Measurement of Insulin-Like Growth Factor II by a Specific Radioreceptor Assay in Serum of Normal Individuals, Patients with Abnormal Growth Hormone Secretion, and Patients with Tumor-Associated Hypoglycemia*
A radioreceptor assay for insulin-like growth factor II (IGF-II) is described in which [125I]IGF-II is bound by rat placental membranes. IGF-I and other insulin-related peptides have less than 1% of the potency of IGF-II in displacing [125I]-IGF-II. IGF-II is extracted from its serum binding protein with a simple acid-ethanol step. After neutralization with Tris base, the extract is introduced directly into the radioreceptor assay. Results are expressed in terms of a reference serum with an assigned potency of 1 U⁄ml. By this method, serum from normal adults, 20–69 yr of age, had a mean (±SE) IGF-II activity of 0.73 ± 0.03; higher concentrations were noted in adults more than 70 years of age (1.05 ± 0.05), in cord serum (1.55 ± 0.24), and in short children with normal GH secretion (0.88 ± 0.42). In hypopituitary dwarfism, serum IGF-II activity was reduced (0.50 ± 0.05), but in acromegaly, it was not increased (0.77 ± 0.06). In 10 of 14 serum samples from patients with tumor-related hypoglycemia, the IGF-II exceeded the normal 97.5% confidence limits. In 8 of these sera, IGF-I by RIA was low, and in 5, it was essentially unmeasurable. These results with a new radioreceptor assay for IGF-II provide additional evidence that the regulation of this serum peptide differs from that of IGF-I.