Quantification of urinary insulin-like growth factors (IGFs) and IGF binding protein 3 in healthy volunteers before and after stimulation with recombinant human growth hormone

Abstract

Tönshoff B, Blum WF, Vickers M, Kurilenko S, Mehls 0, Ritz E. Quantification of urinary insulin-like growth factors (IGFs) and IGF binding protein 3 in healthy volunteers before and after stimulation with recombinant human growth hormone. Eur J Endocrinol 1995;132:433–7. ISSN 0804–4643 We examined excretion of urinary insulin-like growth factors I and II (IGF-I and IGF-II) and their major binding protein IGFBP-3 in comparison to their respective serum concentration in nine healthy female volunteers (median age 25 years, range 22–27) under baseline conditions and after stimulation with recombinant human growth hormone (rhGH), 4.5 IU twice daily subcutaneously for a period of 3 days. The IGFs were measured in unconcentrated urine by use of recently developed, highly sensitive radioimmunoassays. The IGFBP-3 was measured by a specific radioimmunoassay. The mean (±sd) urinary concentrations of IGF-I (0.08 ± 0.07 μg/l), IGF-II (1.02 ± 0.47 μg/l) and IGFBP-3 (19.1 ± 6.9 μg/l) were two to three orders of magnitude lower than in serum. The ratio of IGF-II over IGF-I concentration in urine (13:1) was five times higher than in serum (2.5:1), and the ratio of IGFBP-3 over the sum of IGF-I and IGF-II in urine (17:1) was four times higher than in serum (4:1). Urinary excretion was 63.3 ± 46.6 ng·m−2 · 24 h−1 for IGF-I, 1002 ± 598 ng·m−2 · 24 h−1 for IGFII and 18039 ± 4983 ng·m−2·24 h−1 for IGFBP-3. Using fast protein liquid exclusion chromatography, only immunoreactive IGFBP-3 components of less than 60 kD were detected in urine, with a major peak at 20kD. Urinary IGFBP-3 excretion correlated with serum IGFBP-3 (r = 0.61, p < 0.01) and the glomerular filtration rate (r = 0.56, p < 0.05) measured by steady-state inulin infusion clearances. Administration of rhGH stimulated significantly (p < 0.005) the serum IGF-I concentration by 50%, but not the urinary IGF-I excretion. In conclusion: the considerably higher ratio of IGF-II to IGF-I in urine compared to serum indicates that urinary IGF excretion does not represent only filtered IGFs, urinary IGF-I is a less sensitive indicator of GH activity than serum IGF-I, and as urinary IGFBP-3 excretion is in proportion to the glomerular filtration rate and serum IGFBP-3, it presumably reflects renal filtration of small immunoreactive IGFBP-3 fragments from the circulation. Burkhard Tönshoff, University Children's Hospital, Im Neuenheimer Feld 150, 69120 Heidelberg, Germany