Quantitation of Urinary Somatomedin-C in Children With Normal and Abnormal Growth

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Abstract
The renal excretion of radioimmunoassayable somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) was measured in 12-h overnight urine samples obtained from 88 subjects, aged 3-19 yr. The participants included 34 healthy children (group 1), 29 children with idiopathic growth failure and normal GH stimulation tests (group 2), and 25 GH-deficient subjects (group 3). The mean (.+-. SEM) urinary Sm-C/IGF-I excretion in group 1 (28.4 .+-. 2.1 mU/kg) was significantly greater than that in group 2 (8.1 .+-. 1.6 mU/kg) or group 3 (8.6 .+-. 1.3 mU/kg). Twenty-two of the 29 subjects in group 2 had urinary Sm-C/IGF-I values less than 8 mU/kg. After the administration of biosynthetic GH to 12 GH-deficient subjects, urinary Sm-C/IGF-I excretion rose from 10.3 .+-. 2.3 to 21.4 .+-. 4.2 mU/kg within 12 h (P < 0.05), indicating that renal excretion of Sm-C/IGF-I is GH dependent. One woman with acromegaly had markedly elevated urinary Sm-C/IGF-I excretion (420 mU/kg). The authenticity of urinary Sm-C/IGF-I was confirmed by high pressure liquid chromatography (HPLC). Assay of serial dilutions of urinary Sm-C/IGF-I demonstrated a direct proportionality between concentration and dilution. Although it is not possible to identify whether urinary Sm-C/IGF-I reflects local or generalized systhesis of the peptide, we hypothesize that qualitation of Sm-C/IGF-I in timed urine collections will yield additional information about GH production and action in children with normal and abnormal growth.