ± 11%, P < 0.001) and glucose utilization was more stimulated (68 ± 18 vs. 38 ± 19%, P < 0.05) with IGF-I. Lactate and pyruvate increased significantly with IGF-I (by 85 ± 28 and 123 ± 83%, respectively) but not with insulin. Glycerol and free-fatty acid levels decreased much less with IGF-I than insulin (29 ± 16 vs. 52 ± 5%, P < 0.05, and 36 ± 10 vs. 56 ± 8%, P < 0.01). β-Hydroxybutyrate, alanine, and glucagon decreased similarly with IGF-I or insulin by 60, 20, and 20%, respectively. Plasma insulin was not affected by IGF-I administration (92.6 ± 8.5 vs. 92.6 ± 10.1 pM). In conclusion, in insulin-infused depancreatized dogs, 1) IGF-I is 8–11% as potent as insulin as a glucose-lowering agent on a molar basis, 2) IGF-I does not affect insulin clearance at doses effective in lowering plasma glucose, and 3) equipotent glucose-lowering doses of IGF-I and insulin have very different effects on both glucose kinetics and lipolysis. This suggests that the IGF-I-insulin potency ratio in vivo is higher in muscle than liver or adipose tissue. Therefore, insulinlike effects of supraphysiological doses of IGF-I can be mediated not only through the insulin but also the IGF-I receptor.

Serum Somatomedin a and Non-Dialyzable Urinary Hydroxyproline in Girls with Idiopathic Scoliosis

Serum somatomedin A was determined by radioreceptor assay in 31 girls with idiopathic scoliosis and 30 controls, with ages ranging from 9.7-16.2 years. In the material as a whole no significant difference could be found between scoliotics and controls. Similarly no differences were found in the girls under 13 years of age. However, in girls older than 13 years the controls had significantly higher somatomedin A levels than the scoliotics. In the second part of the study 26 girls with idiopathic scoliosis and 22 controls were examined with regard to the urinary excretion of non-dialyzable hydroxyproline,. which has been suggested to be correlated with collagen synthesis. The percentage non-dialyzable fraction of the total hydroxyproline was found to be significantly higher in the control girls. The mean age was 12.8 years for both groups. The results suggest a higher growth rate in the controls than in the scoliotics for girls over 12 years. There is, however, no definite evidence as to the function of the somatomedins or of the relationship between non-dialyzable hydroxyproline and growth.